Artist's impression of the dwarf planet Haumea and its moons, Hi'aka and Namaka. Credit: NASA
The Trans-Neptunian region has become a veritable treasure trove of discoveries in recent years. Since 2003, the dwarf planets and “plutoids” of Eris, Sedna, Makemake, Quaoar, and Orcus were all observed beyond the orbit of Pluto. And in between all of these, Haumea – that odd, oblong-shaped dwarf planet that has its own system of moons – was also discovered.
In addition to being the largest member of its particular family of Trans-Neptunian Objects (TNOs), Haumea is unique amongst known dwarf planets. This is due to its elongation, an unusually rapid rotation, two known moons, high density, and high albedo – all of which make Haumea something of an oddity when it comes to dwarf planets.
Discovery and Naming:
While bodies that are designated as dwarf planets tend to attract their share of controversy, dissension over Haumea began as soon as it was discovered. In fact, two teams claim credit for its discovery: Mike Brown and his team at Caltech and Jose Luis Ortiz Moreno and his team from the Instituto de Astrofísica de Andalucía at Sierra Nevada Observatory in Spain.
The former discovered Haumea in December of 2004 from images they had taken on May 6th, 2004 from the W.M. Keck Observatory. They published an online abstract about their discovery on July 20th, 2005, and announced their discovery at a conference in September of that year. Meanwhile, Ortiz and his team emailed the IAU Minor Planet Center of the discovery of Haumea on July 27th, 2005, claiming they had found it on images taken from March 7th to 10th, 2003.
Keck image of 2003 EL61 Haumea, with moons Hi’iaka and Naumaka. Credit: CalTech/Mike Brown et al.
The IAU announcement on September 17th, 2008, that Haumea had been accepted as a dwarf planet, did not mention a discoverer. The location of discovery was listed as the Sierra Nevada Observatory of the Spanish team, but the chosen name, Haumea, was proposed by the Caltech team.
The name Haumea comes from Hawaiian mythology, specifically from the goddess of fertility who is also the matron goddess of the island of Hawaii where the W. M. Keck Observatory is located. Hence, the name was not only consistent with IAU guidelines – that classical Kuiper Belt Objects (KBOs) be given names of mythological beings associated with creation – but was also an homage to the facility that made the discovery.
Ortiz’s team had proposed “Ataecina”, named for the ancient Iberian goddess of Spring; but not meet the IAU requirements since she is not a creation goddess, and hence was rejected. Until it was given a permanent name, the Caltech discovery team used the nickname “Santa” among themselves, because they had discovered Haumea on December 28th, 2004, just after Christmas.
Because the Spanish team had filed their claim with the Minor Planet Center first, Haumea was given the provisional designation 2003 EL61 (based on the date of the Spanish discovery image) on July 29th, 2005.
Size, Mass and Orbit:
Calculating Haumeau’s size, mass and density is somewhat complicated. Whereas it is large enough and bright enough for its albedo (and thus its size) to be measured, the calculations of its dimensions are made difficult by its rapid rotation. However, several ellipsoid-model calculations have been conducted using the Keck telescopes, the Spitzer Space Telescope, and the Herschel Space Telescope that have provided estimates.
The first calculations, conducted by Brown et al., provided the approximate dimensions of 2,000 x 1,500 x 1,000 km. Meanwhile, the Spitzer measurements gave it a diameter of 1050 – 1400 km, while subsequent light-curve analyses suggested an equivalent circular diameter of 1,450 km. In 2010 an analysis of measurements taken by Herschel Space Telescope together with the older Spitzer Telescope measurements yielded a new estimate of ~1300 km.
These independent size estimates overlap at an average geometric mean diameter of roughly 1,400 km. In essence, this means that Haumea is comparable in diameter to Pluto along its longest axis and about half that at its poles. It’s mass, meanwhile, is estimated to be approximately 4.0 ×1021 kg – one-third the mass of Pluto and 1/1400th that of Earth.
This makes Haumea one of the largest trans-Neptunian objects discovered, smaller than Eris, Pluto, probably Makemake, and possibly 2007 OR10, but larger than Sedna, Quaoar, and Orcus. Combined with estimates of its density, Haumea is massive enough to have achieved hydrostatic equilibrium. Although Haumea appears to be far from spherical, its ellipsoidal shape is thought to result from its rapid rotation.
Haumea has a typical orbit for a classical KBO, with an eccentric orbit that takes it from 34.952 AU (5.23 billion km) at perihelion to 51.483 AU (7.7 billion km) at aphelion. Also consistent with other KBOs, it has an orbital period of 284 Earth years, an orbital inclination of 28°, and completes a sidereal rotation every 3.9 hours (0.163 Earth days).
Composition:
Much like its size, Haumea’s rotation and the amplitude of its light curve make judging its composition rather difficult. If its density were consistent with Pluto and other KBOs (2.0 g/cm³) then its rapid rotation would have elongated it to a greater extent than current estimates allow for. As such, Haumea’s density is believed to range between 2.6 – 3.3 g/cm³, which is comparable to Earth’s Moon (also 3.3 g/cm³).
Haumea’s possible density covers the values for silicate minerals such as olivine and pyroxene, which make up many of the rocky objects in the Solar System. This suggests that the bulk of Haumea is rock covered with a relatively thin layer of ice. It is possible that a thicker ice mantle that is more typical of Kuiper belt objects existed in the past, but was blasted off during the impact that formed the Haumean collisional family.
Haumea is as bright as snow, with an high albedo that is consistent with crystalline ice. Spectral modelling of the surface suggested that 66% to 80% of the Haumean surface appears to be pure crystalline water ice, with the possible presence of hydrogen cyanide or phyllosilicate clays. Inorganic cyanide salts such as copper potassium cyanide may also be present.
A large dark red area on Haumea’s bright white surface, possibly an impact feature, has also been observed which could indicate an area rich in minerals and organic (carbon-rich) compounds – or possibly a higher proportion of crystalline ice. Thus Haumea may have a mottled surface similar to that of Pluto.
Classification:
Haumea has been classified as a plutoid and dwarf planet residing beyond Neptune’s orbit. This classification means that it is presumed to be massive enough to have been rounded by its own gravity, but not to have cleared its neighborhood of similar objects.
Although Haumea appears to be far from spherical, its ellipsoidal shape is thought to result from its rapid rotation and not from a lack of sufficient gravity to overcome the compressive strength of its material. Haumea was initially listed as a classical Kuiper Belt Object in 2006 by the Minor Planet Center, but that has since been revised.
Moons:
Haumea has two known moons, which are named after the daughters of the Hawaiian goddess – Hi’iaka and Namaka. Both were discovered in 2005 by Brown’s team while conducting observations of Haumea at the W.M. Keck Observatory. Hi’iaka, which was initially nicknamed “Rudolph” by the Caltech team, was discovered January 26th, 2005.
It is the outer and – at roughly 310 km in diameter – the larger and brighter of the two, and orbits Haumea in a nearly circular path every 49 days. Infrared observations indicate that its surface is almost entirely covered by pure crystalline water ice. Because of this, Brown and his team have speculated that the moon is a fragment of Haumea that broke off during a collision.
Comparison of Sedna with the other largest TNOs and with Earth (all to scale). Credit: NASA/Lexicon
Namaka, the smaller and innermost of the two, was discovered on June 30th, 2005, and nicknamed “Blitzen”. It is a tenth the mass of Hi‘iaka and orbits Haumea in 18 days in a highly elliptical orbit. Both moons circle Haumea is highly eccentric orbits. No estimates have been made yet as to their mass.
Exploration:
So far, no missions have been mounted to Haumea and none are currently planned. However, numerous scenarios have been calculated using hypothetical launch dates. For example, if a probe were launched on September 25th, 2025, a flyby mission could take place within 14.25 years, when Haumea would be 48.18 AU from the Sun. Based on a launch date of Nov. 1st, 2026, September 23rd, 2037, and October 29th, 2038, a flyby mission would take 16.45 years to get to Haumea.
So if the budget environment remains stable and scientists decide to make close-up observations of Haumea a priority, a flyby could be taking place no sooner than December of 2039. And with luck, we might learn more about this distant and odd little ball of rock and ice that stands out from its peers.
Why is landing on a comet so difficult and what does this tell us about future missions to comets and asteroids?
Us nerds were riveted by the coverage of the ESA’s Rosetta mission and its arrival at Comet 67/P in 2014. One such nerd is Paco Juarez, friend of the show and patron. He wanted to know why is it so darned hard to land on a comet?
In 2014, the tiny Philae Lander detached from the spacecraft and slowly descended down to the surface of the comet. If everything went well, it would have gracefully touched down and then sent back a pile of information about this filthy roving snowball.
As you know, the landing didn’t go according to plan. Instead of gently touching down on 67/P, Philae bounced off the surface of the comet like a tennis ball dropped from a tower, and rose a kilometer off the surface. Then more descending, and more bouncing, finally settling down on rugged terrain, surrounded by crevices and large boulders. At that point, engineers lost contact with the lander, and so much science went undone.
If I recorded this video a few months ago, that would have been the end of the story. You know how this goes, space exploration is hard and dangerous, don’t be surprised when your missions fail and space unfeelingly smashes up your pretty little robot probes with their little gold foil 27 pieces of flair.
Rosetta
Fortunately, I’m able to report that ESA regained contact with the Philae lander on June 13, 2015, resuming its mission, and scientific operations.
But why is landing on a comet so difficult and what does this tell us about future robotic and human missions to smaller comets and asteroids? When ESA engineers designed Philae, they knew it was going to be very difficult to land on a comet like 67/P because they have a such a low gravity. And they have low gravity because they’re little.
Illustration of the Rosetta Missions Philae lander on final approach to a comet surface. (Photo: ESA)
On Earth, 6 septillion tonnes of rock and metal give you an escape velocity of 11.2 km/s. That’s how fast you need to be able to jump in order to leave the planet entirely. But the escape velocity of 67/P is only 1 m/s. You could trip off the comet and never return. Whilst small children threw rocks at you from the surface as you drifted away.
Philae was built with harpoon drills in its landing struts. The moment the lander touched the surface of the comet, those harpoons were supposed to fire, securing the lander. The surface of the comet was softer than scientists had anticipated, and the harpoons didn’t fire. Or possibly they were broken and couldn’t fire. Space is hard. Whatever the case, without being able to grab onto the surface, it used the comet as a bouncy castle.
We’re learning what it takes to land on lower mass objects like comets and asteroids. NASA’s OSIRIS-REx mission will visit Comet Bennu, and send a lander down to the surface of the asteroid. From there it’ll pick up a few samples, and return them back to Earth. It’ll be Philae, all over again.
An artist concept of the Philae lander on comet 67P/Churyumov-Gerasimenko. Credit: Astrium – E. Viktor/ESA
In the future, we’re told, humans will be visiting asteroids to study them for science and their potential for ice and minerals. You can imagine it’ll be a harrowing descent, but even just walking around on the surface will be dangerous when every step could throw an astronaut into an escape trajectory. They’ll need to learn lessons from rock climbers and Rorschach.
As we learned with Philae, landings on low mass objects is really tough. We’re going to need to get more practice and develop new techniques and technologies before we’re ready to add asteroid mining to our list of “stuff we just do, NBD”.
What are some unusual worlds you’d like humanity to visit? Put your suggestions in the comments below.
A view of Ceres in natural colour, pictured by the Dawn spacecraft in May 2015. Credit: NASA/ JPL/Planetary Society/Justin Cowart
The Asteroid Belt is a pretty interesting place. In addition to containing between 2.8 and 3.2 quintillion metric tons of matter, the region is also home to many minor planets. The largest of these, known as Ceres, is not only the largest minor planet in the Inner Solar System, but also the only body in this region to be designated as a “dwarf planet” by the International Astronomical Union (IAU).
Due to its size and shape, when it was first observed, Ceres was thought to be a planet. While this belief has since been revised, Ceres is alone amongst objects in the Asteroid Belt in that it is the only object massive enough to have become spherical in shape. And like most of the dwarf planets in our Solar System, its status remains controversial, and our knowledge of it limited.
Discovery and Naming:
Ceres was discovered by Giuseppe Piazzi on January 1st, 1801, while searching for zodiacal stars. However, the existence of Ceres had been predicted decades before by Johann Elert Bode, a German astronomer who speculated that there had to be a planet between Mars and Jupiter. The basis for this assumption was the now defunct Bode-Titus law, which was first proposed by Johann Daniel Titius in 1766.
This law stated that there existed a regular pattern in the semi-major axes of the orbits of known planets, the only exception of which was the large gap between Mars and Jupiter. In an attempt to resolve this, in 1800, German astronomer Franz Xaver von Zach sent requests to twenty-four experienced astronomers (dubbed the “Celestial Police”) to combine their their efforts to located this missing planet.
Comparison of HST and Dawn FC images of Ceres taken nearly 11 years apart. Credit: NASA.
One of these astronomers was Giuseppe Piazzi at the Academy of Palermo, who had made the discovery shortly before his invitation to join the group had arrived. At the time of his discovery, he mistook it for a comet, but subsequent observations led him to declare that it could be something more. He officially shared his observations with friends and colleagues by April of 1801, and sent the information to von Zach to be published in September.
Unfortunately, due to its change in its apparent position, Ceres was too close to the Sun’s glare to be visible to astronomers. It would not be until the end of the year that it would be spotted again, thanks in large part to German astronomer Carl Freidrich Gauss and the predictions he made of its orbit. On December 31st, von Zach and his colleague Heinrich W.M. Olbers found Ceres near the position predicted by Gauss, and thus recovered it.
Piazzi originally suggesting naming his discovery Cerere Ferdinandea, after the Roman goddess of agriculture Ceres (Cerere in Italian) and King Ferdinand of Sicily. The name Ferdinand was dropped in other nations, but Ceres was eventually retained. Ceres was also called Hera for a short time in Germany; whereas in Greece, it is still called Demeter after the Greek equivalent of the Roman goddess Ceres.
Classification:
The classification of Ceres has changed more than once since its discovery, and remains the subject of controversy. For example, Johann Elert Bode – a contemporary of Piazzi – believed Ceres to be the “missing planet” he had proposed to exist between Mars and Jupiter. Ceres was assigned a planetary symbol, and remained listed as a planet in astronomy books and tables (along with 2 Pallas, 3 Juno, and 4 Vesta) until the mid-19th century.
Ceres compared to asteroids visited to date, including Vesta, Dawn’s mapping target in 2011. Credit: NASA/ESA/Paul Schenck.
As other objects were discovered in the neighborhood of Ceres, it was realized that Ceres represented the first of a new class of objects. In 1802, with the discovery of 2 Pallas, William Herschel coined the term asteroid (“star-like”) for these bodies. As the first such body to be discovered, Ceres was given the designation 1 Ceres under the modern system of minor-planet designations.
By the 1860s, the existence of a fundamental difference between asteroids such as Ceres and the major planets was widely accepted, though a precise definition of “planet” was never formulated. The 2006 debate surrounding Eris, Pluto, and what constitutes a planet led to Ceres being considered for reclassification as a planet.
The definition that was adopted on August 24th, 2006 carried the requirements that a planet have sufficient mass to assume hydrostatic equilibrium, be in orbit around a star and not be a satellite, and have cleared the neighborhood around its orbit. As it is, Ceres does not dominate its orbit, but shares it with the thousands of other asteroids, and constitutes only about a third of the mass of the Asteroid Belt. Bodies like Ceres that met some of these qualification, but not all, were instead classified as “dwarf planets”.
In addition to the controversy surrounding the use of this term, there is also the question of whether or not Ceres status as a dwarf planet means that it can no longer be considered an asteroid. The 2006 IAU decision never addressed whether Ceres is an asteroid or not. In fact, the IAU has never defined the word ‘asteroid’ at all, having preferred the term ‘minor planet’ until 2006, and the terms ‘small Solar System body’ and ‘dwarf planet’ thereafter.
Size, Mass and Orbit:
Early observations of Ceres were only able to calculate its size to within an order of magnitude. In 1802, English astronomer William Herschel underestimated its diameter as 260 km, whereas in 1811 Johann Hieronymus Schröter overestimated it as 2,613 km. Current estimates place its mean radius at 473 km, and its mass at roughly 9.39 × 1020 kg (the equivalent of 0.00015 Earths or 0.0128 Moons).
Size comparison of Vesta, Eros and Ceres. Credit: NASA/JPL
With this mass, Ceres comprises approximately a third of the estimated total mass of the asteroid belt (which is in turn approximately 4% of the mass of the Moon). The next largest objects are Vesta, Pallas and Hygiea, which have mean diameters of more than 400 km and masses of 2.6 x 1020 kg, 2.11 x 1020 kg, and 8.6 ×1019 kg respectively. The mass of Ceres is large enough to give it a nearly spherical shape, which makes it unique amongst objects and minor planets in the Asteroid Belt.
Ceres follows a slightly inclined and moderately eccentric orbit, ranging from 2.5577 AU (382.6 million km) from the Sun at perihelion and 2.9773 AU (445.4 million km) at aphelion. It has an orbital period of 1,680 Earth days (4.6 years) and takes 0.3781 Earth days (9 hours and 4 minutes) to complete a sidereal rotation.
Composition and Atmosphere:
Based on its size and density (2.16 g/cm³), Ceres is believed to be differentiated between a rocky core and an icy mantle. Based on evidence provided by the Keck telescope in 2002, the mantle is estimated to be 100 km-thick, and contains up to 200 million cubic km of water – which is more fresh water than exists on Earth. Infrared data on the surface also suggests that Ceres may have an ocean beneath its icy mantle.
If true, it is possible that this ocean could harbor microbial extraterrestrial life, similar to what has been proposed about Mars, Titan, Europa and Enceladus. It has further been hypothesized that ejecta from Ceres could have sent microbes to Earth in the past.
Other possible surface constituents include iron-rich clay minerals (cronstedtite) and carbonate minerals (dolomite and siderite), which are common minerals in carbonaceous chondrite meteorites. The surface of Ceres is relatively warm, with the maximum temperature estimated to reach approximately 235 K (-38 °C, -36 °F) when the Sun is overhead.
Assuming the presence of sufficient antifreeze (such as ammonia), the water ice would become unstable at this temperature. Therefore, it is possible that Ceres may have a tenuous atmosphere caused by outgassing from water ice on the surface. The detection of significant amounts of hydroxide ions near Ceres’ north pole, which is a product of water vapor dissociation by ultraviolet solar radiation, is another indication of this.
However, it was not until early 2014 that several localized mid-latitude sources of water vapor were detected on Ceres. Possible mechanisms for the vapor release include sublimation from exposed surface ice (as with comets), cryovolcanic eruptions resulting from internal heat, and subsurface pressurization. The limited amount of data suggests that the vaporization is more consistent with cometary-style sublimation.
Origin:
Multiple theories exist as to the origin of Ceres. On the one hand, it is widely believed that Ceres is a surviving protoplanet which formed 4.57 billion year ago in the Asteroid Belt. Unlike other inner Solar System protoplanets, Ceres neither merged with others to form a terrestrial planet and avoided being ejected from the Solar System by Jupiter. However, there is an alternate theory that proposes that Ceres formed in the Kuiper belt and later migrated to the asteroid belt.
The geological evolution of Ceres is dependent on the heat sources that were available during and after its formation, which would have been provided by friction from planetesimal accretion and decay of various radionuclides. These are thought to have been sufficient to allow Ceres to differentiate into a rocky core and icy mantle soon after its formation. This icy surface would have gradually sublimated, leaving behind various hydrated minerals like clay minerals and carbonates.
Today, Ceres appears to be a geologically inactive body, with a surface sculpted only by impacts. The presence of significant amounts of water ice in its composition is what has led scientists to the possible conclusion that Ceres has or had a layer of liquid water in its interior.
Exploration:
Until recently, very few direct observations had been made of Ceres and little was known about its surface features. In 1995, the Hubble Space Telescope captured high-resolutions images that showed a dark spot in the surface that was thought to be a crater – and nicknamed “Piazzi” after its founder.
The near-infrared images taken by the Keck telescope in 2002 showed several bright and dark features moving with Ceres’s rotation. Two of the dark features had circular shapes and were presumed to be craters. One was identified as the “Piazzi” feature, while the other was observed to have a bright central region. In 2003 and 2004, visible-light images were taken by Hubble during a full rotation that showed 11 recognizable surface features, the natures of which are yet undetermined.
With the launch of the Dawn mission, with which NASA intends to conduct a nearly decade-long study of Ceres and Vesta, much more has been learned about this dwarf planet. For instance, after achieving orbit around the asteroid in March of 2015, Dawn revealed a large number of surface craters with low relief, indicating that they mark a relatively soft surface, most likely made of water ice.
Several bright spots have also been observed by Dawn, the brightest of which (“Spot 5”) is located in the middle of an 80 km (50 mi) crater called Occator. These bright features have an albedo of approximately 40% that are caused by a substance on the surface, possibly ice or salts, reflecting sunlight. A haze periodically appears above Spot 5, supporting the hypothesis that some sort of outgassing or sublimating ice formed the bright spots.
The Dawn spacecraft also noted the presence of a towering 6 kilometer-tall mountain (4 miles or 20,000 feet) in early August, 2015. This mountain, which is roughly pyramidal in shape and protrudes above otherwise smooth terrain, appears to be the only mountain of its kind on Ceres.
Like so many celestial bodies in our Solar System, Ceres is a mystery that scientists and astronomers are working to slowly unravel. In time, our exploration of this world will likely teach us much about the history and evolution of our Solar System, and may even lead to the discovery of life beyond Earth.
We have many interesting articles on Ceres here at Universe Today. For example, here are some articles on the many bright spots captured by the Dawn probe, and what they likely are.
The layout of the solar system, including the Oort Cloud, on a logarithmic scale. Credit: NASA
For thousands of years, astronomers have watched comets travel close to Earth and light up the night sky. In time, these observations led to a number of paradoxes. For instance, where were these comets all coming from? And if their surface material vaporizes as they approach the Sun (thus forming their famous halos), they must formed farther away, where they would have existed there for most of their lifespans.
In time, these observations led to the theory that far beyond the Sun and planets, there exists a large cloud of icy material and rock where most of these comets come from. This existence of this cloud, which is known as the Oort Cloud (after its principal theoretical founder), remains unproven. But from the many short and long-period comets that are believed to have come from there, astronomers have learned a great deal about it structure and composition.
Definition:
The Oort Cloud is a theoretical spherical cloud of predominantly icy planetesimals that is believed to surround the Sun at a distance of up to around 100,000 AU (2 ly). This places it in interstellar space, beyond the Sun’s Heliosphere where it defines the cosmological boundary between the Solar System and the region of the Sun’s gravitational dominance.
Like the Kuiper Belt and the Scattered Disc, the Oort Cloud is a reservoir of trans-Neptunian objects, though it is over a thousands times more distant from our Sun as these other two. The idea of a cloud of icy infinitesimals was first proposed in 1932 by Estonian astronomer Ernst Öpik, who postulated that long-period comets originated in an orbiting cloud at the outermost edge of the Solar System.
In 1950, the concept was resurrected by Jan Oort, who independently hypothesized its existence to explain the behavior of long-term comets. Although it has not yet been proven through direct observation, the existence of the Oort Cloud is widely accepted in the scientific community.
Structure and Composition:
The Oort Cloud is thought to extend from between 2,000 and 5,000 AU (0.03 and 0.08 ly) to as far as 50,000 AU (0.79 ly) from the Sun, though some estimates place the outer edge as far as 100,000 and 200,000 AU (1.58 and 3.16 ly). The Cloud is thought to be comprised of two regions – a spherical outer Oort Cloud of 20,000 – 50,000 AU (0.32 – 0.79 ly), and disc-shaped inner Oort (or Hills) Cloud of 2,000 – 20,000 AU (0.03 – 0.32 ly).
The outer Oort cloud may have trillions of objects larger than 1 km (0.62 mi), and billions that measure 20 kilometers (12 mi) in diameter. Its total mass is not known, but – assuming that Halley’s Comet is a typical representation of outer Oort Cloud objects – it has the combined mass of roughly 3×1025 kilograms (6.6×1025 pounds), or five Earths.
Based on the analyses of past comets, the vast majority of Oort Cloud objects are composed of icy volatiles – such as water, methane, ethane, carbon monoxide, hydrogen cyanide, and ammonia. The appearance of asteroids thought to be originating from the Oort Cloud has also prompted theoretical research that suggests that the population consists of 1-2% asteroids.
Earlier estimates placed its mass up to 380 Earth masses, but improved knowledge of the size distribution of long-period comets has led to lower estimates. The mass of the inner Oort Cloud, meanwhile, has yet to be characterized. The contents of both Kuiper Belt and the Oort Cloud are known as Trans-Neptunian Objects (TNOs), because the objects of both regions have orbits that that are further from the Sun than Neptune’s orbit.
A belt of comets called the Oort Cloud is theorized to encircle the Solar system (image credit: NASA/JPL).
Origin:
The Oort cloud is thought to be a remnant of the original protoplanetary disc that formed around the Sun approximately 4.6 billion years ago. The most widely accepted hypothesis is that the Oort cloud’s objects initially coalesced much closer to the Sun as part of the same process that formed the planets and minor planets, but that gravitational interaction with young gas giants such as Jupiter ejected them into extremely long elliptic or parabolic orbits.
Recent research by NASA suggests that a large number of Oort cloud objects are the product of an exchange of materials between the Sun and its sibling stars as they formed and drifted apart. It is also suggested that many – possibly the majority – of Oort cloud objects were not formed in close proximity to the Sun.
Alessandro Morbidelli of the Observatoire de la Cote d’Azur has conducted simulations on the evolution of the Oort cloud from the beginnings of the Solar System to the present. These simulations indicate that gravitational interaction with nearby stars and galactic tides modified cometary orbits to make them more circular. This is offered as an explanation for why the outer Oort Cloud is nearly spherical in shape while the Hills cloud, which is bound more strongly to the Sun, has not acquired a spherical shape.
A comparison of the Solar System and its Oort Cloud. 70,000 years ago, Scholz’s Star and companion passed along the outer boundaries of our Solar System. Credit: NASA, Michael Osadciw/University of Rochester
Recent studies have shown that the formation of the Oort cloud is broadly compatible with the hypothesis that the Solar System formed as part of an embedded cluster of 200–400 stars. These early stars likely played a role in the cloud’s formation, since the number of close stellar passages within the cluster was much higher than today, leading to far more frequent perturbations.
Comets:
Comets are thought to have two points of origin within the Solar System. They start as infinitesimals in the Oort Cloud and then become comets when passing stars knock some of them out of their orbits, sending into a long-term orbit that take them into the inner solar system and out again.
Short-period comets have orbits that last up to two hundred years while the orbits of long-period comets can last for thousands of years. Whereas short-period comets are believed to have emerged from either the Kuiper Belt or the scattered disc, the accepted hypothesis is that long-period comets originate in the Oort Cloud. However, there are some exceptions to this rule.
For example, there are two main varieties of short-period comet: Jupiter-family comets and Halley-family comets. Halley-family comets, named for their prototype (Halley’s Comet) are unusual in that although they are short in period, they are believed to have originated from the Oort cloud. Based on their orbits, it is suggested they were once long-period comets that were captured by the gravity of a gas giant and sent into the inner Solar System.
Evolution of a comet as it orbits the sun. Credit: Laboratory for Atmospheric and Space Sciences/ NASA
Exploration:
Because the Oort Cloud is so much farther out than the Kuiper Belt, the region remained unexplored and largely undocumented. Space probes have yet to reach the area of the Oort cloud, and Voyager 1 – the fastest and farthest of the interplanetary space probes currently exiting the Solar System – is not likely to provide any information on it.
At its current speed, Voyager 1 will reach the Oort cloud in about 300 years, and will will take about 30,000 years to pass through it. However, by around 2025, the probe’s radioisotope thermoelectric generators will no longer supply enough power to operate any of its scientific instruments. The other four probes currently escaping the Solar System –Voyager 2, Pioneer 10 and 11, and New Horizons – will also be non-functional when they reach the Oort cloud.
Exploring the Oort Cloud presents numerous difficulties, most of which arise from the fact that it is incredible distant from Earth. By the time a robotic probe could actually reach it and begin exploring the area in earnest, centuries will have passed here on Earth. Not only would those who had sent it out in the first place be long dead, but humanity will have most likely invented far more sophisticated probes or even manned craft in the meantime.
Still, studies can be (and are) conducted by examining the comets that it periodically spits out, and long-range observatories are likely to make some interesting discoveries from this region of space in the coming years. It’s a big cloud. Who knows what we might find lurking in there?
We talk about stellar mass and supermassive black holes. What are the limits? How massive can these things get?
Without the light pressure from nuclear fusion to hold back the mass of the star, the outer layers compress inward in an instant. The star dies, exploding violently as a supernova.
All that’s left behind is a black hole. They start around three times the mass of the Sun, and go up from there. The more a black hole feeds, the bigger it gets.
Terrifyingly, there’s no limit to much material a black hole can consume, if it’s given enough time. The most massive are ones found at the hearts of galaxies. These are the supermassive black holes, such as the 4.1 million mass nugget at the center of the Milky Way. Astronomers figured its mass by watching the movements of stars zipping around the center of the Milky Way, like comets going around the Sun.
There seems to be supermassive black holes at the heart of every galaxy we can find, and our Milky Way’s black hole is actually puny in comparison. Interstellar depicted a black hole with 100 million times the mass of the Sun. And we’re just getting started.
The giant elliptical galaxy M87 has a black hole with 6.2 billion times the mass of the Sun. How can astronomers possibly know that? They’ve spotted a jet of material 4,300 light-years long, blasting out of the center of M87 at relativistic speeds, and only black holes that massive generate jets like that.
Most recently, astronomers announced in the Journal Nature that they have found a black hole with about 12 billion times the mass of the Sun. The accretion disk here generates 429 trillion times more light than the Sun, and it shines clear across the Universe. We see the light from this region from when the Universe was only 6% into its current age.
Somehow this black hole went from zero to 12 billion times the mass of the Sun in about 875 million years. Which poses a tiny concern. Such as how in the dickens is it possible that a black hole could build up so much mass so quickly? Also, we’re seeing it 13 billion years ago. How big is it now? Currently, astronomers have no idea. I’m sure it’s fine. It’s fine right?
We’ve talked about how massive black holes can get, but what about the opposite question? How teeny tiny can a black hole be?
An illustration that shows the powerful winds driven by a supermassive black hole at the centre of a galaxy. The schematic figure in the inset depicts the innermost regions of the galaxy where a black hole accretes, that is, consumes, at a very high rate the surrounding matter (light grey) in the form of a disc (darker grey). At the same time, part of that matter is cast away through powerful winds. (Credits: XMM-Newton and NuSTAR Missions; NASA/JPL-Caltech;Insert:ESA)
Astronomers figure there could be primordial black holes, black holes with the mass of a planet, or maybe an asteroid, or maybe a car… or maybe even less. There’s no method that could form them today, but it’s possible that uneven levels of density in the early Universe might have compressed matter into black holes.
Those black holes might still be out there, zipping around the Universe, occasionally running into stars, planets, and spacecraft and interstellar picnics. I’m sure it’s the stellar equivalent of smashing your shin on the edge of the coffee table.
Astronomers have never seen any evidence that they actually exist, so we’ll shrug this off and choose to pretend we shouldn’t be worrying too much. And so it turns out, black holes can get really, really, really massive. 12 billion times the mass of the Sun massive.
What part about black holes still make you confused? Suggest some topics for future episodes of the Guide to Space in the comments below.
An artist's conception of Sedna. this assumes that Sedna has a tiny as yet undiscovered moon. Image credit; NASA/JPl-Caltech
There has been quite a bit of buzz about dwarf planets lately. Ever since the discovery of Eris in 2005, and the debate that followed over the proper definition of the word “planet”, this term has been adopted to refer to planets beyond Neptune that rival Pluto in size. Needless to say, it has been a controversial subject, and one which is not likely to be resolved anytime soon.
In the meantime, the category has been used tentatively to describe many Trans-Neptunian objects that were discovered before or since the discovery of Eris. Sedna, which was discovered in the outer reaches of the Solar System in 2003, is most likely a dwarf planet. And as the furthest known object from the Sun, and located within the hypothetical Oort Cloud, it is quite the fascinating find.
Discovery and Naming:
Much like Eris, Haumea and Makemake, Sedna was co-discovered by Mike Brown of Caltech, with assistance from Chad Trujillo of the Gemini Observatory, and David Rabinowitz of Yale University on November 14th, 2003. Initially designated as 2003 VB12, the discovery was part of a survey that commenced in 2001 using the Samuel Oschin Telescope at the Palomar Observatory near San Diego, California.
Observations at the time indicated the presence of an object at a distance of approximately 100 AU from the Sun. Follow-up observations made in November and December of 2003 by the Cerro Tololo Inter-American Observatory in Chile and the W. M. Keck Observatory in Hawaii revealed that the object was moving along a distant highly eccentric orbit.
Comparison of Sedna with the other largest TNOs and with Earth (all to scale). Credit: NASA/Lexicon
It was later learned that the object had been previously observed by the Samual Oschin telescope as well as the Jet Propulsion Laboratory’s Near Earth Asteroid Tracking (NEAT) consortium. Comparisons with these previous observations have since allowed for a more precise calculation of Sedna’s orbit and orbital arc.
According to Mike Brown’s website, the planet was named Sedna after the Inuit Goddess of the sea. According to legend, Sedna was once mortal but became immortal after drowning in the Arctic Ocean, where she now resides and protects all the creatures of the sea. This name seemed appropriate to Brown and his team because Sedna is currently the farthest (and hence coldest) object from the Sun.
The team made the name public before the object had been officially numbered; and while this represented a breach in IAU protocol, no objections were raised. In 2004, the IAU’s Committee on Small Body Nomenclature formally accepted the name.
Classification:
Astronomers remain somewhat divided when it comes to Sedna’s proper classification. On the one hand, its discovery resurrected the question of which astronomical objects should be considered planets and which ones could not. Under the IAU’s definition of a planet, which was adopted on August 24th, 2006 (in response to the discovery of Eris), a planet needs to have cleared its orbit. Hence, Sedna does not qualify.
However, to be a dwarf planet, a celestial body must be in hydrostatic equilibrium – meaning that it is symmetrically rounded into a spheroid or ellipsoid shape. With a surface albedo of 0.32 ± 0.06 – and an estimated diameter of between 915 and 1800 km (compared to Pluto’s 1186 km) – Sedna is bright enough, and also large enough, to be spheroid in shape.
Therefore, Sedna is believed by many astronomers to be a dwarf planet, and is often referred to confidently as such. One reason why astronomers are reluctant to definitively place it in that category is because it is so far away that it is difficult to observe.
Size, Mass and Orbit:
In 2004, Mike Brown and his team placed an upper limit of 1,800 km on its diameter, but by 2007 this was revised downward to less than 1,600 km after observations were made by the Spitzer Space Telescope. In 2012, measurements from the Herschel Space Observatory suggested that Sedna’s diameter was between 915 and 1075 km, which would make it smaller than Pluto’s moon Charon.
Because Sedna has no known moons, determining its mass is currently impossible without sending a space probe. Nevertheless, many astronomers think that Sedna is the fifth largest trans-Neptunian object (TNO) and dwarf planet – after Eris, Pluto, Makemake, and Haumea, respectively.
Sedna has a highly elliptical orbit around the Sun, which means it ranges in distance from 76 astronomical units (AU) at perihelion (114 billion km/71 billion mi) to 936 AU (140 billion km/87 billion mi) at aphelion.
Sedna’s orbit, compared to other bodies in the Solar System, the Kuiper Belt and the Oort Cloud. Credit: web.gps.caltech.edu
Estimations on how long it takes Sedna to orbit the Sun vary, although it is known to be more than 10,000 years. Some astronomers calculate the orbital period could be as long as 12,000 years. Although astronomers believed at first that Sedna had a satellite, they have not been able to prove it.
Composition:
At the time of its discovery, Sedna was the intrinsically brightest object found in the Solar System since Pluto in 1930. In terms of color, Sedna appears to be almost as red as Mars, which some astronomers believe is caused by hydrocarbon or tholin. Its surface is also rather homogeneous in terms of color and spectrum, which may the result of Sedna’s distance from the Sun.
Unlike planets in the Inner Solar System, Sedna experiences very few surface impacts from meteors or stray objects. As a result, it does not have as many exposed bright patches of fresh icy material. Sedna, and the entire Oort Cloud, is freezing at temperatures below 33 Kelvin (-240.2°C).
Models have been constructed of Sedna that place an upper limit of 60% for methane ice and 70% for water ice. This is consistent with the existence of tholins on it’s surface, since they are produced by the irradiation of methane. Meanwhile, M. Antonietta Barucci and colleagues compared Sedna’s spectrum to that of Triton and came up with a model that included 24% Triton-type tholins, 7% amorphous carbon, 10% nitrogen, 26% methanol and 33% methane.
Artist’s concept of the surface of Sedna. Credit: NASA/ESA/Adolf Schaller
The presence of nitrogen on the surface suggests the possibility that, at least for a short time, Sedna may have a tenuous atmosphere. During a 200-year period near perihelion, the maximum temperature on Sedna would likely exceed 35.6 K (-237.6 °C), which would be just warm enough for some of the nitrogen ice to sublimate. Models of internal heating via radioactive decay suggest that, like many bodies in the Outer Solar System, Sedna might be capable of supporting a subsurface ocean of liquid water.
Origin:
When he and his colleagues first observed Sedna, they claimed that it was part of the Oort Cloud – the hypothetical cloud of comets believed to exist a light-year’s distance from the Sun. This was based on the fact that Sedna’s perihelion (76 AUs) made it too distant to be scattered by the gravitational influence of Neptune.
Because it was also closer to the Sun than was expected from on Oort cloud object, and has an inclination in line with the planets and Kuiper Belt, they described it as being an “inner Oort Cloud object”. Brown and his colleagues have proposed that Sedna’s orbit is best explained by the Sun having formed in an open cluster of several stars that gradually disassociated over time.
In this scenario, Sedna was lifted into its current orbit by a star that was part of this cluster rather than it having been formed in its current location. This hypothesis has also been confirmed by computer simulations that suggest that multiple close passes by young stars in such a cluster would pull many objects into Sedna-like orbits.
The layout of the solar system, including the Oort Cloud, on a logarithmic scale. Credit: NASA
On the other hand, if Sedna formed in its current location, then it would mean that the Sun’s original protoplanetary disc would have extended farther than previously expected – approximately 75 AUs into space. Also, Sedna’s initial orbit would have been approximately circular, otherwise its formation by the accretion of smaller bodies into a whole would not have been possible.
Therefore, it must have been tugged into its current eccentric orbit by a gravitational interaction with another body – which could have been another planet in the Kuiper Belt, a passing star, or one of the young stars embedded with the Sun in the stellar cluster in which it formed.
Another possibility is the Sedna’s orbit is the result of influence by a large binary companion thousands of AU distant from our Sun. One such hypothetical companion is Nemesis, a dim companion to the Sun. However, to date no direct evidence of Nemesis has been found, and many lines of evidence have thrown its existence into doubt.
Astronomers believe that they will find more objects in the Oort Cloud in years to come, especially as ground-based and space telescopes become more advanced and sensitive. Most likely, we will also see Sedna officially christened a “dwarf planet” by the IAU. As with other astronomical bodies that have been designated as such, we can expect some controversy to follow!
So just what would it take to terraform Jupiter, the largest planet in our solar system?
Just a few videos ago, I blew minds with a “How to” on terraforming the Moon. Once we’ve developed a Solar System spanning civilization and have claimed mastery over the laws of physics, and have common-place technology which staggers and dwarf our current comprehension of what’s possible it should be easy enough.
In fact, it might even be easier than terraforming Mars or Venus, as long as you keep a steady flow of gas to the Moon replenishing the constantly escaping atmosphere.
And in the comments on that video, ABitOfTheUniverse threw down, he wants to know what it would take to terraform Jupiter. All right “ABitOfTheUniverse”, if that is your real name… I’m up for it.
On the surface, this is madness. We already explained how Jupiter is completely and totally inhospitable to life. An alien started a “Build a star kit” and stopped a ? of the way through, because he got bored and wandered away. Just like his Mom said he would.
Jupiter is a ball of hydrogen and helium, which compresses these gasses to almost starlike temperatures and pressures. Fine, Jupiter is the absolute worst. It makes traveling to Venus look like a spa visit.
Jupiter does have something we can work with. Astronomers think below the septillions tons of hydrogen and gas, there’s actually a rocky core. The mass of the core is still a mystery, but recent computer simulations put it at somewhere between 7 and 45 times the mass of the Earth, complete with plenty of water ices and other chemicals you might require on an Earthlike planet.
Furthermore, this core may contain similar constituents as the internal structure of Earth. This means a central core of iron and nickel, surrounded by liquid metal, surrounded by rock.
The problem is you need to strip away 95% of the planet’s mass. It’s all that hydrogen and helium, and that’s pretty much impossible. And almost completely impossible, is still very slightly completely possible.
Cutaway of Jupiter. Credit: Kevinsong
Jupiter is made of fuel. It’s like looking at a pool of gasoline and wondering if there was some way to get rid of it all. What good Solar System-spanning civilization hasn’t worked out hydrogen fusion? It’s a technology that’s probably only 30 years away from us now.
You could fly a spacecraft down into Jupiter’s gravity well and scoop up hydrogen fuel from the clouds. Or you could create fusion-powered dirigibles filled with hot hydrogen, which float around the cloud tops of Jupiter, using their fusion reactors to spew hydrogen off into space.
Over untold lengths of time, you could get at that rocky juicy center, once you stripped it of its hydrogen. Then you’ll need to do all that other stuff I mentioned in previous videos, to turn it into a habitable world.
Sure, it’s a world with much higher gravity than Earth, but that’s not my problem. You said “Earthlike”. That’ll teach you to make wishes with a monkey’s paw!
What if you need to move Jupiter first, perhaps a little closer to the Sun. There’s an awesome idea cooked up by Larry Niven in his book, “A World Out of Time”. It’s a fusion candle, and it lets you shift gas giants around.
A Star Trek-inspired space station.
You take a long space station, and light up fusion thrusters on both ends. You dip one end down into the upper clouds, where it siphons hydrogen fuel. Both ends of the space station start blasting. One end keeps it from dropping down into the planet, and the other end pushes on the entire planet, pushing it around the Solar System.
Instead of trying to terraform Jupiter, we could just push the planet closer to the Sun, where its icy moons warm up and become habitable themselves.
Well, ABitOfTheUniverse, that sounds a little easier. What do you think? I’ll admit, trying to figure out how to terraform Jupiter was a good exercise in tomfoolery. Fortunately, my imagination is a limitless and renewable source of energy. We’ve done Mars, Venus, the Moon and now Jupiter. What should we terraform next? Tell us in the comments below.
Mercury, as imaged by the MESSENGER spacecraft, revealing parts of the never seen by human eyes. Image Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
Mercury is the closest planet to our Sun, the smallest of the eight planets, and one of the most extreme worlds in our Solar Systems. Named after the Roman messenger of the gods, the planet is one of a handful that can be viewed without the aid of a telescope. As such, it has played an active role in the mythological and astrological systems of many cultures.
In spite of that, Mercury is one of the least understood planets in our Solar System. Much like Venus, its orbit between Earth and the Sun means that it can be seen at both morning and evening (but never in the middle of the night). And like Venus and the Moon, it also goes through phases; a characteristic which originally confounded astronomers, but eventually helped them to realize the true nature of the Solar System.
Size, Mass and Orbit:
With a mean radius of 2440 km and a mass of 3.3022×1023 kg, Mercury is the smallest planet in our Solar System – equivalent in size to 0.38 Earths. And while it is smaller than the largest natural satellites in our system – such as Ganymede and Titan – it is more massive. In fact, Mercury’s density (at 5.427 g/cm3) is the second-highest in the Solar System, only slightly less than Earth’s (5.515 g/cm3).
Mercury has the most eccentric orbit of any planet in the Solar System (0.205). Because of this, its distance from the Sun varies between 46 million km (29 million mi) at its closest (perihelion) to 70 million km (43 million mi) at its farthest (aphelion). And with an average orbital velocity of 47.362 km/s (29.429 mi/s), it takes Mercury a total of 87.969 Earth days to complete a single orbit.
Mercury and Earth, size comparison. Credit: NASA / APL (from MESSENGER)
With an average rotational speed of 10.892 km/h (6.768 mph), Mercury also takes 58.646 days to complete a single rotation. This means that Mercury has a spin-orbit resonance of 3:2, which means that it completes three rotations on its axis for every two rotations around the Sun. This does not, however, mean that three days last the same as two years on Mercury.
In fact, its high eccentricity and slow rotation mean that it takes 176 Earth days for the Sun to return to the same place in the sky (aka. a solar day). This means that a single day on Mercury is twice as long as a single year. Mercury also has the lowest axial tilt of any planet in the Solar System – approximately 0.027 degrees compared to Jupiter’s 3.1 degrees (the second smallest).
Composition and Surface Features:
As one of the four terrestrial planets of the Solar System, Mercury is composed of approximately 70% metallic and 30% silicate material. Based on its density and size, a number of inferences can be made about its internal structure. For example, geologists estimate that Mercury’s core occupies about 42% of its volume, compared to Earth’s 17%.
The interior is believed to be composed of molten iron which is surrounded by a 500 – 700 km mantle of silicate material. At the outermost layer is Mercury’s crust, which is believed to be 100 – 300 km thick. The surface is also marked by numerous narrow ridges that extend up to hundreds of kilometers in length. It is believed that these were formed as Mercury’s core and mantle cooled and contracted at a time when the crust had already solidified.
Mercury’s core has a higher iron content than that of any other major planet in the Solar System, and several theories have been proposed to explain this. The most widely accepted theory is that Mercury was once a larger planet which was struck by a planetesimal measuring several thousand km in diameter. This impact could have then stripped away much of the original crust and mantle, leaving behind the core as a major component.
Internal structure of Mercury, consisting of the crust (100–300 km thick), mantle (600 km thick) and core (1,800 km radius). Credit: MASA/JPL
Another theory is that Mercury may have formed from the solar nebula before the Sun’s energy output had stabilized. In this scenario, Mercury would have originally been twice its present mass but would have been subjected to temperatures of 25,000 to 35,000 K (or as high as 10,000 K) as the protosun contracted. This process would have vaporized much of Mercury’s surface rock, reducing it to its current size and composition.
A third hypothesis is that the solar nebula created drag on the particles from which Mercury was accreting, which meant that lighter particles were lost and not gathered to form Mercury. Naturally, further analysis is needed before any of these theories can be confirmed or ruled out.
At a glance, Mercury looks similar to the Earth’s moon. It has a dry landscape pockmarked by asteroid impact craters and ancient lava flows. Combined with extensive plains, these indicate that the planet has been geologically inactive for billions of years. However, unlike the Moon and Mars, which have significant stretches of similar geology, Mercury’s surface appears much more jumbled. Other common features include dorsa (aka. “wrinkle-ridges”), Moon-like highlands, montes (mountains), planitiae (plains), rupes (escarpments), and valles (valleys).
Names for these features come from a variety of sources. Craters are named for artists, musicians, painters, and authors; ridges are named for scientists; depressions are named after works of architecture; mountains are named for the word “hot” in different languages; planes are named for Mercury in various languages; escarpments are named for ships of scientific expeditions, and valleys are named after radio telescope facilities.
Enhanced-color image of Munch, Sander, and Poe craters amid volcanic plains (orange) near Caloris Basin. Credit: NASA/Johns Hopkins University/Carnegie Institution
During and following its formation 4.6 billion years ago, Mercury was heavily bombarded by comets and asteroids, and perhaps again during the Late Heavy Bombardment period. During this period of intense crater formation, the planet received impacts over its entire surface, thanks in part to the lack of any atmosphere to slow impactors down. During this time, the planet was volcanically active, and released magma would have produced the smooth plains.
Craters on Mercury range in diameter from small bowl-shaped cavities to multi-ringed impact basins hundreds of kilometers across. The largest known crater is Caloris Basin, which measures 1,550 km in diameter. The impact that created it was so powerful that it caused lava eruptions on the other side of the planet and left a concentric ring over 2 km tall surrounding the impact crater. Overall, about 15 impact basins have been identified on those parts of Mercury that have been surveyed.
Despite its small size and slow 59-day-long rotation, Mercury has a significant, and apparently global, magnetic field that is about 1.1% the strength of Earth’s. It is likely that this magnetic field is generated by a dynamo effect, in a manner similar to the magnetic field of Earth. This dynamo effect would result from the circulation of the planet’s iron-rich liquid core.
Mercury’s magnetic field is strong enough to deflect the solar wind around the planet, thus creating a magnetosphere. The planet’s magnetosphere, though small enough to fit within Earth, is strong enough to trap solar wind plasma, which contributes to the space weathering of the planet’s surface.
Mercury’s Magnetic Field. Credit: NASA
Atmosphere and Temperature:
Mercury is too hot and too small to retain an atmosphere. However, it does have a tenuous and variable exosphere that is made up of hydrogen, helium, oxygen, sodium, calcium, potassium, and water vapor, with a combined pressure level of about 10-14 bar (one-quadrillionth of Earth’s atmospheric pressure). It is believed this exosphere was formed from particles captured from the Sun, volcanic outgassing and debris kicked into orbit by micrometeorite impacts.
Because it lacks a viable atmosphere, Mercury has no way to retain the heat from the Sun. As a result of this and its high eccentricity, the planet experiences considerable variations in temperature. Whereas the side that faces the Sun can reach temperatures of up to 700 K (427° C), while the side in shadow dips down to 100 K (-173° C).
Despite these highs in temperature, the existence of water ice and even organic molecules has been confirmed on Mercury’s surface. The floors of deep craters at the poles are never exposed to direct sunlight, and temperatures there remain below the planetary average.
These icy regions are believed to contain about 1014–1015 kg of frozen water and may be covered by a layer of regolith that inhibits sublimation. The origin of the ice on Mercury is not yet known, but the two most likely sources are from outgassing of water from the planet’s interior or deposition by the impacts of comets.
Images of Mercury’s north pole, provided by MESSENGER. Red indicates shaded regions while yellow indicates the presence of ice. Credit: NASA/JPL
Historical Observations:
Much like the other planets that are visible to the naked eye, Mercury has a long history of being observed by human astronomers. The earliest recorded observations of Mercury are believed to be from the Mul Apin tablet, a compendium of Babylonian astronomy and astrology.
The observations, which were most likely made during the 14th century BCE, refer to the planet as “the jumping planet”. Other Babylonian records, which refer to the planet as “Nabu” (after the messenger to the gods in Babylonian mythology) date back to the first millennium BCE. The reason for this has to do with Mercury being the fastest-moving planet across the sky.
To the ancient Greeks, Mercury was known variously as “Stilbon” (a name which means “the gleaming”), Hermaon, and Hermes. As with the Babylonians, this latter name came from the messenger of the Greek pantheon. The Romans continued this tradition, naming the planet Mercurius after the swift-footed messenger of the gods, which they equated with the Greek Hermes.
Ibn al-Shatir’s model for the appearances of Mercury, showing the multiplication of epicycles using the Tusi couple, thus eliminating the Ptolemaic eccentrics and equant. Credit: Wikipedia Commons
In his book Planetary Hypotheses, Greco-Egyptian astronomer Ptolemy wrote about the possibility of planetary transits across the face of the Sun. For both Mercury and Venus, he suggested that no transits had been observed because the planet was either too small to see or because the transits are too infrequent.
To the ancient Chinese, Mercury was known as Chen Xing (“the Hour Star”) and was associated with the direction of north and the element of water. Similarly, modern Chinese, Korean, Japanese, and Vietnamese cultures refer to the planet literally as the “water star” based on the Five Elements. In Hindu mythology, the name Budha was used for Mercury – the god that was thought to preside over Wednesday.
The same is true of the Germanic tribes, who associated the god Odin (or Woden) with the planet Mercury and Wednesday. The Maya may have represented Mercury as an owl – or possibly four owls, two for the morning aspect and two for the evening – that served as a messenger to the underworld.
In medieval Islamic astronomy, the Andalusian astronomer Abu Ishaq Ibrahim al-Zarqali in the 11th century described Mercury’s geocentric orbit as being oval, although this insight did not influence his astronomical theory or his astronomical calculations. In the 12th century, Ibn Bajjah observed: “two planets as black spots on the face of the Sun”, which was later suggested as the transit of Mercury and/or Venus.
Mercury’s path across the solar disk as seen from the Solar and Heliospheric Observatory (SOHO) on November 8th, 2006. The transit was visible in eastern Europe and the eastern hemisphere. Credit: NASA.
In India, the Kerala school astronomer Nilakantha Somayaji in the 15th century developed a partially heliocentric planetary model in which Mercury orbits the Sun, which in turn orbits Earth, similar to the system proposed by Tycho Brahe in the 16th century.
The first observations using a telescope took place in the early 17th century by Galileo Galilei. Although he had observed phases when looking at Venus, his telescope was not powerful enough to see Mercury going through similar phases. In 1631, Pierre Gassendi made the first telescopic observations of the transit of a planet across the Sun when he saw a transit of Mercury, which had been predicted by Johannes Kepler.
In 1639, Giovanni Zupi used a telescope to discover that the planet had orbital phases similar to Venus and the Moon. These observations demonstrated conclusively that Mercury orbited around the Sun, which helped to definitively prove that the Copernican Heliocentric model of the universe was the correct one.
In the 1880s, Giovanni Schiaparelli mapped the planet more accurately and suggested that Mercury’s rotational period was 88 days, the same as its orbital period due to tidal locking. The effort to map the surface of Mercury was continued by Eugenios Antoniadi, who published a book in 1934 that included both maps and his own observations. Many of the planet’s surface features, particularly the albedo features, take their names from Antoniadi’s map.
Map of Mercury prepared by E.M. Antoniadi during the 1920s. Credit: airandspace.si.edu
In June of 1962, Soviet scientists at the USSR Academy of Sciences became the first to bounce a radar signal off Mercury and receive it, which began the era of using radar to map the planet. Three years later, Americans Gordon Pettengill and R. Dyce conducted radar observations using the Arecibo Observatory’s radio telescope. Their observations demonstrated conclusively that the planet’s rotational period was about 59 days and the planet did not have a synchronous rotation (which was widely believed at the time).
Ground-based optical observations did not shed much further light on Mercury, but radio astronomers using interferometry at microwave wavelengths – a technique that enables removal of the solar radiation – were able to discern physical and chemical characteristics of the subsurface layers to a depth of several meters.
In 2000, high-resolution observations were conducted by the Mount Wilson Observatory which provided the first views that resolved surface features on previously unseen parts of the planet. Most of the planet has been mapped by the Arecibo radar telescope, with 5 km resolution, including polar deposits in shadowed craters of what was believed to be water ice.
Exploration:
Prior to the first space probes flying past Mercury, many of its most fundamental morphological properties remained unknown. The first of these was NASA’s Mariner 10, which flew past the planet between 1974 and 1975. During the course of its three close approaches to the planet, it was able to capture the first close-up images of Mercury’s surface, which revealed heavily cratered terrain, giant scarps, and other surface features.
Mariner 10 space probe to Venus and Mercury. Credit: NASA
Unfortunately, due to the length of Mariner 10‘s orbital period, the same face of the planet was lit at each of Mariner 10‘s close approaches. This made observation of both sides of the planet impossible and resulted in the mapping of less than 45% of the planet’s surface.
During its first close approach, instruments also detected a magnetic field, to the great surprise of planetary geologists. The second close approach was primarily used for imaging, but at the third approach, extensive magnetic data were obtained. The data revealed that the planet’s magnetic field is much like Earth’s, which deflects the solar wind around the planet.
On March 24th, 1975, just eight days after its final close approach, Mariner 10 ran out of fuel, prompting its controllers to shut the probe down. Mariner 10 is thought to be still orbiting the Sun, passing close to Mercury every few months.
The second NASA mission to Mercury was the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (or MESSENGER) space probe. The purpose of this mission was to clear up six key issues relating to Mercury, namely – its high density, its geological history, the nature of its magnetic field, the structure of its core, whether it has ice at its poles, and where its tenuous atmosphere comes from.
To this end, the probe carried imaging devices that gathered much higher-resolution images of much more of the planet than Mariner 10, assorted spectrometers to determine abundances of elements in the crust, and magnetometers and devices to measure velocities of charged particles.
The MESSENGER spacecraft has been in orbit around Mercury since March 2011. Credit: NASA/JHU APL/Carnegie Institution of Washington
Having launched from Cape Canaveral on August 3rd, 2004, it made its first flyby of Mercury on January 14th, 2008, a second on October 6th, 2008, and a third on September 29th, 2009. Most of the hemisphere not imaged by Mariner 10 was mapped during these fly-bys. On March 18th, 2011, the probe successfully entered an elliptical orbit around the planet and began taking images by March 29th.
After finishing its one-year mapping mission, it then entered a one-year extended mission that lasted until 2013. MESSENGER’s final maneuver took place on April 24th, 2015, which left it without fuel and an uncontrolled trajectory that inevitably led it to crash into Mercury’s surface on April 30th, 2015.
In 2016, the European Space Agency and the Japan Aerospace and Exploration Agency (JAXA) plan to launch a joint mission called BepiColombo. This robotic space probe, which is expected to reach Mercury by 2024, will orbit Mercury with two probes: a mapper probe and a magnetosphere probe.
The magnetosphere probe will be released into an elliptical orbit, then fire its chemical rockets to deposit the mapper probe into a circular orbit. The mapper probe will then go on to study the planet in many different wavelengths – infrared, ultraviolet, X-ray, and gamma-ray – using an array of spectrometers similar to those on MESSENGER.
An enhanced-color view of Mercury, assembled from images taken at various wavelengths by the cameras on board the MESSENGER spacecraft. Credit: NASA / Johns Hopkins University Applied Physics Laboratory / Carnegie Institution of Washington
Yes, Mercury is a planet of extremes and is riddled with contradictions. It ranges from extreme hot to extreme cold; it has a molten surface but also has water ice and organic molecules on its surface, and it has no discernible atmosphere but possessing an exosphere and magnetosphere. Combined with its proximity to the Sun, it is little wonder why we don’t know much about this terrestrial world.
One can only hope that the technology exists in the future for us to get closer to this world and study its extremes more thoroughly.
In the meantime, here are some articles on Mercury that we hope you find interesting, illuminating, and fun to read:
This portrait looking down on Saturn and its rings was created from images obtained by NASA's Cassini spacecraft on Oct. 10, 2013. Credit: NASA/JPL-Caltech/Space Science Institute/G. Ugarkovic
The farthest planet from the Sun that can be observed with the naked eye, the existence of Saturn has been known for thousands of years. And much like all celestial bodies that can be observed with the aid of instruments – i.e. Mercury, Venus, Mars, Jupiter and the Moon – it has played an important role in the mythology and astrological systems of many cultures.
Saturn is one of the four gas giants in our Solar System, also known as the Jovian planets, and the sixth planet from the Sun. It’s ring system, which is it famous for, is also the most observable – consisting of nine continuous main rings and three discontinuous arcs.
Saturn’s Size, Mass and Orbit:
With a polar radius of 54364±10 km and an equatorial radius of 60268±4 km, Saturn has a mean radius of 58232±6 km, which is approximately 9.13 Earth radii. At 5.6846×1026 kg, and a surface area, at 4.27×1010 km2, it is roughly 95.15 as massive as Earth and 83.703 times it’s size. However, since it is a gas giant, it has significantly greater volume – 8.2713×1014 km3, which is equivalent to 763.59 Earths.
The sixth most distant planet, Saturn orbits the Sun at an average distance of 9 AU (1.4 billion km; 869.9 million miles). Due to its slight eccentricity, the perihelion and aphelion distances are 9.022 (1,353.6 million km; 841.3 million mi) and 10.053 AU (1,513,325,783 km; 940.13 million mi), on average respectively.
Saturn Compared to Earth. Image credit: NASA/JPL
With an average orbital speed of 9.69 km/s, it takes Saturn 10,759 Earth days to complete a single revolution of the Sun. In other words, a single Cronian year is the equivalent of about 29.5 Earth years. However, as with Jupiter, Saturn’s visible features rotate at different rates depending on latitude, and multiple rotation periods have been assigned to various regions.
The latest estimate of Saturn’s rotation as a whole are based on a compilation of various measurements from the Cassini, Voyager and Pioneer probes. Saturn’s rotation causes it to have the shape of an oblate spheroid; flattened at the poles but bulging at the equator.
Saturn’s Composition:
As a gas giant, Saturn is predominantly composed of hydrogen and helium gas. With a mean density of 0.687 g/cm3, Saturn is the only planet in the Solar System that is less dense than water; which means that it lacks a definite surface, but is believed to have a solid core. This is due to the fact that Saturn’s temperature, pressure, and density all rise steadily toward the core.
Standard planetary models suggest that the interior of Saturn is similar to that of Jupiter, having a small rocky core surrounded by hydrogen and helium with trace amounts of various volatiles. This core is similar in composition to the Earth, but more dense due to the presence of metallic hydrogen, which as a result of the extreme pressure.
Diagram of Saturn’s interior. Credit: Kelvinsong/Wikipedia Commons
Saturn has a hot interior, reaching 11,700 °C at its core, and it radiates 2.5 times more energy into space than it receives from the Sun. This is due in part to the Kelvin-Helmholtz mechanism of slow gravitational compression, but may also be attributable to droplets of helium rising from deep in Saturn’s interior out to the lower-density hydrogen. As these droplets rise, the process releases heat by friction and leaves Saturn’s outer layers depleted of helium. These descending droplets may have accumulated into a helium shell surrounding the core.
In 2004, French astronomers Didier Saumon and Tristan Guillot estimated that the core must 9-22 times the mass of Earth, which corresponds to a diameter of about 25,000 km. This is surrounded by a thicker liquid metallic hydrogen layer, followed by a liquid layer of helium-saturated molecular hydrogen that gradually transitions to a gas with increasing altitude. The outermost layer spans 1,000 km and consists of gas.
Saturn’s Atmosphere:
The outer atmosphere of Saturn contains 96.3% molecular hydrogen and 3.25% helium by volume. The gas giant is also known to contain heavier elements, though the proportions of these relative to hydrogen and helium is not known. It is assumed that they would match the primordial abundance from the formation of the Solar System.
Trace amounts of ammonia, acetylene, ethane, propane, phosphine and methane have been also detected in Saturn’s atmosphere. The upper clouds are composed of ammonia crystals, while the lower level clouds appear to consist of either ammonium hydrosulfide (NH4SH) or water. Ultraviolet radiation from the Sun causes methane photolysis in the upper atmosphere, leading to a series of hydrocarbon chemical reactions with the resulting products being carried downward by eddies and diffusion.
NASA’s Cassini spacecraft captures a composite near-true-color view of the huge storm churning through the atmosphere in Saturn’s northern hemisphere. Image credit: NASA/JPL-Caltech/SSI
Saturn’s atmosphere exhibits a banded pattern similar to Jupiter’s, but Saturn’s bands are much fainter and wider near the equator. As with Jupiter’s cloud layers, they are divided into the upper and lower layers, which vary in composition based on depth and pressure. In the upper cloud layers, with temperatures in range of 100–160 K and pressures between 0.5–2 bar, the clouds consist of ammonia ice.
Water ice clouds begin at a level where the pressure is about 2.5 bar and extend down to 9.5 bar, where temperatures range from 185–270 K. Intermixed in this layer is a band of ammonium hydrosulfide ice, lying in the pressure range 3–6 bar with temperatures of 290–235 K. Finally, the lower layers, where pressures are between 10–20 bar and temperatures are 270–330 K, contains a region of water droplets with ammonia in an aqueous solution.
On occasion, Saturn’s atmosphere exhibits long-lived ovals, similar to what is commonly observed on Jupiter. Whereas Jupiter has the Great Red Spot, Saturn periodically has what’s known as the Great White Spot (aka. Great White Oval). This unique but short-lived phenomenon occurs once every Saturnian year, roughly every 30 Earth years, around the time of the northern hemisphere’s summer solstice.
These spots can be several thousands of kilometers wide, and have been observed in 1876, 1903, 1933, 1960, and 1990. Since 2010, a large band of white clouds called the Northern Electrostatic Disturbance have been observed enveloping Saturn, which was spotted by the Cassini space probe. If the periodic nature of these storms is maintained, another one will occur in about 2020.
The huge storm churning through the atmosphere in Saturn’s northern hemisphere overtakes itself as it encircles the planet in this true-color view from NASA’s Cassini spacecraft. Image credit: NASA/JPL-Caltech/SSI
The winds on Saturn are the second fastest among the Solar System’s planets, after Neptune’s. Voyager data indicate peak easterly winds of 500 m/s (1800 km/h). Saturn’s northern and southern poles have also shown evidence of stormy weather. At the north pole, this takes the form of a hexagonal wave pattern, whereas the south shows evidence of a massive jet stream.
The persisting hexagonal wave pattern around the north pole was first noted in the Voyager images. The sides of the hexagon are each about 13,800 km (8,600 mi) long (which is longer than the diameter of the Earth) and the structure rotates with a period of 10h 39m 24s, which is assumed to be equal to the period of rotation of Saturn’s interior.
The south pole vortex, meanwhile, was first observed using the Hubble Space Telescope. These images indicated the presence of a jet stream, but not a hexagonal standing wave. These storms are estimated to be generating winds of 550 km/h, are comparable in size to Earth, and believed to have been going on for billions of years. In 2006, the Cassini space probe observed a hurricane-like storm that had a clearly defined eye. Such storms had not been observed on any planet other than Earth – even on Jupiter.
Saturn’s Moons:
Saturn has at least 150 moons and moonlets, but only 53 of these moons have been given official names. Of these moons, 34 are less than 10 km in diameter and another 14 are between 10 and 50 km in diameter. However, some of its inner and outer moons are rather large, ranging from 250 to over 5000 km.
Moons of Saturn (from left to right): Mimas, Enceladus, Tethys, Dione, Rhea, Titan in the background; Iapetus (top) and irregularly shaped Hyperion (bottom). Credit: NASA/JPL/Space Science Institute
Traditionally, most of Saturn’s moons have been named after the Titans of Greek mythology, and are grouped based on their size, orbits, and proximity to Saturn. The innermost moons and regular moons all have small orbital inclinations and eccentricities and prograde orbits. Meanwhile, the irregular moons in the outermost regions have orbital radii of millions of kilometers, orbital periods lasting several years, and move in retrograde orbits.
The Inner Large Moons, which orbit within the E Ring (see below), includes the larger satellites Mimas, Enceladus, Tethys, and Dione. These moons are all composed primarily of water ice, and are believed to be differentiated into a rocky core and an icy mantle and crust. With a diameter of 396 km and a mass of 0.4×1020 kg, Mimas is the smallest and least massive of these moons. It is ovoid in shape and orbits Saturn at a distance of 185,539 km with an orbital period of 0.9 days.
Enceladus, meanwhile, has a diameter of 504 km, a mass of 1.1×1020 km and is spherical in shape. It orbits Saturn at a distance of 237,948 km and takes 1.4 days to complete a single orbit. Though it is one of the smaller spherical moons, it is the only Cronian moon that is endogenously active – and one of the smallest known bodies in the Solar System that is geologically active. This results in features like the famous “tiger stripes” – a series of continuous, ridged, slightly curved and roughly parallel faults within the moon’s southern polar latitudes.
Large geysers have also been observed in the southern polar region that periodically release plumes of water ice, gas and dust which replenish Saturn’s E ring. These jets are one of several indications that Enceladus has liquid water beneath it’s icy crust, where geothermal processes release enough heat to maintain a warm water ocean closer to its core. With a geometrical albedo of more than 140%, Enceladus is one of the brightest known objects in the Solar System.
Artist’s rendering of possible hydrothermal activity that may be taking place on and under the seafloor of Enceladus. Image Credit: NASA/JPL
At 1066 km in diameter, Tethys is the second-largest of Saturn’s inner moons and the 16th-largest moon in the Solar System. The majority of its surface is made up of heavily cratered and hilly terrain and a smaller and smoother plains region. Its most prominent features are the large impact crater of Odysseus, which measures 400 km in diameter, and a vast canyon system named Ithaca Chasma – which is concentric with Odysseus and measures 100 km wide, 3 to 5 km deep and 2,000 km long.
With a diameter and mass of 1,123 km and 11×1020 kg, Dione is the largest inner moon of Saturn. The majority of Dione’s surface is heavily cratered old terrain, with craters that measure up to 250 km in diameter. However, the moon is also covered with an extensive network of troughs and lineaments which indicate that in the past it had global tectonic activity.
The Large Outer Moons, which orbit outside of the Saturn’s E Ring, are similar in composition to the Inner Moons – i.e. composed primarily of water ice and rock. Of these, Rhea is the second largest – measuring 1,527 km in diameter and 23 × 1020 kg in mass – and the ninth largest moon of the Solar System. With an orbital radius of 527,108 km, it is the fifth-most distant of the larger moons, and takes 4.5 days to complete an orbit.
Like other Cronian satellites, Rhea has a rather heavily cratered surface, and a few large fractures on its trailing hemisphere. Rhea also has two very large impact basins on its anti-Saturnian hemisphere – the Tirawa crater (similar to Odysseus on Tethys) and an as-yet unnamed crater – that measure 400 and 500 km across, respectively.
A composite image of Titan’s atmosphere, created using blue, green and red spectral filters to create an enhanced-color view. Image Credit: NASA/JPL/Space Science Institute
At 5150 km in diameter, and 1,350×1020 kg in mass, Titan is Saturn’s largest moon and comprises more than 96% of the mass in orbit around the planet. Titan is also the only large moon to have its own atmosphere, which is cold, dense, and composed primarily of nitrogen with a small fraction of methane. Scientists have also noted the presence of polycyclic aromatic hydrocarbons in the upper atmosphere, as well as methane ice crystals.
The surface of Titan, which is difficult to observe due to persistent atmospheric haze, shows only a few impact craters, evidence of cryo-volcanoes, and longitudinal dune fields that were apparently shaped by tidal winds. Titan is also the only body in the Solar System beside Earth with bodies of liquid on its surface, in the form of methane–ethane lakes in Titan’s north and south polar regions.
With an orbital distance of 1,221,870 km, it is the second-farthest large moon from Saturn, and completes a single orbit every 16 days. Like Europa and Ganymede, it is believed that Titan has a subsurface ocean made of water mixed with ammonia, which can erupt to the surface of the moon and lead to cryovolcanism.
Hyperion is Titan’s immediate neighbor. At an average diameter of about 270 km, it is smaller and lighter than Mimas. It is also irregularly shaped and quite odd in composition. Essentially, the moon is an ovoid, tan-colored body with an extremely porous surface (which resembles a sponge). The surface of Hyperion is covered with numerous impact craters, most of which are 2 to 10 km in diameter. It also has a highly unpredictable rotation, with no well-defined poles or equator.
The two sides of Iapetus, which is known as “Saturn’s yin yang moon” because of the contrast in its color composition. Credit: NASA/JPL
At 1,470 km in diameter and 18×1020 kg in mass, Iapetus is the third-largest of Saturn’s large moons. And at a distance of 3,560,820 km from Saturn, it is the most distant of the large moons, and takes 79 days to complete a single orbit. Due to its unusual color and composition – its leading hemisphere is dark and black whereas its trailing hemisphere is much brighter – it is often called the “yin and yang” of Saturn’s moons.
Beyond these larger moons are Saturn’s Irregular Moons. These satellites are small, have large-radii, are inclined, have mostly retrograde orbits, and are believed to have been acquired by Saturn’s gravity. These moons are made up of three basic groups – the Inuit Group, the Gallic Group, and the Norse Group.
The Inuit Group consists of five irregular moons that are all named from Inuit mythology – Ijiraq, Kiviuq, Paaliaq, Siarnaq, and Tarqeq. All have prograde orbits that range from 11.1 to 17.9 million km, and from 7 to 40 km in diameter. They are all similar in appearance (reddish in hue) and have orbital inclinations of between 45 and 50°.
The Gallic group are a group of four prograde outer moons named for characters in Gallic mythology -Albiorix, Bebhionn, Erriapus, and Tarvos. Here too, the moons are similar in appearance and have orbits that range from 16 to 19 million km. Their inclinations are in the 35°-40° range, their eccentricities around 0.53, and they range in size from 6 to 32 km.
Saturns rings and moons, shown to scale. Credit: NASA
Last, there is the Norse group, which consists of 29 retrograde outer moons that take their names from Norse mythology. These satellites range in size from 6 to 18 km, their distances from 12 and 24 million km, their inclinations between 136° and 175°, and their eccentricities between 0.13 and 0.77. This group is also sometimes referred to as the Phoebe group, due to the presence of a single larger moon in the group – which measures 240 km in diameter. The second largest, Ymir, measures 18 km across.
Within the Inner and Outer Large Moons, there are also those belonging to Alkyonide group. These moons – Methone, Anthe, and Pallene – are named after the Alkyonides of Greek mythology, are located between the orbits of Mimas and Enceladus, and are among the smallest moons around Saturn.
Some of the larger moons even have moons of their own, which are known as Trojan moons. For instance, Tethys has two trojans – Telesto and Calypso, while Dione has Helene and Polydeuces.
Saturn’s Ring System:
Saturn’s rings are believed to be very old, perhaps even dating back to the formation of Saturn itself. There are two main theories as to how these rings formed, each of which have variations. One theory is that the rings were once a moon of Saturn whose orbit decayed until it came close enough to be ripped apart by tidal forces.
In version of this theory, the moon was struck by a large comet or asteroid – possible during the Late Heavy Bombardment – that pushed it beneath the Roche Limit. The second theory is that the rings were never part of a moon, but are instead left over from the original nebular material from which Saturn formed billions of years ago.
The structure is subdivided into seven smaller ring sets, each of which has a division (or gap) between it and its neighbor. The A and B Rings are the densest part of the Cronian ring system and are 14,600 and 25,500 km in diameter, respectively. They extend to a distance of 92,000 – 117,580 km (B Ring) and 122,170 – 136,775 km (A Ring) from Saturn’s center, and are separated by the 4,700 km wide Cassini Division.
Saturn’s rings. Credit: NASA/JPL/Space Science Institute.
The C Ring, which is separated from the B Ring by the 64 km Maxwell Gap, is approximately 17,500 km in width and extends 74,658 – 92,000 from Saturn’s center. Together with the A and B Rings, they comprise the main rings, which are denser and contain larger particles than the “dusty rings”.
These tenuous rings are called “dusty” due to the small particles that make them up. They include the D Ring, a 7,500 km ring that extends inward to Saturn’s cloud tops (66,900 – 74,510 km from Saturn’s center) and is separated from the C Ring by the 150 km Colombo Gap. On the other end of the system, the G and E Rings are located, which are also “dusty” in composition.
The G Ring is 9000 km in width and extends 166,000 – 175,000 km from Saturn’s center. The E Ring, meanwhile, is the largest single ring section, measuring 300,000 km in width and extending 166,000 to 480,000 km from Saturn’s center. It is here where the majority of Saturn’s moons are located (see above).
The narrow F Ring, which sits on the outer edge of the A Ring, is more difficult to categorize. While some parts of it are very dense, it also contains a great deal of dust-size particles. For this reason, estimates on its width range from 30 to 500 km, and it extends roughly 140,180 km from Saturn’s center.
History of Observing Saturn:
Because it is visible to the naked eye in the night sky, human beings have been observing Saturn for thousands of years. In ancient times, it was considered the most distant of five known the planets, and thus was accorded special meaning in various mythologies. The earliest recorded observations come from the Babylonians, where astronomers systematically observed and recorded its movements through the zodiac.
Roman astrological calendar, from the stone plate of the 3rd—4th centuries CE, Rome. Credit: Museo della civiltà romana
To the ancient Greeks, this outermost planet was named Cronus (Kronos), after the Greek god of agriculture and youngest of the Titans. The Greek scientist Ptolemy made calculations of Saturn’s orbit based on observations of the planet while it was in opposition.The Romans followed in this tradition, identifying it with their equivalent of Cronos (named Saturnus).
In ancient Hebrew, Saturn is called ‘Shabbathai’, whereas in Ottoman Turkish, Urdu and Malay, its name is ‘Zuhal’, which derived is from the original Arabic. In Hindu astrology, there are nine astrological objects known as Navagrahas. Saturn, which is one of them, is known as “Shani”, who judges everyone based on the good and bad deeds performed in life. In ancient China and Japan, the planet was designated as the “earth star” – based on the Five Elements of earth, air, wind, water and fire.
However, the planet was not directly observed until 1610, when Galileo Galilee first discerned the presence of rings. At the time, he mistook them for two moons that were located on either side. It was not until Christiaan Huygens used a telescope with greater magnification that this was corrected. Huygens also discovered Saturn’s moon Titan, and Giovanni Domenico Cassini later discovered the moons of Iapetus, Rhea, Tethys and Dione.
No further discoveries of significance were made again until the 181th and 19th centuries. The first occurred in 1789 when William Herschel discovered the two distant moons of Mimas and Enceladus, and then in 1848 when a British team discovered the irregularly-shaped moon of Hyperion.
Drawing of Saturn by Robert Hook, taken from Philosophical Transactions (1666). Credit: Wikipedia Commons
In 1899 William Henry Pickering discovered Phoebe, noting that it had a highly irregular orbit that did not rotate synchronously with Saturn as the larger moons do. This was the first time any satellite had been found to move about a planet in retrograde orbit. And by 1944, research conducted throughout the early 20th century confirmed that Titan has a thick atmosphere – a feature unique among the Solar System’s moons.
Exploration of Saturn:
By the late 20th century, unmanned spacecraft began to conduct flybys of Saturn, gathering information on its composition, atmosphere, ring structure, and moons. The first flyby was conducted by NASA using the Pioneer 11 robotic space probe, which passed Saturn at a distance of 20,000 km in September of 1979.
Images were taken of the planet and a few of its moons, although their resolution was too low to discern surface detail. The spacecraft also studied Saturn’s rings, revealing the thin F Ring and the fact that dark gaps in the rings are bright when facing towards the Sun, meaning that they contain fine light-scattering material. In addition, Pioneer 11 measured the temperature of Titan.
The next flyby took place in November of 1980 when the Voyager 1 space probe passed through the Saturn system. It sent back the first high-resolution images of the planet, its rings and satellites – which included features of various moons that had never before been seen.
These six narrow-angle color images were made from the first ever ‘portrait’ of the solar system taken by Voyager 1 in November 1980. Credit: NASA/JPL
In August 1981, Voyager 2 conducted its flyby and gathered more close-up images of Saturn’s moons, as well as evidence of changes in the atmosphere and the rings. The probes discovered and confirmed several new satellites orbiting near or within the planet’s rings, as well as the small Maxwell Gap and Keeler gap (a 42 km wide gap in the A Ring).
In June of 2004, the Cassini–Huygens space probe entered the Saturn system and conducted a close flyby of Phoebe, sending back high-resolution images and data. By July 1st, 2004, the probe entered orbit around Saturn, and by December, it had completed two flybys of Titan before releasing the Huygens probe. This lander reached the surface and began transmitting data on the atmospheric and surface by by Jan. 14th, 2005. Cassini has since conducted multiple flybys of Titan and other icy satellites.
In 2006, NASA reported that Cassini had found evidence of liquid water reservoirs that erupt in geysers on Saturn’s moon Enceladus. Over 100 geysers have since been identified, which are concentrated around the southern polar region. In May 2011, NASA scientists at an Enceladus Focus Group Conference reported that Enceladus’ interior ocean may be the most likely candidate in the search for extra-terrestrial life.
In addition, Cassini photographs have revealed a previously undiscovered planetary ring, eight new satellites, and evidence of hydrocarbon lakes and seas near Titan’s north pole. The probe was also responsible for sending back high-resolution images of the intense storm activity at Saturn’s northern and southern poles.
Cassini’s primary mission ended in 2008, but the probe’s mission has been extended twice since then – first to September 2010 and again to 2017. In the coming years, NASA hopes to use the probe to study a full period of Saturn’s seasons.
Artist Illustration of the Cassini space probe to Saturn and Titan, a joint NASA, ESA mission. Credit: NASA/JPL
From being a very important part of the astrological systems of many cultures to becoming the subject of ongoing scientific fascination, Saturn continues to occupy a special place in our hearts and minds. Whether it’s Saturn’s fantastically large and beautiful ring system, its many many moons, its tempestuous weather, or its curious composition, this gas giant continues to fascinate and inspire.
In the coming years and decades, additional robotic explorer missions will likely to be sent to investigate Saturn, its rings and its system of moons in greater detail. What we find may constitute some of the most groundbreaking discoveries of all time, and will likely teach us more about the history of our Solar System.
It’s a staple of scifi, and a requirement if we’re going to travel long-term in space. Will we ever develop artificial gravity?
It’s safe to say we’ve spent a significant amount of our lives consuming science fiction.
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Science fiction is great for the imagination, it’s rich in iron and calcium, and takes us to places we could never visit. It also helps us understand and predict what might happen in the future: tablet computers, cloning, telecommunication satellites, Skype, magic slidey doors, and razors with 5 blades.
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Then there are a whole bunch of predictions that have yet to happen, but still might, Fun things like the climate change apocalypse, regular robot apocalypse, the giant robot apocalypse, the alien invasion apocalypse, the apocalypse apocalypse, comet apocalypse, and the great Brawndo famine of 2506. Continue reading “Could We Make Artificial Gravity?”
