Phobos and Deimos, photographed here by the Mars Reconnaissance Orbiter, are tiny, irregularly-shaped moons that are probably strays from the main asteroid belt. Credit: NASA - See more at: http://astrobob.areavoices.com/2013/07/05/rovers-capture-loony-moons-and-blue-sunsets-on-mars/#sthash.eMDpTVPT.dpuf
Many of the planets in our Solar System have a system of moons. But among the rocky planets that make up the inner Solar System, having moons is a privilege enjoyed only by two planets: Earth and Mars. And for these two planets, it is a rather limited privilege compared to gas giants like Jupiter and Saturn which each have several dozen moons.
Whereas Earth has only one satellite (aka. the Moon), Mars has two small moons in orbit around it: Phobos and Deimos. And whereas the vast majority of moons in our Solar System are large enough to become round spheres similar to our own Moon, Phobos and Deimos are asteroid-sized and misshapen in appearance.
A brilliant Geminid flashes below Sirius and Orion over Mount Balang in China. Credit: NASA/Alvin Wu
2015 looks like a fantastic year for the Geminids. With the Moon just 3 days past new and setting at the end of evening twilight, conditions couldn’t be more ideal. Provided the weather cooperates! But even there we get a break. With a maximum of 120 meteors per hour, the shower is expected to peak around 18:00 UT (1 p.m. EST, 10 a.m. PST) December 14th, making for two nights of approximately equal activity: Sunday night Dec. 13-14 and Monday night Dec. 14-15. Continue reading “Viewing Guide to the 2015 Geminid Meteor Shower”
At one time, astronomers believed the surface of Mars was crisscrossed by canal systems. This in turn gave rise to speculation that Mars was very much like Earth, capable of supporting life and home to a native civilization. But as human satellites and rovers began to conduct flybys and surveys of the planet, this vision of Mars quickly dissolved, replaced by one in which the Red Planet was a cold, desiccated and lifeless world.
However, over the past few decades, scientists have come to learn a great deal about the history of Mars that has altered this view as well. We now know that though Mars may currently be very cold, very dry, and very inhospitable, this wasn’t always the case. What’s more, we have come to see that even in its current form, Mars and Earth actually have a lot in common.
Between the two planets, there are similarities in size, inclination, structure, composition, and even the presence of water on their surfaces. That being said, they also have a lot of key differences that would make living on Mars, a growing preoccupation among many humans (looking at you, Elon Musk and Bas Lansdorp!), a significant challenge. Let’s go over these similarities and the difference in an orderly fashion, shall we?
Sizes, Masses and Orbits:
In terms of their size and mass, Earth and Mars are quite different. With a mean radius of 6371 km and a mass of 5.97×1024 kg, Earth is the fifth largest and fifth most-massive planet in the Solar System, and the largest of the terrestrial planets. Mars, meanwhile, has a radius of approximately 3,396 km at its equator (3,376 km at its polar regions), which is the equivalent of roughly 0.53 Earths. However, it’s mass is just 6.4185 x 10²³ kg, which is around 10.7% that of Earth’s.
Artistic representation of the orbits of Earth and Mars. Credit: NASA
Similarly, Earth’s volume is a hefty 1.08321 x 1012 km3, which works out 1,083 billion cubic kilometers. By comparison, Mars has a volume of 1.6318 x 10¹¹ km³ (163 billion cubic kilometers) which is the equivalent of 0.151 Earths. Between this difference in size, mass, and volume, Mars’s surface gravity is 3.711 m/s², which works out to 37.6% of Earths (0.376 g).
In terms of their orbits, Earth and Mars are also quite different. For instance, Earth orbits the Sun at an average distance (aka. semi-major axis) of 149,598,261 km – or one Astronomical Unit (AU). This orbit has a very minor eccentricity (approx. 0.0167), which means its orbit ranges from 147,095,000 km (0.983 AU) at perihelion to 151,930,000 km (1.015 AU) at aphelion.
At its greatest distance from the Sun (aphelion), Mars orbits at a distance of approximately 249,200,000 km (1.666 AU). At perihelion, when it is closest to the Sun, it orbits at a distance of approximately 206,700,000 km (1.3814 AU). At these distances, the Earth has an orbital period of 365.25 days (1.000017 Julian years) while Mars has an orbital period of 686.971 days (1.88 Earth years).
However, in terms of their sidereal rotation (time it takes for the planet to complete a single rotation on its axis) Earth and Mars are again in the same boat. While Earth takes precisely 23h 56m and 4 s to complete a single sidereal rotation (0.997 Earth days), Mars does the same in about 24 hours and 40 minutes. This means that one Martian day (aka. Sol) is very close to single day on Earth.
Earth’s axial tilt (or obliquity) and its relation to the rotation axis and plane of orbit. Credit: Wikipedia Commons
Mars’s axial tilt is very similar to Earth’s, being inclined 25.19° to its orbital plane (whereas Earth’s axial tilt is just over 23°). This means that Mars also experiences seasons and temperature variations similar to that of Earth (see below).
Structure and Composition:
Earth and Mars are similar when it comes to their basic makeups, given that they are both terrestrial planets. This means that both are differentiated between a dense metallic core and an overlying mantle and crust composed of less dense materials (like silicate rock). However, Earth’s density is higher than that of Mars – 5.514 g/cm3 compared to 3.93 g/cm³ (or 0.71 Earths) – which indicates that Mars’ core region contains more lighter elements than Earth’s.
Earth’s core region is made up of a solid inner core that has a radius of about 1,220 km and a liquid outer core that extends to a radius of about 3,400 km. Both the inner and outer cores are composed of iron and nickel, with trace amounts of lighter elements, and together, they add to a radius that is as large as Mars itself. Current models of Mars’ interior suggest that its core region is roughly 1,794 ± 65 kilometers (1,115 ± 40 mi) in radius, and is composed primarily of iron and nickel with about 16-17% sulfur.
Both planets have a silicate mantle surrounding their cores and a surface crust of solid material. Earth’s mantle – consisting of an upper mantle of slightly viscous material and a lower mantle that is more solid – is roughly 2,890 km (1,790 mi) thick and is composed of silicate rocks that are rich in iron and magnesium. The Earth’s crust is on average 40 km (25 mi) thick, and is composed of rocks that are rich in iron and magnesium (i.e. igneous rocks) and granite (rich in sodium, potassium, and aluminum).
Artist’s impression of the interior of Mars. Credit: NASA/JPL
Comparatively, Mars’ mantle is quite thin, measuring some 1,300 to 1,800 kilometers (800 – 1,100 mi) in thickness. Like Earth, this mantle is believed to be composed of silicate rock that are rich in minerals compared to the crust, and to be partially viscous (resulting in convection currents which shaped the surface). The crust, meanwhile, averages about 50 km (31 mi) in thickness, with a maximum of 125 km (78 mi). This makes it about three times as hick as Earth’s crust, relative to the sizes of the two planets.
Ergo, the two planets are similar in composition, owing to their common status as terrestrial planets. And while they are both differentiated between a metallic core and layers of less dense material, there is some variance in terms of how proportionately thick their respective layers are.
Surface Features:
When it comes to the surfaces of Earth and Mars, things once again become a case of contrasts. Naturally, it is the differences that are most apparent when comparing Blue Earth to the Red Planet – as the nicknames would suggest. Unlike other planet’s in our Solar System, the vast majority of Earth is covered in liquid water, about 70% of the surface – or 361.132 million km² (139.43 million sq mi) to be exact.
The surface of Mars is dry, dusty, and covered in dirt that is rich iron oxide (aka. rust, leading to its reddish appearance). However, large concentrations of ice water are known to exist within the polar ice caps – Planum Boreum and Planum Australe. In addition, a permafrost mantle stretches from the pole to latitudes of about 60°, meaning that ice water exists beneath much of the Martian surface. Radar data and soil samples have confirmed the presence of shallow subsurface water at the middle latitudes as well.
As for the similarities, Earth and Mars’ both have terrains that varies considerably from place to place. On Earth, both above and below sea level, there are mountainous features, volcanoes, scarps (trenches), canyons, plateaus, and abyssal plains. The remaining portions of the surface are covered by mountains, deserts, plains, plateaus, and other landforms.
Mars is quite similar, with a surface covered by mountain ranges, sandy plains, and even some of the largest sand dunes in the Solar System. It also has the largest mountain in the Solar System, the shield volcano Olympus Mons, and the longest, deepest chasm in the Solar System: Valles Marineris.
Earth and Mars have also experienced many impacts from asteroids and meteors over the years. However, Mars’ own impact craters are far better preserved, with many dating back billions of years. The reason for this is the low air pressure and lack of precipitation on Mars, which results in a very slow rate of erosion. However, this was not always the case.
Mars has discernible gullies and channels on its surface, and many scientists believe that liquid water used to flow through them. By comparing them to similar features on Earth, it is believed that these were were at least partially formed by water erosion. Some of these channels are quite large, reaching 2,000 kilometers in length and 100 kilometers in width.
Color mosaic of Mars’ greatest mountain, Olympus Mons, viewed from orbit. Credit NASA/JPL
So while they look quite different today, Earth and Mars were once quite similar. And similar geological processes occurred on both planets to give them the kind of varied terrain they both currently have.
Atmosphere and Temperature:
Atmospheric pressure and temperatures are another way in which Earth and Mars are quite different. Earth has a dense atmosphere composed of five main layers – the Troposphere, the Stratosphere, the Mesosphere, the Thermosphere, and the Exosphere. Mars’ is very thin by comparison, with pressure ranging from 0.4 – 0.87 kPa – which is equivalent to about 1% of Earth’s at sea level.
Earth’s atmosphere is also primarily composed of nitrogen (78%) and oxygen (21%) with trace concentrations of water vapor, carbon dioxide, and other gaseous molecules. Mars’ is composed of 96% carbon dioxide, 1.93% argon and 1.89% nitrogen along with traces of oxygen and water. Recent surveys have also noted trace amounts of methane, with an estimated concentration of about 30 parts per billion (ppb).
Because of this, there is a considerable difference between the average surface temperature on Earth and Mars. On Earth, it is approximately 14°C, with plenty of variation due to geographical region, elevation, and time of year. The hottest temperature ever recorded on Earth was 70.7°C (159°F) in the Lut Desert of Iran, while the coldest temperature was -89.2°C (-129°F) at the Soviet Vostok Station on the Antarctic Plateau.
Space Shuttle Endeavor silhouetted against the atmosphere. The orange layer is the troposphere, the white layer is the stratosphere and the blue layer the mesosphere. Credit: NASABecause of its thin atmosphere and its greater distance from the Sun, the surface temperature of Mars is much colder, averaging at -46 °C (-51 °F). However, because of its tilted axis and orbital eccentricity, Mars also experiences considerable variations in temperature. These can be seen in the form of a low temperature of -143 °C (-225.4 °F) during the winter at the poles, and a high of 35 °C (95 °F) during summer and midday at the equator.
The atmosphere of Mars is also quite dusty, containing particulates that measure 1.5 micrometers in diameter, which is what gives the Martian sky a tawny color when seen from the surface. The planet also experiences dust storms, which can turn into what resembles small tornadoes. Larger dust storms occur when the dust is blown into the atmosphere and heats up from the Sun.
So basically, Earth has a dense atmosphere that is rich in oxygen and water vapor, and which is generally warm and conducive to life. Mars, meanwhile, is generally very cold, but can become quite warm at times. It’s also quite dry and very dusty.
Magnetic Fields:
When it comes to magnetic fields, Earth and Mars are in stark contrast to each other. On Earth, the dynamo effect created by the rotation of Earth’s inner core, relative to the rotation of the planet, generates the currents which are presumed to be the source of its magnetic field. The presence of this field is of extreme importance to both Earth’s atmosphere and to life on Earth as we know it.
Map from the Mars Global Surveyor of the current magnetic fields on Mars. Credit: NASA/JPL
Essentially, Earth’s magnetosphere serves to deflect most of the solar wind’s charged particles which would otherwise strip away the ozone layer and expose Earth to harmful radiation. The field ranges in strength between approximately 25,000 and 65,000 nanoteslas (nT), or 0.25–0.65 Gauss units (G).
Today, Mars has weak magnetic fields in various regions of the planet which appear to be the remnant of a magnetosphere. These fields were first measured by the Mars Global Surveyor, which indicated fields of inconsistent strengths measuring at most 1500 nT (~16-40 times less than Earth’s). In the northern lowlands, deep impact basins, and the Tharsis volcanic province, the field strength is very low. But in the ancient southern crust, which is undisturbed by giant impacts and volcanism, the field strength is higher.
This would seem to indicate that Mars had a magnetosphere in the past, and explanations vary as to how it lost it. Some suggest that it was blown off, along with the majority of Mars’ atmosphere, by a large impact during the Late Heavy Bombardment. This impact, it is reasoned, would have also upset the heat flow in Mars’ iron core, arresting the dynamo effect that would have produced the magnetic field.
Another theory, based on NASA’s MAVEN mission to study the Martian atmosphere, has it that Mars’ lost its magnetosphere when the smaller planet cooled, causing its dynamo effect to cease some 4.2 billion years ago. During the next several hundred million years, the Sun’s powerful solar wind stripped particles away from the unprotected Martian atmosphere at a rate 100 to 1,000 times greater than that of today. This in turn is what caused Mars to lose the liquid water that existed on its surface, as the environment to become increasing cold, desiccated, and inhospitable.
Satellites:
Earth and Mars are also similar in that both have satellites that orbit them. In Earth’s case, this is none other than The Moon, our only natural satellite and the source of the Earth’s tides. It’s existence has been known of since prehistoric times, and it has played a major role in the mythological and astronomical traditions of all human cultures. In addition, its size, mass and other characteristics are used as a reference point when assessing other satellites.
The Moon is one of the largest natural satellites in the Solar System and is the second-densest satellite of those whose moons who’s densities are known (after Jupiter’s satellite Io). Its diameter, at 3,474.8 km, is one-fourth the diameter of Earth; and at 7.3477 × 1022 kg, its mass is 1.2% of the Earth’s mass. It’s mean density is 3.3464 g/cm3 , which is equivalent to roughly 0.6 that of Earth. All of this results in our Moon possessing gravity that is about 16.54% the strength of Earth’s (aka. 1.62 m/s2).
The Moon varies in orbit around Earth, going from 362,600 km at perigee to 405,400 km at apogee. And like most known satellites within our Solar System, the Moon’s sidereal rotation period (27.32 days) is the same as its orbital period. This means that the Moon is tidally locked with Earth, with one side is constantly facing towards us while the other is facing away.
Thanks to examinations of Moon rocks that were brought back to Earth, the predominant theory states that the Moon was created roughly 4.5 billion years ago from a collision between Earth and a Mars-sized object (known as Theia). This collision created a massive cloud of debris that began circling our planet, which eventually coalesced to form the Moon we see today.
Mars has two small satellites, Phobos and Deimos. These moons were discovered in 1877 by the astronomer Asaph Hall and were named after mythological characters. In keeping with the tradition of deriving names from classical mythology, Phobos and Deimos are the sons of Ares – the Greek god of war that inspired the Roman god Mars. Phobos represents fear while Deimos stands for terror or dread.
Phobos measures about 22 km (14 mi) in diameter, and orbits Mars at a distance of 9,234.42 km when it is at periapsis (closest to Mars) and 9,517.58 km when it is at apoapsis (farthest). At this distance, Phobos is below synchronous altitude, which means that it takes only 7 hours to orbit Mars and is gradually getting closer to the planet. Scientists estimate that in 10 to 50 million years, Phobos could crash into Mars’ surface or break up into a ring structure around the planet.
Meanwhile, Deimos measures about 12 km (7.5 mi) and orbits the planet at a distance of 23,455.5 km (periapsis) and 23,470.9 km (apoapsis). It has a longer orbital period, taking 1.26 days to complete a full rotation around the planet. Mars may have additional moons that are smaller than 50- 100 meters (160 to 330 ft) in diameter, and a dust ring is predicted between Phobos and Deimos.
Scientists believe that these two satellites were once asteroids that were captured by the planet’s gravity. The low albedo and the carboncaceous chondrite composition of both moons – which is similar to asteroids – supports this theory, and Phobos’ unstable orbit would seem to suggest a recent capture. However, both moons have circular orbits near the equator, which is unusual for captured bodies.
So while Earth has a single satellite that is quite large and dense, Mars has two satellites that are small and orbit it at a comparatively close distance. And whereas the Moon was formed from Earth’s own debris after a rather severe collision, Mars’ satellites were likely captured asteroids.
Conclusion:
Okay, let’s review. Earth and Mars have their share of similarities, but also some rather stark differences.
Mean Radius: 6,371 km 3,396 km
Mass: 59.7×1023 kg 6.42 x 10²³ kg
Volume: 10.8 x 1011 km3 1.63 x 10¹¹ km³
Semi-Major Axis: 0.983 – 1.015 AU 1.3814 – 1.666 AU
Air Pressure: 101.325 kPa 0.4 – 0.87 kPa
Gravity: 9.8 m/s² 3.711 m/s²
Avg. Temperature: 14°C (57.2 °F) -46 °C (-51 °F)
Temp. Variations: ±160 °C (278°F) ±178 °C (320°F)
Axial Tilt: 23° 25.19°
Length of Day: 24 hours 24h 40m
Length of Year: 365.25 days 686.971 days
Water: Plentiful Intermittent (mostly frozen)
Polar Ice Caps: Yep Yep
In short, compared to Earth, Mars is a pretty small, dry, cold, and dusty planet. It has comparatively low gravity, very little atmosphere and no breathable air. And the years are also mighty long, almost twice that of Earth, in fact. However, the planet does have its fair share of water (albeit mostly in ice form), has seasonal cycles similar to Earth, temperature variations that are similar, and a day that is almost as long.
All of these factors will have to be addressed if ever human beings want to live there. And whereas some can be worked with, others will have to be overcome or adapted to. And for that, we will have to lean pretty heavily on our technology (i.e. terraforming and geoengineering). Best of luck to those who would like to venture there someday, and who do not plan on coming home!
The Earth's layers, showing the Inner and Outer Core, the Mantle, and Crust. Credit: discovermagazine.com
There is more to the Earth than what we can see on the surface. In fact, if you were able to hold the Earth in your hand and slice it in half, you’d see that it has multiple layers. But of course, the interior of our world continues to hold some mysteries for us. Even as we intrepidly explore other worlds and deploy satellites into orbit, the inner recesses of our planet remains off limit from us.
However, advances in seismology have allowed us to learn a great deal about the Earth and the many layers that make it up. Each layer has its own properties, composition, and characteristics that affects many of the key processes of our planet. They are, in order from the exterior to the interior – the crust, the mantle, the outer core, and the inner core. Let’s take a look at them and see what they have going on.
Modern Theory:
Like all terrestrial planets, the Earth’s interior is differentiated. This means that its internal structure consists of layers, arranged like the skin of an onion. Peel back one, and you find another, distinguished from the last by its chemical and geological properties, as well as vast differences in temperature and pressure.
Our modern, scientific understanding of the Earth’s interior structure is based on inferences made with the help of seismic monitoring. In essence, this involves measuring sound waves generated by earthquakes, and examining how passing through the different layers of the Earth causes them to slow down. The changes in seismic velocity cause refraction which is calculated (in accordance with Snell’s Law) to determine differences in density.
Model of a flat Earth, with the continents modeled in a disk-shape and Antarctica as an ice wall. Credit: Wikipedia Commons
These are used, along with measurements of the gravitational and magnetic fields of the Earth and experiments with crystalline solids that simulate pressures and temperatures in the Earth’s deep interior, to determine what Earth’s layers looks like. In addition, it is understood that the differences in temperature and pressure are due to leftover heat from the planet’s initial formation, the decay of radioactive elements, and the freezing of the inner core due to intense pressure.
History of Study:
Since ancient times, human beings have sought to understand the formation and composition of the Earth. The earliest known cases were unscientific in nature – taking the form of creation myths or religious fables involving the gods. However, between classical antiquity and the medieval period, several theories emerged about the origin of the Earth and its proper makeup.
Most of the ancient theories about Earth tended towards the “Flat-Earth” view of our planet’s physical form. This was the view in Mesopotamian culture, where the world was portrayed as a flat disk afloat in an ocean. To the Mayans, the world was flat, and at it corners, four jaguars (known as bacabs) held up the sky. The ancient Persians speculated that the Earth was a seven-layered ziggurat (or cosmic mountain), while the Chinese viewed it as a four-side cube.
By the 6th century BCE, Greek philosophers began to speculate that the Earth was in fact round, and by the 3rd century BCE, the idea of a spherical Earth began to become articulated as a scientific matter. During the same period, the development of a geological view of the Earth also began to emerge, with philosophers understanding that it consisted of minerals, metals, and that it was subject to a very slow process of change.
Illustration of Edmond Halley’s model of a Hallow Earth, one that was made up of concentric spheres. Credit: Wikipedia Commons/Rick Manning
However, it was not until the 16th and 17th centuries that a scientific understanding of planet Earth and its structure truly began to advance. In 1692, Edmond Halley (discoverer of Halley’s Comet) proposed what is now known as the “Hollow-Earth” theory. In a paper submitted to Philosophical Transactions of Royal Society of London, he put forth the idea of Earth consisting of a hollow shell about 800 km thick (~500 miles).
Between this and an inner sphere, he reasoned there was an air gap of the same distance. To avoid collision, he claimed that the inner sphere was held in place by the force of gravity. The model included two inner concentric shells around an innermost core, corresponding to the diameters of the planets Mercury, Venus, and Mars respectively.
Halley’s construct was a method of accounting for the values of the relative density of Earth and the Moon that had been given by Sir Isaac Newton, in his Philosophiæ Naturalis Principia Mathematica (1687) – which were later shown to be inaccurate. However, his work was instrumental to the development of geography and theories about the interior of the Earth during the 17th and 18th centuries.
Another important factor was the debate during the 17th and 18th centuries about the authenticity of the Bible and the Deluge myth. This propelled scientists and theologians to debate the true age of the Earth, and compelled the search for evidence that the Great Flood had in fact happened. Combined with fossil evidence, which was found within the layers of the Earth, a systematic basis for identifying and dating the Earth’s strata began to emerge.
The growing importance of mining in the 17th and 18th centuries, particularly for precious metals, led to further developments in geology and Earth sciences. Credit: minerals.usgs.gov
The development of modern mining techniques and growing attention to the importance of minerals and their natural distribution also helped to spur the development of modern geology. In 1774, German geologist Abraham Gottlob Werner published Von den äusserlichen Kennzeichen der Fossilien (On the External Characters of Minerals) which presented a detailed system for identifying specific minerals based on external characteristics.
In 1741, the National Museum of Natural History in France created the first teaching position designated specifically for geology. This was an important step in further promoting knowledge of geology as a science and in recognizing the value of widely disseminating such knowledge. And by 1751, with the publication of the Encyclopédieby Denis Diderot, the term “geology” became an accepted term.
By the 1770s, chemistry was starting to play a pivotal role in the theoretical foundation of geology, and theories began to emerge about how the Earth’s layers were formed. One popular idea had it that liquid inundation, like the Biblical Deluge, was responsible for creating all the geological strata. Those who accepted this theory became known popularly as the Diluvianists or Neptunists.
Another thesis slowly gained currency from the 1780s forward, which stated that instead of water, strata had been formed through heat (or fire). Those who followed this theory during the early 19th century referred to this view as Plutonism, which held that the Earth formed gradually through the solidification of molten masses at a slow rate. These theories together led to the conclusion that the Earth was immeasurably older than suggested by the Bible.
HMS Beagle in the Galapagos Islands, painted by John Chancellor. Credit: hmsbeagleproject.otg
In the early 19th century, the mining industry and Industrial Revolution stimulated the rapid development of the concept of the stratigraphic column – that rock formations were arranged according to their order of formation in time. Concurrently, geologists and natural scientists began to understand that the age of fossils could be determined geologically (i.e. that the deeper the layer they were found in was from the surface, the older they were).
During the imperial period of the 19th century, European scientists also had the opportunity to conduct research in distant lands. One such individual was Charles Darwin, who had been recruited by Captain FitzRoy of the HMS Beagle to study the coastal land of South America and give geological advice.
Darwin’s discovery of giant fossils during the voyage helped to establish his reputation as a geologist, and his theorizing about the causes of their extinction led to his theory of evolution by natural selection, published in On the Origin of Species in 1859.
During the 19th century, the governments of several countries including Canada, Australia, Great Britain and the United States began funding geological surveys that would produce geological maps of vast areas of the countries. Thought largely motivated by territorial ambitions and resource exploitation, they did benefit the study of geology.
The Earth’s Tectonic Plates. Credit: msnucleus.org
By this time, the scientific consensus established the age of the Earth in terms of millions of years, and the increase in funding and the development of improved methods and technology helped geology to move farther away from dogmatic notions of the Earth’s age and structure.
By the early 20th century, the development of radiometric dating (which is used to determine the age of minerals and rocks), provided the necessary the data to begin getting a sense of the Earth’s true age. By the turn of the century, geologists now believed the Earth to be 2 billion years old, which opened doors for theories of continental movement during this vast amount of time.
In 1912, Alfred Wegener proposed the theory of Continental Drift, which suggested that the continents were joined together at a certain time in the past and formed a single landmass known as Pangaea. In accordance with this theory, the shapes of continents and matching coastline geology between some continents indicated they were once attached together.
The super-continent Pangea during the Permian period (300 – 250 million years ago). Credit: NAU Geology/Ron Blakey
Research into the ocean floor also led directly to the theory of Plate Tectonics, which provided the mechanism for Continental Drift. Geophysical evidence suggested lateral motion of continents and that oceanic crust is younger than continental crust. This geophysical evidence also spurred the hypothesis of paleomagnetism, the record of the orientation of the Earth’s magnetic field recorded in magnetic minerals.
Then there was the development of seismology, the study of earthquakes and the propagation of elastic waves through the Earth or through other planet-like bodies, in the early 20th century. By measuring the time of travel of refracted and reflected seismic waves, scientists were able to gradually infer how the Earth was layered and what lay deeper at its core.
For example, in 1910, Harry Fielding Ried put forward the “elastic rebound theory”, based on his studies of the 1906 San Fransisco earthquake. This theory, which stated that earthquakes occur when accumulated energy is released along a fault line, was the first scientific explanation for why earthquakes happen, and remains the foundation for modern tectonic studies.
Earth viewed from the Moon by the Apollo 11 spacecraft. Credit: NASA
Then in 1926, English scientist Harold Jeffreys claimed that below the crust, the core of the Earth is liquid, based on his study of earthquake waves. And then in 1937, Danish seismologist Inge Lehmann went a step further and determined that within the earth’s liquid outer core, there is a solid inner core.
By the latter half of the 20th century, scientists developed a comprehensive theory of the Earth’s structure and dynamics had formed. As the century played out, perspectives shifted to a more integrative approach, where geology and Earth sciences began to include the study of the Earth’s internal structure, atmosphere, biosphere and hydrosphere into one.
This was assisted by the development of space flight, which allowed for Earth’s atmosphere to be studied in detail, as well as photographs taken of Earth from space. In 1972, the Landsat Program, a series of satellite missions jointly managed by NASA and the U.S. Geological Survey, began supplying satellite images that provided geologically detailed maps, and have been used to predict natural disasters and plate shifts.
Earth’s Layers:
The Earth can be divided into one of two ways – mechanically or chemically. Mechanically – or rheologically, meaning the study of liquid states – it can be divided into the lithosphere, asthenosphere, mesospheric mantle, outer core, and the inner core. But chemically, which is the more popular of the two, it can be divided into the crust, the mantle (which can be subdivided into the upper and lower mantle), and the core – which can also be subdivided into the outer core, and inner core.
The inner core is solid, the outer core is liquid, and the mantle is solid/plastic. This is due to the relative melting points of the different layers (nickel–iron core, silicate crust and mantle) and the increase in temperature and pressure as depth increases. At the surface, the nickel-iron alloys and silicates are cool enough to be solid. In the upper mantle, the silicates are generally solid but localized regions of melt exist, leading to limited viscosity.
In contrast, the lower mantle is under tremendous pressure and therefore has a lower viscosity than the upper mantle. The metallic nickel–iron outer core is liquid because of the high temperature. However, the intense pressure, which increases towards the inner core, dramatically changes the melting point of the nickel–iron, making it solid.
The differentiation between these layers is due to processes that took place during the early stages of Earth’s formation (ca. 4.5 billion years ago). At this time, melting would have caused denser substances to sink toward the center while less-dense materials would have migrated to the crust. The core is thus believed to largely be composed of iron, along with nickel and some lighter elements, whereas less dense elements migrated to the surface along with silicate rock.
Earth’s Crust:
The crust is the outermost layer of the planet, the cooled and hardened part of the Earth that ranges in depth from approximately 5-70 km (~3-44 miles). This layer makes up only 1% of the entire volume of the Earth, though it makes up the entire surface (the continents and the ocean floor).
The Earth’s layers (strata) shown to scale. Credit: pubs.usgs.gov
The thinner parts are the oceanic crust, which underlies the ocean basins at a depth of 5-10 km (~3-6 miles), while the thicker crust is the continental crust. Whereas the oceanic crust is composed of dense material such as iron magnesium silicate igneous rocks (like basalt), the continental crust is less dense and composed of sodium potassium aluminum silicate rocks, like granite.
The uppermost section of the mantle (see below), together with the crust, constitutes the lithosphere – an irregular layer with a maximum thickness of perhaps 200 km (120 mi). Many rocks now making up Earth’s crust formed less than 100 million (1×108) years ago. However, the oldest known mineral grains are 4.4 billion (4.4×109) years old, indicating that Earth has had a solid crust for at least that long.
Upper Mantle:
The mantle, which makes up about 84% of Earth’s volume, is predominantly solid, but behaves as a very viscous fluid in geological time. The upper mantle, which starts at the “Mohorovicic Discontinuity” (aka. the “Moho” – the base of the crust) extends from a depth of 7 to 35 km (4.3 to 21.7 mi) downwards to a depth of 410 km (250 mi). The uppermost mantle and the overlying crust form the lithosphere, which is relatively rigid at the top but becomes noticeably more plastic beneath.
Compared to other strata, much is known about the upper mantle, thanks to seismic studies and direct investigations using mineralogical and geological surveys. Movement in the mantle (i.e. convection) is expressed at the surface through the motions of tectonic plates. Driven by heat from deeper in the interior, this process is responsible for Continental Drift, earthquakes, the formation of mountain chains, and a number of other geological processes.
Computer simulation of the Earth’s field in a period of normal polarity between reversals. Credit: science.nasa.govThe mantle is also chemically distinct from the crust, in addition to being different in terms of rock types and seismic characteristics. This is due in large part to the fact that the crust is made up of solidified products derived from the mantle, where the mantle material is partially melted and viscous. This causes incompatible elements to separate from the mantle, with less dense material floating upward and solidifying at the surface.
The crystallized melt products near the surface, upon which we live, are typically known to have a lower magnesium to iron ratio and a higher proportion of silicon and aluminum. These changes in mineralogy may influence mantle convection, as they result in density changes and as they may absorb or release latent heat as well.
In the upper mantle, temperatures range between 500 to 900 °C (932 to 1,652 °F). Between the upper and lower mantle, there is also what is known as the transition zone, which ranges in depth from 410-660 km (250-410 miles).
Lower Mantle:
The lower mantle lies between 660-2,891 km (410-1,796 miles) in depth. Temperatures in this region of the planet can reach over 4,000 °C (7,230 °F) at the boundary with the core, vastly exceeding the melting points of mantle rocks. However, due to the enormous pressure exerted on the mantle, viscosity and melting are very limited compared to the upper mantle. Very little is known about the lower mantle apart from that it appears to be relatively seismically homogeneous.
The internal structure of Earth. Credit: Wikipedia Commons/Kelvinsong
Outer Core:
The outer core, which has been confirmed to be liquid (based on seismic investigations), is 2300 km thick, extending to a radius of ~3,400 km. In this region, the density is estimated to be much higher than the mantle or crust, ranging between 9,900 and 12,200 kg/m3. The outer core is believed to be composed of 80% iron, along with nickel and some other lighter elements.
Denser elements, like lead and uranium, are either too rare to be significant or tend to bind to lighter elements and thus remain in the crust. The outer core is not under enough pressure to be solid, so it is liquid even though it has a composition similar to that of the inner core. The temperature of the outer core ranges from 4,300 K (4,030 °C; 7,280 °F) in the outer regions to 6,000 K (5,730 °C; 10,340 °F) closest to the inner core.
Because of its high temperature, the outer core exists in a low viscosity fluid-state that undergoes turbulent convection and rotates faster than the rest of the planet. This causes eddy currents to form in the fluid core, which in turn creates a dynamo effect that is believed to influence Earth’s magnetic field. The average magnetic field strength in Earth’s outer core is estimated to be 25 Gauss (2.5 mT), which is 50 times the strength of the magnetic field measured on Earth’s surface.
Inner Core:
Like the outer core, the inner core is composed primarily of iron and nickel and has a radius of ~1,220 km. Density in the core ranges between 12,600-13,000 kg/m³, which suggests that there must also be a great deal of heavy elements there as well – such as gold, platinum, palladium, silver and tungsten.
Artist’s illustration of Earth’s core, inner core, and inner-inner core. Credit: Huff Post Science
The temperature of the inner core is estimated to be about 5,700 K (~5,400 °C; 9,800 °F). The only reason why iron and other heavy metals can be solid at such high temperatures is because their melting temperatures dramatically increase at the pressures present there, which ranges from about 330 to 360 gigapascals.
Because the inner core is not rigidly connected to the Earth’s solid mantle, the possibility that it rotates slightly faster or slower than the rest of Earth has long been considered. By observing changes in seismic waves as they passed through the core over the course of many decades, scientists estimate that the inner core rotates at a rate of one degree faster than the surface. More recent geophysical estimates place the rate of rotation between 0.3 to 0.5 degrees per year relative to the surface.
Recent discoveries also suggest that the solid inner core itself is composed of layers, separated by a transition zone about 250 to 400 km thick. This new view of the inner core, which contains an inner-inner core, posits that the innermost layer of the core measures 1,180 km (733 miles) in diameter, making it less than half the size of the inner core. It has been further speculated that while the core is composed of iron, it may be in a different crystalline structure that the rest of the inner core.
What’s more, recent studies have led geologists to conjecture that the dynamics of deep interior is driving the Earth’s inner core to expand at the rate of about 1 millimeter a year. This occurs mostly because the inner core cannot dissolve the same amount of light elements as the outer core.
The freezing of liquid iron into crystalline form at the inner core boundary produces residual liquid that contains more light elements than the overlying liquid. This in turn is believed to cause the liquid elements to become buoyant, helping to drive convection in the outer core. This growth is therefore likely to play an important role in the generation of Earth’s magnetic field by dynamo action in the liquid outer core. It also means that the Earth’s inner core, and the processes that drive it, are far more complex than previously thought!
Yes indeed, the Earth is a strange and mysteries place, titanic in scale as well as the amount of heat and energy that went into making it many billions of years ago. And like all bodies in our universe, the Earth is not a finished product, but a dynamic entity that is subject to constant change. And what we know about our world is still subject to theory and guesswork, given that we can’t examine its interior up close.
As the Earth’s tectonic plates continue to drift and collide, its interior continues to undergo convection, and its core continues to grow, who knows what it will look like eons from now? After all, the Earth was here long before we were, and will likely continue to be long after we are gone.
An artist's image of Hayabusa leaving Earth. Hayabusa was a Japanese sample return mission to the asteroid 25143 Itokawa. The mission was a partial success. A sample mission to Earth's sister planet is the holy-grail for the exploration of Venus. Image credit: JAXA
A space-faring friend pays our fair planet a visit this week on the morning of December 3rd, as the Japanese Space Agency’s Hayabusa 2 spacecraft passes the Earth.
The Flyby
Rick Baldridge on the SeeSat-L message board notes that Hayabusa-2 will pass 9,520 kilometers from the Earth’s center or 3,142 kilometers/1,885 miles from the Earth’s surface at 10:08 UT/5:08 AM EST on Thursday, December 3rd, passing from north-to-south above latitude 18.7 north, longitude 189.8 east just southwest of the Hawaiian Islands.
Unfortunately, the sighting opportunities for Hayabusa-2 aren’t stellar: even at its closest, the 1.5 meter-sized spacecraft is about nine times more distant than the International Space Station and satellites in low Earth orbit. To compound the challenge, Hayabusa-2 passes into the Earth’s shadow from 9:58 UT to 10:19 UT.
The path of Hayabusa-2 past the Earth. Image credit: JAXA
Still, skilled observers with large telescopes and sophisticated tracking rigs based along the Pacific Rim of North America might just catch sight of Hayabusa-2 as it speeds by. The JPL Horizons ephemeris generator is a great resource to create a customized positional chart in right ascension and declination for spacecraft for your given location, including Hayabusa-2.
The Earth-Moon pair snapped by Hayabusa-2 on November 26th from about three million kilometers distant. Image credit: JAXA
Hayabusa-2 won’t crack 20 degrees elevation for observers along the U.S. West Coast, putting it down in the atmospheric murk of additional air mass low to the horizon. This also tends to knock the brightness of objects down a magnitude or so… estimates place Hayabusa-2 at around magnitude +13 shortly before entering the Earth’s shadow. That’s pretty faint, but still, there are some dedicated observers with amazing rigs out there, and it’s quite possible someone could nab it. Hawaii-based observers should have the best shot at it, though again, it’ll be in the Earth’s shadow at its very closest…
Amateur radio satellite trackers are also on the hunt for the carrier-wave signal of the inbound Hayabusa-2 mission. You can also virtually fly along with the spacecraft until December 5th: (H/T @ImAstroNix):
A simulation of tomorrow’s flyby. Image Credit: JAXA
Probably the best eye-candy images will come from the spacecraft itself: already, Hayabusa-2 has already snapped some great images of the Earth-Moon pair using its ONC-T optical navigation camera during its inbound leg.
A close-up of Hayabusa-2’s view of the Earth and Moon. Image credit: JAXA
Other notable missions used Earth flybys en route to their final destinations, including Cassini in 1999, and Juno in 2013. Cassini’s return caused a bit of a stir as it has a plutonium-powered RTG aboard, though Earth and its inhabitants were never in danger. A nuclear RTG actually reentered during the return of Apollo 13, with no release of radioactive material. Meant for the ALSEP science package on the surface of the Moon, it was deposited on the reentry of the Lunar Module over the Marinas Trench in the South Pacific. And no, Hayabusa-2 carries no radioactive material, and in any event, it’s missing the Earth by about a quarter of its girth.
The successor to the Hayabusa (‘Peregrine Falcon’ in Japanese) mission which carried out a historic asteroid sample return from 25143 Itokawa in 2010, Hayabusa-2 launched atop an H-IIA rocket from Tanegashima, Japan exactly a year ago tomorrow on a six year mission to asteroid 162173 Ryugu. This week’s Earth flyby will boost the spacecraft an additional 1.6 kilometers per second to an outbound velocity towards its target of 31.9 kilometers per second post-flyby.
Launch of an H-IIA rocket with Hayabusa-2. Image credit: JAXA
Like its predecessor, Hayabusa-2 is a sample return mission. Unlike the original Hayabusa, however, Hayabusa-2 is more ambitious, also carrying the MASCOT (Mobile Asteroid Surface Scout) lander and an explosive seven kilogram impactor. Hayabusa-2 will deploy a secondary camera in orbit to watch the detonation and will briefly touch down at the impact site to collect material.
If all goes as planned, Hayabusa-2 will return to Earth in late 2020.
NASA has its own future asteroid sample return mission planned, named OSIRIS-REx. This mission will launch in September of next year to rendezvous with asteroid 101955 Bennu in September 2019 and return to Earth in September 2023.
An artist’s conception on Hayabusa 2 at asteroid 162173 Ryugu. Image credit: JAXA
We’re entering the golden age of asteroid exploration, for sure. And this all comes about as the U.S. authorized asteroid mining just last week (or at least, as stated, ‘asteroid utilization’) under the controversial U.S. Commercial Space Launch Competitiveness Act. But the original Hayabusa mission brought back mere micro-meter-sized dust grains, highlighting just how difficult asteroid mining is using present technology…
Perhaps, for now, its more cost effective to simply wait for the asteroids to come to us as meteorites and just scoop ’em up. We’ll be keeping an eye out over the next few days for images of Hayabusa-2 as it speeds by, and more postcards of the Earth-Moon system from the spacecraft as it heads towards its 2018 rendezvous with destiny.
A chart of the constellations and signs that make up the zodiac. Credit: NASA
Did you know that there are 88 constellations in the night sky? Over the course of several thousand years, human beings have cataloged and named them all. But only 12 of them are particularly famous and continue to play an active role in our astrological systems. These are known as the zodiac signs, 12 constellations that correspond to the different months of the year.
Each of these occupies a sector of the sky which makes up 30° of the ecliptic, starting at the vernal equinox – one of the intersections of the ecliptic with the celestial equator. The order of these astrological signs is Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius and Pisces. Here are all the zodiac signs and their dates. If your birthday falls within one of those date ranges, that’s your zodiac sign.
Granted, modern science has shown astrology to be an ancient fallacy, a way of connecting patterns in celestial movements to events and behaviors here on Earth. But for ancient people, such patterns were necessary given the fact that they lacked an understanding of human psychology, astronomy, and that Earth was not the center of the universe.
The concept of the zodiac originated in Babylon in the 2nd millennium BCE, and was later influenced by Hellenistic (Ancient Greek), Roman, and Egyptian culture. This resulted in a mix of traditions, where the 12 zodiac symbols were associated with the 12 Houses – different fields of experience associated with the various planets – and the four classical elements (Earth, Wind, Water and Fire).
The symbols of the zodiac. Credit: what-is-astrology.com
In essence, astrology maintains that celestial phenomena are related to human activity, so the signs are held to represent certain characteristics of behavior and personality traits. What we know today as astrology comes from the 2nd century AD, as it was formally described by Ptolemy in his work, Tetrabiblos.
This book was responsible for the spread of astrology’s as we know it across Europe and the Middle East during the time of the Roman Empire. These traditions have remained relatively unchanged for over seventeen centuries, though some alterations have been made due to the subsequent discoveries of the other planets in our Solar System.
Naturally, the birth of the modern psychology, biology and astronomy has completely discredited the notion that our personalities are determined by birth signs, the position of the stars or the planets. Given what we know today of the actual elements, the movements of the planets, and the forces that govern the universe, astrology is now known for being little more than superstition.
What’s more, the dates of the ‘star signs’ were assigned over 2,000 years ago, when the zodiac was first devised. At that time, astronomers believed that the Earth’s position was fixed in the universe, and did not understand that the Earth is subject to precession – where Earth’s rotational and orbital parameters slowly change with time. As such, the zodiac signs no longer correspond to constellations of stars that appear in night sky.
The constellations Ophiuchus, represented as a man grasping a snake. Credit: chandra.harvard.edu
And last, but certainly not least, there is the issue of the missing 13th sign, which corresponds to the constellation Ophiuchus. Over 2000 years ago, this constellation was deliberately left out, though the Sun clearly passes in front of it after passing in front of Scorpius (aka. Scorpio) and before reaching Sagittarius.
It is unclear why ancient astrologers would do this, but it is a safe bet that they wanted to divide the 360° path of the Sun into 12 equal parts. But the true boundaries that divide the constellations, as defined by the International Astronomical Union (IAU), are not exact. And Ophiuchus actually spends more time behind the Sun than its immediate neighbor (19 days compared to Scorpius’ 12).
To find out what zodiac sign you were really born under, check out this story from BBC’s iWonder. And in the meantime, here are the zodiac signs, listed in order along with what they mean, and some interesting facts associated with their respective constellations:
Aries: March 21 – April 19
The sign of Aries, which covers 0° to 30° of celestial longitude, is represented by The Ram, which is based on the Chrysomallus – the flying ram that provided the Golden Fleece in Greek mythology. Aries is associated with the First House, known traditionally as Vita (Latin for life) and in the modern context as the “House of Self”. Aries is associated with Fire, and the ruling celestial body of Aries is Mars.
The Aries constellation is also home to Teegarden’s Star, one of Sun’s closest neighbors, located approximately 12 light years away. It appears to be a red dwarf, a class of low temperature and low luminosity stars. And then there’s Alpha Areitis, which is easily spotted by the naked eye. Also known as “Hamal” – literally “head of the sheep” in Arabic – this star is located at the point where constellations angles downward to form an arc.
The constellation Aries. Credit: iau.org
For those with telescopes, several galaxies can be spotted within the Aries constellation as well. These include the spiral galaxy NGC 772 and the large 13th magnitude NGC 697 spiral galaxy. NGC 972 is another, which is faint (at magnitude 12) and part of a galaxy group. And then there’s the dwarf irregular galaxy NGC 1156, which is considered a Magellanic-type galaxy with a larger than average core.
Aries is also home to several meteor showers, such as the May Arietids. This daylight meteor shower begins between May 4th and June 6th with maximum activity happening on May 16th. The Epsilon Arietids are also a daylight occurrence, and are active between April 25th to May 27th with peak activity on May 9th. And then there are the Daytime Arietids, which occur from May 22nd to July 2nd with a maximum rate of one a minute on June 8th.
To top it off, the Aries constellation contains several stars with extrasolar planets. For example, HIP 14810, a G5 type star, is orbited by three confirmed exoplanets, all of them giant planets (all Super-Earths). HD 12661, also a G-type main sequence star, has two orbiting planets (which appear to be Super-Jupiters). And HD 20367, a G0 type star, has one orbiting gas giant that roughly the same size as Jupiter.
Taurus: April 20 – May 20
The sign of Taurus, which covers 30° to 60° of celestial longitude, is represented by The Bull – which is based on the Cretan Bull that fathered the Minotaur and was killed by Theseus. Taurus is associated with the Second House, known by the Latin name of Lucrum (wealth) and by the modern name, “House of Value”, and the element Earth. The ruling celestial body of Taurus is Venus.
The constellation Taurus. Credit: iau.org
Taurus’ brightest star, Alpha Tauri, is also known by its traditional name, Al Dabaran (which was Latinized to become Aldebaran). The name, which is Arabic, literally means “the Follower” because of the way the Taurus constellation appears to follow the Pleiades star cluster across the sky. In Latin, it was traditionally known as Stella Dominatrix, but to Medieval English astronomers, it was known as Oculus Tauri – literally the “eye of Taurus.”
There is one major annual meteor shower associated with the constellation of Taurus: the annual Taurids, which peak on or about November 5th of each year and have a duration period of about 45 days. The maximum fall rate for this meteor shower is about 10 meteors per hour, with many bright fireballs often occurring when the parent comet – Encke – has passed near perihelion.
And speaking of Pleiades (aka. Messier 45, The Seven Sisters) this cluster of stars is located perpendicular to Aldebaran in the night sky, and is visible to the unaided eye. Although it is made up of over 1000 confirmed stars, this object is identifiable by its seven particularly bright blue stars (though as many as 14 up can be seen with the naked eye depending on local observing conditions).
Gemini: May 21 – June 20
The sign Gemini covers 60° to 90° of the celestial longitude, and is represented by The Twins. These are based on the Dioscuri of Greek mythology, two mortals that were granted shared godhood after death. Gemini is part of the Third House, traditionally named Fratres (Brothers) and currently known as the House of Communications. The associated element for Geminis is Air, and the ruling celestial body is Mercury.
The constellation Gemini. Credit: iau.org
Gemini’s alpha and beta stars – aka. Castor and Pollux (“The Twins”) – are the easiest to recognize and can be spotted with the naked eye. Pollux is the brighter of the two, an orange-hued giant star of magnitude 1.2 that is 34 light-years from Earth. Pollux has an extrasolar planet revolving around it, as do two other stars in Gemini, a super-Jupiter which was confirmed in 2006.
There are two annual meteor showers associated with the constellation of Gemini. The first is the March Geminids, which peaks on or around March 22nd. The average fall rate is generally about 40 per hour (but this varies) and the meteors appear to be very slow, entering our atmosphere unhurriedly and leaving lasting trails.
The second meteor shower are the Geminids themselves, which peak on or near the date of December 14th, with activity beginning up to two weeks prior and lasting for several days. The Geminids are one of the most beautiful and mysterious showers, with a rate of about 110 per hour during a moonless night.
The Gemini constellation is also associated with Messier 35, a galactic open star cluster that is easily spotted with the naked eye. The star cluster is quite young, having formed some 100 million years ago, and is quite bright due to it having blown away most of its leftover material (i.e. nebular dust and gas) that went into the star formation process. Other open clusters in Gemini include NGC 2158, which lies directly southwest of M35 in the night sky.
The open star clusters Messier 35 and NGC 2158, photographed at La Palma, Roque de los Muchachos. Credit: estelar.de/Oliver Stein
Cancer: June 21 – July 22
Cancer, which covers 90° to 120° of celestial longitude, is represented by The Crab – or Karkinos, a giant crab from Greek mythology that harassed Hercules during his fight with the Hydra. The sign is associated with the Fourth House – Genitor (Parent) in Latin, or the House of Home and Family in modern times. In terms of the elements, Cancers are characterized by the element of Water, and the ruling celestial body of Cancer is The Moon.
Cancer’s best known star is Beta Cancri, also known by its Arab name Altarf (“the End”). This 3.5 magnitude star is located 290 light-years from Earth and is a binary star system that consists of a spectral type K4III orange giant and a magnitude 14 red dwarf. This system is also home to a confirmed exoplanet, beta Cancri b, which is a Super-Jupiter with an orbital period of over 600 days.
In terms of deep-sky objects, Cancer is best known as being the home of Messier Object 44 (aka. Praesepe, or the Beehive Cluster), an open cluster located in the center of the constellation. Located 577 light-years from Earth, it is one of the nearest open clusters to our Solar System. M44 contains about 50 stars, the brightest of which are of the sixth magnitude.
The smaller, denser open cluster of Messier Object 67 can also be found in Cancer, which is 2500 light-years from Earth and contains approximately 200 stars. And so can the famous quasar, QSO J0842+1835, which was used to measure the speed of gravity in the VLBI experiment conducted by Edward Fomalont and Sergei Kopeikin in September 2002.
The location of the Caner constellation. Credit: IAU
The active galaxy OJ 287 is also found in the Cancer constellation. Located 3.5 billion light years away from Earth, this galaxy has a central supermassive black hole that is one of the largest known (with 18 billion solar masses), and produces quasi-periodic optical outbursts. There is only one meteor shower associated with the constellation of Cancer, which is the Delta Cancrids. The peak date for this shower is on or about January 16t, and has been known to average only about 4 comets per hour (and the meteors are very swift).
Leo: July 23 – Aug. 22
Those born under the sign of Leo, which covers 120° to 150° of celestial longitude, carry the sign of The Lion – which is based on the Nemean Lion of Greek mythology, a lion that had an impenetrable hide. The sign of Leo is tied to the Fifth House, known in Latin as Nati (Children), or by its modern name, House of Pleasure. The sign of Leo is also associated with the element of Fire and the ruling celestial body of Leo is The Sun.
There are five annual meteor showers associated with the constellation Leo. The first is the Delta Leonid meteor stream, which begins between February 5th through March 19th every year. The activity peaks in late February, and the maximum amount of meteors averages around 5 per hour. The next is the Sigma Leonid meteor shower, which begins on April 17th. This is a very weak shower, with activity rates no higher than 1 to 2 per hour.
The next is the November Leonids, the largest and most dependable meteor shower associated with the Leo constellation. The peak date is November 17th, but activity occurs around 2 days on either side of the date. The radiant is near Regulus and this is the most spectacular of modern showers.
The constellation Leo. Credit: iau.org
The shower is made more spectacular by the appearance of the Temple-Tuttle comet, which adds fresh material to the stream when it is at perihelion. The last is the Leo Minorids, which peak on or about December 14th, which is believed to produce around 10 faint meteors per hour.
Leo is also home to some of the largest structures in the observable universe. This includes many bright galaxies, which includes the Leo Triplet (aka. the M60 group). This group of objects is made up of three spiral galaxies – Messier 65, Messier 66, and NGC 3628.
The Triplet is at a distance of 37 million light-years from Earth and has a somewhat distorted shape due to gravitational interactions with the other members of the Triplet, which are pulling stars away from M66. Both M65 and M66 are visible in large binoculars or small telescopes, but seeing them in all of their elongated glory requires a telescope.
In addition, it is also home to the famous objects Messier 95, Messier 96, and Messier 105. These are spiral galaxies, in the case of M95 and M96 (with M95 being a barred spiral), while Messier 105 is an elliptical galaxy which is known to have a supermassive black hole at its center. Then there is the Leo Ring (aka. Cosmic Horseshoe) a cloud of hydrogen and helium gas, that orbits two galaxies found within this constellation.
The notable gravitational lens known as the Cosmic Horseshoe is found in Leo. Credit: NASA/ESA/Hubble
Virgo: Aug. 23 – Sept. 22
The sign of Virgo, which covers 150° to 180° of celestial longitude, is represented by the The Maiden. Based on Astraea from Greek mythology, the maiden was the last immortal to abandon Earth at the end of the Silver Age, when the gods fled to Olympus. Virgo is part of the Sixth House – Valetudo (Health) in Latin, or House of Health in modern times. They are also associated with the element of Earth and the ruling celestial body of Virgo is Mercury.
The brightest star in the Virgo constellation is Spica, a binary and rotating ellipsoidal variable – which means the two stars are so close together that they are egg-shaped instead of spherical – located between 240 and 260 light years from Earth. The primary is a blue giant and a variable star of the Beta Cephei type.
The star 70 Virginis was one of the first planetary systems to have a confirmed exoplanet discovered orbiting it, which is 7.5 times the mass of Jupiter. The star Chi Virginis has one of the most massive planets ever detected, at a mass of 11.1 times that of Jupiter. The sun-like star 61 Virginis has three planets: one is a super-Earth and two are Neptune-mass planets.
The constellation Virgo. Credit: iau.org
Libra: Sept. 23 – Oct. 22
The sign of Libra covers 180° to 210° of celestial longitude. It is represented by the symbol of The Scales, which is based on the Scales of Justice held by Themis, the Greek personification of divine law and custom and the inspiration for modern depictions of Lady Justice. Libra is part of the Seventh House – Uxor (Spouse) or House of Partnership, are associated with the element of Air, and the ruling celestial body is Venus.
Two notable stars in the Libra constellation are Alpha and Beta Librae – also known as Zubenelgenubi and Zubeneschamali, which translates to “The Southern Claw” and “The Northern Claw”. Alpha Libae is a double star consisting of an A3 primary star with a slight blue tinge and a fainter type F4 companion, both of which are located approximately 77 light years from our Sun.
Beta Librae is the brighter of the two, and the brightest star in the Virgo constellation. This is a blue star of spectral type B8 (but which appears somewhat greenish) which is located roughly 160 light years from Earth. Then there’s Gamma Librae (also called Zubenelakrab, which means “the Scorpion’s Claw”) which completes the Scorpion sign. It is an orange giant of magnitude 3.9, and is located 152 light-years from Earth.
The constellation Libra. Credit: iau.org
Libra is home to the star Gliese 581, which has a planetary system consisting of at least 6 planets. Both Gliese 581 d and Gliese 581 g are considered to be some of the most promising candidates for life. Gliese 581 c is considered to be the first Earth-like exoplanet to be found within its parent star’s habitable zone. All of these exoplanets are of significance for establishing the likelihood of life outside of the Solar System.
Libra is also home to one bright globular cluster, NGC 5897. It is a fairly large and loose cluster, has an integrated magnitude of 9, and is located 40,000 light-years from Earth.
Scorpio: Oct. 23 – Nov. 21
The sign of Scorpio covers 210° to 240° of celestial longitude. Scorpio is represented by The Scorpion, which is based on Scorpius – a giant scorpion in Greek mythology sent by Gaia to kill Orion. Scorpio is part of the Eighth House – Mors (Death), known today as the House of Reincarnation – and is associated with the element of Water. Traditionally, the ruling celestial body of Scorpio was Mars, but has since become Pluto.
The Scorpius constellations includes many bright stars, the brightest being Alpha Scorpii (aka. Antares). The name literally means “rival of Mars” because of its distinct reddish hue. Other stars of note include Beta Scorpii (Acrab, or “the scorpion”), Delta Scorpii (Dschubba, or “the forehead”), Xi Scorpii (Girtab, also “the scorpion”), and Sigma and Tau Scorpii (Alniyat, “the arteries”).
Lambda Scorpii (Shaula) and Upsilon Scorpii (Lesath) – whose names both mean “sting”- mark the tip of the scorpion’s curved tail. Given their proximity to one another, Lambda Scorpii and Upsilon Scorpii are sometimes referred to as “the Cat’s Eyes”.
The constellation Scorpius. Credit: iau.org
The Scorpius constellation, due to its position within the Milky Way, contains many deep-sky objects. These include the open clusters Messier 6 (the Butterfly Cluster) and Messier 7 (the Ptolemy Cluster), the open star cluster NGC 6231 (aka. Northern Jewel Box), and the globular clusters Messier 4 and Messier 80 (NGC 6093).
The constellation is also where the Alpha Scorpiids and Omega Scorpiids meteor showers take place. The Alphas begin on or about April 16th and end around May 9th, with a peak date of most activity on or about May 3rd. The second meteor shower, the Omega (or June) Scorpiids peaks on or about June 5th of each year. The radiant for this particular shower is closer to the Ophiuchus border and the activity rate on the peak date is high – with an average of about 20 meteors per hour and many reported fireballs.
Sagittarius: Nov. 22 – Dec. 21
The sign of Sagittarius covers 240° to 270° of celestial longitude and is represented by The Archer. This symbol is based on the centaur Chiron, who according to Greek mythology mentored Achilles in the art of archery. Sagittarius is part of the Ninth House – known as Iter (Journeys) or the House of Philosophy. Sagittarius’ associated element is Fire (positive polarity), and the ruling celestial body is Jupiter.
Stars of note in the Sagittarius constellation include Alpha Sagittarii, which is also known as Alrami or Rukbat (literally “the archer’s knee”). Then there is Epsilon Sagittarii (“Kaus Australis” or “southern part of the bow”), the brightest star in the constellation – at magnitude 1.85. Beta Sagittarii, located at a position associated with the forelegs of the centaur, has the traditional name Arkab, which is Arabic for “achilles tendon.”
The Sagittarius constellation. Credit: iau.org
The second-brightest star is Sigma Sagittarii (“Nunki”), which is a B2V star at magnitude 2.08, approximately 260 light years from our Sun. Nunki is the oldest star name currently in use, having been assigned by the ancient Babylonians, and thought to represent the sacred Babylonian city of Eridu. Then there’s Gamma Sagittarii, otherwise known as Alnasl (the “arrowhead”). This is actually two star systems that share the same name, and both stars are actually discernible to the naked eye.
The Milky Way is at its densest near Sagittarius, since this is the direction in which the galactic center lies. Consequently, Sagittarius contains many star clusters and nebulae. This includes Messier 8 (the Lagoon Nebula), an emission (red) nebula located 5,000 light years from Earth which measures 140 by 60 light years.
Though it appears grey to the unaided eye, it is fairly pink when viewed through a telescope and quite bright (magnitude 3.0). The central area of the Lagoon Nebula is also known as the Hourglass Nebula, so named for its distinctive shape. Sagittarius is also home to the M17 Omega Nebula (also known as the Horseshoe or Swan Nebula).
This nebula is fairly bright (magnitude 6.0) and is located about 4890 light-years from Earth. Then there’s the Trifid Nebula (M20 or NGC 6514), an emission nebula that has reflection regions around the outside, making its exterior bluish and its interior pink. NGC 6559, a star forming region, is also associated with Sagittarius, located about 5000 light-years from Earth and showing both emission and reflection regions (blue and red).
Capricorn: Dec. 22 – Jan. 19
The sign of Capricorn spans 270° to 300° of celestial longitude and is represented by the Mountain Sea-Goat. This sign is based on Enki, the Sumerian primordial god of wisdom and waters who has the head and upper body of a mountain goat, and the lower body and tail of a fish. The sign is part of the Tenth House – Regnum (Kingdom), or The House of Social Status. Capricorns are associated with the element Earth, and the ruling body body is Saturn.
The constellation Capricornus. Credit: iau.org
The brightest star in Capricornus is Delta Capricorni, also called Deneb Algedi. Like other stars such as Denebola and Deneb, it is named for the Arabic word for “tail”, which in this case translates to “the tail of the goat’. Deneb Algedi is a eclipsing binary star with a magnitude of 2.9, and which is located 39 light-years from Earth.
Another bright star in the Capricorni constellation is Alpha Capricorni (Algedi or Geidi, Arabic for “the kid”), which is an optical double star (two stars that appear close together) – both o which are binaries. It’s primary (Alpha² Cap) is a yellow-hued giant of magnitude 3.6, located 109 light-years from Earth, while its secondary (Alpha¹ Cap) is a yellow-hued supergiant of magnitude 4.3, located 690 light-years from Earth.
Beta Capricorni is a double star known as Dabih, which comes from the Arabic phrase “the lucky stars of the slaughter” a reference to ritual sacrifices performed by ancient Arabs. Its primary is a yellow-hued giant star of magnitude 3.1, 340 light-years from Earth, while the secondary is a blue-white hued star of magnitude 6.1. Another visible star is Gamma Capricorni (aka. Nashira, “bringing good tidings”), which is a white-hued giant star of magnitude 3.7, 139 light-years from Earth.
Several galaxies and star clusters are contained within Capricornus. This includes Messier 30 (NGC 7099) a centrally-condensed globular cluster of magnitude 7.5. Located approximately 30,000 light-years from our Sun, it cannot be seen with the naked eye, but has chains of stars extending to the north that can be seen with a telescope.
The globular cluster Messier 30, imaged by the Hubble Telescope. Credit: NASA/Wikisky
And then there is the galaxy group known as HCG 87, a group of at least three galaxies located 400 million light-years from Earth. It contains a large elliptical galaxy, a face-on spiral galaxy, and an edge-on spiral galaxy. These three galaxies are interacting, as evidenced by the high amount of star formation in the face-on spiral, and the connecting stream of stars and dust between edge-on spiral and elliptical galaxy.
The constellation of Capricornus has one meteor shower associated with it. The Capricornid meteor stream peaks on or about July 30th and is active for about a week before and after, with an average fall rate is about 10 to 30 per hour.
Aquarius: Jan. 20 – Feb. 18
Aquarius, which spans 300° to 330° of celestial longitude, is represented by the Water Bearer. In ancient Greek mythology, Aquarius is Ganymede, the beautiful Phrygian youth who was snatched up by Zeus to become the cup-bearer of the Gods. Aquarius is part of the Eleventh House – Benefacta (Friendship), or House of Friendship, is associated with the element of Air. Traditionally, the ruling celestial body of Aquarius was Saturn, but has since changed to Uranus.
While Aquarius has no particularly bright stars, recent surveys have shown that there are twelve exoplanet systems within the constellation (as of 2013). Gliese 876, one of the nearest stars (15 light-years), was the first red dwarf start to be found to have a planetary system – which consists of four planets, one of which is a terrestrial Super-Earth. 91 Aquarii is an orange giant star orbited by one planet, 91 Aquarii b, a Super-Jupiter. And Gliese 849 is a red dwarf star orbited by the first known long-period Jupiter-like planet, Gliese 849 b.
The constellation Aquarius. Credit: iau.org
Aquarius is also associated with multiple Messier objects. M2 (NGC 7089) is located in Aquarius, which is an incredibly rich globular cluster located approximately 37,000 light-years from Earth. So is the four-star asterism M73 (which refers to a group of stars that appear to be related by their proximity to each other). Then there’s the small globular cluster M72, a globular cluster that lies a degree and half to the west of M73.
Aquarius is also home to several planetary nebulae. NGC 7293, also known as the Helix Nebula, is located at a distance of about 650 light years away, making it the closest planetary nebula to Earth. Then there’s the Saturn Nebula (NGC 7009) so-named because of its apparent resemblance to the planet Saturn through a telescope, with faint protrusions on either side that resemble Saturn’s rings.
There are five meteor showers associated with the constellation of Aquarius. The Southern Iota Aquarids begin around July 1st and end around September 18th, with the peak date occurring on August 6th with an hourly rate of 7-8 meteors average. The Northern Iota Aquarids occur between August 11th to September 10th, their maximum peak occurring on or about August 25th with an average of 5-10 meteors per hour.
Image of the Helix Nebula, combining from information from NASA’s Spitzer Space Telescope and the Galaxy Evolution Explorer (GALEX). Credit: NASA
The Southern Delta Aquarids begin about July 14th and end around August 18th with a maximum hourly rate of 15-20 peaking on July 29th. The Northern Delta Aquarids usually begin around July 16th, and last through September 10th. The peak date occurs on or around August 13th with a maximum fall rate of about 10 meteors per hour.
Then there is the Eta Aquarid meteor shower, which begins about April 21th and ends around May 12th. It reaches its maximum on or about May 5th with a peak fall rate of up to 20 per hour for observers in the northern hemisphere and perhaps 50 per hour for observers in the southern hemisphere. Last, there is the March Aquarids, a daylight shower that may be associated with the Northern Iota Aquarid stream.
Pisces: Feb. 19 – March 20
The sign of Pisces covers 330° to 360° of celestial longitude and is represented by the The Fish. This symbol is derived from the ichthyocentaurs – a pair of centaurian sea-gods that had the upper body of a male human, the lower front of a horse, and the tail of a fish – who aided Aphrodite when she was born from the sea. Pisces is part of the Twelfth House of Carcer (Prison), or The House of Self-Undoing, and are associated with the element of Water. The ruling celestial body of Pisces is traditionally Jupiter, but has since come to be Neptune.
The constellation Pisces. Credit: iau.org
Beta Piscium, also known as Samakah (the “Fish’s Mouth”), is a B-class hydrogen fusing dwarf star in the Pisces constellation. Located 495 light years from Earth, this star produces 750 times more than light than our own Sun and is believed to be 60 million years old. The brightest star in the constellation, Eta Piscium, is a bright class B star that is located 294 years away from our Solar System.
This star is also known by its Babylonian name, Kullat Nunu (which translates to “cord of the fish”), the Arab name Al Pherg (“pouring point of water”), and the Chinese name Yòu Gèng – which means “Official in Charge of the Pasturing“, referring to an asterism consisting of Eta Piscium and its immediate neighbors – Rho Piscium, Pi Piscium, Omicron Piscium, and 104 Piscium.
And then there’s van Maanen’s Star (aka. Van Maanen 2) a white dwarf that is located about 14 light years from our Sun, making it the third closest star of its kind to our system (after Sirius B and Procyon B). Gamma Piscium is a yellow-orange giant star located about 130 light years away, and is visible with just binoculars.
The Pisces constellationis also home to a number of deep-sky objects. These include M74, a loosely-wound spiral galaxy that lies at a distance of 30 million light years from our Sun. It has many clusters of young stars and the associated nebulae, showing extensive regions of star formation. Also, there’s CL 0024+1654, a massive galaxy cluster that is primarily made up of yellow elliptical and spiral galaxies. CL 0024+1654 lies at a distance of 3.6 billion light-years from Earth and lenses the galaxy behind it (i.e. it creates arc-shaped images of the background galaxy).
Last, there the active galaxy and radio source known as 3C 31. Located at a distance of 237 million light-years from Earth, this galaxy has a supermassive black hole at its center. In addition to being the source of its radio waves, this black hole is also responsible for creating the massive jets that extend several million light-years in both directions from its center – making them some of the largest objects in the universe.
There is one annual meteor shower associated with Pisces which peaks on or about October 7 of each year. The Piscid meteor shower has a radiant near the Aries constellation and produces an average of 15 meteors per hour which have been clocked at speeds of up to 28 kilometers per second. As always, the meteoroid stream can begin a few days earlier and end a few days later than the expected peak and success on viewing depends on dark sky conditions.
Currently, the Vernal Equinox is currently located in Pisces. In astronomy, equinox is a moment in time at which the vernal point, celestial equator, and other such elements are taken to be used in the definition of a celestial coordinate system. Due to the precession of the equinoxes, the Vernal Equinox is slowly drifting towards Aquarius.
Astrology is a tradition that has been with us for thousands of years and continues to be observed by many people and cultures around the world. Today, countless people still consult their horoscope to see what the future has in store, and many more consider their birth sign to be of special significance.
And the fact that many people still consider it to be valid is an indication that superstitious and “magical” thinking is something we have yet to completely outgrow. But this goes to show how some cultural traditions are so enduring, and how people still like to ascribe supernatural powers to the universe.
We have a complete guide to all 88 constellations here at Universe Today. Research them at your leisure, and be sure to check out more than just the “zodiac sign” ones!
We also have a comprehensive list of all the Messier Objects in the night sky.
Flashback to 1995: Clinton was in the White House, Star Trek Voyager premiered, we all carried pagers in the pre-mobile phone era, and Windows 95 and the Internet itself was shiny and new to most of us. It was also on this day in late 1995 when our premier eyes on the Sun—The SOlar Heliospheric Observatory (SOHO)—was launched. A joint mission between NASA and the European Space Agency, SOHO lit up the pre-dawn sky over the Florida Space Coast as it headed space-ward atop an Atlas IIAS rocket at 3:08 AM EST from launch complex 39B at Cape Canaveral Air Force Station.
Envisioning SOHO
SOHO on Earth
There aren’t a whole lot of 20th century spacecraft still in operation; SOHO joins the ranks of Hubble and the twin Voyager spacecraft as platforms from another era that have long exceeded their operational lives. Seriously, think back to what YOU were doing in 1995, and what sort of technology graced your desktop. Heck, just thinking of how many iterations of mobile phones spanned the last 20 years is a bit mind-bending. A generation of solar astronomers have grown up with SOHO, and the space-based observatory has consistently came through for researchers and scientists, delivering more bang for the buck.
“SOHO has been truly extraordinary and revolutionary in countless ways,” says astrophysicist Karl Battams at the Naval Research Laboratory in Washington D.C. “SOHO has completely changed our way of thinking about the Sun, solar active regions, eruptive events, and so much more. I honestly can’t think of a more broadly influential space mission than SOHO.”
SOHO has monitored the Sun now for the complete solar cycle #23 and well into the ongoing solar cycle #24. SOHO is a veritable Swiss Army Knife for solar astrophysics, not only monitoring the Sun across optical and ultraviolet wavelengths, but also employing the Michelson Doppler Imager to record magnetogram data and the Large Angle Spectrometric Coronograph (LASCO) able to create an artificial solar eclipse and monitor the pearly white corona of the Sun.
Peering into the solar interior.
SOHO observes the Sun from its perch one million miles sunward located at the L1 Sun-Earth point. It actually circles this point in space in what is known as a lissajous, or ‘halo’ orbit.
SOHO has revolutionized solar physics and the way we perceive our host star. We nearly lost SOHO early on in its career in 1998, when gyroscope failures caused the spacecraft to lose a lock on the Sun, sending it into a lazy one revolution per minute spin. Quick thinking by engineers led to SOHO using its reaction wheels as a virtual gyroscope, the first spacecraft to do so. SOHO has used this ad hoc method to point sunward ever since. SOHO was also on hand to document the 2003 Halloween flares, the demise of comet ISON on U.S. Thanksgiving Day 2013, and the deep and strangely profound solar minimum that marked the transition from solar cycle 23 to 24.
What was your favorite SOHO moment?
A massive sunspot witnessed by SOHO in 2000, compared to the Earth.
SOHO is also a champion comet hunter, recently topping an amazing 3000 comets and counting. Though it wasn’t designed to hunt for sungrazers, SOHO routinely sees ’em via its LASCO C2 and C3 cameras, as well as planets and background stars near the Sun. The effort to hunt for sungrazing comets crossing the field of view of SOHO’s LASCO C3 and C2 cameras represents one of the earliest crowd-sourced efforts to do volunteer science online. SOHO has discovered enough comets to characterize and classify the Kreutz family of sungrazers, and much of this effort is volunteer-based. SOHO grew up with the internet, and the images and data made publicly available are an invaluable resource that we now often take for granted.
A ‘neat’ image… Comet NEAT photobombs the view of SOHO’s LASCO C3 camera.
NASA/ESA has extended SOHO’s current mission out to the end of 2016. With any luck, SOHO will complete solar cycle 24, and take us into cycle 25 to boot.
“Right now, it (SOHO) is operating in a minimally funded mode, with the bulk of its telemetry dedicated solely to the LASCO coronagraph,” Battams told Universe Today. “Many of its instruments have now been superseded by instruments on other missions. As of today it remains healthy, and I think that’s a testament to the amazing collaboration between ESA and NASA. Together, they’ve kept a spacecraft designed for a two-year mission operating for twenty years.”
Today, missions such as the Solar Dynamics Observatory, Hinode, and Proba-2 have joined SOHO in watching the Sun around the clock. The solar occulting disk capabilities of SOHO’s LASCO C2 and C3 camera remains unique, though ESA’s Proba-3 mission launching in 2018 will feature a free-flying solar occulting disk.
Happy 20th SOHO… you’ve taught us lots about our often tempestuous host star.
This illustration shows Earth surrounded by filaments of dark matter called “hairs. A hair is created when a stream of dark matter particles goes through the planet. A new study proposes that Earth and the other planets are filled with “hair”. Credit: NASA/JPL-Caltech
I’m losing mine, but the Solar System may be way hairier than we ever thought, with thick crops of filamentary dark matter streaming through Earth’s core and back out again even as you read this.
Estimated distribution of matter and energy in the universe. Credit: NASA
A new study publishing this week in the Astrophysical Journal by Gary Prézeau of NASA’s Jet Propulsion Laboratory proposes the existence of long filaments of dark matter, or “hairs.” Dark matter is a hypothetical form of matter that emits no light, thereby resisting our attempts to see and photograph it, but based on many observations of its gravitational pull on ordinary matter, astronomers have measured the amount of dark matter to an accuracy of 1%.
Massive amounts of it formed a tangled web of filaments after the Big Bang and ensuing epoch of cosmic inflation that served as sites for the “condensation” of bright matter galaxies. We likely owe our existence to this stuff, whatever it is, which has yet to be directly detected. Along with dark energy, it remains one of the greatest mysteries of our age.
This Hubble image shows the distribution of dark matter in the center of the giant galaxy cluster Abell 1689, containing about 1,000 galaxies and trillions of stars. Researchers used the observed positions of 135 lensed images of 42 background galaxies to calculate the location and amount of dark matter in the cluster. They superimposed a map of these inferred dark matter concentrations, tinted blue, on an image of the cluster. The greastest concentration of dark matter is in the cluster’s center. Credit: NASA, ESA, D. Coe, N. Benitez , T. Broadhurst
As if that weren’t enough, dark matter comprises 85% of all the known matter reserves in the universe and 27% of the entire matter-energy cosmic budget. Ordinary stuff like stars, baseball bats and sushi constitute just 4.9% of the the total. The leading theory is that dark matter is “cold,” meaning it moves slowly compared to the speed of light, and it’s “dark” because it doesn’t produce or interact with light. The axion, a hypothetical elementary particle, appears to be good candidate for dark matter as do WIMPs or weakly interacting massive particles, but again, these exist only on paper.
According to calculations done in the 1990s and simulations performed in the last decade, dark matter forms “fine-grained streams” of particles that move at the same velocity and orbit galaxies such as ours. Streams can be much larger than our Solar System and criss-cross the galaxy. Prézeau compares the formation of fine-grained streams of dark matter to mixing chocolate and vanilla ice cream. Swirl a scoop of each together a few times and you get a mixed pattern, but you can still see the individual colors.
“When gravity interacts with the cold dark matter gas during galaxy formation, all particles within a stream continue traveling at the same velocity,” Prézeau said.
This illustration zooms in to show what dark matter hairs would look like around Earth. The hairs in this illustration are not to scale. Simulations show that the roots of such hairs can be 600,000 miles (1 million km) from Earth. Credit: NASA /JPL-Caltech
But a different scenario unfolds when a stream passes by an obstacle like the Earth or a moon. Prézeau used computer simulations to discover that when dark matter stream passes through a planet — dark matter passes right through us unlike ordinary matter — it’s focused into an ultra-dense filament or hair. Not a solo strand but a luxuriant crop bushy as a brewer’s beard.
According to Prézeau, hairs emerging from planets have both “roots,” the densest concentration of dark matter particles in the hair, and “tips,” where the hair ends. When particles of a dark matter stream pass through Earth’s core, they focus at the “root” of a hair, where the density of the particles is about a billion times more than average. The root of such a hair should be around 600,000 miles (1 million km) away from the surface, or a little more than twice as far as the moon. The stream particles that graze Earth’s surface will form the tip of the hair, about twice as far from Earth as the hair’s root.
The root of a dark matter hair produced from particles going through Jupiter’s core would be about 1 trillion times denser than average. Credit: NASA/JPL-Caltech
A stream passing through more massive Jupiter would have roots a trillion times denser than the original stream. Naturally, these dense concentrations would make ideal places to send a probe to study dark matter right here in the neighborhood.
The computer simulations reveal that changes in Earth’s density from inner core to outer core to mantle and crust are reflected in the shape of the hairs, showing up as “kinks” that correspond to transitions from one zone to the next. If it were possible to get our hands on this kind of information, we could use it to map to better map Earth’s interior and even the depth of oceans inside Jupiter’s moon Europa and Saturn’s Enceladus.
Earth getting its roots done. What’ll they think of next?
We’ve featured the photography of André van der Hoeven here many times, and all of his photos are wonderful. Well, now you can get them all in one big book, titled Treasures of the Universe.
This 150+ page book contains photos of most of the major objects in the Solar System as well as deep sky objects, like galaxies, star clusters and nebulae. van der Hoeven provides many of the pictures in the book, and then fills out the rest with the highest quality photos from the Hubble Space Telescope, Spitzer, Subaru and many of the top observatories around the world. There are also great photos from rovers and spacecraft sent to distant worlds (including the latest pictures of Pluto from New Horizons). If you want a coffee table book with great images of space, it’s a great choice. Treasures of the Universe by André van der Hoeven
The book is currently being run as a Kickstarter, but unlike most campaigns, this book is complete and ready to go to the printers, so you’re really just deciding if you want a copy or not – a printed, signed copy or an electronic PDF.
At the time I’m writing this, there are just 5 days left in the Kickstarter, which is already fully funded. This project is already happening, but you can help André reach the stretch goal of 25,000 Euros.
![Space Shuttle Endeavour sillouetted against the atmosphere. The orange layer is the troposphere, the white layer is the stratosphere and the blue layer the mesosphere.[1] (The shuttle is actually orbiting at an altitude of more than 320 km (200 mi), far above all three layers.) Credit: NASA](https://www.universetoday.com/wp-content/uploads/2012/03/Endeavour_silhouette_STS-130.jpg)
![Computer simulation of the Earth's field in a period of normal polarity between reversals.[1] The lines represent magnetic field lines, blue when the field points towards the center and yellow when away. The rotation axis of the Earth is centered and vertical. The dense clusters of lines are within the Earth's core](https://www.universetoday.com/wp-content/uploads/2010/03/Geodynamo_Between_Reversals-e1449100829326.gif)
