Astronomy Archives

October 20, 2015November 6, 2017 by Matt Williams

Saturn’s Icy Moon Enceladus

"Tiger stripes" -- sources of ice spewing -- in this image of Saturn's Enceladus taken by the Cassini spacecraft in 2009. Credit: Cassini Imaging Team, SSI, JPL, ESA, NASA

In the ongoing drive to unlock the secrets of Saturn and its system of moons, some truly fascinating and awe-inspiring things have been discovered. In addition to things like methane lakes and propane-rich atmospheres (Titan) to moon’s that resemble the Death Star (Mimas), it is also becoming abundantly clear that planet’s beyond Earth may harbor interior oceans and even the extra-terrestrial organisms.

Nowhere is this more apparent than on Enceladus, Saturn’s sixth largest moon, which also possesses some of the most interesting characteristics in the outer Solar System. These include long veins of blue ice that resemble stripes, not to mention amazing plumes of water ice that have been spotted periodically blasting out of the moon’s southern pole. These, in turn, raise the possibility of liquid water beneath the surface, and possibly even life!

Discovery and Naming:

Discovered in 1789 by William Herschel, Enceladus is named after one of the giants in Greek mythology. In fact, all of the large moons of Saturn are named after the Titans, as suggested by William Herschel’s son, John Herschel. He chose these names because Saturn (known in Greek mythology as Kronos) was the father of the Titans.

In contrast, in accordance with the IAU naming conventions for Enceladus, features are named after characters and places from the classic story One Thousand and One Nights (aka. Arabian Nights). Impact craters are named after characters, whereas other feature types – such as fossae (long, narrow depressions), dorsa (ridges), planitia (plains), and sulci (long parallel grooves), are named after places.

iameter comparison of the Saturnian moon Enceladus, Moon, and Earth. Credit: NASA/JPL-Caltech/Tom Reding — *Size comparison between the Cronian moon Enceladus, the Moon, and Earth. Credit: NASA/JPL-Caltech/Tom Reding*

Size, Mass and Orbit:

With a mean radius of 252 km, Enceladus is equivalent in size to 0.0395 Earths (or 0.1451 Moons). But with a mass of 1.08 ×10²⁰ kg, it is only 0.000018 as massive. The planet has a very minor eccentricity (0.0047) and orbits Saturn at an average distance (semi-major axis) of 237,948 km, between the orbits of Mimas and Tethys.

Enceladus takes 32.9 hours (1.37 days) to complete a single orbit around Saturn, and is currently in a 2:1 mean-motion orbital resonance with Dione; meaning that it completes two orbits of Saturn for every orbit completed by Dione. This forced resonance is what maintains Enceladus’s orbital eccentricity and results in tidal deformation, and the resulting heat dissipation is the main heating source for Enceladus’s geologic activity.

Like most of the larger natural satellites of Saturn, Enceladus rotates synchronously with its orbital period, keeping one face pointed toward Saturn. The planet also experiences forced libration, where it appears to oscillate relative to Saturn’s other moons – which may also provide Enceladus with an internal heat source.

Composition and Surface Features:

Enceladus has a density of 1.61 g/cm³, which is higher than Saturn’s other mid-sized, icy satellites, suggesting a composition that includes a greater percentage of silicates and iron. It is also believed to be largely differentiated between a geologically active core and an icy mantle, with a liquid water ocean nestled between.

Gravity measurements by NASA's Cassini spacecraft and Deep Space Network suggest that Saturn's moon Enceladus, which has jets of water vapor and ice gushing from its south pole, also harbors a large interior ocean beneath an ice shell, as this illustration depicts. Image Credit: NASA/JPL-Caltech — *Gravity measurements by NASA’s Cassini spacecraft and Deep Space Network suggest that Saturn’s moon Enceladus harbors a large interior ocean beneath it’s south pole.* *Credit:* *NASA/JPL-Caltech*

The existence of this liquid water ocean has been the subject of scientific debate since 2005, when scientists first observed plumes containing water vapor spewing from Enceladus’s south polar surface. These jets are capable of dispensing 250 kg of water vapor every second at speeds of up to 2,189 km/h (1,360 mph), and reaching 500 km into space.

In 2006, it was determined that Enceladus’s plumes are the source of Saturn’s E Ring and actively replenish it. According to measurements made by the Cassini-Huygens probe, these emissions are composed mostly of water vapor, as well as minor components like molecular nitrogen, methane, and carbon dioxide. Further observations noted the presence of simple hydrocarbons such as methane, propane, acetylene and formaldehyde.

The combined analysis of imaging, mass spectrometry, and magnetospheric data suggests that the observed south polar plume emanates from pressurized subsurface chambers. The intensity of the eruptions varies significantly due to changes in Enceladus’s orbit. Basically, the plumes are about four times brighter when Enceladus is at apoapsis (farthest from Saturn), which is consistent with geophysical calculations that predict that the south polar fissures will be under less compression, thus opening them wider.

The existence of subsurface water was confirmed thanks to evidence provided by the Cassini mission in 2014. This included gravity measurements obtained during the flybys of 2010-2012, which confirmed the existence of a south polar subsurface ocean of liquid water within Enceladus with a thickness of around 10 km.

Artist's rendering of possible hydrothermal activity that may be taking place on and under the seafloor of Enceladus. Image Credit: NASA/JPL — *Artist’s rendering of possible hydrothermal activity that may be taking place on and under the seafloor of Enceladus. Credit: NASA/JPL*

In addition, during the July 14, 2005 flyby, the Cassini probe also detected the presence of escaping internal heat in the southern polar region. These temperatures were too high to be attributed to solar heating, and combined with the geyser activity, seemed to indicate that the interior of the planet is still geologically active.

Further studies from measurements of Enceladus’s libration as it orbits Saturn strongly suggest that the entire icy crust is detached from the rocky core, which would mean that the ocean beneath its surface is planet-wide. The amount of libration implies that this global ocean is about 26 to 31 kilometers in depth (compared to Earth’s average ocean depth of 3.7 kilometers).

Observations of Enceladus’ surface has revealed five types of terrain – cratered terrain, smooth (young) terrain, ridged terrain (often bordering on smooth areas), linear cracks, scarps, troughs, and grooves. Surveys of the cratered terrain, smooth plains, and other features indicate a level of resurfacing that suggests that tectonics are an important factor in the geological history of Enceladus.

Recent observations by Cassini have provided a closer look at the crater distribution and size. These features have been named by the IAU after characters and places from Burton’s translation of The Book of One Thousand and One Nights – i.e. the Shahrazad crater, the Diyar plains, the Anbar depression.

Artist impression of the view of Saturn from its moon Enceladus (Michael Carroll) — *Artist impression of the view of Saturn from Enceladus, with geysers erupting at the right in the foreground. Credit: Michael Carroll*

The smooth plains are dominated by fresh clean ice, which gives Enceladus what is possibly the most reflective surface in the Solar System (with a visual geometric albedo of 1.38). These areas have few craters, which indicate that they are likely younger than a few hundred million years old. In addition, the relative youthfulness of these regions are an indication that cryovolcanism and other processes actively renew the surface.

The older terrain is not only cratered, but numerous fractures have also been observed – suggesting that the surface has been subject to extensive deformation since the craters formed. Some areas show regions with no craters, indicating major resurfacing events in the geologically recent past. The fissures, plains, corrugated terrain and other crustal deformations also indicate that Enceladus is geologically active.

One of the more dramatic types of tectonic features found on Enceladus are its rift canyons. These canyons can be up to 200 km long, 5–10 km wide, and 1 km deep. Such features are geologically young, because they cut across other tectonic features and have sharp topographic relief with prominent outcrops along the cliff faces.

Evidence of tectonics on Enceladus is also derived from grooved terrain, consisting of lanes of curved formations and ridges that often separate smooth plains from cratered regions. Deep fractures are another, which are often found in bands cutting across cratered terrain, and which were probably influenced by the formation of weakened regolith produced by impact craters.

Enceladus. Credit: NASA/JPL/Space Science Institute — *Enceladus, showing the famous “Tiger Stripes” feature – a series of fractures bound on either side by colorful ice. Credit: NASA/JPL/Space Science Institute*

Linear grooves can also be seen cutting across other terrain types, like the groove and ridge belts. Like the deep rifts, they are among the youngest features on Enceladus. However, some linear grooves have been softened like the craters nearby, suggesting that they are older. Ridges have also been observed on Enceladus, though they are relatively limited in extent and are up to one kilometer tall.

Other interesting features include the “Tiger stripes“: a series of fractures bounded on either side by ridges in the southern polar region that are are surrounded by mint-green-colored, coarse-grained water ice. These fractures appear to be the youngest features in this region, and combined with a lack of impact craters in this area, are further evidence of geological activity.

Atmosphere:

Saturn’s moon Enceladus has an atmosphere greater than that of all others in the Solar System, with the exception of Titan. The source of the atmosphere is attributed to the periodic cryovolcanism, which leads to gases and vapor escaping from the surface or the interior. Evidence of a tenuous atmosphere came from magnetometer readings provided by the Cassini‘s probe in 2005.

This consisted of an increased detection in the power of ion cyclotron waves, which are produced by the interaction of ionized particles and magnetic fields. During the next two encounters, the magnetometer team determined that gases in Enceladus’s atmosphere are concentrated over the south polar region, with atmospheric density away from the pole being much lower.

Water vapour geysers erupting from Enceladus' south pole. Credit: NASA/JPL — *Water vapour geysers erupting from Enceladus’ south pole. Credit: NASA/JPL*

Much like the content of the jet plumes, this atmosphere is composed primarily of water vapor (91%), but also shows signs of minor components like molecular nitrogen (4%) and carbon dioxide (3.2%). There has also been evidence of simple hydrocarbons, which take the form of methane (1.7%) as well as trace amounts of propane, acetylene and formaldehyde.

Habitability:

Ever since the discovery of Enceladus’s geysers and evidence that suggested an interior ocean, scientists have speculated about the possibility of there being life on Enceladus. Because it reflects so much sunlight, the mean surface temperature at noon only reaches -198 °C, making it somewhat colder than other Cronian satellites. However, within the core, multiple indications of life exist.

It’s resonance with Dione excites its orbital eccentricity, which tidal forces damp, resulting in tidal heating of its interior. This offers a possible explanation for its geological activity, and also suggests that its interior oceans are warmer closer to the core. In addition, geological models have indicated that the large rocky core is porous, allowing water to flow through it to pick up heat.

A model of Enceladus’s ocean created by Christopher R. Glein et al. (2015) suggests that it has an alkaline pH of 11 to 12. This high pH (alkaline) is interpreted to be a consequence of serpentinization of chondritic rock, which leads to the generation of molecular hydrogen (H²). This geochemical source of energy can be metabolized by methanogen microbes to provide energy for life.

The presence of an internal salty ocean with an energy source and simple organic compounds are all strong indications that microbes may exist closer to the core, where the water is warm and the basic building blocks of life exist.

Exploration:

Although it was first discovered in the late 18th century, astronomers didn’t know much about this moon for many centuries. It was not until it was first visited in a series of flybys by NASA’s two Voyager spacecraft in the 1980’s that certain things began to become apparent about Enceladus.

Voyager 1 has traveled far past the realm of the gas or even ice giants and is now in uncharted territory where scientists are learning more and more about the dynamic environment at the far-flung edges of our solar system. Image Credit: NASA/JPL - Caltech — *Artist’s impression of Voyager 1 reaching Saturn and its system of moons. Image Credit: NASA/JPL – Caltech*

For starters, the Voyager missions showed that the planet has a diameter of only 500 km (310 miles), which makes it less than one-tenth the diameter of Saturn’s largest moon of Titan. They also noted that most of the surface is covered in fresh, clean ice; giving it a pure, snow-white appearance that also attracts close to 100% of the sunlight that strikes its surface.

The Voyager 1 mission also confirmed that Enceladus was embedded in the densest part of Saturn’s diffuse E-ring. Combined with the apparent youthful appearance of the surface, Voyager scientists suggested that the E-ring consisted of particles vented from Enceladus’s surface. The Voyager 2 mission provided better photographs than its predecessor, confirming the presence of a youthful surface, but also other features.

By 2005, the Cassini spacecraft began performing multiple close flybys of Enceladus, revealing its surface and environment in greater detail. In particular, Cassini discovered the water-rich plumes venting from the south polar region of Enceladus, which became the subject of much research and speculation.

Cassini has provided strong evidence that Enceladus has an ocean with an energy source, nutrients and organic molecules, making Enceladus one of the best places for the study of potentially habitable environments for extraterrestrial life. By contrast, the water thought to be on Jupiter’s moon Europa is located under a much thicker layer of ice.

Cassini-Huygens Mission — *An artist illustration of the Cassini spacecraft. Image Credit: NASA/JPL*

Cassini’s latest flyby took place on October 14th, 2015, passing the moon at an altitude of 1,839 kilometers (1,142 miles) above the northern polar region. This was the first time that Cassini had been able to observe the northern polar region, due to the fact that on all previous occasions, the northern region was experiencing its winter cycle and was concealed by darkness.

Cassini’s instruments took pictures of multiple surface features, including craters (many of which look like they are melting), fractures and wrinkles. The latter features are believed to be an indication that the moon’s spin rate has changed, which may be another indication that the surface has undergone multiple episodes of geologic activity over the course of much of its lifetime.

The discoveries Cassini has made at Enceladus have prompted studies into follow-up mission concepts. In 2013, NASA proposed a possible sample-return mission to Enceladus that would involve a low-cost orbiter. This mission would launch during the 2020s and last 15 years.

Another proposal for a probe flyby, known as Journey to Enceladus and Titan (JET) would analyze plume contents in-situ. Proposed in response to NASA’s 2010 Discovery Announcement of Opportunity, the mission would involve an orbiter conducting high-resolution mass spectroscopy surveys of Enceladus and Titan, assessing them for biological potential.

The German Aerospace Center has also proposed studying the habitability of Enceladus’s subsurface ocean using an Enceladus Explorer, and two astrobiology-oriented mission concepts (the Enceladus Life Finder and Life Investigation For Enceladus). In 2007, the European Space Agency (ESA) proposed sending a probe to Enceladus in a mission to be combined with studies of Titan – known as TandEM (Titan and Enceladus Mission).

Additionally, there’s the Titan Saturn System Mission (TSSM), a joint NASA/ESA flagship-class proposal to explore Saturn’s moons (with a focus on Enceladus). TSSM was competing against the Europa Jupiter System Mission (EJSM) proposal for funding. In February 2009, it was announced that NASA/ESA had given the EJSM mission priority ahead of TSSM, although TSSM will continue to be studied and evaluated.

Enceladus is a tempting target for future research and exploration, and for good reason. For starters, it is one of the few Solar System bodies (alongside with Earth, Io, and Triton) to have confirmed contemporary volcanic activity. Second is the distinct possibility that life exists beneath its icy surface, much like Europa. But with Enceladus, getting to a place where we could study that life would be much easier.

As such, it is almost certain that any missions to Saturn and/or the outer Solar System in the coming years will likely involve a close flyby of Enceladus. Maybe we’ll even pop in a lander and an aquatic explorer to examine the surface and peak underneath it!

We’ve written many articles about Enceladus for Universe Today. Here’s an article about salt found in the plumes from Enceladus, and the possibility of a liquid ocean on Enceladus.

And here is a rundown of Cassini’s Most Interesting Discoveries.

If you’d like more information on Enceladus, check out NASA’s Solar System Exploration Guide, and here’s a link to a cool mosaic image of Enceladus.

We’ve recorded an episode of Astronomy Cast all about Saturn’s moons. Listen here, Episode 61: Saturn’s Moons.

Sources:

October 19, 2015December 23, 2015 by David Dickinson

The 2015 Orionids: Watch the Meteors Fly from the Club of Orion

(Note: we’ll be posting this article as a running blog with updates over the next few mornings, as the Orionids are already moderately active for this week. Watch this space for info as it is added after our first meteor vigil tomorrow morning and Wednesday, weather and clear skies willing…)

The month of October is about midway through meteor shower season for the northern hemisphere, and one of the annual sure-fire best bets is the Orionid meteor shower. One of two meteor showers emanating from debris shed by that most famous of all periodic comets –1/P Halley—the Orionids generate a typical zenithal hourly rate of around 20 per hour, though surges heading towards a ZHR of in the range 70 are not unknown on some years.

Orionid radiant — The radiant for the Orionid meteors, looking eastward around 2 AM local. Image credit: Stellarium

2015 sees the shower peaking right around the morning of Wednesday, October 21^st. This will place the Moon at a 59% illuminated waxing gibbous phase and setting around local midnight, setting the stage for dark AM skies, perfect for meteor hunting.

Like the springtime Eta Aquarids also generated by Halley’s Comet, the Orionids are swift movers, striking the atmosphere at about 66 kilometers per second. The shower’s radiant drifts across the club of Orion the Hunter toward the astronomical constellation Gemini near its peak, and the radiant rides highest around 4 AM local time. This is also typically the best time to carry out a meteor vigil, as early morning hours places an Earth-bound observer facing forward into the oncoming meteor stream.

The twin Spring and Fall showers hailing from Halley’s are a product of the geometry of its elliptical orbit: Halley’s Comet spends most of its 75.3 year orbit south of the ecliptic, and only briefly ‘pops up’ northward above the Earth’s orbit for northern hemisphere viewers for a few months around perihelion, which next occurs on July 28th, 2061.

Let’s see, I’ll be 90-something next time Halley’s Comet comes ‘round next…

Like many meteor showers, researchers (with the tongue-twisting title of meteoriticists) are still working to precisely model the debris streams of showers such as the Orionids. There’s evidence to suggest an 11 year periodicity for the Orionid meteor stream undergoing modification by the gravitational influence of the giant planet Jupiter, a period which we’re approximately passing the mid-point low for in 2015.

Rates for the Orionids from previous years seem to support this trend: over the last few years, observers saw the Orionids top out at a ZHR of 21(2014), 45(2012), 33(2011) and 38(2010) as per the International Meteor Organization (IMO).

The next few mornings are key for a successful Orionid vigil. The Orionids also display a broad swath of activity, typically running from early October, to the first week of November before falling back down to levels below the background sporadic rate.

And while 2015 may be an off year for the Orionids, another shower may prompt a once a decade fireball swarm to rival the Full Moon this Halloween right into early November…

Will the Northern Taurids of 2015 perform? Stay tuned!

Update: Well, skies were indeed clear over the Central Florida peninsula this AM, allowing for a brief one hour vigil before dawn ensued. We counted three swift Orionids for about 40 minutes of total effective observing time, suggesting the the current rate is already well above the background sporadic rate, not bad. A late season Draconid meteor, and a curious unidentified tumbling satellite on a retrograde (read ‘Earth observing’ or spy satellite) orbit also joined the fray, along with the current cavalcade of dawn planets. A nice pass of the Hubble Space Telescope capped off the session as dawn broke, not bad.

The Orionids are noticeably speedy, flitting briefly in and out of view. Another great plus with this shower: The radiant is almost directly at the zenith for Florida residents at around 5 AM or so. This means that the Orionids can be enjoyed fairly far southward as well… has anyone ever studied just why a majority of major meteor shower radiants reside in the northern hemisphere?

The IMO hasn’t yet put up their live tracker yet, but hey, you can still report those Orionid rates worldwide…. We’ll see what Wednesday and Thursday morning brings as the Orionid meteor peak arrives. Unlike many showers, the Orionids have a very broad peak, and should be active all week into late October.

And don’t forget the tweet those sightings to #Meteorwatch!

October 17, 2015April 21, 2016 by Matt Williams

Jupiter’s Moon Callisto

Callisto has many more craters than Europa and a thicker icy crust. Image credit: NASA/JPL

With 67 confirmed satellites, Jupiter has the largest system of moons in the Solar System. The greatest of these are the four major moons of Io, Europa, Ganymede and Callisto – otherwise known as the Galilean Moons. Named in honor of their founder, these moons are not only comparable in size to some planets (such as Mercury), they are also some of the few places outside of Earth where liquid water exists, and perhaps even life.

But it is Callisto, the fourth and farthest moon of Jupiter, that may be the most rewarding when it comes to scientific research. In addition to the possibility of a subsurface ocean, this moon is the only Galilean far enough outside of Jupiter’s powerful magnetosphere that it does not experience harmful levels of radiation. This, and the prospect of finding life, make Callisto a prime candidate for future exploration.

Discovery and Naming:

Along with Io, Europa and Ganymede, Callisto was discovered in January of 1610 by Galileo Galilei using a telescope of his own design. Like all the Galilean Moons, it takes its name from one of Zeus’ lovers in classic Greek mythology. Callisto was a nymph (or the daughter of Lycaon) who was associated with the goddess of the hunt, Artemis.

The name was suggested by German astronomer Simon Marius, apparently at the behest of Johannes Kepler. However, Galileo initially refused to use them, and the moons named in his honor were designed as Jupiter I through IV, based on their proximity to their parent planet. Being the farthest planet from Jupiter, Callisto was known as Jupiter IV until the 20th century, by which time, the names suggested by Marius were adopted.

Galilean Family Portrait — *The Galilean moons to scale, with Callisto in the bottom left corner. Credit: NASA/JPL*

Size, Mass and Orbit:

With a mean radius of 2410.3 ± 1.5 km (0.378 Earths) and a mass of 1.0759 × 10²³ kg (0.018 Earths), Callisto is the second largest Jupiter’s moons (after Ganymede) and the third largest satellite in the solar system. Much like Ganymede, it is comparable in size to Mercury – being 99% as large – but due to its mixed composition, it has less than one-third of Mercury mass.

Callisto orbits Jupiter at an average distance (semi-major axis) of 1,882,700 km. It has a very minor eccentricity (0.0074) and ranges in distance from 1,869,000 km at periapsis to 1,897,000 km at apoapsis. This distance, which is far greater than Ganymede’s, means that Callisto does not take part in the mean-motion resonance that Io, Europa and Ganymede do.

Much like the other Galileans, Callisto’s rotation is synchronous with its orbit. This means that it takes the same amount of time (16.689 days) for Callisto to complete a single orbit of Jupiter and a single rotation on its axis. Its orbit is very slightly eccentric and inclined to the Jovian equator, with the eccentricity and inclination changing over the course of centuries due to solar and planetary gravitational perturbations.

*Size comparison of Earth, Moon and Callisto. Credit: NASA/JPL/DLR/Gregory H. Revera*

Unlike the other Galileans, Callisto’s distant orbit means that it has never experienced much in the way of tidal-heating, which has had a profound impact on its internal structure and evolution. Its distance from Jupiter also means that the charged particles from Jupiter’s magnetosphere have had a very minor influence on its surface.

Composition and Surface Features:

The average density of Callisto, at 1.83 g/cm³, suggests a composition of approximately equal parts of rocky material and water ice, with some additional volatile ices such as ammonia. Ice is believed to constitute 49-55% of the moon, with the rock component likely made up of chondrites, silicates and iron oxide.

Callisto’s surface composition is thought to be similar to its composition as a whole, with water ice constituting 25-50% of its overall mass. High-resolution, near-infrared and UV spectra imaging have revealed the presence of various non-ice materials, such as magnesium and iron-bearing hydrated silicates, carbon dioxide, sulfur dioxide, and possibly ammonia and various organic compounds.

Model of Callisto's internal structure showing a surface ice layer, a possible liquid water layer, and an ice–rock interior. Credit: NASA/JPL — *Model of Callisto’s internal structure showing a surface ice layer, a possible liquid water layer, and an ice–rock interior. Credit: NASA/JPL*

Beneath the surface is an icy lithosphere that is between 80-150 m thick. A salty ocean 50–200 km deep is believed to exist beneath this, thanks to the presence of radioactive elements and the possible existence of ammonia. Evidence of this ocean include Jupiter’s magnetic field, which shows no signs of penetrating Callisto’s surface. This suggests a layer of highly conductive fluid that is at least 10 km in depth. However, if this water contains ammonia, which is more likely, than it could be up to 250-300 km.

Beneath this hypothetical ocean, Callisto’s interior appears to be composed of compressed rocks and ices, with the amount of rock increasing with depth. This means, in effect, that Callisto is only partially differentiated, with a small silicate core no larger than 600 km (and a density of 3.1-3.6 g/cm³) surrounded by a mix of ice and rock.

Spectral data has also indicated that Callisto’s surface is extremely heterogeneous at the small scale. Basically, the surface consists of small, bright patches of pure water ice, intermixed with patches of a rock–ice mixture, and extended dark areas made of a non-ice material.

Compared to the other Galilean Moons, Callisto’s surface is quite dark, with a surface albedo of about 20%. Another difference is the nature of its asymmetric appearance. Whereas with the other Galileans, the leading hemisphere is lighter than the trailing one, with Callisto the opposite is true.

*Interior density structures created by an outer solar system late heavy bombardment onto Ganymede (top row) and Callisto (bottom row). Credit: SwRI*

An immediately obvious feature about Callisto’s surface is the ancient and heavily cratered nature of it. In fact, the surface is the most cratered in the Solar System and is almost entirely saturated by craters, with newer ones having formed over older ones. What’s more, impact craters and their associated structures are the only large features on the surface. There are no mountains, volcanoes or other endogenic tectonic features.

Callisto’s impact craters range in size from 0.1 km to over 100 km, not counting the multi-ring structures. Small craters, with diameters less than 5 km, have simple bowl or flat-floored shapes, whereas those that measure 5–40 km usually have a central peak.

Larger impact features, with diameters that range from 25–100 km have central pits instead of peaks. Those with diameters over 60 km can have central domes, which are thought to result from central tectonic uplift after an impact.

The largest impact features on Callisto’s surface are multi-ring basins, which probably originated as a result of post-impact concentric fracturing which took place over a patch of lithosphere that overlay a section of soft or liquid material (possibly a patch of the interior ocean). The largest of these are Valhalla and Asgard, whose central, bright regions measure 600 and 1600 km in diameter (respectively) with rings extending farther outwards.

*Voyager 1 image of Valhalla, a multi-ring impact structure 3800 km in diameter. Credit: NASA/JPL*

The relative ages of the different surface units on Callisto can be determined from the density of impact craters on them – the older the surface, the denser the crater population. Based on theoretical considerations, the cratered plains are thought to be ~4.5 billion years old, dating back almost to the formation of the Solar System.

The ages of multi-ring structures and impact craters depend on chosen background cratering rates, and are estimated by different researchers to vary between 1 and 4 billion years of age.

Atmosphere:

Callisto has a very tenuous atmosphere composed of carbon dioxide which has an estimated surface pressure of 7.5 × 10-¹² bar (0.75 micro Pascals) and a particle density of 4 × 10⁸ cm-³. Because such a thin atmosphere would be lost in only about 4 days, it must be constantly replenished, possibly by slow sublimation of carbon dioxide ice from Callisto’s icy crust.

While it has not been directly detected, it is believed that molecular oxygen exists in concentrations 10-100 times greater than CO². This is evidenced by the high electron density of the planet’s ionosphere, which cannot be explained by the photoionization of carbon dioxide alone. However, condensed oxygen has been detected on the surface of Callisto, trapped within its icy crust.

Habitability:

Much like Europa and Ganymede, and Saturn’s moons of Enceladus, Mimas, Dione, Titan, the possible existence of a subsurface ocean on Callisto has led many scientists to speculate about the possibility of life. This is particularly likely if the interior ocean is made up of salt-water, since halophiles (which thrive in high salt concentrations) could live there.

In addition, the possibility of extra-terrestrial microbial life has also been raised with respect to Callisto. However, the environmental conditions necessary for life to appear (which include the presence of sufficient heat due to tidal flexing) are more likely on Europa and Ganymede. The main difference is the lack of contact between the rocky material and the interior ocean, as well as the lower heat flux in Callisto’s interior.

In essence, while Callisto possesses the necessary pre-biotic chemistry to host life, it lacks the necessary energy. Because of this, the most likely candidate for the existence of extra-terrestrial life in Jupiter’s system of moons remains Europa.

Exploration:

The first exploration missions to Callisto were the Pioneer 10 and 11 spacecrafts, which conducted flybys of the Galilean moon in 1973 and 1974, respectively, But these missions provided little additional information beyond what had already learned through Earth-based observations. In contrast, the Voyager 1 and 2 spacecraft, which conducted flybys of the moon in 1979, managed to image more than half the surface and precisely measured Callisto’s temperature, mass and shape.

Capturing Callisto — *New Horizons Long Range Reconnaissance Imager (LORRI) captured these two images of Jupiter’s outermost large moon, Callisto, during its flyby in February 2007. Credit: NASA/JPL*

Further exploration took place between 1994 and 2003, when the Galileo spacecraft performed eight close flybys with Callisto. The orbiter completed the global imaging of the surface and delivered a number of pictures with a resolution as high as 15 meters. In 2000, while en route to Saturn, the Cassini spacecraft acquired high-quality infrared spectra of the Galilean satellites, including Callisto.

In February–March 2007, while en route to Pluto, the New Horizons probe obtained new images and spectra of Callisto. Using its Linear Etalon Imaging Spectral Array (LEISA) instrument, the probe was able to reveal how lighting and viewing conditions affect infrared spectrum readings of its surface water ice.

The next planned mission to the Jovian system is the European Space Agency’s Jupiter Icy Moon Explorer (JUICE), due to launch in 2022. Ostensibly geared towards exploring Europa and Ganymede, the mission profile also includes several close flybys of Callisto.

Colonization:

Compared to the other Galileans, Callisto presents numerous advantages as far as colonization is concerned. Much like the others, the moon has an abundant supply of water in the form of surface ice (but also possibly liquid water beneath the surface). But unlike the others, Callisto’s distance from Jupiter means that colonists would have far less to worry about in terms of radiation.

In 2003, NASA conducted a conceptual study called Human Outer Planets Exploration (HOPE) regarding the future human exploration of the outer Solar System. The target chosen to consider in detail was Callisto, for the purposes of investigating the possible existence of life forms embedded in the ice crust on this moon and on Europa.

Artist's impression of a base on Callisto. Credit: NASA — *Artist’s impression of a base on the icy surface of Callisto. Credit: NASA*

The study proposed a possible surface base on Callisto where a crew could “teleoperate a Europa submarine and excavate Callisto surface samples near the impact site”. In addition, this base could extract water from Callisto’s ample supply of water ices to produce rocket propellant for further exploration of the Solar System.

The advantages of a base on Callisto include low radiation (due to its distance from Jupiter) and geological stability. Such a base could facilitate exploration on other Galilean Moons, and be an ideal location for a Jovian system way station, servicing spacecraft heading farther into the outer Solar System – which would likely take the form of craft using a gravity assist from a close flyby of Jupiter.

Reports filed by NASA’s Glenn Research Center and Langley Research Center – in December and February of 2003, respectively – both outlined possible manned missions to Callisto, as envisioned by HOPE. According to these reports, a mission that would likely involve a ship using a Mangetoplasmadynamic (MPD) or Nuclear-Electric Propulsion (NEP) drive system, and equipped to generate artificial gravity, could be mounted in the 2040s.

So while Callisto may not be the best target in the search for extra-terrestrial life, it may be the most hospitable of Jupiter’s moons for human life. In either case, any future missions to Jupiter will likely include a stopovers to Callisto, with the intent of investigating both of these possibilities.

We have many great articles on Callisto, Jupiter, and its system of moons here at Universe Today. Here’s one about how impacts effected Callisto’s interior, And here is one on all of the Galilean Moons.

For more information, check out NASA’s Solar System Exploration page on Callisto.

Astronomy Cast offers has a good episode on the subject, titled Episode 57: Jupiter’s Moons.

October 16, 2015December 23, 2015 by Bob King

Thousands of Pits Punctuate Pluto’s Forbidding Plains in Latest Photos

This image was taken by the Long Range Reconnaissance Imager (LORRI) on NASA's New Horizons spacecraft shortly before closest approach to Pluto on July 14, 2015; it resolves details as small as 270 yards (250 meters). The scene shown is about 130 miles (210 kilometers) across. The sun illuminates the scene from the left, and north is to the upper left. Credits: NASA/JHUAPL/SwRI

A brand new batch of Pluto and Charon photos showed up today on the New Horizons LORRI (LOng-Range Reconnaissance Imager) site. The photos were taken during the close flyby of the system on July 14, 2015 and show rich detail including craters and parallel cracks on Charon and thousands of small pits punctuating Pluto’s nitrogen ice landscape. Have at ’em!

This wider view shows the textured surface of Pluto's icy plains riddled with small pits. It almost looks like the dark areas in the sinuous channels between the mounds were once covered with frost or ice that has since sublimated away. They look similar to the polar regions on Mars where carbon dioxide frost burns off in the spring to reveal darker material beneath. Credit: NASA/JHUAPL/SwRI — This wider view shows the snakeskin-like textured surface of Pluto’s icy plains riddled with small pits. It almost looks like the dark areas in the sinuous channels between the mounds were once covered with frost or ice that has since sublimated away. They look similar to the polar regions on Mars where carbon dioxide frost burns off in the spring to reveal darker material beneath. Credit: NASA/JHUAPL/SwRI

The first couple images feature the region informally known as Sputnik Planum. According to a release from NASA today, scientists think the region is composed of volatile ices such as solid nitrogen. They theorize that the pits and troughs – typically hundreds of meters across and tens of meters deep – are possibly formed by sublimation or evaporation of these ices in Pluto’s thin atmosphere. Still, their curious shapes and alignments remain a mystery. Adding to the intrigue is that even when seen up close, no impact craters are visible, testifying to the icy plain’s extreme geologic youth.

By the way, there are more images at the LORRI link at top. I picked a representative selection but I encourage you to visit and explore.

Now that's what I call getting a photo in low light. Sunlight scrapes across rugged mountains as well as highlight the ubiquitous pits. Credit: — Now that’s what I call getting a photo in low light. Sunlight scrapes across rugged mountains as well as highlight the ubiquitous pitted terrain. Credit: NASA/JHUAPL/SwRI

Life's definitely the pits on Pluto's Tombaugh Regio. This photo shows the fainter "ghost" pits well. Is ice filling them in or are we seeing the start of a pit's formation? Credit: — Life’s definitely the pits on Pluto’s Tombaugh Regio. This photo shows the fainter “ghost” pits well. Is ice filling them in or are we seeing the beginning of a pit’s formation? Credit: NASA/JHUAPL/SwRI

A fine view of Pluto's largest moon Charon and its vast canyon system. Credit: — A fine view of Pluto’s largest moon Charon and its vast canyon system. Credit: NASA/JHUAPL/SwRI

Looking over Charon's dark north polar region, the border of which is highlighted by several beautiful rayed craters. Not that it's necessarily related, but the dark spot reminds me of a lunar mare or sea. On the moon, cracks in the crust allowed lava to fill gigantic basins to create the maria. Could material from beneath Charon have bubbled up to fill an ancient impact? Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute — Looking over Charon’s dark north polar region, the border of which is highlighted by several beautiful rayed craters. Not that it’s necessarily related, but the dark spot reminds me of a lunar mare or sea. On the moon, cracks in the crust allowed lava to fill gigantic basins to create the maria. Could material from beneath Charon have bubbled up to fill an ancient impact? Credit: NASA/JHUAPL/SwRI

Speaking of the Moon, these cracks resembles lunar rills, some of which formed through faulting / fracturing and others as conduits for lava flows. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute — Speaking of the Moon, the large cracks at left resemble lunar rills, some of which formed through faulting / fracturing and others as conduits for lava flows. The multiple, fine cracks are interesting. Credit: NASA/JHUAPL/SwRI

Splendid rayed crater, each with its own set of tones. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute — Splendid rayed crater with an interesting contrast between dark and light ejecta. Credit: NASA/JHUAPL/SwRI

A busy region on Charon, the meeting place of different terrains. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute — A busy region on Charon, the meeting place of different terrains. Credit: NASA/JHUAPL/SwRI

October 16, 2015December 23, 2015 by Bob King

What’s Orbiting KIC 8462852 – Shattered Comet or Alien Megastructure?

Something other than a transiting planet makes the Kepler star KIC fluctuate wildly and unpredictably in brightness. Astronomers suspect a shattered comet, but who knows? Credit: NASA

“Bizarre.” “Interesting.” “Giant transit”. That were the reactions of Planet Hunters project volunteers when they got their first look at the light curve of the otherwise normal sun-like star KIC 8462852 nearly.

Of the more than 150,000 stars under constant observation during the four years of NASA’s primary Kepler Mission (2009-2013), this one stands alone for the inexplicable dips in its light. While almost certainly naturally-caused, some have suggested we consider other possibilities.

Kepler-11 is a sun-like star around which six planets orbit. At times, two or more planets pass in front of the star at once, as shown in this artist's conception of a simultaneous transit of three planets observed by NASA's Kepler spacecraft on Aug. 26, 2010. Image credit: NASA/Tim Pyle — Kepler-11, a sun-like star orbited by six planets. At times, two or more planets pass in front of the star at once, as shown in this artist’s conception of a simultaneous transit of three planets observed by the Kepler spacecraft on Aug. 26, 2010. During each pass or transit, the star’s light fades in a periodic way.
Credit: NASA/Tim Pyle

You’ll recall that the orbiting Kepler observatory continuously monitored stars in a fixed field of view focused on the constellations Lyra and Cygnus hoping to catch periodic dips in their light caused by transiting planets. If a drop was seen, more transits were observed to confirm the detection of a new exoplanet.

And catch it did. Kepler found 1,013 confirmed exoplanets in 440 star systems as of January 2015 with 3,199 unconfirmed candidates. Measuring the amount of light the planet temporarily “robbed” from its host star allowed astronomers to determine its diameter, while the length of time between transits yielded its orbital period.

Graph showing the big dip in brightness of KIC 8462852 around 800 days (center) followed after 1500 days whole series of dips of varying magnitude. Credit: Boyajian et. all — Graph showing the big dip in brightness of KIC 8462852 around 800 days (center) followed after 1500 days whole series of dips of varying magnitude up to 22%. The usual drop in light when an exoplanet transits its host star is a *fraction of a percent*. The star’s normal brightness has been set to “1.00” as a baseline. Credit: Boyajian et. all

Volunteers with the Planet Hunters project, one of many citizen science programs under the umbrella of Zooniverse, harness the power of the human eye to examine Kepler light curves (a graph of a star’s changing light intensity over time), looking for repeating patterns that might indicate orbiting planets. They were the first to meet up with the perplexing KIC 8462852.

dsafad — A detailed look at a small part of the star’s light curve reveals an unknown, regular variation of its light every 20 days. Superimposed on that is the star’s 0.88 day rotation period. Credit: Boyajian et. all

This magnitude +11.7 star in Cygnus, hotter and half again as big as the Sun, showed dips all over the place. Around Day 800 during Kepler’s run, it faded by 15% then resumed a steady brightness until Days 1510-1570, when it underwent a whole series of dips including one that dimmed the star by 22%. That’s huge! Consider that an exo-Earth blocks only a fraction of a percent of a star’s light; even a Jupiter-sized world, the norm among extrasolar planets, soaks up about a percent.

Exoplanets also show regular, repeatable light curves as they enter, cross and then exit the faces of their host stars. KIC 8462852’s dips are wildly a-periodic.

Could a giant comet breakup followed by those pieces crumbling into even smaller comets be the reason for KIC's erratic changes in brightness? Credit: NASA — Could a giant comet breakup and subsequent cascading breakups of those pieces be behind KIC 8462852’s erratic changes in brightness? Credit: NASA

Whatever’s causing the flickering can’t be a planet. With great care, the researchers ruled out many possibilities: instrumental errors, starspots (like sunspots but on other stars), dust rings seen around young, evolving stars (this is an older star) and pulsations that cover a star with light-sucking dust clouds.

What about a collision between two planets? That would generate lots of material along with huge clouds of dust that could easily choke off a star’s light in rapid and irregular fashion.

A great idea except that dust absorbs light from its host star, warms up and glows in infrared light. We should be able to see this “infrared excess” if it were there, but instead KIC 8462852 beams the expected amount of infrared for a star of its class and not a jot more. There’s also no evidence in data taken by NASA’s Wide-field Infrared Survey Explorer (WISE) several years previously that a dust-releasing collision happened around the star.

Our featured star shines around 12th magnitude in the constellation Cygnus the Swan (Northern Cross) high in the southern sky at nightfall this month. A 6-inch or larger telescope will easily show it. Use this map to get oriented and the map below to get there. Source: Stellarium — Our featured star shines at magnitude +11.7 in the constellation Cygnus the Swan (Northern Cross) high in the southern sky at nightfall this month. A 6-inch or larger telescope will easily show it. Use this map to get oriented and the map below to get there. Source: Stellarium

After examining the options, the researchers concluded the best fit might be a shattered comet that continued to fragment into a cascade of smaller comets. Pretty amazing scenario. There’s still dust to account for, but not as much as other scenarios would require.

Detailed map showing stars to around magnitude 12 with the Kepler star identified. It's located only a short distance northeast of the open cluster NGC 6886 in Cygnus. North is up. Source: Chris Marriott's SkyMap — Detailed map showing stars to around magnitude 12 with the Kepler star identified. It’s located only a short distance northeast of the open cluster NGC 6886 in Cygnus. North is up. Click to enlarge. Source: Chris Marriott’s SkyMap

Being fragile types, comets can crumble all by themselves especially when passing exceptionally near the Sun as sungrazing comets are wont to do in our own Solar System. Or a passing star could disturb the host star’s Oort comet cloud and unleash a barrage of comets into the inner stellar system. It so happens that a red dwarf star lies within about 1000 a.u. (1000 times Earth’s distance from the Sun) of KIC 8462852. No one knows yet whether the star orbits the Kepler star or happens to be passing by. Either way, it’s close enough to get involved in comet flinging.

So much for “natural” explanations. Tabetha Boyajian, a postdoc at Yale, who oversees the Planet Hunters and the lead author of the paper on KIC 8462852, asked Jason Wright, an assistant professor of astronomy at Penn State, what he thought of the light curves. “Crazy” came to mind as soon he set eyes on them, but the squiggles stirred a thought. Turns out Wright had been working on a paper about detecting transiting megastructures with Kepler.

There are Dyson rings and spheres and this, an illustration of a Dyson swarm. Could this or a variation of it be what we're detecting around KIC? Not likely, but a fun thought experiment. Credit: Wikipedia — There are Dyson rings and spheres and a Dyson swarm depicted here. Could this or a variation of it be what we’re seeing around KIC 8462852? Not likely, but a fun thought experiment. Credit: Wikipedia

In a recent blog, he writes: “The idea is that if advanced alien civilizations build planet-sized megastructures — solar panels, ring worlds, telescopes, beacons, whatever — Kepler might be able to distinguish them from planets.” Let’s assume our friendly aliens want to harness the energy of their home star. They might construct enormous solar panels by the millions and send them into orbit to beam starlight down to their planet’s surface. Physicist Freeman Dyson popularized the idea back in the 1960s. Remember the Dyson Sphere, a giant hypothetical structure built to encompass a star?

From our perspective, we might see the star flicker in irregular ways as the giant panels circled about it. To illustrate this point, Wright came up with a wonderful analogy:

“The analogy I have is watching the shadows on the blinds of people outside a window passing by. If one person is going around the block on a bicycle, their shadow will appear regularly in time and shape (like a regular transiting planet). But crowds of people ambling by — both directions, fast and slow, big and large — would not have any regularity about it at all. The total light coming through the blinds might vary like — Tabby’s star.”

The Green Bank Telescope is the world's largest, fully-steerable telescope. The GBT's dish is 100-meters by 110-meters in size, covering 2.3 acres of space. — The Green Bank Telescope is the world’s largest, fully-steerable telescope. The GBT’s dish is 100-meters by 110-meters in size, covering 2.3 acres of space. Credit: NRAO/AUI/NSF

Even Wright admits that the “alien hypothesis” should be seen as a last resort. But to make sure no stone goes unturned, Wright, Boyajian and several of the Planet Hunters put together a proposal to do a radio-SETI search with the Green Bank 100-meter telescope. In my opinion, this is science at its best. We have a difficult question to answer, so let’s use all the tools at our disposal to seek an answer.

Star with a mystery, KIC 8462852, photographed on Oct. 15, 2015. Credit: Gianluca Masi — KIC 8462852, photographed on Oct. 15, 2015. It’s an F3 V star (yellow-white dwarf) located about 1,480 light years from Earth. Credit: Gianluca Masi

In the end, it’s probably not an alien megastructure, just like the first pulsar signals weren’t sent by LGM-1 (Little Green Men). But whatever’s causing the dips, Boyajian wants astronomers to keep a close watch on KIC 8462852 to find out if and when its erratic light variations repeat. I love a mystery, but answers are even better.

October 16, 2015December 23, 2015 by David Dickinson

ExoMars Heads to the Red Planet in 2016

An artist's concept of the S EDM Lander separating from the ExoMars Trace Gas Orbiter. Image Credit: ESA

The 2016 launch window for Mars missions is fast approaching along with opposition, and ESA is refining its target window for ExoMars. Mars launch season offers the optimal time to make the trip from Earth to Mars, as missions prepare to break the surly bonds and head towards the Red Planet next spring. NASA’s InSight lander will also make the trip.

ExoMars is the first joint European Space Agency (ESA) Roscosmos mission to the Red Planet. The ExoMars Trace Gas Orbiter is under contract to Thales Alenia Space, and the EDM stationary lander dubbed Schiaparelli after the 19th century Italian astronomer is being constructed by Airbus Defense and Space. This would be Russia’s first successful Mars lander mission for over a dozen tries if successful.

The ExoMars Trace gas Orbiter in the lab. Image credit: ESA — The ExoMars Trace Gas Orbiter in the lab. Image credit: ESA

The ExoMars project is a two-part mission, and will culminate in an ExoMars rover in 2018. The key objective for the Trace Gas Orbiter, lander and rover to follow in 2018 is to seek out the controversial source of methane on Mars. A product of biology—think bovine flatulence—on Earth, researchers have proposed various sources—inorganic and otherwise—as a source of the anomalous methane seen in the Martian atmosphere. The Trace Gas Orbiter will remain on-station in orbit through 2018 to relay communications from the ExoMars rover. The Entry, Descent and Landing Demonstrator Module Schiaparelli will demonstrate key technologies for landing, including a hybrid Buck Rodgers fins-first style retro-rocket landing reminiscent of Viking, along with a deformable underside meant to absorb impact.

The landing sequence for the EDL Lander. Image credit: ESA

The landing with be a dramatic one on Meridiani Planum at the expected height of dust storm season, and we may get some interesting footage from the onboard descent camera. Along with weather and atmospheric measurements, the EDM Lander will also make the first electrical field measurements from the surface of Mars.

Image credit: MOLA Science Team and NASA/JPL/Arizona State Unversity — The landing ellipse for EDL. Note that its very close to the NASA rover Opportunity. Image credit: MOLA Science Team and NASA/JPL/Arizona State University

Unfortunately, EDM’s life will be short; Roscosmos originally intended to supply a 100-watt plutonium-powered RTG for the lander, but later opted due to export control to use an on-board battery. The EDM’s lifespan will be measured in a few days, at best.

Heading to Mars in 2016

An issue related to two propulsion system sensors aboard the EDM Lander recently prompted mission planners to opt for a launch for ExoMars at the end of the window next year, with liftoff set for March 14^th atop a Proton rocket from the Baikonur Cosmodrome in Kazakhstan instead of January, as originally intended. NASA’s Mars InSight will depart Earth for the Red Planet just ten days earlier on March 4^th from Vandenberg AFB in a rare dramatic night shot of an Atlas 5 rocket deploying an interplanetary mission from the US West Coast. InSight’s primary objective is to study seismic activity and the Martian interior, and will land in one of four selected sites (1 primary and 3 backup) in Elysium Planitia on September 28^th, 2016.

Naturally, ESA and Roscomos are taking every precaution to assure the success of ExoMars and EDM. The 2011 failure of Phobos-Grunt highlighted the perils of tempting the ‘Great Martian Ghoul’ with more tasty spacecraft. Space is hard, and landing on Mars even more so.

Opposition 2016 for Mars occurs on May 22^nd, 2016. Mars is always high in the early morning sky a few months prior to opposition, presenting an optimal window to send spacecraft to the Red Planet on the most efficient in trajectory in terms of fuel and time. This 3-month wide window comes around every 26 months leading up to opposition season. Oppositions of Mars are now getting more favorable, and the next opposition after 2016 in 2018 will be nearly as favorable as the historic 2003 event.

Typical Earth to Mars launch trajectories, in this case, for NASA’s twin Mars Exploration Rovers. Image credit: NASA/JPL

Our robots are swiftly colonizing Mars on our behalf. Here’s a Who’s Who scorecard of functioning spacecraft. On the surface: NASA’s Opportunity and Mars Curiosity rovers. In orbit: Mars Odyssey, (Since 2001!) Mars Express, HiRISE, India’s Mars Orbiter, and MAVEN. Add the ExoMars 2016 and 2018 missions, InSight and the Mars 2020 rover for NASA, and we’ve truly established a redundant sort of ‘telepresence’ on and around Mars.

An artist's conception of the ExoMars 2018 rover on the Red Planet. Image credit: ESA — An artist’s conception of the ExoMars 2018 rover on the Red Planet. Image credit: ESA

Will the EDM Lander become the first successful non-NASA lander to approach the Red Planet? Keep an eye on the Insight and the first of two ExoMars missions, as Earth invades Mars in 2016!

October 15, 2015April 21, 2016 by Matt Williams

Jupiter’s Moon Ganymede

In 1610, Galileo Galilei looked up at the night sky through a telescope of his own design. Spotting Jupiter, he noted the presence of several “luminous objects” surrounding it, which he initially took for stars. In time, he would notice that these “stars” were orbiting the planet, and realized that they were in fact Jupiter’s moons – which would come to be named Io, Europa, Ganymede and Callisto.

Of these, Ganymede is the largest, and boasts many fascinating characteristics. In addition to being the largest moon in the Solar System, it is also larger than even the planet Mercury. It is the only satellite in the Solar System known to possess a magnetosphere, has a thin oxygen atmosphere, and (much like its fellow-moons, Europa and Callisto) is believed to have an interior ocean.

Continue reading “Jupiter’s Moon Ganymede”

October 14, 2015December 23, 2015 by David Dickinson

Is This Month’s Jupiter-Venus Pair Really a Star of Bethlehem Stand In?

Image credit and copyright: Clapiotte Astro

Eclipse tetrads of doom. Mars, now bigger than the Full Moon each August. The killer asteroid of the month that isn’t. Amazing Moons of all stripes, Super, Blood, Black and Blue…

Image credit and copyright: @TaviGrainer(ck) — Venus, Mars, Jupiter and the Moon from October 9th. Image credit and copyright: @TaviGreiner

The internet never lets reality get in the way of a good meme, that’s for sure. Here’s another one we’ve caught in the wild this past summer, one that now appears to be looking for a tenuous referent to grab onto again next week.

You can’t miss Jupiter homing in on Venus this month, for a close 61.5’ pass on the morning on Oct 25^th. -1.4 magnitude Jupiter shows a 33” disk on Sunday’s pass, versus -4 magnitude Venus’ 24” disk.

Oct 26 Stellarium — Looking east on the morning of October 26th. Credit: Stellarium

We also had a close pass on July 1^st, which prompted calls of ‘the closest passage of Venus and Jupiter for the century/millennia/ever!’ (spoiler alert: it wasn’t) Many also extended this to ‘A Star of Bethlehem convergence’ which, again, set the web a-twittering.

Will the two brightest planets in the sky soon converge every October, in the minds of Internet hopefuls?

This idea seems to come around every close pass of Jupiter and Venus as of late, and may culminate next year, when an extra close 4’ pass occurs on August 27^th, 2016. But the truth is, close passes of Venus and Jupiter are fairly common, occurring 1-2 times a year. Venus never strays more than 47 degrees from the Sun, and Jupiter moves roughly one astronomical constellation eastward every Earth year.

Much of the discussion in astrological circles stems from the grouping of Jupiter, Venus and the bright star Regulus this month. Yes, this bears a resemblance to a grouping of the same seen in dawn skies on August 12^th, 2 BCE. This was part of a series of Jupiter-Venus conjunctions that also occurred on May 24^th, 3 BCE and June 17^th, 1 BCE. The 2 BCE event was located in the constellation Leo the Lion, and Regulus rules the sign of kings in the minds of many…

Stall — Looking eastward on the morning of August 12th, 2 BCE. Credit: Stellarium

But even triple groupings are far from uncommon over long time scales. Pairings of Jupiter, Venus in any given zodiac constellation come back around every 11-12 years. Many great astronomical minds over the centuries have gone broke trying to link ‘The Star’ seen by the Magi to the latest astronomical object in vogue, from conjunctions, to comets, to supernovae and more. If there’s any astronomical basis to the allegorical tale, we’ll probably never truly know.

The October 25th pass of Venus vs Jupiter. Created using Starry Night Education software.

Aaron Adair, the author of The Star of Bethlehem: A Skeptical View has this to say to Universe Today:

“The 3/2 BCE conjunctions don’t fit the time of Jesus’ birth. There is also no evidence that these sorts of conjunctions were considered all that good; I even found evidence that they were bad news for a king, especially if Jupiter was circling around Regulus. And of course, none of this even comes close to doing the things the Star of Bethlehem was claimed to have done.

So, we have a not terribly rare situation in the sky that conforms to something that doesn’t really fit the Gospel story in a time frame that doesn’t fit the Jesus chronology which doesn’t really have anything all that auspicious about that to ancient observers.”

The dance of the planets also gives us a brief opening teaser on Saturday morning, as Mars passes just 0.38 degrees NNE of Jupiter on Oct 17^thlooking like a fifth pseudo-moon.

Finally, the crescent Moon joins the scene once again on November 7^th, passing 1.9 degrees SSW of Jupiter and 1.2 SSW of Venus, a great time to attempt to spy both in the daytime using the crescent Moon as a guide. And keep an eye on Venus, as the next passage of the crescent Moon on December 7^th features a close grouping with binocular Comet C/2013 US10 Catalina as well.

How close can the two planets get?

Stick around ‘til November 22^nd 2065, and you can watch Venus actually transit the face of Jupiter:

Though rare, such an occlusion involving the two brightest planets happens every other century or so… we ran a brief simulation, and uncovered 11 such events over the next three millennia:

Bruce McCurdy of the Royal Canadian Astronomical Society posed a further challenge: how often does Venus fully occult Jupiter? We ran a simulation covering 9000 BC to 9000 AD, and found no such occurrence, though the July 14^th, 4517 AD meeting of Jupiter and Venus is close.

Let’s see, I’ll be on my 3^rd cyborg body, in the post- Robot Apocalypse by then…

This sort of total occlusion of Jupiter by Venus turns out to be rarer than any biblical conjunction. Why?

Well, for one thing, Venus is generally smaller in apparent size than Jupiter. When Jupiter is near Venus, it’s also near the Sun and in the 30-35” size range. Venus only breaks 30” in size for about 20% of its 584 synodic period. But we suspect a larger cycle may be in play, keeping the occurrence of a large Venus meeting and covering a shrunken Jove in our current epoch.

A Moon, a star, three planets and... a space station? A close pass of Tiangong-1 (arrowed) near this month's grouping. Image credit: Dave Dickinson — A Moon, a star, three planets and… a space station? A close pass of Tiangong-1 (arrowed) near this month’s grouping. Image credit: Dave Dickinson

Astronomy makes us ponder the weirdness of our skies gracing our backyard over stupendously long time scales. Whatever your take on the tale and the modern hype, be sure to get out and enjoy the real show on Sunday morning October 25th, as the brightest of planets make for a brilliant pairing.

October 13, 2015December 23, 2015 by Vanessa Janek

Shape-shifting neutrinos earn physicists the 2015 Nobel

Super-Kamiokande, a neutrino detector in Japan, holds 50,000 tons of ultrapure water surrounded by light tubes. Credit: Super-Kamiokande Observatory

What do Albert Einstein, Neils Bohr, Paul Dirac, and Marie Curie have in common? They each won the Nobel prize in physics. And today, Takaaki Kajita and Arthur McDonald have joined their ranks, thanks to a pioneering turn-of-the-century discovery: in defiance of long-held predictions, neutrinos shape-shift between multiple identities, and therefore must have mass.

The neutrino, a slight whiff of a particle that is cast off in certain types of radioactive decay, nuclear reactions, and high-energy cosmic events, could be called… shy. Electrically neutral but enormously abundant, half the time a neutrino could pass through a lightyear of lead without interacting with a single other particle. According to the Standard Model of particle physics, it has a whopping mass of zero.

As you can imagine, neutrinos are notoriously difficult to detect.

But in 1956, scientists did exactly that. And just a few years later, a trio of physicists determined that neutrinos came in not just one, not two, but three different types, or flavors: the electron neutrino, the muon neutrino, and the tau neutrino.

The first annotated neutrino event. Image credit: — The neutrino was first detected in 1956 by Clyde Cowan and Frederick Reines. In 1970, scientists captured the first image of a neutrino track in a hydrogen bubble chamber. *Image: Argonne National Laboratory*

But there was a problem. Sure, scientists had figured out how to detect neutrinos—but they weren’t detecting enough of them. In fact, the number of electron neutrinos arriving on Earth due to nuclear reactions in the Sun’s core was only one-third to one-half the number their calculations had predicted. What, scientists wondered, was happening to the rest?

Kajita, working at the Super-Kamiokande detector in Japan in 1998, and McDonald, working at the Sudbury Neutrino Observatory in Canada in 1999, determined that the electron neutrinos were not disappearing at all; rather, these particles were changing identity, spontaneously oscillating between the three flavor-types as they traveled through space.

Moreover, the researchers proclaimed, in order for neutrinos to make such transformations, they must have mass.

This is due to some quantum funny business having to do with the oscillations themselves. Grossly simplified, a massless particle, which always travels at the speed of light, does not experience time—Einstein’s theory of special relativity says so. But change takes time. Any particle that oscillates between identities needs to experience time in order for its state to evolve from one flavor to the the next.

The interior structure of the Sun. Credit: Wikipedia Commons/kelvinsong — Neutrinos are produced in abundance during fusion reactions at the center of our Sun, and oscillate between three different types, or *flavors*, on their way to Earth. *Image: Wikipedia Commons/kelvinsong*

Kajita and McDonald’s work showed that neutrinos must have a mass, albeit a very small one. But neutrinos are abundant in the Universe, and even a small mass has a large effect on all sorts of cosmic phenomena, from solar nuclear physics, where neutrinos are produced en masse, to the large-scale evolution of the cosmos, where neutrinos are ubiquitous.

The neutrino, no longer massless, is now considered to play a much larger role in these processes than scientists had originally believed.

What is more, the very existence of a massive neutrino undermines the theoretical basis of the Standard Model. In fact, Kajita’s and McDonald’s discovery provided some of the first evidence that the Standard Model might not be as airtight as had been previously believed, nudging scientists ever more in the direction of so-called “new physics.”

This is not the first time physicists have been awarded a Nobel prize for research into the nature of neutrinos. In 1988, Leon Lederman, Melvin Schwartz, and Jack Steinberger were awarded the prize for their discovery that neutrinos come in three flavors; in 1995, Frederick Reines won a Nobel for his detection of the neutrino along with Clyde Cowan; and in 2002, a Nobel was awarded to Raymond David Jr., the oldest person ever to receive a the prize in physics, and Masatoshi Koshiba for their detection of cosmic neutrinos.

Kajita, of the University of Tokyo, and McDonald, of Queen’s University in Canada, were awarded the prestigious prize this morning at a news conference in Stockholm.

October 13, 2015December 23, 2015 by Bob King

Hubble Sees Changes in Jupiter’s Red Spot, a Weird Wisp and Rare Waves

This new image from the largest planet in the Solar System, Jupiter, was made during the Outer Planet Atmospheres Legacy (OPAL) programme. The images from this programme make it possible to determine the speeds of Jupiter’s winds, to identify different phenomena in its atmosphere and to track changes in its most famous features. The map shown was observed on 19 January 2015, from 2:00 UT to 12:30 UT. Credit: NASA, ESA, A. Simon (GSFC), M. Wong (UC Berkeley), and G. Orton (JPL-Caltech)

Jupiter global map created from still images from the Hubble Space Telescope

It’s been widely reported, including at Universe Today, that the apple of Jupiter’s eye, the iconic Great Red Spot (GRS), has been shrinking for decades. Even the rate of shrinkage has been steadily increasing.

Back in the late 1800s you could squeeze three Earths inside the GRS. Those were the days. Last May it measured just 10,250 miles (16,496 km) across, big enough for only 1.3 of us.

And while new photos from the Hubble Space Telescope show that Jupiter’s swollen red eye has shrunk an additional 150 miles (240 km) since 2014, the good news is that the rate of shrinkage appears to be well, shrinking. The contraction of the GRS has been studied closely since the 1930s; even as recently as 1979, the Voyager spacecraft measured it at 14,500 miles (23,335 km) across. But the alarm sounded in 2012, when amateur astronomers discovered sudden increase in the rate of 580 miles (933 km) a year along with a shift in shape from oval to roughly circular.

For the moment, it appears that the GRS is holding steady, making for an even more interesting Jupiter observing season than usual. Already, the big planet dominates the eastern sky along with Venus on October mornings. Consider looking for changes in the Spot yourself in the coming months. A 6-inch or larger scope and determination are all you need.

Hubble photos of the Great Red Spot taken at on a first rotation (left frames) and 10 hours later (right frames) show the counterclockwise rotation of the newly-discovered filament or wisp inside the GRS. Credit: — Hubble photos of the Great Red Spot taken on a first rotation (left frames) and 10 hours later (right frames) show the counterclockwise rotation of the newly-discovered filament or wisp inside the GRS. Credit: NASA, ESA, A. Simon (GSFC), M. Wong (UC Berkeley), and G. Orton (JPL-Caltech)

New imagery from the Hubble OPAL program also shows a curious wisp at the center of the Great Red Spot spanning almost the entire width of the hurricane-like vortex. This filamentary streamer rotates and twists throughout the 10-hour span of the Great Red Spot image sequence, drawn out by winds that are blowing at 335 mph (540 km/hr). Color-wise, the GRS remains orange, not red. Currently, the reddest features on the planet are the North Equatorial Belt and the occasional dark, oval “barges” (cyclonic storms) in the northern hemisphere.

The newly-found waves in Jupiter's atmosphere are located in regions where cyclones are common. They look like dark eyelashes. Credit: — The newly-found waves in Jupiter’s atmosphere are located in regions where cyclones and anticyclones are common. They look like dark eyelashes. A cyclone is a storm or system of winds that rotates around an area of low pressure. Anticyclones spin around areas of high pressure. Credit: NASA, ESA, A. Simon (GSFC), M. Wong (UC Berkeley), and G. Orton (JPL-Caltech)

That’s not all. The photos uncovered a rare wave structure just north of Jupiter’s equator that’s only been seen once before and with difficulty by the Voyager 2 spacecraft in 1979. The scientists, whose findings are described in this just-published Astrophysical Journal paper, say it resembles an earthly atmospheric feature called a baroclinic wave, a large-scale meandering of the jet stream associated with developing storms.

Hubble view of Jupiter's barocyclonic clouds and those recorded earlier by Voyager 2. Credit: — Hubble view of Jupiter’s baroclinic waves on January 19, 2015 (top) and our only other view of them photographed by Voyager 2 in 1979. Credit: NASA, ESA, A. Simon (GSFC), M. Wong (UC Berkeley), and G. Orton (JPL-Caltech)

Jupiter’s “current wave” riffles across a region rich with cyclonic and anticyclonic storms. The wave may originate in a clear layer beneath Jupiter’s clouds, only becoming visible when it propagates up into the cloud deck, according to the researchers. While it’s thought to be connected to storm formation in the Jovian atmosphere, it’s a mystery why the wave hasn’t been observed more often.

The OPAL program focuses on long-term observation of the atmospheres of Jupiter, Uranus and Neptune until the end of the Saturn Cassini Mission and all four planets afterwords. We have to keep watch from Earth as no missions to Saturn and beyond are expected for quite some time. To date, Neptune and Uranus have already been observed with photos to appear (hopefully) soon in a public archive.