Jupiter’s Moon Callisto

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
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
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
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.

Amazing Impact Crater Where a Triple Asteroid Smashed into Mars

At first glance, you many not guess that this feature on Mars is an impact crater. The reason it looks so unusual is that it likely is a triple impact crater, formed when three asteroids struck all at once in the Elysium Planitia region.

Why do planetary scientists think the three craters did not form independently at different times?

“The ejecta blanket appears to be uniform around the triple-crater showing no signs of burial or overlapping ejecta from overprinting craters,” write scientists Eric Pilles, Livio Tornabene, Ryan Hopkins, and Kayle Hansen on the HiRISE website. “The crater rims are significantly stunted where the craters overlap.”

This oblong-shaped crater could have been created from a triple asteroid, or it could have been a binary asteroid, and one broke apart, creating the three overlapping craters. The team says the two larger craters must have been produced by asteroids of approximately the same size, probably on the order of a few hundred meters across.

“The northern crater might have been created by a smaller asteroid, which was orbiting the larger binary pair, or when one of the binary asteroids broke up upon entering the atmosphere,” the team explained. “The shape of the triple-crater is oblong, suggesting an oblique impact; therefore, another alternative would be that the asteroid split upon impact and ricocheted across the surface, creating additional craters.”

Studying craters on Mars — and there are lots of them, thanks to Mars’ sparse atmosphere — can help estimate the ages of different terrains, as well as revealing materials such as ice or minerals that get exposed from the impact.

HiRISE is the amazing camera on board the Mars Reconnaissance Orbiter.

Citizen Scientists Hunt for Impact Craters in Persia

Citizen scientists have discovered planets beyond our Solar System and established morphological classifications for thousands of galaxies (e.g., the Planet Hunters and Galaxy Zoo projects).  At an upcoming meeting of planetary scientists, Hamed Pourkhorsandi from the University of Tehran will present his efforts to mobilize citizens to identify impact craters throughout Persia.   Pourkhorsandi said he is recruiting volunteers to identify craters using Google Earth, while continuing to seek sightings of fireballs cited in ancient books and among rural folk.  Discovering impact craters is an important endeavour, since it helps astronomers estimate how many asteroids of a particular size strike Earth over a given time (i.e., the impact frequency).  Indeed, that is especially relevant in light of the recent meteor explosion over Russia this past February (see the UT article here), which hints at the potentially destructive nature of such occurrences.

Satellite images have facilitated the detection of impact sites such as the Kamil and Puka craters, which were identified by V. de Michele and D. Hamacher using Google Earth, respectively (see the UT article here).  Pourkhorsandi noted that, “Free access to satellite images has led to the investigation of earth’s surface by specialists and nonspecialists, attempts that have led to the discovery of new impact craters around the globe.   [Yet] few researches on this topic have been done in the Middle East.”  Incidentally, citizens are likewise being recruited to classify craters and features on other bodies in the Solar System (e.g., the Moon Zoo project).

kamilnatgeo
The Kamil impact crater in Egypt was discovered by V. de Michele using Google Earth, and H. Pourkhorsandi is recruiting volunteers to discover such structures throughout Persia following a similar approach (image credit: L. Folco).

In his paper, Pourkhorsandi describes examples of two targets investigated thus far: “1. a circular structure with a diameter of 200 m (33°21’57”N 58°14’24”E).  [However,] there is no sign of … meteoritic fragments in the region that are primary diagnostic indicators for small size impact craters.”  The second target is tied to an old tale, and note that the Puka crater in Australia was identified by following-up on an old Aboriginal story.  However, Pourkhorsandi states that a field study of the second target (28°24’52” N 60°34’44” E) revealed that the crater is not associated with an impactor from space.

“Beside these structures, field studies on other craters in Persia are in progress, the outcomes of which will be announced in the near future,” said Pourkhorsandi.



View Larger Map
Pourkhorsandi underscores that numerous meteorites have been found in desert regions throughout the world, yet scant attention has been given to Persian deserts (e.g., the Lut desert).  The Lut desert in Persia extends over several thousand square kilometres and is one of the hottest places on Earth (featuring land surface temperatures upwards of 70 degrees Celsius).  Pourkhorsandi noted that in 2005 a ‘curious stone’ was recovered in the Lut desert and subsequent work revealed its extraterrestrial origin.

He went on to remark that, “Three recent short field trips to the central Lut desert led to the collection of several meteoritic fragments, which points to large concentrations of meteoritic materials in the area.”  Some of those fragments are shown in the figure below, and the broader region is likely a pertinent place for citizen scientists to continue the hunt for impact craters in Persia.

Pourkhorsandi concluded by telling the Universe Today, “In the future we aim to expand our efforts with the help of additional people, and will direct individuals to scan other regions of the planet.  Simultaneously, we have commenced a comprehensive analysis of meteorites in the Lut desert with fellow European scientists.”

"Fragments of a H5 chondrite in the field. The scale." from Pour/arXiv.
H chondrite fragments found in the Lut desert (in Persia) are argued to be extraterrestrial in origin (image credit: Fig. 3 in Pourkhorsandi 2013/LPI).

H. Pourkhorsandi’s findings were shown at the 44th Lunar and Planetary conference in Texas, and will be presented at the upcoming Large Meteorite Impact and Evolution V conference.  That latter conference will feature the latest results concerning the cratering process, and a description of the science program is available here.  Copies of H. Pourkhorsandi and H. Mirnejad’s conference submissions are available via the LPI and arXiv.   Those readers interested in joining H. Pourkhorsandi’s effort, or desiring additional information, may also find the following pertinent: the Earth Impact DatabaseRampino and Haggerty 1996, “Collision Earth! The Threat from Outer Space” by P. Grego, NASA’s projects for Citizen Scientists.

Oldest Impact Crater on Earth Discovered in Greenland

Artistic expression of large meteorite impact

With shifting continents, rain, and wind, finding traces of ancient impact craters on Earth has been, literally, astronomically low. Now, an international team of scientists say they have found a massive impact crater in Greenland a billion years older than other known asteroid impact on Earth.

Scientists found the remains of the giant 100-kilometer (62 mile) wide crater near the Maniitsoq region of West Greenland and they believe it’s three billion years old. The largest and previously oldest known crater is the 300 kilometer-wide Vredefort crater in South Africa. Tipped on its side, the edges of the Maniitsoq crater would extend from the surface of the Earth to the edge of space.


“This single discovery means that we can study the effects of cratering on the Earth nearly a billion years further back in time than was possible before,” according to Dr. Iain McDonald of the School of Earth and Ocean Sciences at Cardiff University, who was part of the team.

Finding the crater wasn’t an easy task. Today, the Moon still shows marks of the massive bombardment that took place between three and four billion years ago. The early Earth, with its greater gravitational attraction, would have experienced even more collisions. But the land around Maniitsoq has been eroded over the eons to expose crust that originally was 25 kilometers (16 miles) below the surface. Effects of the immense shockwave produced on impact penetrated deep into the crust and remain visible.

Evidence at that depth had never been observed before, says McDonald. “The process was rather like a Sherlock Holmes story,” said McDonald. “We eliminated the impossible in terms of any conventional terrestrial processes, and were left with a giant impact as the only explanation for all of the facts.”

Only about 180 impact craters have been discovered on Earth. Around 30 percent of them contain important natural resources, including nickel, gold, oil and natural gas. It was during an exploration of natural resources that evidence for the crater was discovered. “It has taken us nearly three years to convince our peers in the scientific community,” said McDonald. “But the mining industry was far more receptive. A Canadian exploration company has been using the impact model to explore for deposits of nickel and platinum metals at Maniitsoq since the autumn of 2011.”

The international team, led by Adam Garde, a senior research scientist at the Geological Survey of Denmark and Greenland, or GEUS, contains members from Cardiff, Lund University in Sweden, and the Institute of Planetary Science in Moscow. Their work was recently published in the jounal Earth and Planetary Science Letters.

Image caption: An artistic expression of how a large meteorite impact into the sea might have looked in the first second of the impacting. We do not know if the area that was hit was actually covered by water or if there was just a sea nearby. Source: Carsten Egestal Thuesen, GEUS

Map caption: Black circle on map shows the location of the meteorite impact structure near the town Maniitsoq in Greenland.

Read more about the Maniitsoq structure.

Impact in Latvia Creates 20-Meter Crater

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A possible meteorite fall near in northern Latvia on Sunday left a crater approximately 20 meters (66 feet) in diameter and 10 meters (33 feet) deep. UPDATE: Many reports now say the impact was a fake; The Bad Astronomer says “shovel” marks were found around the perimeter of the crater; additionally, a burning impactor is highly unlikely (see video below). And here’s an article from the Associated press. , and another from Yahoo news, where a phone company in Latvia admits the “crater” was a publicity stunt.


Our earlier report:
No one was injured, as the impact occurred outside the small town of Mazsalaca, although houses were nearby. Early reports said it was not clear whether it was an asteroid or a space satellite, but later news indicated it was a meteorite strike. Another account said it might be a hoax, as a cover-up of illegal weapons tests. One report said a witness saw the object falling through the sky, leaving a burning trail behind, and said it was making a noise similar to the one of an aircraft flying at a low altitude. See a video of the crater below.

A spokesperson for the Latvian State Fire and Rescue Service said that rescuers and soldiers immediately cordoned off the territory, as they wanted to guard against any radioactive contamination if it was a satellite.

See this link for more images of the crater.

We’ll post more news about the crater as it becomes available.

Sources: RiaNovosti, ITAR-TASS

Hat tip to @cosmos4U on Twitter