Seeing Double: Jupiter Returns at Dawn

double shadow transit

Missing Jove? The largest planet in our solar system is currently on the far side of the Sun and just passed solar conjunction on September 26th, 2016. October now sees Jupiter slowly return to the dawn sky. Follow that gas giant, as an interesting set of double shadow transits transpires in late October leading in to early November.

This particular cycle of double shadow transits involves the large Jovian moons of Europa and Ganymede.

The scene on October 24th at 23:55 UT. Image credit: Created using Starry Night Education software.
The scene on October 24th at 23:55 UT. Image credit: Created using Starry Night Education software.

Europa and Ganymede double shadow transit season begins later this month, as both cast shadows on the Jovian cloud tops. This series of simultaneous shadow transits runs from October 17th to November 8th, and includes four weekly events.

The inner three large moons Io, Europa and Ganymede are in a 4:2:1 resonance. Europa orbits Jove once every 3.6 days and makes two circuits for Ganymede’s one. This means there’s a double shadow transit once every week in the current season:

The double shadow transit season of 2016. Created by author.
The double shadow transit season of 2016. Created by author.

When can you first spy Jupiter, post solar conjunction? Catching this particular series of double shadow transits is challenging this time around, owing to the planet’s position low in the dawn twilight. The first event on October 17th starts with Jupiter just 16 degrees west of the Sun, and the cycle ends with Jove 38 degrees west of the Sun on November 8th.

Keep in mind, it is possible to track Jupiter up in to the daytime sky, post sunrise. To do this, you’ll need a ‘scope with a solid equatorial mount and good sidereal tracking. The trick is to lock on to Jupiter before sunrise and track it up in to the dawn sky. Be sure to physically block that dazzling rising Sun out of view behind a hill or building, and NEVER aim your telescope at the Sun!

Using this method opens up the possibility of nabbing a given double shadow event to longitudes due east of the quoted locales above.

The waning crescent Moon also passes 1.4 degrees NNE of Jupiter on October 28th, offering another chance to spy the gas giant in the dawn sky, using the nearby crescent Moon as a guide.

The Moon and Jupiter in the daytime skies on Novemebr.
The Moon and Jupiter in the daytime skies on October 28th. Image credit: Stellarium.

And another interesting pairing is coming right up on the morning of Tuesday, October 11th, when Mercury passes just 0.8 degrees (48′) NNE of Jupiter. Both are only 12 degrees west of the Sun at closest approach, which occurs around 10:00 UT. Still, both will appear as an interesting pseudo-double star, with Mercury shining at magnitude -1.1 and Jupiter only half a magnitude fainter at -1.6.

You can even see Jupiter coming off of solar conjunction and headed toward dawn skies courtesy of SOHO’s LASCO C3 camera:

Jupiter (arrowed) exiting the 15 degree wide field of view of SOHO's LASCO C3 camera on October 5th. Image credit: NASA/ESA/SOHO.
Jupiter (arrowed) exiting the 15 degree wide field of view of SOHO’s LASCO C3 camera on October 5th. Image credit: NASA/ESA/SOHO.

Callisto, the outermost large moon of Jupiter, ceased casting its shadow on Jupiter earlier this year on September 1st 2016. Callisto is the only large moon that can ‘miss’ the gas giant’s cloud tops. Callisto must be involved for a triple shadow transit to occur, and the moon resumes regularly casting its shadow on Jove on December 4th, 2019.

Callisto can also experience total solar eclipses similar to those seen from the Earth during the mutual eclipse season for Jupiter’s moons, albeit shorter in duration:

And don ‘t forget: we’ve got a spacecraft currently exploring Jupiter for the next year and a half: NASA’s very own Juno.

Be sure to check out the Jovian action over the next month, gracing a dawn sky near you.

7 Days Out From Orbital Insertion, NASA’s Juno Images Jupiter and its Largest Moons

This annotated color view of Jupiter and its four largest moons -- Io, Europa, Ganymede and Callisto -- was taken by the JunoCam camera on NASA's Juno spacecraft on June 21, 2016, at a distance of 6.8 million miles (10.9 million kilometers) from Jupiter. Image credit: NASA/JPL-Caltech/MSSS
This annotated color view of Jupiter and its four largest moons — Io, Europa, Ganymede and Callisto — was taken by the JunoCam camera on NASA’s Juno spacecraft on June 21, 2016, at a distance of 6.8 million miles (10.9 million kilometers) from Jupiter. Image credit: NASA/JPL-Caltech/MSSS

Now just 7 days out from a critical orbital insertion burn, NASA’s Jupiter-bound Juno orbiter is closing in fast on the massive gas giant. And as its coming into focus the spacecraft has begun snapping a series of beautiful images of the biggest planet and its biggest moons.

In a newly released color image snapped by the probes educational public outreach camera named Junocam, banded Jupiter dominates a spectacular scene that includes the giant planet’s four largest moons — Io, Europa, Ganymede and Callisto.

Junocam’s image of the approaching Jovian system was taken on June 21, 2016, at a distance of 6.8 million miles (10.9 million kilometers) and hints at the multitude of photos and science riches to come from Juno.

“Juno on Jupiter’s Doorstep,” says a NASA description. “And the alternating light and dark bands of the planet’s clouds are just beginning to come into view,” revealing its “distinctive swirling bands of orange, brown and white.”

This color view of Jupiter and its four largest moons -- Io, Europa, Ganymede and Callisto -- was taken by the JunoCam camera on NASA's Juno spacecraft on June 21, 2016, at a distance of 6.8 million miles (10.9 million kilometers) from Jupiter. Image credit: NASA/JPL-Caltech/MSSS
This color view of Jupiter and its four largest moons — Io, Europa, Ganymede and Callisto — was taken by the JunoCam camera on NASA’s Juno spacecraft on June 21, 2016, at a distance of 6.8 million miles (10.9 million kilometers) from Jupiter. Image credit: NASA/JPL-Caltech/MSSS

Rather appropriately for an American space endeavor, the fate of the entire mission hinges on do or die ‘Independence Day’ fireworks.

On the evening of July 4, Juno must fire its main engine for 35 minutes.

The Joy of JOI – or Jupiter Orbit Insertion – will place NASA’s robotic explorer into a polar orbit around the gas giant.

The approach over the north pole is unlike earlier probes that approached from much lower latitudes nearer the equatorial zone, and thus provide a perspective unlike any other.

After a five-year and 2.8 Billion kilometer (1.7 Billion mile) outbound trek to the Jovian system and the largest planet in our solar system and an intervening Earth flyby speed boost, the moment of truth for Juno is now inexorably at hand.

This colorized composite shows more than half of Earth’s disk over the coast of Argentina and the South Atlantic Ocean as the Juno probe slingshotted by on Oct. 9, 2013 for a gravity assisted acceleration to Jupiter. The mosaic was assembled from raw images taken by the Junocam imager. Credit: NASA/JPL/SwRI/MSSS/Ken Kremer/Marco Di Lorenzo
This colorized composite shows more than half of Earth’s disk over the coast of Argentina and the South Atlantic Ocean as the Juno probe slingshotted by on Oct. 9, 2013 for a gravity assisted acceleration to Jupiter. The mosaic was assembled from raw images taken by the Junocam imager. Credit: NASA/JPL/SwRI/MSSS/Ken Kremer/Marco Di Lorenzo

And preparations are in full swing by the science and engineering team to ensure a spectacular Fourth of July fireworks display.

The team has been in contact with Juno 24/7 since June 11 and already uplinked the rocket firing parameters.

Signals traveling at the speed of light take 10 minutes to reach Earth.

The protective cover that shields Juno’s main engine from micrometeorites and interstellar dust was opened on June 20.

“And the software program that will command the spacecraft through the all-important rocket burn was uplinked,” says NASA.

The pressurization of the propulsion system is set for June 28.

“We have over five years of spaceflight experience and only 10 days to Jupiter orbit insertion,” said Rick Nybakken, Juno project manager from NASA’s Jet Propulsion Laboratory in Pasadena, California, said in a statement.

“It is a great feeling to put all the interplanetary space in the rearview mirror and have the biggest planet in the solar system in our windshield.”

On the night of orbital insertion, Juno will fly within 2,900 miles (4,667 kilometers) of the Jovian cloud tops.

All instruments except those critical for the JOI insertion burn on July 4, will be tuned off on June 29. That includes shutting down Junocam.

“If it doesn’t help us get into orbit, it is shut down,” said Scott Bolton, Juno’s principal investigator from the Southwest Research Institute in San Antonio.

“That is how critical this rocket burn is. And while we will not be getting images as we make our final approach to the planet, we have some interesting pictures of what Jupiter and its moons look like from five-plus million miles away.”

During a 20 month long science mission – entailing 37 orbits lasting 11 days each – the probe will plunge to within about 3000 miles of the turbulent cloud tops and collect unprecedented new data that will unveil the hidden inner secrets of Jupiter’s origin and evolution.

“Jupiter is the Rosetta Stone of our solar system,” says Bolton. “It is by far the oldest planet, contains more material than all the other planets, asteroids and comets combined and carries deep inside it the story of not only the solar system but of us. Juno is going there as our emissary — to interpret what Jupiter has to say.”

During the orbits, Juno will probe beneath the obscuring cloud cover of Jupiter and study its auroras to learn more about the planet’s origins, structure, atmosphere and magnetosphere.

Junocam has already taken some striking images during the Earth flyby gravity assist speed boost on Oct. 9, 2013.

For example the dazzling portrait of our Home Planet high over the South American coastline and the Atlantic Ocean.

For a hint of what’s to come, see our colorized Junocam mosaic of land, sea and swirling clouds, created by Ken Kremer and Marco Di Lorenzo.

NASA's Juno probe captured the image data for this composite picture during its Earth flyby on Oct. 9 over Argentina,  South America and the southern Atlantic Ocean. Raw imagery was reconstructed and aligned by Ken Kremer and Marco Di Lorenzo, and false-color blue has been added to the view taken by a near-infrared filter that is typically used to detect methane. Credit: NASA/JPL/SwRI/MSSS/Ken Kremer/Marco Di Lorenzo
NASA’s Juno probe captured the image data for this composite picture during its Earth flyby on Oct. 9 over Argentina, South America and the southern Atlantic Ocean. Raw imagery was reconstructed and aligned by Ken Kremer and Marco Di Lorenzo, and false-color blue has been added to the view taken by a near-infrared filter that is typically used to detect methane. Credit: NASA/JPL/SwRI/MSSS/Ken Kremer/Marco Di Lorenzo

As Juno sped over Argentina, South America and the South Atlantic Ocean it came within 347 miles (560 kilometers) of Earth’s surface.

During the flyby, the science team observed Earth using most of Juno’s nine science instruments since the slingshot also serves as an important dress rehearsal and key test of the spacecraft’s instruments, systems and flight operations teams.

Juno soars skyward to Jupiter on Aug. 5, 2011 from launch pad 41 at Cape Canaveral Air Force Station at 12:25 p.m. EDT. View from the VAB roof. Credit: Ken Kremer/kenkremer.com
Juno soars skyward to Jupiter on Aug. 5, 2011 from launch pad 41 at Cape Canaveral Air Force Station at 12:25 p.m. EDT. View from the VAB roof. Credit: Ken Kremer/kenkremer.com

The $1.1 Billion Juno was launched on Aug. 5, 2011 from Cape Canaveral, Florida atop the most powerful version of the Atlas V rocket augmented by 5 solid rocket boosters and built by United Launch Alliance (ULA). That same Atlas V 551 version just launched MUOS-5 for the US Navy on June 24.

Stay tuned here for Ken’s continuing Earth and Planetary science and human spaceflight news.
Ken Kremer

Juno spacecraft and its science instruments. Image credit: NASA/JPL
Juno spacecraft and its science instruments. Image credit: NASA/JPL
Juno graphic
Juno orbital graphic

How Do We Terraform Jupiter’s Moons?

Continuing with our “Definitive Guide to Terraforming“, Universe Today is happy to present to our guide to terraforming Jupiter’s Moons. Much like terraforming the inner Solar System, it might be feasible someday. But should we?

Fans of Arthur C. Clarke may recall how in his novel, 2010: Odyssey Two (or the movie adaptation called 2010: The Year We Make Contact), an alien species turned Jupiter into a new star. In so doing, Jupiter’s moon Europa was permanently terraformed, as its icy surface melted, an atmosphere formed, and all the life living in the moon’s oceans began to emerge and thrive on the surface.

As we explained in a previous video (“Could Jupiter Become a Star“) turning Jupiter into a star is not exactly doable (not yet, anyway). However, there are several proposals on how we could go about transforming some of Jupiter’s moons in order to make them habitable by human beings. In short, it is possible that humans could terraform one of more of the Jovians to make it suitable for full-scale human settlement someday.

Continue reading “How Do We Terraform Jupiter’s Moons?”

Saturn’s Moon Rhea

The Cronian system (i.e. Saturn and its system of rings and moons) is breathtaking to behold and intriguing to study. Besides its vast and beautiful ring system, it also has the second-most satellites of any planet in the Solar System. In fact, Saturn has an estimated 150 moons and moonlets – and only 53 of them have been officially named – which makes it second only to Jupiter.

For the most part, these moons are small, icy bodies that are believed to house interior oceans. And in all cases, particularly Rhea, their interesting appearances and compositions make them a prime target for scientific research. In addition to being able to tell us much about the Cronian system and its formation, moons like Rhea can also tell us much about the history of our Solar System.

Discovery and Naming:

Rhea was discovered by Italian astronomer Giovanni Domenico Cassini on December 23rd, 1672. Together with the moons of Iapetus, Tethys and Dione, which he discovered between 1671 and 1672, he named them all Sidera Lodoicea (“the stars of Louis”) in honor of his patron, King Louis XIV of France. However, these names were not widely recognized outside of France.

In 1847, John Herschel (the son of famed astronomer William Herschel, who discovered Uranus, Enceladus and Mimas) suggested the name Rhea – which first appeared in his treatise Results of Astronomical Observations made at the Cape of Good Hope. Like all the other Cronian satellites, Rhea was named after a Titan from Greek mythology, the “mother of the gods” and one the sisters of Cronos (Saturn, in Roman mythology).

The moons of Saturn, from left to right: Mimas, Enceladus, Tethys, Dione, Rhea; Titan in the background; Iapetus (top) and irregularly shaped Hyperion (bottom). Some small moons are also shown. All to scale. Credit: NASA/JPL/Space Science Institute
The moons of Saturn, from left to right: Mimas, Enceladus, Tethys, Dione, Rhea; Titan (background), Iapetus (top), and Hyperion (bottom). Credit: NASA/JPL/Space Science Institute

Size, Mass and Orbit:

With a mean radius of 763.8±1.0 km and a mass of 2.3065 ×1021 kg, Rhea is equivalent in size to 0.1199 Earths (and 0.44 Moons), and about 0.00039 times as massive (or 0.03139 Moons). It orbits Saturn at an average distance (semi-major axis) of 527,108 km, which places it outside the orbits of  Dione and Tethys, and has a nearly circular orbit with a very minor eccentricity (0.001).

With an orbital velocity of about 30,541 km/h, Rhea takes approximately 4.518 days to complete a single orbit of its parent planet. Like many of Saturn’s moons, its rotational period is synchronous with its orbit, meaning that the same face is always pointed towards it.

Composition and Surface Features:

With a mean density of about 1.236 g/cm³, Rhea is estimated to be composed of 75% water ice (with a density of roughly 0.93 g/cm³) and 25% of silicate rock (with a density of around 3.25 g/cm³). This low density means that although Rhea is the ninth-largest moon in the Solar System, it is also the tenth-most massive.

In terms of its interior, Rhea was originally suspected of being differentiated between a rocky core and an icy mantle. However, more recent measurements would seem to indicate that Rhea is either only partly differentiated, or has a homogeneous interior – likely consisting of both silicate rock and ice together (similar to Jupiter’s moon Callisto).

Views of Saturn's moon Rhea. Credit: NASA/JPL/Space Science Institute
Views of Saturn’s moon Rhea, with false-color image showing elevation data at the right. Credit: NASA/JPL/Space Science Institute

Models of Rhea’s interior also suggest that it may have an internal liquid-water ocean, similar to Enceladus and Titan. This liquid-water ocean, should it exist, would likely be located at the core-mantle boundary, and would be sustained by the heating caused by from decay of radioactive elements in its core.

Rhea’s surface features resemble those of Dione, with dissimilar appearances existing between their leading and trailing hemispheres – which suggests that the two moons have similar compositions and histories. Images taken of the surface have led astronomers to divide it into two regions – the heavily cratered and bright terrain, where craters are larger than 40 km (25 miles) in diameter; and the polar and equatorial regions where craters are noticeably smaller.

Another difference between Rhea’s leading and trailing hemisphere is their coloration. The leading hemisphere is heavily cratered and uniformly bright while the trailing hemisphere has networks of bright swaths on a dark background and few visible craters. It had been thought that these bright areas (aka. wispy terrain) might be material ejected from ice volcanoes early in Rhea’s history when its interior was still liquid.

However, observations of Dione, which has an even darker trailing hemisphere and similar but more prominent bright streaks, has cast this into doubt. It is now believed that the wispy terrain are tectonically-formed ice cliffs (chasmata) which resulted from extensive fracturing of the moon’s surface. Rhea also has a very faint “line” of material at its equator which was thought to be deposited by material deorbiting from its rings (see below).

Hemispheric color differences on Saturn's moon Rhea are apparent in this false-color view from NASA's Cassini spacecraft. This image shows the side of the moon that always faces the planet. Image Credit: NASA/JPL/SSI
Hemispheric color differences on Saturn’s moon Rhea are apparent in this false-color view of the anti-Cronian side, from NASA’s Cassini spacecraft. Image Credit: NASA/JPL/SSI

Rhea has two particularly large impact basins, both of which are situated on Rhea’s anti-Cronian side (aka. the side facing away from Saturn). These are known as Tirawa and Mamaldi basins, which measure roughly 360 and 500 km (223.69 and 310.68 mi) across. The more northerly and less degraded basin of Tirawa overlaps Mamaldi – which lies to its southwest – and is roughly comparable to the Odysseus crater on Tethys (which gives it its “Death-Star” appearance).

Atmosphere:

Rhea has a tenuous atmosphere (exosphere) which consists of oxygen and carbon dioxide, which exists in a 5:2 ratio. The surface density of the exosphere is from 105 to 106 molecules per cubic centimeter, depending on local temperature. Surface temperatures on Rhea average 99 K (-174 °C/-281.2 °F) in direct sunlight, and between 73 K (-200 °C/-328 °F) and 53 K (-220 °C/-364 °F) when sunlight is absent.

The oxygen in the atmosphere is created by the interaction of surface water ice and ions supplied from Saturn’s magnetosphere (aka. radiolysis). These ions cause the water ice to break down into oxygen gas (O²) and elemental hydrogen (H), the former of which is retained while the latter escapes into space. The source of the carbon dioxide is less clear, and could be either the result of organics in the surface ice being oxidized, or from outgassing from the moon’s interior.

Saturn's second-largest moon Rhea, in front of the rings and a blurred Epimetheus (or Janus) whizzing behind. Acquired March 29, 2012.
Saturn’s second-largest moon Rhea, pictured by the Cassini probe on March 29, 2012. Credit: NASA/JPL

Rhea may also have a tenuous ring system, which was inferred based on observed changes in the flow of electrons trapped by Saturn’s magnetic field. The existence of a ring system was temporarily bolstered by the discovered presence of a set of small ultraviolet-bright spots distributed along Rhea’s equator (which were interpreted as the impact points of deorbiting ring material).

However, more recent observations made by the Cassini probe have cast doubt on this. After taking images of the planet from multiple angles, no evidence of ring material was found, suggesting that there must be another cause for the observed electron flow and UV bright spots on Rhea’s equator. If such a ring system were to exist, it would be the first instance where a ring system was found orbiting a moon.

Exploration:

The first images of Rhea were obtained by the Voyager 1 and 2 spacecraft while they studied the Cronian system, in 1980 and 1981, respectively. No subsequent missions were made until the arrival of the Cassini orbiter in 2005. After it’s arrival in the Cronian system, the orbiter made five close targeted fly-bys and took many images of Saturn from long to moderate distances. 

The Cronian system is definitely a fascinating place, and we’ve really only begun to scratch its surface in recent years. In time, more orbiters and perhaps landers will be traveling to the system, seeking to learn more about Saturn’s moons and what exists beneath their icy surfaces. One can only hope that any such mission includes a closer look at Rhea, and the other “Death Star Moon”, Dione.

We have many great articles on Rhea and Saturn’s system of moons here at Universe Today. Here is one about its possible ring system, its tectonic activity, it’s impact basins, and images provided by Cassini’s flyby.

Astronomy Cast also has an interesting interview with Dr. Kevin Grazier, who worked on the Cassini mission.

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

Io, Jupiter’s Volcanic Moon

This global view of Jupiter's moon, Io, was obtained during the tenth orbit of Jupiter by NASA's Galileo spacecraft. Credit: NASA

Exploring the Solar System is like peeling an onion. With every layer removed, one finds fresh mysteries to ponder over, each one more confounding than the last. And this is certainly the case when it comes to Jupiter’s system of moons, particularly its four largest – Io, Europa, Ganymede and Callisto. Known as the Galilean Moons, in honor of their founder, these moons possess enough natural wonders to keep scientists busy for centuries.

As Jupiter’s innermost moon, it is also the fourth-largest moon in the Solar System, has the highest density of any known moon, and is the driest known object in the Solar System. It is also one of only four known bodies that experiences active volcanism and – with over 400 active volcanoes – it is the most geologically active body in the Solar System.

Continue reading “Io, Jupiter’s Volcanic Moon”

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.

Jupiter’s Moon Ganymede

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”

Jupiter’s Moon Europa

Europa

Jupiter‘s four largest moons – aka. the Galilean Moons, consisting of Io, Europa, Ganymede and Callisto – are nothing if not fascinating. Ever since their discovery over four centuries ago, these moons have been a source of many great discoveries. These include the possibility of internal oceans, the presence of atmospheres, volcanic activity, one has a magnetosphere (Ganymede), and possibly having more water than even Earth.

But arguably, the most fascinating of the Galilean Moons is Europa: the sixth closest moon to Jupiter, the smallest of the four, and the sixth largest moon in the Solar System. In addition to having an icy surface and a possible warm-water interior, this moon is considered to be one of the most-likely candidates for possessing life outside of Earth.

Continue reading “Jupiter’s Moon Europa”

What are the Galilean Moons?

It’s no accident that Jupiter shares its name with the king of the gods. In addition to being the largest planet in our Solar System – with two and a half times the mass of all the other planets combined – it is also home to some of the largest moons of any Solar planet. Jupiter’s largest moons are known as the Galileans, all of which were discovered by Galileo Galilei and named in his honor.

They include Io, Europa, Ganymede, and Callisto, and are the Solar System’s fourth, sixth, first and third largest satellites, respectively. Together, they contain almost 99.999% of the total mass in orbit around Jupiter, and range from being 400,000 and 2,000,000 km from the planet. Outside of the Sun and eight planets, they are also among the most massive objects in the Solar System, with radii larger than any of the dwarf planets.

Continue reading “What are the Galilean Moons?”