NASA Galileo Image: Jupiter’s cratered moon, Callisto

The speckled object depicted here is Callisto, Jupiter’s second largest moon. 

This image was taken in May 2001 by NASA’s Galileo spacecraft, which studied Jupiter and its moons from 1995 until 2003.

Similar in appearance to a golf ball, Callisto is covered almost uniformly with pockmarks and craters across its surface, evidence of relentless collisions.

In fact, Callisto is the most heavily cratered object in the Solar System.

The moon is made up of equal parts of rock and ice, the brighter parts of Callisto’s surface are thought to be mainly water ice, whereas the darker patches are regions of highly eroded and ice-poor rocky material.

Callisto is roughly the same size as the planet Mercury, but only about a third of the mass. It is the outermost of Jupiter’s four large Galilean satellites, a group consisting of Io, Europa, Ganymede and Callisto.

It orbits relatively far away from Jupiter compared to these other satellites: it lies 1 880 000 km from the planet, roughly 26 times the radius of the planet itself.

While this in itself is not unusual, our Moon orbits at some 60 times Earth’s radius, the important thing is Callisto’s isolation from its neighbouring moons.

Callisto’s closest neighbour is Ganymede, which orbits 800 000 km closer to Jupiter.

This isolation means that Callisto does not experience any significant tidal forces from Jupiter that would tear at its structure.

It also does not show any signs of geological processes such as volcanism or plate tectonics, which we clearly see on moons that are involved in violent cosmic tugs-of-war with Jupiter, such as Io, Europa and Ganymede.

Callisto remains relatively intact and is a witness of the early Solar System: its surface is the oldest terrain, at a truly ancient four billion years.

This image is the only complete full-colour view of Callisto obtained by Galileo.

The spacecraft provided us with a great deal of information about the jovian system: as well as sending the first probe into the atmosphere of Jupiter, and measuring Jupiter’s composition and dynamics, it observed Io’s volcanism, sent back data supporting the idea of a liquid ocean on Europa, and probed the properties of Ganymede and the subject of this image,

Callisto. It also managed to observe the famous Comet Shoemaker–Levy 9 colliding with Jupiter in 1994.

The jovian system will be visited again in the not-too-distant future. In 2016, NASA’s Juno spacecraft will arrive at Jupiter and start to beam back images of the planet’s poles.

Later, ESA’s Juice, short for JUpiter ICy moons Explorer, planned for launch in 2022, will tour the system with the aim of making a breakthrough in our knowledge of the giant gaseous planet and its environs, especially the intriguing moons Ganymede, Europa and Callisto.

Radio Signals from Jupiter Aids Search for Life and Liquid Water

This artist's impression shows Jupiter and its moon Europa using captured Jupiter and Europa images in visible light. 

The Hubble ultraviolet images showing the faint emission from the water vapour plumes have been superimposed, respecting the size but not the brightness of the plumes. 

Image courtesy NASA, ESA, and M. Kornmesser, University of California, Santa Cruz.

Powerful radio signals that Jupiter generates could be used to help researchers scan its giant moons for oceans that could be home to extraterrestrial life, according to a recent study submitted to the journal Icarus (In PDF format).

Jupiter, the largest planet in the Solar System, possesses 67 known moons, including three giant icy moons that might possess liquid oceans underneath their frozen surfaces.

Astrobiologists want to investigate Europa, Ganymede and Callisto for extraterrestrial life, as there is life virtually wherever there is liquid water on Earth.

Of Jupiter's three largest icy moons, Europa, which is roughly the size of Earth's moon, is favored as having the greatest potential to sustain life.

Magnetic readings captured by NASA's Galileo spacecraft provided compelling hints that it has an ocean, and radio scans by the probe suggest a water-rich layer beneath the surface between 50 to 105 miles (80 to 170 kilometers) thick.

Recent findings even suggest its ocean could be loaded with enough oxygen to support millions of tons worth of marine life.

Scientists would like to analyze Europa's ocean directly, perhaps with missions to bore into Europa's icy shell using heat to melt through the ice, whirling blades to clear away rocks, and robot subs to explore the ocean.

However, it remains uncertain how thick this shell is, complicating any plans to penetrate it.

Models of its thickness, based on the amount of heat the shell receives from the Sun and Europa itself, predict it to be roughly 18 miles (30 kilometers) thick.

In contrast, analyses of the Galileo spacecraft's data suggest the shell is no more than 9 miles (15 kilometers) thick, and maybe as little as 2.5 miles (4 kilometers) thick.

True colour and feature-highlighted photos of Europa. 

The bright feature towards the lower right of the disk is the 45 km diameter crater Pwyll. 

Credit: NASA.

Ice-penetrating radar is currently the most promising technique to directly confirm the existence of any ocean hidden within Jupiter's icy moons.

Radar works by transmitting radio signals, detecting any radio signals that reflect back, and analyzing these signals to deduce details about what they reflected off of, much like how a person might use a flashlight to illuminate objects hidden in the dark.

Ice and ground-penetrating radar systems look for signals that indicate buried objects and boundaries between layers.

In Europa's case, this means looking for the boundaries between the icy crust and any hidden ocean, and between such an ocean and Europa's rocky core.

To detect these oceans with ice-penetrating radar, low-frequency signals of less than 30 megahertz are needed to overcome radio wave absorption by the ice, as well as the unpredictable scattering of radio waves by the crinkled surfaces of these moons.

The low-frequency radio waves that researchers would like to use are decametric, meaning they have wavelengths tens of meters long.



Jupiter's Decametric waves
One problem with attempting ice-penetrating decametric radar on Jupiter's moons has to do with the powerful decametric radio bursts coming from Jupiter itself.

Altogether, these signals are more than 3,000 times stronger than any leaking into the Solar System from the rest of the galaxy.

Jupiter's decametric waves come from clouds of electrically charged particles trapped in Jupiter's magnetic field.

To overcome Jupiter's loud radio signals, a mission probing Jupiter's moons would need a relatively strong transmitter, a massive device that might be difficult to power and fit aboard the limited confines of a spacecraft.

Read the full article about how the research team plan to overcome the difficulties of Jupiter's natural emanation and generation of decametric waves.

More Information: A Passive Probe for Subsurface Oceans and Liquid Water in Jupiter's Icy Moons - Authors: Andrew Romero-Wolf, Steve Vance, Frank Maiwald, Essam Heggy, Paul Ries, Kurt Liewer

ESA JUICE Mission to Jupiter's Icy Moons

An artist's illustration of the JUpiter ICy moons Explorer spacecraft in the Jovian system. The mission will launch in 2022 and arrive at Jupiter in 2030 to study the planet and its largest moons.

CREDIT: ESA/AOES

An ambitious European mission that will launch a robotic probe to explore Jupiter's icy moons in 2022 has got its science gear.

The European Space Agency has picked 11 instruments for the planned JUpiter ICy moons Explorer, or JUICE, spacecraft.

The mission is expected to reach Jupiter, the largest planet in the solar system, in 2030 and spend at least three years studying the gas giant's major moons Callisto, Europa, and Ganymede.

The Jovian satellites are intriguing to scientists because they are thought to have vast oceans beneath their icy outer crust.

"Jupiter and its icy moons constitute a kind of mini-Solar System in their own right, offering European scientists and our international partners the chance to learn more about the formation of potentially habitable worlds around other stars," said Dmitrij Titov, JUICE study scientist for ESA, in a Feb. 21 statement.

The JUICE mission will observe Jupiter's atmosphere and magnetosphere, as well all four Galilean moons: Europa, Callisto, Ganymede and the volcanic Io.

The spacecraft is expected to make 12 flybys of crater-covered Callisto, as well as two close passes of Europa in an attempt to gather the first-ever measurements of the thickness of that moon's frozen crust, ESA officials said.



The spacecraft will eventually end up orbiting Ganymede, the largest moon in our solar system, to study its surface and internal structure. Ganymede is also the only known moon in the solar system with its own magnetic field, and JUICE will closely observe the moon's interactions with Jupiter's magnetosphere, ESA officials said.

The collection of approved instruments to help scientists complete these tasks includes cameras, spectrometers, a laser altimeter and an ice-penetrating radar, as well as a magnetometer, plasma and particle monitors, and radio science hardware, ESA officials said. Teams from 15 European countries and the United States and Japan will develop the tools.

"The suite of instruments addresses all of the mission's science goals, from in-situ measurements of Jupiter's vast magnetic field and plasma environment, to remote observations of the surfaces and interiors of the three icy moons," Luigi Colangeli, coordinator of ESA's solar system missions, said in a statement.

Jupiter: Difference Between Ganymede And Callisto

The Difference Between Ganymede And Callisto

Jupiter (right) and the Galilean satellites (right to left) Io, Europa, Ganymede, and Callisto. Cutaways show the interior states of Ganymede and Callisto after many impacts by icy planetesimals during the late heavy bombardment. Colors represent density, with black showing the rocky core (with a density 3 g/cm^3), blue showing mixed ice and rock (densities 1.8 to 1.9 g/cm^3) and white showing rock-free ice.

Differences in the number and speed of cometary impacts onto Jupiter's large moons Ganymede and Callisto some 3.8 billion years ago can explain their vastly different surfaces and interior states, according to research by scientists at the Southwest Research Institute appearing online in Nature Geoscience Jan. 24, 2010.

Ganymede and Callisto are similar in size and are made of a similar mixture of ice and rock, but data from the Galileo and Voyager spacecraft show that they look different at the surface and on the inside. A conclusive explanation for the differences between Ganymede and Callisto has eluded scientists since the Voyager Jupiter encounters 30 years ago.

Dr. Amy C. Barr and Dr. Robin M. Canup of the SwRI Planetary Science Directorate created a model of melting by cometary impacts and rock core formation to show that Ganymede and Callisto's evolutionary paths diverged about 3.8 billion years ago during the Late Heavy Bombardment, the phase in lunar history dominated by large impact events.

"Impacts during this period melted Ganymede so thoroughly and deeply that the heat could not be quickly removed. All of Ganymede's rock sank to its center the same way that all the chocolate chips sink to the bottom of a melted carton of ice cream," says Barr. "Callisto received fewer impacts at lower velocities and avoided complete melting."