NASA Cassini image of Mimas: A Subtle Colour Difference in Herschel's Crater

A Subtle Difference
Image Credit: NASA/JPL/Space Science Institute

Subtle colour differences on Saturn's moon Mimas are apparent in this false-colour view of Herschel Crater captured by NASA's Cassini spacecraft during its closest-ever flyby of that moon.

The image shows terrain-dependent color variations, particularly the contrast between the bluish materials in and around Herschel Crater and the greenish cast on older, more heavily cratered terrain elsewhere.

The origin of the color differences is not yet understood, but may be caused by differences in the surface composition between the two terrains. False color images from Cassini's previous closest encounter, in 2005, also showed such variations.

The natural color of Mimas visible to the human eye may be a uniform gray or yellow color, but this mosaic has been contrast-enhanced and shows differences at other wavelengths of light.

During this flyby on Feb. 13, 2010, Cassini came within about 5,900 miles of Mimas and these images were obtained with Cassini's narrow-angle camera on that day at a distance of approximately 10,000 miles from Mimas.

The images were re-projected into an orthographic map projection. A black and white image, taken in visible light with the wide-angle camera, is used to fill in parts of the mosaic.

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."