High Resolution Imaging Science Experiment (HiRISE) camera on NASA's
Mars Reconnaissance Orbiter took two images of the larger of Mars' two
moons, Phobos, within 10 minutes of each other on 23rd March,
2008. This is the second, taken from a distance of about 5,800
kilometers (about 3,600 miles). The colour was produced by
combining data from the camera's blue-green, red, and near-infrared
The illuminated part of Phobos seen in the images is about 21 kilometers
(13 miles) across. The most prominent feature in the images is the
large crater Stickney in the lower right. With a diameter of 9
kilometers (5.6 miles), it is the largest feature on Phobos.
The colour data accentuate details not apparent in black-and-white
images. For example, materials near the rim of Stickney appear
bluer than the rest of Phobos. Based on analogy with materials on
our own moon, this could mean this surface is fresher, and therefore
younger, than other parts of Phobos.
A series of troughs and crater chains is obvious on other parts of the
moon. Although many appear radial to Stickney in this image,
recent studies from the European Space Agency's Mars Express orbiter
indicate that they are not related to Stickney. Instead, they may
have formed when material ejected from impacts on Mars later collided
with Phobos. The lineated textures on the walls of Stickney and
other large craters are landslides formed from materials falling into
the crater interiors in the weak Phobos gravity (less than one
one-thousandth of the gravity on Earth).