Phobos

Credit: Malin Space Science Systems/NASA

Mars, of course, has two moons, Phobos (“Fear”) and Deimos (“Panic”). The photo above is of Phobos, the innermost and larger of the two moons, taken by the Mars Global Surveyor on August 19, 1998. Phobos and Deimos were discovered in 1877 by the American astronomer Asalph Hall. He had been on the verge of giving up his search to discover if Mars had any moons, but his wife, Chloe Angeline Stickney Hall, persuaded him to continue. As a result, when he did discover Phobos and Deimos, the largest crater on Phobos (visible above) was named after her, using her maiden name, Stickney.

The crater Stickney is about ten kilometers (six miles) in diameter. Individual boulders are visible on the near rim of the crater, and are presumed to be ejecta blocks from the impact that formed Stickney. Some of these boulders are enormous -- over 50 meters (160 feet) across. Also crossing at and near the rim of Stickney are shallow grooves that was originally thought to have been formed when the crater was made; however, further research from data by the Mars Express spacecraft have revealed that the grooves are independent of Stickney and are deposits of material thrown into space by impacts on the Martian surface.

The far wall of the crater shows lighter and darker streaks going down the slopes. The presence of material of different brightness on the far crater slopes, and in some of the grooves, shows that the satellite is made from different types of materials.