MARTIAN SATELLITES
Only three satellites are known for the terrestrial planets. One is the moon, which we discussed in Chapter 7. The other two are the little satellites of Mars, which were discovered in 1877. Phobos, the inner one, and Deimos, the outer one, are potato-shaped bodies with cratered surfaces. Phobos orbits eastward, just as our moon does, and in the same direction that Mars rotates, in a period of 7.5 hours at a distance of about 6000 kilometers from the surface of Mars. This gives it an angular size, as seen from the surface of Mars, of about half that of ou r moon. Since it revolves about Mars much faster than the planet rotates, it rises on the western horizon and sets on the eastern horizon 5.5 hours later. This is counter to any other natural satellite in the solar system, as observed from its primary.
Phobos is about 27 kilometers long, 21 kilometers high, and 19 kilometers wide. Both Phobos and Deimos have been shaped by high-velocity impacts, which appear to have sheared off large sections of each satellite. In addition, both have many craters but no ejecta or craters with central peaks, a reasonable feature since their gravitational attraction is very small. Material ejected during cratering impact simply escapes and does not fall back to the satellites' surface.
Phobos seems more heavily cratered than does Deimos, the largest crater, Stickney, being about 10 kilometers across. It also has mysterious, long, parallel grooves across a large part of its surface. They are a few hundred meters wide and a few tens of meters deep and may have been formed by the same impact that caused the crater Stickney.
Deimos is about half the size of Phobos-about 15 kilometers by 12 kilometers. It orbits Mars some 20,000 kilometers from the planet's surface in a period of 30.3 hours. Its angular size, as seen from Mars, is quite small, roughly equivalent to a quarter viewed at a distance of about 40 meters. Its orbital period is somewhat longer than the rotational period of Mars; so it rises on the eastern horizon and sets on the western horizon nearly 3 days later, while going through its phases twice.
The darkness of the surface of both satellites is probably due to carbon-rich and water-rich minerals, such as are found in black, crumbly meteorites known as carbonaceous chondrites. A number of the asteroids appear to have similar surfaces. This has led to the speculation that Mars's satellites may be captured asteroids, acquired early in the planet's life.
