The Solar System

CHAPTER 22 | COMPARATIVE PLANETOLOGY OF VENUS AND MARS 489

The Moons of Mars

If you could camp overnight on Mars, you might notice its
two small moons, Phobos and Deimos. Phobos, shaped like a
fl attened loaf of bread measuring 20 km  23 km  28 km in
size, would appear less than half as large in angular diameter as
Earth’s full moon. Deimos, only 12 km in diameter and three
times farther from Mars, would look only __ 151 the diameter of
Earth’s moon.
Both moons are tidally locked to Mars, keeping the same side
facing the planet as they orbit. Also, both moons revolve around
Mars in the same direction that Mars rotates, but Phobos follows
such a small orbit that it revolves faster than Mars rotates. If you
camped overnight on Mars, you would see Phobos rise in the west,
drift eastward across the sky, and set in the east 6 hours later.

Origin and Evolution

Deimos and Phobos are typical of the small, rocky moons in our
solar system (■ Figure 22-23). Th eir albedos are only about 0.06,
making them look as dark as coal. Th ey have low densities, about
2 g/cm^3.
Many of the properties of these moons suggest that they are
captured asteroids. In the outer parts of the asteroid belt, almost

22-3 all asteroids are dark, low-density objects like Phobos and

Deimos. Massive Jupiter, orbiting just outside the asteroid belt,

can scatter such bodies throughout the solar system, so you

should not be surprised if a number of them have encountered

Mars, the closest Terrestrial planet to the asteroid belt.

However, capturing a passing asteroid into a closed orbit is

not so easy that it can be expected to happen often. An asteroid

approaches a planet along a hyperbolic (open) orbit and, if it is

unimpeded, swings around the planet and disappears back into

space. To change the hyperbolic orbit into a closed orbit, the

asteroid must somehow be slowed down as it passes. Tidal forces

might do this, but in the case of Mars they would be rather weak.

Interactions with other moons or grazing collisions with a thick

atmosphere might also slow the asteroid enough so it could be

captured.

Both Phobos and Deimos have been photographed by

nearby spacecraft, and those photos show that the satellites are

heavily cratered. Such cratering could have occurred either

while the moons were still in the asteroid belt or while they were

in orbit around Mars. In any case, the heavy battering has bro-

ken the satellites into irregular chunks of rock, and they cannot

pull themselves into smooth spheres because their gravity is too

weak to overcome the structural strength of the rock. You will

Visual-wavelength images

Deimos looks smoother

because it has more dust

and debris on its surface.

Phobos is marked by

the large crater Stickney,

which is 10 km in diameter.

■ Figure 22-23

The moons of Mars are too small to pull themselves into spherical shape.

Deimos is about half the size of Phobos. The two moons were named for the

mythical attendants of Mars, the god of war. Phobos was the god of fear,

and Deimos was the god of dread. (Phobos: Damon Simonelli and Joseph Ververka,

Cornell University/NASA; Deimos: NASA)