374 Encyclopedia of the Solar System
moral satireGullliver’s Travels(published in 1726), a fanci-
ful but coincidentally accurate prediction of the existence
and orbital characteristics of two small Martian satellites
was made. The two bodies are barely visible in the scat-
tered light from Mars in Earth-based telescopes. Most of
what is known about Phobos and Deimos was obtained from
theMariner 9and theViking 1and 2 missions to Mars (see
Table 2). Their physical and orbital properties are listed in
Table 1. Both satellites are shaped approximately like el-
lipsoids, and they are in synchronous rotation. Phobos, and
possibly Deimos, has a regolith of dark material similar to
that found on C-type (carbonaceous) asteroids common in
the outer asteroid belt. Thus the satellites may have been as-
teroids or asteroidal fragments, which were perturbed into
a Mars-crossing orbit and captured. The orbital period of
Phobos around Mars is only 7.7 hours: An observer on Mars
would see the moon rise and set twice in a single day.
Both satellites are heavily cratered, which indicates that
their surfaces are at least 3 billion years old (Fig. 2). But
only Deimos appears to be covered with a fine dust, which
gives its surface a smoother appearance. This dust may exist
because the surface is more easily pulverized by impacts, or
it may be the result of a large impact the shaped the moon’s
southern hemisphere. The surface of Phobos is extensively
scored by linear grooves that are deepest near the huge
impact crater Stickney (named after the Asaph Hall’s wife,
Angeline Stickney Hall, who collaborated with him in many
of his astronomical observations) but that define planes cut-
ting through the satellite and parallel to its intermediate axis
that points along its direction of orbital motion. The grooves
are probably fractures, enhanced by the collision that pro-
duced Stickney. There is some evidence that tidal action
is bringing Phobos, which is already inside Roche’s limit,
closer to Mars. The satellite will either disintegrate (per-
haps to form a ring) or crash into Mars in about 100 million
years.
4.2 The Small Satellites of Jupiter
Ten years ago, Jupiter had 12 known small satellites, in-
cluding three discovered by theVoyagermission. As of July
2006, the planet had 63 known satellites (59 of them small);
most of the new small satellites were discovered by sensitive
CCD cameras on large telescopes. The small satellites are
irregular in shape, and many of the outer satellites may be
captured objects.
Within the orbit of Io are at least four satellites: Adrastea,
Metis, Amalthea, and Thebe (see Fig. 3). Adrastea and
Metis, both discovered byVoyager, are the closest known
satellites to Jupiter and move in nearly identical orbits just
outside the outer edge of the thin Jovian ring, for which
they may be a source of particles. Between Amalthea and
Io lies the orbit of Thebe, also discovered byVoyager.Little
is known about the composition of these satellites, but they
are most likely primarily rock–ice mixtures. The three inner
FIGURE 2 The two moons of Mars: (a) Phobos and (b) Deimos.
Both pictures were obtained by theVikingspacecraft.satellites sweep out particles in the jovian magnetosphere
to form voids at their orbital positions.
Amalthea is a dark, reddish, heavily cratered object re-
flecting less than 5% of the visible radiation it receives;
the red color is probably due to contamination by sulfur
particles from Io. Little else is known about its composi-
tion except that the dark material may be carbonaceous.
In addition to two large craters, Pan (100 km wide) and
Gaea (80 km wide), Amalthea has mountains that are about
20 km high. In 2002, theGalileospacecraft swooped to
within 150 km of the moon’s surface to find that its density
is anomalously low, about that of water ice (∼1 gm/cm^3 ).