Overview
Mars has two small moons, Deimos, and Phobos.
Their orbits are illustrated in the
adjacent right figure, with the orbital radii and the radius of Mars drawn to scale.
Minor Satellites
Deimos and Phobos are examples of
minor satellites: small chunks of rock in orbit around planets.
Such minor satellites are typically very small and very irregular in shape
as compared with large satellites like the Earth's Moon. Phobos and Deimos have irregular shapes,
small sizes (less than 30 km in the longest dimensions), very low
albedos (around 0.06), and low densities (2 g/cm3).
These properties are
similar to those of many darker asteroids and the minor satellites like
Deimos and Phobos
are probably
captured asteroids (but see the right panel).
Appearance As Viewed from Mars
From the
surface of Mars, Phobos would appear to be about half the size of Earth's Moon as viewed from Earth,
while Deimos (smaller and further away) would appear to be about 12 times smaller than Earth's Moon as viewed
in our sky. Because of its high orbital velocity, Phobos has the unusual
behavior of rising in the west and
setting in the east as viewed from the surface of Mars.
This Phobos-Deimos orbit
animation illustrates why Phobos has this apparent
retrograde orbital motion,
whereas Deimos has the more normal
behavior of rising in the east and setting in the west.
Here is an
animation
that also illustrates the motion of the Martian system.