Mars is the fourth planet from the Sun and the second smallest
planet in the Solar System. Named after the Roman god of war, it
is often described as the "Red Planet", as the iron oxide
prevalent on its surface gives it a reddish appearance. Mars is
a terrestrial planet with a thin atmosphere, having surface
features reminiscent both of the impact craters of the Moon and
the volcanoes, valleys, deserts, and polar ice caps of Earth.
The rotational period and seasonal cycles of Mars are likewise
similar to those of Earth, as is the tilt that produces the
seasons. Mars is the site of Olympus Mons, the second highest
known mountain within the Solar System (the tallest on a
planet), and of Valles Marineris, one of the largest canyons.
The smooth Borealis basin in the northern hemisphere covers 40%
of the planet and may be a giant impact feature. Mars has two
known moons, Phobos and Deimos, which are small and irregularly
shaped. These may be captured asteroids, similar to 5261 Eureka,
a Martian trojan asteroid. A