Figure 25.17: The Martian canyon Valles Marineris is the largest canyon in the solar system.
( 19 )
Although Mars has mountains, canyons, and other features similar to Earth, it doesn’t have
as much geological activity as Earth. There is no evidence of plate tectonics on Mars. There
are also more craters on Mars than on Earth, though fewer than on the Moon.
Is There Water on Mars?
Water cannot stay in liquid form on Mars because the pressure of the atmosphere is too
low. However, there is a lot of water in the form of ice.Figure25.15shows a prominent ice
cap at the south pole of Mars. Scientists also believe there is also a lot of ice water present
just under the Martian surface. This ice can melt when volcanoes erupt, and water can flow
across the surface temporarily.
Scientists have reason to think that water once flowed over the surface of Mars because they
can see surface features that look like water-eroded canyons, and the Mars rover collected
round clumps of crystals that, on Earth, usually form in water. The presence of water on
Mars, even though it is now frozen as ice, suggests that it might have been possible for life
to exist on Mars in the past.
Two Martian Moons
Mars has two very small moons, Phobos and Deimos. As you can see inFigure25.18, these
moons are not spherical in shape, but instead just look like irregular rocks. Phobos and
Deimos were discovered in 1877. They are named after characters in Greek mythology—the
two sons of Ares, who followed their father into war. Ares is equivalent to the Roman god
Mars.
Lesson Summary
- The four inner planets, or terrestrial planets, have solid, rocky surfaces.