Mars

Mars is the next planet outward from Earth and becomes especially rewarding to observe near opposition, when Earth passes between Mars and the Sun. At those times its disk is larger and surface markings are easier to study. The planet has long carried a strong cultural fascination because its reddish color and changing features encouraged speculation about life beyond Earth.

Although Mars is far smaller than Earth, it is larger than the Moon and has enough gravity to keep a thin atmosphere. That atmosphere is far too tenuous for people to breathe, and the planet's lower escape velocity makes it easier for gases to leak away into space over geological time. Stable liquid water cannot persist on the surface today, despite the historic idea of Martian seas and channels.

Spacecraft images and measurements show valleys, canyons, and other landforms that strongly point to flowing liquid in Mars's past. Planetary science therefore treats early Mars as a world that may once have had abundant water and perhaps conditions suitable for life, especially during periods when volcanism and internal activity released large amounts of gas into the atmosphere.

Mars is still an active environment shaped by ice, wind, weather, and dust. Its year lasts a little less than 700 Earth days, and its axial tilt is close enough to Earth's to produce clear seasons. The polar caps grow in winter and shrink in summer; they contain frozen carbon dioxide as well as water ice. Large dust storms can hide surface markings for weeks, while orbiters continue to map the planet and monitor Martian weather.

Surface pressure is roughly one hundredth of Earth's sea-level pressure, and the atmosphere is dominated by carbon dioxide, making present-day Mars a hostile place for familiar forms of life. Mars has two small moons, Phobos and Deimos, measuring about 27 km and 15 km across; both are too faint and small for typical amateur telescopes.