Mars

Mars is the fourth planet from the Sun and the most thoroughly explored extraterrestrial world in human history. The "Red Planet" gets its distinctive rust color from iron oxide minerals coating the surface — Mars is literally rusting. The planet is approximately half the size of Earth in diameter and one-tenth the mass.

Mars's surface preserves evidence of a wetter past. Ancient river valleys, lake beds, and what appear to be ancient ocean shorelines suggest that Mars had significant surface water in its first billion years. Today the planet is cold and dry, with thin atmosphere (1% of Earth's atmospheric pressure) of mostly carbon dioxide. Surface temperatures range from -195°F at the winter pole to about 70°F at the equator on summer afternoons. Liquid water cannot exist stably on the surface today — water either freezes into permafrost or sublimates directly to vapor — but subsurface water ice is abundant.

The planet has the largest known mountain in the solar system: Olympus Mons, a shield volcano 14 miles tall and 370 miles across — three times the height of Mount Everest. The Valles Marineris canyon system extends 2,500 miles across the equator with depths exceeding 4 miles, dwarfing the Grand Canyon by factors of 10 in length and depth.

Mars has been the focus of more spacecraft exploration than any other planet besides Earth. NASA's Viking, Pathfinder, Opportunity, Spirit, Curiosity, and Perseverance rovers have provided ground-truth observations across multiple landing sites. The Perseverance rover (active since 2021) is collecting carefully chosen rock and soil samples that future missions plan to return to Earth for laboratory analysis — a multi-decade effort that may finally answer whether Mars hosted past life.

For amateur observers, Mars is most spectacular at opposition (when Earth passes between Mars and the Sun, every 2 years and 50 days). At opposition, Mars rises at sunset, shines brilliantly orange-red, and shows visible surface detail in modest telescopes. The next favorable Mars opposition is in February 2027.

The two small moons Phobos and Deimos are likely captured asteroids; both are too small to be spherical and orbit very close to Mars. Phobos will eventually crash into Mars or break up into a ring system within 50 million years.