Mars
Mars is the fourth planet from the Sun and one of Earth’s closest planetary neighbors. Its rusty red color comes from iron oxide covering its dry, rocky surface. Billions of years ago, Mars was likely much wetter and warmer, with a thicker atmosphere — today it’s the only planet we know of inhabited entirely by robots.
| Distance from Sun | 142 million miles (228 million km) — 1.5 AU |
| Diameter | 4,212 miles (6,780 km) |
| Length of day | 24.6 hours |
| Length of year | 687 Earth days (669.6 Martian sols) |
| Moons | 2 (Phobos and Deimos) |
| Temperature | 70°F (20°C) max to -225°F (-153°C) min |
| Atmosphere | Mostly carbon dioxide, with nitrogen and argon |
Interesting facts
- Mars is currently home to multiple active NASA spacecraft and rovers, making it the only planet known to be inhabited entirely by robots.
- Evidence from rovers and orbiters shows Mars once had rivers, lakes, and possibly oceans of liquid water.
- Mars’s moon Phobos is slowly spiraling inward and is expected to crash into the planet or break apart in about 50 million years.
- Mars has the largest volcano and canyon in the solar system: Olympus Mons and Valles Marineris.
- NASA’s Curiosity rover has found organic molecules on Mars, and Perseverance is collecting samples for eventual return to Earth.