Mars is the fourth planet from the Sun and the second-smallest in the solar system, about half the diameter of Earth. It gets its nickname, the Red Planet, from iron oxide — rust — coating its surface, the same chemical reaction that turns iron reddish-brown here on Earth.
The basics
A day on Mars, called a sol, is close to Earth's — about 24 hours and 37 minutes. A year is a different story: Mars takes roughly 687 Earth days to orbit the Sun, nearly twice as long as an Earth year. Its axis is tilted at almost the same angle as Earth's, which means Mars has seasons too, just longer ones.
Temperatures swing wildly. A summer afternoon near the equator can reach a relatively pleasant 20°C (68°F), while winter nights near the poles can drop below -125°C (-195°F). The atmosphere is thin — less than 1% the pressure of Earth's — and made almost entirely of carbon dioxide, which is why liquid water can't stay on the surface for long; it either freezes or boils away almost immediately.
Two small moons
Mars has two moons, Phobos and Deimos, both tiny and irregularly shaped — more like captured asteroids than proper moons. Phobos is slowly spiraling inward and is expected to either crash into Mars or break apart into a ring in tens of millions of years.
Who's there right now
Mars is the most explored planet in the solar system besides Earth. As of 2026, two NASA rovers are still active on its surface: Curiosity, exploring Gale Crater since 2012, and Perseverance, which has been searching Jezero Crater for signs of ancient microbial life since 2021 and has collected rock samples for a future return to Earth — though the mission to actually bring those samples home was cancelled by Congress in early 2026, leaving their return plan uncertain for now. Several orbiters, including NASA's Mars Reconnaissance Orbiter and ESA's Mars Express, continue to study the planet from above and relay data from the surface back to Earth.
Why people care
Mars is the most Earth-like planet we can actually reach. Evidence strongly suggests it once had rivers, lakes, and maybe an ocean, billions of years ago — which makes it the best candidate in the solar system for having once hosted, or still hosting in some hidden form, microbial life. It's also the planet most often discussed as a future destination for human exploration, though as of 2026 no crewed mission has a confirmed launch date.