Mars

Mars once had rivers, lakes and a thicker atmosphere. Today it is a cold desert. How Mars was built, what it lost and why it remains the top target in the search for past life.

Diameter: 6,792 km (53% of Earth)
Mass: 0.107 × Earth
Surface gravity: 0.38 × Earth
Distance from Sun: 1.52 AU (about 228 million km)
Year: 687 Earth days
Day: 24 hours 40 minutes
Moons: 2 (Phobos and Deimos)
Average temperature: about −65 °C
Surface pressure: less than 1% of Earth's

The build: a planet that stopped growing

Mars has only about a tenth of Earth's mass. Planet-formation models have trouble explaining why it is so small — the region where it formed seems to have been short of building material. Some models link this to the early movements of Jupiter, though that idea is still debated. Mars may be a planetary embryo that never went through the final round of giant impacts that built Earth.

A wetter past

Orbiters and rovers have found clear signs that liquid water once flowed on Mars: dried river valleys, ancient deltas, minerals that only form in water, and layered rocks laid down in lakes. NASA's Curiosity rover (landed 2012) found that Gale Crater once held a long-lived lake. Perseverance (landed 2021) is exploring Jezero Crater, an old lake with a river delta, and collecting rock samples that may one day be returned to Earth.

For liquid water to be stable, early Mars needed a thicker, warmer atmosphere than it has today.

What went wrong

Mars shows what happens when a planet is too small:

The interior cooled quickly. A small planet loses heat faster.
The magnetic field shut down. Rocks show Mars once had a global magnetic field, but it faded roughly four billion years ago as the core cooled.
The atmosphere leaked away. With weak gravity and no global magnetic shield, the solar wind stripped gas from the upper atmosphere. NASA's MAVEN spacecraft has measured this loss directly.
Water froze or escaped. As the air thinned, surface water could no longer stay liquid. Much of what remains is frozen in polar caps and underground ice.

Record-breaking landscapes

Low gravity and the lack of moving plates let Martian volcanoes grow enormous. Olympus Mons rises about 22 km — two and a half times the height of Mount Everest above sea level. Valles Marineris is a canyon system about 4,000 km long, long enough to stretch across the United States.

Seasons and dust

Mars is tilted about 25°, similar to Earth, so it has seasons — each about twice as long as ours. In some years, dust storms grow to cover the entire planet. Over millions of years, without a large moon to steady it, Mars's tilt has varied much more than Earth's, shifting where ice collects.

Lessons for planet builders

Being in the habitable zone is not enough; Mars sits near its outer edge.
Small planets cool fast, lose their magnetic fields and then their air.
Mass is one of the most important decisions in step 3.