Mars Has the Biggest Volcano in the Solar System. And Its Moon Rises in the West.

Mars is half the diameter of Earth. It has the largest volcano in the solar system, the largest canyon, and a moon that goes the wrong way. There is more happening on this small red planet than its size suggests.

A volcano you cannot see the top of

Olympus Mons is 22 km tall. Everest, on Earth, is 8.8 km. The base of Olympus Mons covers 600 km across, roughly the area of Italy.

What makes it strange is the slope. Olympus Mons is gentle. The sides rise at an average angle of about 5 degrees, which is less than a wheelchair ramp. If you were standing on it, you would not feel like you were on a mountain. You would just be on a slightly tilted plain. Walk for two days in one direction and the ground would still be tilted at five degrees. The planet curves away beneath you before the slope ever feels steep.

It is so wide that from the base, you cannot see the summit. The horizon, on Mars's curved surface, gets in the way. The summit caldera at the top is itself the size of London.

This works because Mars has no plate tectonics. On Earth, hotspots in the mantle move across the surface as the crust drifts, building chains of smaller volcanoes (Hawaii is the textbook example). On Mars, the crust does not drift. So a single hotspot can keep producing lava in the same place for billions of years, piling layer upon layer until the result is a mountain three times the height of Everest and the width of a country. The full Mars planet guide covers what else this static crust produces.

A moon that rises in the west

Mars has two moons. The larger, Phobos, orbits at a distance of just 6,000 km, closer than any other moon orbits its planet anywhere in the solar system.

It orbits so close that it goes around Mars in 7 hours 39 minutes. Mars itself rotates in 24 hours 37 minutes. Phobos, in other words, is moving faster than the surface of Mars beneath it. To an observer standing on Mars, the rest of the sky moves east to west as the planet rotates. Phobos, going the other way relative to the rotation, appears to rise in the west, cross the sky, and set in the east. It does this twice in a Martian day.

Venus does something similar with the Sun for a different reason. Venus rotates backwards, so the Sun rises in the west. Phobos rises in the west because Mars rotates normally but Phobos is faster. Two routes to the same backwards view.

There is one more thing about Phobos. It is slowly spiralling inward. Tidal forces are tightening its orbit by about two metres per century. In roughly 50 million years it will either crash into Mars or break apart and form a ring. Mars currently has no ring. It probably will.

Mars has more weather than people think, and most of it ends up pinker than expected, or bluer than expected. The mountains and moons are doing their own thing entirely.