Science Puzzle
Why Is the Air on Mars So Thin?
Mars was once much wetter. Its surface is carved with dried-up river valleys and old lake beds, so it probably had a thicker atmosphere long ago. Today its air is so thin that the pressure at the surface is less than 1 per cent of Earth's, and any water left out would quickly freeze or boil away.
Where did the air on Mars go?
The Answer
Much of it was stripped away into space, mostly by the solar wind. Mars lost the magnetic field that could have shielded it, and its weak gravity couldn't hold on to its air as well as Earth's does.
The Sun constantly blows out a stream of charged particles called the solar wind. Earth is protected by its magnetic field, which is made by molten iron churning in its core. The field turns most of the solar wind aside, and the particles that get through make the Northern and Southern Lights.
Mars is about half as wide as Earth and has only about a tenth of its mass. A small planet cools faster, and around 4 billion years ago the core of Mars cooled enough that its global magnetic field faded away. Without that shield, the solar wind struck the top of the Martian atmosphere directly, knocking gas out into space a little at a time for billions of years. Mars's weaker gravity made escaping easier.
NASA's MAVEN spacecraft, orbiting Mars since 2014, has measured gas being stripped from the top of the atmosphere and found that the loss jumps during storms on the Sun. Not all the water left, though. Some of it is still on Mars, frozen in the polar ice caps and under the ground.
The principle: Atmospheric escape. A small planet with weak gravity and no magnetic field slowly loses its air to space, stripped away by the solar wind.