How did Mars lose its atmosphere?

Earth owes its thick atmosphere to a convecting liquid core that sustains a global magnetic field. This shield deflects the solar wind, the continuous stream of charged particles emitted by the Sun. Mars once had the same mechanism: magnetometers aboard the Mars Global Surveyor probe detected remnant magnetization in the oldest terrains of the southern hemisphere, fossils of a now-extinct dynamo. Smaller than Earth, the planet cooled faster, its core stopped churning, and the field collapsed more than four billion years ago.

From then on, the solar wind struck the upper atmosphere directly. It strips away ions and carries them into space, a process the MAVEN mission, in orbit since 2014, was designed to measure. Its finding: an average loss of about 100 grams per second, multiplied several times over during coronal mass ejections. Extrapolated over billions of years, this erosion is enough to explain the shift from an atmosphere dense enough to sustain liquid water to today's thin film.

A structural handicap compounds the problem. With just over a tenth of Earth's mass, Mars poorly retains light molecules: hydrogen produced by the breakdown of water vapor escapes through simple thermal agitation, leading to the irreversible loss of water. The isotopic ratio measured in the Martian atmosphere, enriched in deuterium, bears the trace of this selective leak.

The current atmosphere, composed of more than 95% carbon dioxide, exerts an average surface pressure of about 6 hectopascals, less than a hundredth of Earth's. Too thin to sustain stable liquid water, but enough to drive planet-wide dust storms and slow down landers.