English: Maven spacecraft : Visualizing Mars' Atmosphere Loss

Mars today is a cold, barren desert, but scientists think it was once a warm and wet planet. The change may have been caused by the loss of an early atmosphere driven into space by the sun’s solar wind. Unlike Earth, Mars lacks a global magnetic field that deflects the stream of charged particles continuously blowing off the sun. Instead, the solar wind crashes into the planet’s upper atmosphere, stripping away electrically charged gas atoms, called ions (depicted here in a simulation).

Charged particles from the Mars upper atmosphere, seen here in color, feel the pull of the electric field generated by the solar wind. These particles, or ions, can pick up sufficient energy from the electric field to escape into space.

In this simulation, green particles represent medium-energy ions swept back in the wake of the solar wind. High-energy ions, in yellow and red, follow the electric field in a "polar plume" above Mars.

NASA’s MAVEN spacecraft, which entered orbit around Mars in September 2014, measures the speed and direction of escaping ions to observe the process of how Mars’ atmosphere is eroded by the solar wind. Measurements show the rate of ion loss increases during periods of extreme solar activity, suggesting a younger and more active sun billions of years ago likely played a significant role in transforming the Red Planet.