A constant stream of charged particles impinges on Mars at roughly one million miles per hour. This solar wind is capable of picking up ions from Mars’ upper atmosphere and stripping them away from the planet out into space. MAVEN has measured this escape rate under present conditions to be about 100 grams (or ¼ lb.) per second.
Unlike Earth, which has the protection of a large, global magnetic field, Mars is directly impacted by the solar wind, although its tenuous atmosphere (< 1% of Earth’s) prevents the solar wind from impacting the planet’s surface.
The complete article courtesy of NASA’s MAVEN Mission to Mars