Charged particles released by the Sun have gradually stripped away Mars's once-rich atmosphere and made it the desolate and lifeless place it is today, Nasa says. Scientists working on the US space agency's Mars Atmosphere and Volatile Evolution (Maven) mission say the solar wind – comprising mainly of streams of electrons and protons – has all but destroyed the planet's upper atmosphere.

This is because unlike the Earth, Mars does not possess a substantial magnetic field to shield its atmosphere from the bombardment of solar particles. The rate of atmospheric erosion was found to increase significantly during solar storms, with Maven observing one such event in March this year. The findings have been detailed in four papers in Science Magazine.

Steady erosion

"Like the theft of a few coins from a cash register every day, the loss becomes significant over time," said Bruce Jakosky, Maven principal investigator at the University of Colorado. "We've seen that the atmospheric erosion increases significantly during solar storms, so we think the loss rate was much higher billions of years ago when the Sun was young and more active."

Charged particles that escape the Sun's powerful gravity travel outward at a speed of about 1.6 million kph (1 million mph) and constantly bombard the Red Planet, knocking its upper atmospheric ions into space. Maven observed solar wind stripping away gas from the Martian atmosphere at a rate of about 100g per second.

This process, played out over billions of years, has led to the chipping away of what was once a thick envelope of gases blanketing the planet, which likely engendered a warm and wet environment. In September, Nasa announced that it had discovered salty streaks on Mars, suggesting liquid salty water seeps seasonally to the surface. However, the planet's thin atmosphere means any liquid water that actually makes it to the surface is instantly vaporised or frozen.

Bruce Jakosky
Maven principal investigator Bruce Jakosky announced the mission's findings at Nasa headquarters in WashingtonReuters

"Solar-wind erosion is an important mechanism for atmospheric loss, and was important enough to account for significant change in the Martian climate," said Joe Grebowsky, Maven project scientist from Nasa's Goddard Space Flight Center. "Maven also is studying other loss processes – such as loss due to impact of ions or escape of hydrogen atoms – and these will only increase the importance of atmospheric escape."