NASA explains how and when Mars lost its atmosphere and bountiful water

Earlier this year, NASA said that Mars — now barren and cold — was once covered with abundant water, perhaps including a mile-deep ocean. In September, NASA scientists reported evidence that there is still liquid water on Mars, if only a seasonal salty trickle. On Thursday, the researchers working on NASA's MAVEN orbiter connected the dots, presenting their theory on how Mars lost its once-dense atmosphere and, thus, its bountiful water. The key, explained Bruce Jakosky, the MAVEN mission's principal investigator, is solar winds, especially strong solar storms that buffeted Mars billions of years ago. "Like the theft of a few coins from a cash register every day, the loss becomes significant over time," he said.

MAVEN was able to quantify how much atmosphere Mars loses every day — about a quarter pound (100 grams) of gas every second. "I can't help but imagine hamburgers flying out of the Martian atmosphere, one per second," joked MAVEN scientist Dave Brain on Thursday. "It's instead oxygen and carbon dioxide that are leaving the planet, which are important both for water and for the climate of the planet overall." But that amount wouldn't explain the wholesale loss of Mars' atmosphere. On March 8, however, the bus-sized MAVEN orbiter observed a big solar storm, and how it made the Martian atmosphere disperse 10 to 20 times faster.

The solar energy pounding Mars during that March 8 storm was equal to about a million tons of TNT an hour, MAVEN scientist Jasper Halekas said. "That's one large nuclear weapon per hour, if you like," he added, and Mars gets a few of those storms a year these days. Billions of years ago, though, the sun was younger and more active, erupting with more frequent solar storms and radiating brighter light. That was probably when Mars took the biggest, decisive hit to its atmosphere, over hundreds of millions of years. You can watch part of the MAVEN team's presentation, with helpful visual aids, in this Associated Press roundup:

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In this excerpt from Thursday's presentation, Jakosky focuses on when the solar winds stripped Mars of its atmosphere and what that did to the Red Planet and any life it may have hosted:

The MAVEN team's findings were published Nov. 5 in four studies in the journal Science and more than 40 papers in Geophysical Research Letters. The MAVEN spacecraft, launched in 2013, concludes its primary science mission on Nov. 16.