Three main theories attempt to explain the puzzle.
Some suggest that water and carbon dioxide still exist on Mars in large reservoirs that are as yet unfound—probably below the planet's surface.
Other theories propose that some type of catastrophic cosmic impact removed much of the Martian atmosphere in a single event.
A third suggestion: Solar wind removed Martian water and carbon dioxide.
"I'm a plasma physicist so I really like the last one," said Barabash, a professor of experimental space physics.
"But our last study shows that escape [of water and carbon dioxide] from this channel is not as intense as we thought before, and that's a very big puzzle."
Some scientists caution that much more data is needed to make sense of the findings from Barabash's team.
"Recent measurements both from Mars Express and from Mars Global Surveyor suggest that we have not yet even described all of the loss processes at the present epoch," said Bruce Jakosky, from the Laboratory for Atmospheric and Space Physics at the University of Colorado.
"This means that we cannot yet determine the total loss rate today, let alone be able to extrapolate to earlier epochs."
Barabash also cautions that the solar wind may function in ways that scientists can't yet measure.
"It's possible that solar wind is far more complex than we think," he said. "So we have to explore other escape channels which are also associated with the solar wind."
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