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Planetary scientists have been watching that process in action through a spacecraft called MAVEN. It entered orbit around Mars five years ago today. And it’s still going.
The craft’s major goals are to measure Mars’s atmosphere and its interaction with the solar wind — a flow of charged particles from the Sun. It’s found that the solar wind strips away hydrogen from the top of the atmosphere — a result that was expected. But it also found something that wasn’t expected: about 10 times more material is stripped away when Mars is closest to the Sun than when it’s farthest.
When Mars was young, it had a much thicker atmosphere. That kept the planet warm enough for liquid water to flow across its surface. But the water has long since disappeared. MAVEN found that most of the atmosphere was gone by 3.7 billion years ago, leaving it too thin to retain the planet’s water. Today, the atmosphere is less than one percent as thick as Earth’s — leaving Mars dry, cold, and desolate.
MAVEN is expected to keep an eye on the Martian atmosphere for quite a while — it has enough propellant to keep going until at least 2030. In the meantime, it’s getting ready for a second mission. It’ll serve as a relay station for the next Mars rover, which is scheduled for launch next year.