A picture has emerged over the last decade of an early Mars that was once much warmer and wetter than it is today. Mars would have needed a thicker atmosphere to keep that water on the surface in liquid form, but today, its atmospheric pressure is less than one percent of Earth’s. What happened to Mars to cause such severe climate change over its history? A clue to this atmospheric loss has been revealed by MAVEN.

NASA’s MAVEN (Mars Atmosphere and Volatile Evolution) mission doesn’t get as much love from the public as the highly celebrated rovers Spirit, Opportunity, and Curiosity. This orbiter still has an important mission, however: to sample and study the tenuous atmosphere of Mars. Launched on time and under budget in 2013, this craft began science operations on November 16, seeking to answer the question of what happened to Mars’ atmosphere and explore its interaction with the solar wind.

While the examination of old rocks by the Curiosity rover has told us something about the timing of Mars' atmospheric loss, MAVEN can observe the processes that have driven it.

MAVEN actually dips into the ionosphere during its orbit of the Red Planet, sampling the charged particles in this outer layer of atmosphere. Mission scientists expected that fast-moving particles in the solar wind would be deflected by the Martian ionosphere, but instead they found that these streams of particles actually penetrate the atmosphere and create streams of particles that dip into its lower layers. In fact, the charged particles, or ions, interact in a way that neutralizes their charge, which somehow reappears as ions travel farther down into the Martian atmosphere.

This deep penetration of solar wind material means that it can transfer energy to atoms and molecules in the lower atmosphere, facilitating their escape from Mars’ gravitational well. This work was done with the Solar Wind Ion Analyzer (SWIA), one of eight instruments aboard MAVEN.

Upon arriving at Mars, another of MAVEN’s instruments, the Suprathermal and Thermal Ion Composition (STATIC) instrument also got busy. SWIA and STATIC work together on MAVEN to characterize what material is coming into the system from the solar wind and what is leaving the atmosphere as a result. STATIC detected a plume of charged particles escaping Mars. The streams of solar wind are one mechanism which might be driving this continual loss. As MAVEN continues to characterize these weird space weather patterns on Mars, a clearer picture of atmospheric loss over time on Mars will begin to emerge.

The Earth has an ionosphere as well, and we have it to thank for protecting us from the charged particles of the solar winds that bombard our planet. In fact, those charged particles are funneled to the poles of the Earth by magnetic fields, where they interact with particles in our atmosphere to create the stunning aurora familiar to those who live at high latitudes.

Mars, however, has no such magnetic field, at least not globally and not anymore. The Earth’s magnetic field is powered by liquid iron and other metals churning in the hot core. Mars, our smaller cousin, cooled off more quickly and has no such internal dynamo. No magnetic field means that the atmosphere is more vulnerable to the solar wind. Mars’ small size also means it has less of a gravitational pull on its atmosphere, and thus can lose its particles more rapidly.

The small size of Mars seems to be the overall driving factor for its current uninhabitability, but the details still need some filling in. The discovery of these deeply penetrating solar wind streams is one piece of the puzzle, and since this is the first time we’ve seen this, we want to know how it’s happening. While Curiosity continues to discover evidence of lake beds and other widespread water features on the surface, MAVEN will continue to study the slow siphoning off of the Martian atmosphere, allowing scientists to walk back the processes that led from a thicker atmosphere of ancient times to today’s tenuous envelope.