Observations from NASA's MAVEN spacecraft reveal that the solar wind causes unexpected interactions with the Red Planet's weak magnetism.

Pity poor little Mars. It's bigger than Mercury, yet planetary scientists struggle to understand why it didn't end up larger than it is. In its infancy, the Red Planet must have been an exciting place in its infancy, with a sizable magnetic Field, a much denser atmosphere, and liquid water gushing across its surface. But the magnetic field mysteriously collapsed, most of the atmosphere and water escaped to space, and the surface went into deep freeze.

These consequences are likely interrelated, and to help researchers puzzle things out NASA dispatched the MAVEN orbiter (short for Mars Atmosphere and Volatile EvolutioN) four years ago. Since arriving at Mars in September 2014, its spectrometers and detectors for fields and charged particles have been gathering observations that bear on how Mars lost its atmosphere.

Because Mars lacks a global magnetic field, researchers thought field lines embedded in the solar wind would simply drape across Mars as they slid by. This forced intimacy would cause some loss of atmospheric gas, a combination of neutral atoms and ions, but the calculated leakage was so slow that over 3½ billion years the planet would have lost the equivalent of a global layer of water 3 meters (10 feet) deep and just 1 millibar of CO₂ — a tiny fraction of what the planet once had. Clearly, the loss rate had to have been more vigorous in the past than it is now.

One clue comes from mission results announced at a recent science meeting in Provo, Utah, which show that the planet's interaction with the outflowing solar wind is more complicated than thought. Gina DiBraccio, MAVEN's project scientist at NASA's Goddard Space Flight Center, explains that field lines from localized pockets of remanent magnetism frozen into the Martian crust can connect with the solar wind's field on the planet's dayside, where the solar wind penetrates deepest.

Once these connections swing around to the night side, they provide a direct escape route for ions from the atmosphere through the cone-shaped magnetotail that extends far behind the planet. "What we're finding with MAVEN," she explains, "is that a majority of the field lines in the magnetotail are open to space." How much more gas might have escaped over time via this mechanism isn't yet clear.

Moreover, the spacecraft found that the collective orientation of field lines within the magnetotail display an unexpected twist. Computer simulations by Yingjuan Ma (University of California, Los Angeles) can reproduce this observed twist — but only if abundant magnetic reconnection events are occurring on the dayside between the bubble-shaped localized fields and the solar-wind field.

Meanwhile, all this reconnection serves to erode the ability of the localized remanent magnetism to fend off solar storms and other charged-particle assaults from deep space — and that's not good news for those who had hoped that these magnetized pockets might be good places to search for evidence of past (or current) primitive life forms on the planet's surface.

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