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Magnetic anomalies
shield the Moon

Posted: 24 September 2010

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Evidence for magnetic anomalies strongly deflecting the solar wind from the lunar surface was presented at the European Planetary Science Congress today.

Unlike the Earth, which has a magnetic field to carve out its protective magnetosphere that shields us from the influx of the solar wind, airless bodies such as the Moon have surfaces exposed directly to the bombardment of solar wind particles. Combined with the Moon's lack of atmosphere – which would normally filter out micro-meteorite impacts – this results in a heavily weathered surface known as the lunar regolith.

Spatial variation of the energetic neutral hydrogen flux over the magnetic anomaly close to the Gerasimovic crater. (a) High energy hydrogen flux with energy indicates a ~50%  flux reduction inside the magnetic anomaly compared to the surrounding area. (b) Hydrogen flux with lower energy of 30-100 eV fills the magnetic anomaly. (c) The albedo (reflectivity) map of the Moon with the spacecraft trajectories (white lines).

Until recently, the solar wind was thought to be absorbed by the regolith, but new data provided by the recent fleet of lunar orbiters – Chang'E-1, Kaguya and Chandrayaan-1– have revealed that there is more to the story. In particular, Chandrayaan-1's Sub-keV Atom Reflecting Analyzer (SARA) mapped for the first time the energetic hydrogen atoms emanating from the Moon, finding that up to one fifth of the solar wind protons reaching the lunar surface are reflected back to space.

"These results may change dramatically the way we understood the solar wind-regolith interaction so far," says Yoshifumi Futaana of the Swedish Institute of Space Physics. "Since the solar wind is one potential source of water on the Moon, we need to make better models of the lunar hydrogen circulation in order to understand how water molecules form in its upper layers. Also, it will be possible to remotely investigate the solar wind-surface interaction on other airless bodies, such as Mercury or the Martian moon Phobos, by imaging the energetic hydrogen atoms that are reflected back to space when the solar wind hits their surface."

Comparison between the reflected proton flux and the magnetic anomaly distribution on the Moon. (top) Proton flux distribution observed by the SWIM sensor mapped on the lunar map (generated by Clementine). The black line shows a contour of the lunar magnetic anomaly. (bottom) Magnetic anomaly distribution model based on Lunar Prospector data.  The same contour as in the left panel is overlaid here.

The plot thickens – when Chandrayaan-1 flew over a magnetic anomaly on the lunar farside near the Gerasimovic crater, significantly less reflected hydrogen atoms were deflected, meaning that the solar wind had not reached the lunar surface. “We detected a strong flux of deflected solar wind protons, which clearly indicates that magnetic anomalies can shield the lunar surface from the incoming solar wind, in the same way as the magnetospheres of several planets in our Solar System," says Futaana.

The 'mini-magnetosphere' at Gerasimovic crater has a footprint of about 360 kilometres at the surface, but the team comment that the shielding effect is linked to the local strength of the solar wind, such that when the solar wind pressure is low, the mini-magnetosphere expands, causing stronger shielding.

The scientists suspect similar behaviour may occur on other airless bodies such as Mercury, asteroids and many of the Solar System's moons. "During the close encounter of the European Mars Express spacecraft with Phobos in 2008, we detected signatures of reflected solar wind protons also from the surface of the Martian moon Phobos," adds Futaana.

For more on the lunar magnetic anomalies see Stirring up the lunar surface, featured in the August 2010 issue of Astronomy Now magazine. Buy online here.