Washington, Jan 8: Powerful solar storms can charge up the surface in frigid, permanently shadowed regions near the poles of the Moon and possibly produce "sparks" that vaporise and melt the lunar soil, a new NASA study has found. This alteration may become evident when analysing future samples from these regions that could hold the key to understanding the history of the Moon and solar system.
The Moon has almost no atmosphere, so its surface is exposed to the harsh space environment. Impacts from small meteoroids constantly churn or "garden" the top layer of the dust and rock, called regolith, on the Moon. "About 10 per cent of this gardened layer has been melted or vaporised by meteoroid impacts," said Andrew Jordan of the University of New Hampshire in the US. "We found that in the Moon's permanently shadowed regions, sparks from solar storms could melt or vaporise a similar percentage," said Jordan.
Explosive solar activity, like flares and coronal mass ejections, blasts highly energetic, electrically charged particles into space. Earth's atmosphere shields us from most of this radiation, but on the Moon, these particles - ions and electrons - slam directly into the surface. They accumulate in two layers beneath the surface; the bulky ions can not penetrate deeply because they are more likely to hit atoms in the regolith, so they form a layer closer to the surface while the tiny electrons slip through and form a deeper layer. The ions have positive charge while the electrons carry negative charge.
Since opposite charges attract, normally these charges flow towards each other and balance out. In August 2014, however, researchers showed that strong solar storms would cause the regolith in the Moon's permanently shadowed regions (PSRs) to accumulate charge in these two layers until explosively released, like a miniature lightning strike.
The PSRs are so frigid that regolith becomes an extremely poor conductor of electricity. Therefore, during intense solar storms, the regolith is expected to dissipate the build-up of charge too slowly to avoid the destructive effects of a sudden electric discharge, called dielectric breakdown. The research estimates the extent that this process can alter the regolith.
"This process is not completely new to space science - electrostatic discharges can occur in any poorly conducting (dielectric) material exposed to intense space radiation and is actually the leading cause of spacecraft anomalies," said Timothy Stubbs of NASA's Goddard Space Flight Center in Greenbelt in the US. The team's analysis was based on this experience.
From spacecraft studies and analysis of samples from NASA's Apollo lunar missions, the researchers knew how often large solar storms occur. The research was published in the journal Icarus.