A miniature magnetic field has been imaged on the surface of the moon, making it a rare, minimally protective lunar refuge from some aspects of the harsh solar wind.
The magnetic region could be a great place to site a lunar base, since tomorrow's lunar colonists will not only need water (check!), but some protection from the heavy radiation in the solar wind.
"You can think of it as kind of a mini umbrella," said Martin Wieser of the Swedish Institute of Space Physics in Kiruna, Sweden. "It will be effective for certain kinds of (space) weather."
But it is very weak in comparison to Earth's planet-wide magnetic field or even compared to the splotchy magnetospheres of Mars.
The 224-mile- (360-kilometer-) wide magnetosphere was detected by an instrument on the Indian Chandrayaan-1 spacecraft. The mini magnetosphere revealed itself by fewer hydrogen atoms scattering off the surface of the moon. Generally when the solar wind rams into the airless surface of the moon, some hydrogen atoms scatter back into space. Any place with fewer of these atoms rising up from the moon is likely to be shielded magnetically, explained Wieser and his colleagues in their paper published in Geophysical Research Letters.
The researchers also noticed that the dust inside the magnetically shielded zone is brighter than the surrounding areas, which might have something to do with the solar wind weathering that surface differently over the eons.
"I think this is a very exciting observation," said lunar researcher Jasper Halekas of the Space Science Laboratory at the University of California at Berkeley. "There have been a number of indications that theses things were taking place. (But) this is the first directed study that really tells you 'they are there.'"
The lunar magnetospheres are a lot like those on Mars, said Halekas, except that on Mars they are almost certainly the remnants of a global magnetic field that was created by the planet's core. A molten core dynamo on the moon is a bit less likely, however.
"It's still quite possible that there was a dynamo on the moon," said Halekas.
Another possibility is that the mini-magnetosphere was generated by large impacts on the direct opposite side of the moon. Exactly how this works is far from clear, but there does seem to be a correlation between large impact craters and magnetic anomalies at their antipodes, said Wieser.
Ironically, the same magnetic shielding could make these the driest places on the moon. One mechanism believed to generate at least the ingredients for water on the moon are solar protons smacking into the surface, forming hydrogen atoms in the lunar soils. Inside a mini magnetosphere that is less likely to happen, Wieser noted.
Without water, maybe this isn't the best lunar real estate after all.