Tiny lightning bolt explosions can vaporise the moon’s thin soil

It’s shocking what goes on in cold lunar soil

NASA

Mini-lightning may flash in the coldest craters on the moon, melting and vapourising soil. All that sparking could have altered the surface as much as impacts from incoming rocks and dust.

The outer layer of the moon is a sort of history book recording the interactions between the moon and the rest of the solar system. To correctly interpret that history, we need to understand the mechanisms that shape it.

We already knew that impacts stir up the fine layer of dust and ground rock that covers the lunar surface. But now Andrew Jordan at the University of New Hampshire in Durham and his colleagues suggest we keep an eye out for another cause: electrical sparks.

The lunar dust is normally an electrical insulator that can’t conduct much current. But high-energy particles thrown off by the sun during solar storms might be able to collect in cold patches of lunar soil.

If enough charged particles accumulate, they could transform the soil into a conductor by opening up vapour-filled channels within the grains that let the current through. That would set off tiny explosions, each one like the spark you might get from touching a metal car door on a dry winter day.

“The electric field keeps building up and building up and then snap, you go from nothing to conducting almost instantaneously,” Jordan says.

The same phenomenon, fuelled by energetic particles in Jupiter’s radiation belts, caused electrical glitches on Voyager and other spacecraft, but hasn’t been directly observed on the moon.

Tiny lightning bolts

Jordan and his colleagues calculated how much of a charge might build up on the lunar surface by considering factors like how often solar storms occur, how many charged particles are usually released and the conductivity of lunar soil at different temperatures. They focused on regions that are permanently in shadow because colder soils are less conductive, allowing electrical charges to accumulate faster than they can dissipate.

Using the melting and vaporisation temperatures for moon soil, the team estimated that between 10 and 25 per cent of the uppermost 1 millimetre of lunar dust in those regions may have been blasted by the tiny lightning bolts over the last 1 million years.

Impacts dig up a similar amount of dust, so electric sparking could be an important, overlooked means of beating up the surface of the moon, Jordan says.

The calculations are convincing, says Paul Lucey at the University of Hawaii in Honolulu. “But we know none of the details,” he adds. The next step is to run tests on simulated lunar soils in the lab, which could give researchers clues for what physical evidence to look for in moon samples brought back by the Apollo missions.

Permanently shadowed regions are particularly interesting because they can trap materials that would otherwise be lost to space. This means they can act as time capsules of the early solar system – and makes understanding how they change all the more important, says Joshua Bandfield at the Space Science Institute in Boulder, Colorado.

“There’s a reasonable chance that if you probe these areas and you look at what’s been stored in them, you might get a nice sampling of pre-biotic chemistry at some level, the organic soup from which things are made,” he says.

But they’re difficult to study, because they’re cold, dark and inaccessible.

“It’s really hard to figure out what’s going on in these permanently shadowed regions,” Jordan says. “There’s always going to be some ambiguity until you can get down and just start scooping up stuff and measuring it there.”

Journal reference: Icarus, DOI: 10.1016/j.icarus.2016.08.027