Electric atmosphere on Martian moon may doom exploration hopes

The Martian moon Phobos has been considered a potentially useful base for astronauts, because its weak gravity makes it an attractive proposition for landing craft and setting up work stations.

With astronauts on the moon, the theory runs, robots on the planet surface could be tasked with exploring – or even mining – and be controlled in very close to real time.

But now a paper published in the journal Advances in Space Research may well have kyboshed that plan once and for all. Research from a team led by William Farrell of NASA’s Goddard Space Flight Centre in Maryland, US, suggests that parts of Phobos may be electrically charged – not enough to hurt visiting humans, but quite possibly enough to fritz all their devices.

Without an atmosphere to protect it, Phobos is directly exposed to solar wind, the combination of ions and electrons that travels through space at about 1,600,000 kilometres an hour.

The wind hits the day side of Phobos and is partially absorbed. This creates an area of low pressure on the night side. Electrons are roughly 1000 times lighter than ions, and so flood past the moon. The ions, meanwhile, are nowhere near as nimble.

“The electrons act like fighter jets – they are able to turn quickly around an obstacle -– and the ions are like big, heavy bombers – they change direction slowly,” explains Farrell. 

“This means the light electrons push in ahead of the heavy ions and the resulting electric field forces the ions into the plasma void behind Phobos, according to our models.”

The models suggest that this effect makes the dark side of Phobos a very risky place to be for an astronaut. 

Friction generated by walking across the moon’s surface would likely result in the astronaut picking up a cargo of electrically charged dirt and dust. Because such bits of debris are generally very poor conductors, the charge would not flow back to the ground but instead build up around the astronaut’s suit.

It would continue to increase – the team’s calculations suggest to as high as 10,000 volts – until the astronaut touched something and caused it to discharge. 

If the thing touched was, say, a sensitive robot-communication device then the chances are that it will be fried by the sudden electrical surge.

The findings may cause a rethink of plans to land humans on Phobos – or the insulation design of Martian equipment.

The research also established that the charging effect probably isn’t confined only to the night side of Phobos. It will also occur during daylight in areas such as the Stickney crater, which have sections in permanent shadow.

“We found that excess charge builds up in these regions during all solar wind conditions, but the charging effect was especially severe in the wake of solar eruptions like coronal mass ejections, which are dense, fast gusts of solar wind,” says Farrell.

Although this research was specific to Phobos, the scientists suggest the same conditions most likely also manifest on Mars’ other moon, Deimos.