A new study published in the journal Advances in Space Research suggests that solar storms create a complex electrical environment around Phobos, giving its night side and shadowed craters a static electric charge.
Phobos, the larger and inner of the two natural satellites of Mars, has been considered as a possible initial base for human exploration of the planet because its weak gravity makes it easier to land spacecraft, astronauts and supplies.
The idea would be to have the astronauts control robots on the Martian surface from the planet’s moons, without the considerable time delay faced by Earth-based operators.
“We found that rovers or astronauts could accumulate significant electric charges when traversing the night side of Phobos — the side facing Mars during the Martian day,” said study lead author Dr. William Farrell, of NASA’s Goddard Space Flight Center.
“While we don’t expect these charges to be large enough to injure an astronaut, they are potentially large enough to affect sensitive equipment, so we would need to design spacesuits and equipment that minimizes any charging hazard.”
Phobos has no atmosphere and is directly exposed to the solar wind — a stream of plasma that’s constantly blowing off the surface of the Sun into space at around a million miles per hour.
When the solar wind strikes the day side of Phobos, the plasma is absorbed by the surface. This creates a void on the night side of Phobos that the plasma flow is obstructed from directly entering.
However, the composition of the wind — made of two types of electrically charged particles, namely ions and electrons — affects the flow. The electrons are over a thousand times lighter than the ions.
“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. 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,” Dr. Farrell said.
The study shows that this plasma void behind Phobos may create a situation where astronauts and rovers build up significant electric charges.
For example, if astronauts were to walk across the night-side surface, friction could transfer charge from the dust and rock on the surface to their spacesuits. This dust and rock is a very poor conductor of electricity, so the charge can’t flow back easily into the surface — and charge starts to build up on the spacesuits.
On the day side, the electrically conducting solar wind and solar UV radiation can remove the excess charge on the suit. But, on the night side, the ion and electron densities in the trailing plasma void are so low they cannot compensate or ‘dissipate’ the charge build-up.
The team’s calculations revealed that this static charge can reach 10,000 volts in some materials, like the Teflon suits used in the Apollo lunar missions.
If the astronaut then touches something conductive, like a piece of equipment, this could release the charge, possibly similar to the discharge you get when you shuffle across a carpet and touch a metal door handle.
The researchers modeled the flow of the solar wind around Phobos and calculated the buildup of charge on the night side, as well as in obstructed regions in shadow, like Stickney crater, the largest crater on Phobos.
“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,” Dr. Farrell said.
Although this study focused on Phobos, similar conditions are expected at Deimos, the smaller of the two Martian moons.
W.M. Farrell et al. Anticipated electrical environment at Phobos: Nominal and solar storm conditions. Advances in Space Research, published online September 5, 2017; doi: 10.1016/j.asr.2017.08.009