Loaded particles charge the night-side of Phobos with hundreds of volts

Attention electric shock: The moon of Mars called Phobos could literally be electrifying, as researchers have found. The surface of the moon is electrically charged by the solar wind and solar storms. On the night-side of the moon a charge separation takes place that could apply power to sensitive electronics of future land probes, but also to shoes and suits of astronauts.

The night-side of Phobos is electrified by the solar wind. © NASA / GSFC, CI Lab

Phobos is rather unimpressive at first sight: it is only 25 km in size, irregularly shaped and quite bare. It also lacks an atmosphere. On the other hand, its low weight, mysterious trenches and signs of high instability are astounding.

Despite its characteristics, the small Phobos could become an important outpost in space for space travel. For its low gravity makes it easy to land, and start again regarding supplies, materials and space probes. It could thus serve as a stopover for the manned exploration of Mars.

Negatively charged

Now, however, William Farrell of the Goddard Space Flight Center of NASA and his colleagues have identified a potential threat to future missions: especially on the Phobos' night-side, the ground is likely to be electrically charged. The surface could build several hundred volts of negative voltage there, as well as in the shadowy interior of the large Stickney crater, as the researchers found.

Data from the NASA space probe MAVEN, which currently measures the atmosphere of the Mars orbit, but also the inflow of the solar wind, showed evidence of this. Combined with model simulations, the measurements showed that Phobos is always charged when it is on the solar side of Mars and is therefore exposed to the solar wind.

How solar winds can charge the night-side of Phobos © NASA/GSFC, CI Lab

Charge separation in the slipstream

The cause of the electric charge is the interaction of the solar wind with the topography of Phobos, as the researchers explain. Because of its small size and steep incline, the charged particles flow around Phobos, but on its back there is a charge separation: Close to the ground, an excess of negatively charged electrons forms, while the positively charged ions travel further outside the solar wind.

The reason for this is the different size and mass of the charged particles:

The electrons are like agile fighter jets - they can turn quickly and fly around an obstacle, says Farrell. The ions, on the other hand, are like big, heavy cargo aircraft, they can only change their flight direction slowly.

As a result, the negative charge predominates on the surface of the sun-facing side.

Energized spacesuits

Astronauts or robots could therefore accumulate significant amounts of electrical charge when they cross the Phobos' side of the night, says Farrell. At each step the astronaut takes more static charge. Although these charges are not strong enough to directly harm an astronaut, they are sufficient to affect sensitive instruments and equipment.

According to the researchers, an extreme astronaut could accumulate up to ten thousand volts as a static charge by the constant friction of his shoes on the Phobos regolith. This is especially true for teflon-based space suits and shoes, as for example the Apollo astronauts carried on the moon. As soon as the astronaut touches a conductive component or instrument, this charge jumps over. This creates a blow as we experience it after walking across a wool carpet touching a door latch.

Equipment should be adapted

We should therefore design spacesuits and equipment to minimize such triboelectric charging, the researchers advise.

This is mainly due to the fact that the electric charge of Phobos' surface during solar storms is much stronger than normally.

The charging effect was particularly strong after solar eruptions such as the coronary mass outbursts, says Farrell.

The researchers suspect that the second moon of Mars called Daimos could be similarly electrified - and possibly other smaller heavenly bodies in our cosmic neighborhood. This could therefore play a role, for example, when mining is operated on asteroids in the future.

Source: Advances in Space Research, 2017; doi: 10.1016 / j.asr.2017.08.009