Human Exploration of Mars Faces New Challenge of Electrical Charge From Solar Wind on Phobos Moon

Mars, at left, and its moon Phobos, center. NASA/JPL-Caltech/GSFC/Univ. of Arizona

Mars' larger moon Phobos isn't nearly as glamorous as the red planet itself—but scientists had hoped the "potato shape" moon could ease the way to putting humans on Mars. According to a recent paper published in the journal Advances in Space Research, we may not be so lucky.

The paper's authors are scientists at the Dynamic Response of the Environments at Asteroids, the Moon, and moons of Mars, a group of scientists who are focused on evaluating the environment on a range of potential targets within the context of potential human exploration. And in the new article, they lay out their fears about Phobos.

One of the big challenges scientists have yet to tackle for a potential human voyage to Mars itself is how to gently land soft, squishy astronauts on its surface. The planet's gravity pulls everything in so strongly toward it that landing is more like crash-landing, and engineers are still working on the braking technology to keep humans safe. Even robots are still a huge risk: NASA referred to the 2012 landing of its Curiosity probe as "Seven Minutes of Terror."

Phobos doesn't have the same problem, since it's so much smaller than Mars, only about 14 miles wide. So space experts have floated the idea of landing the people on Phobos first and having them control equipment on Mars from there. (Coming home would also be easier for the same reason—less gravity means less fuel needed to leave.) Unfortunately, the new research suggests that might be a bad idea.

In the new paper, the team modeled the effects of solar wind on the moon. Solar wind is a flood of the charged particles that make up the outer layer of the Sun escaping from the star at speeds of almost 900,000 miles an hour. Here on Earth, we don't feel the impact of solar wind because the magnetic shield surrounding the planet absorbs it.

Phobos has no such protection, and the scientists calculated that the moon is subject to so much solar wind, it can actually give the surface and objects—or humans—on it an electric charge. Mars' second moon likely has exactly the same problem.

Here's how it works: Whatever side of Phobos is facing the sun is pummeled by the charged particles called plasma that make up solar wind, giving the "day side" a slight electrical charge. But in dark places on the moon, it creates a "plasma void," which also have a slight electrical charge. On the day side, that charge can dissipate, but on the dark side it's trapped.

And almost like an extraterrestrial static electricity, just the action of astronauts walking around the surface of Phobos could build up a charge that would eventually give the astronauts a small shock. The effect would occur across Phobos' dark side and in other places not exposed to direct sunlight, like deep craters.

Worst of all, it would be stronger when the Sun has one of its stellar belches that sends streamers of plasma shooting off its surface. Between the inconvenience to astronauts and the risk to technology, it might be enough to permanently scare explorers off Phobos.