What causes the surface of Phobos to weather?


Enlarge pictureThe two Mars moons Phobos and Deimos.
(Bild: NASA/JPL-Caltech/University of Arizona)

November 30, 2020 – There is no weather in space in our sense, of course, but rocks can also weather in the vacuum of space if they are constantly bombarded by high-energy particles emitted by the sun, for example. The Martian moon Phobos is in a very special situation: It is so close to Mars that not only the solar wind but also the bombardment by particles from Mars plays a decisive role there. A research team from the Vienna University of Technology has now been able to measure this in laboratory experiments. In just a few years, a Japanese space mission is to take rock samples on Phobos and bring them back to Earth. Billions of years of particle bombardment
There are different theories as to how the Mars moon Phobos could have originated, says Paul Szabo, who is working on his dissertation in the research group of Prof. Friedrich Aumayr at the Institute for Applied Physics at Vienna University of Technology. It is conceivable that Phobos was originally an asteroid that was then captured by Mars, but it could also have been formed when a larger celestial body collided with Mars.

TU Vienna

Enlarge picturePaul Szabo
(Image: TU Vienna)

When examining such celestial bodies, one must always bear in mind that their surfaces have completely changed over billions of years due to cosmic particle bombardment. The rock on earth remains unaffected because our atmosphere shields the particles. But the geology of atmospheric celestial bodies, such as our moon or Phobos, can only be understood if it is possible to properly assess the weathering of space. Therefore, elaborate experiments were carried out at the TU Wien: We used rock material, as it also occurs on Phobos, and bombarded it with different charged particles in vacuum chambers, explains Paul Szabo. With extremely precise scales you can measure how much material is removed and which particles affect the rock and how much.

The special properties of the moon Phobos have to be taken into account: Its distance to the surface of Mars is less than 6000 km, which is not even two percent of the distance between our moon and the earth. Just like our moon, it is in a bound rotation around its planet: it always turns the same side towards Mars.

TU Vienna

Enlarge pictureThe research team in the laboratory
(Image: TU Vienna)

Due to the extremely small distance between Mars and Phobos, not only particles that are emitted by the sun play a role on the Phobos surface, but also particles from Mars, says Paul Szabo. The Martian atmosphere consists mainly of carbon dioxide. But there are also larger amounts of oxygen in the outer regions of the atmosphere. If particles of the solar wind penetrate there with great force, oxygen ions can be formed, which then hit Phobos at high speed and change the rock there.

Data for space mission 2024
With our measurement methods, we were able to estimate the erosion much more precisely than was previously possible, says Friedrich Aumayr. Our results show that the effect of the oxygen ions from the Martian atmosphere should not be neglected. It is also important to distinguish between the two sides of Phobos: While the solar wind dominates on the side facing away from Mars, on the other side, when the sun is shielded by Mars, the bombardment from the Mars atmosphere predominates.

These considerations could soon also play an important role in the evaluation of real Phobos samples: As early as 2024, as part of the Japanese space mission MMX (Martian Moon eXploration), a spacecraft should reach Phobos and bring rock samples back to Earth.

P. S. Szabo, et al., Experimental Insights into Space Weathering of Phobos: Laboratory Investigation of Sputtering by Atomic and Molecular Planetary Ions, Journal of Geophysical Research: Planets

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