It is thanks to the earth’s magnetic field that life can exist here at all. If that were not the case, our home planet would have to expose itself to a continuing bombardment of high-energy particles from the sun – and that would have fatal consequences, as scientists from the Vienna University of Technology (TU) explain. The researchers show that the electrically charged particles hit the planet with greater force and have more drastic effects than expected.
“The solar wind consists of charged particles – mainly of hydrogen and helium ions, but also heavier atoms and iron play a role,” explains Friedrich Aumayr from the Institute of Applied Physics at the Vienna University of Technology. These particles strike the surface at a speed of 400 to 800 kilometers per second and can eject numerous other atoms. These particles can rise high before they fall back to the surface, creating an “exosphere” around the Moon or Mercury — an extremely thin atmosphere of atoms.
This exosphere is of great interest to space exploration because its composition suggests the chemical structure of the rock surface – and it is much easier to analyze the exosphere than landing a spacecraft on the surface. The European Space Agency (ESA) will send in October 2018 the probe BepiColombo to Mercury, which is to gain information from the composition of the exosphere on the geological and chemical properties of Mercury.
However, it is necessary to understand the effects of the solar wind on the rocky surfaces exactly – but this is precisely where there were previously crucial knowledge gaps. At the Vienna University of Technology, therefore, the impact of ion bombardment on wollastonite, a typical lunar rock, was investigated. “So far, it was assumed that primarily the kinetic energy of the fast particles is responsible for the atomic atomization of the rock surface,” says Paul Szabo, first author of the study in the journal “Icarus”. “But that’s only half the story: we’ve been able to show that the high electric charge of the particles plays a critical role, which is why the particles on the surface can do far more damage than previously thought.”
If the particles of the solar wind are charged several times, ie if they lack several electrons, then they carry a large amount of energy, which is released in a flash like a lightning strike. “If you do not take that into account, the effects of the solar wind on different rocks are considered wrong,” says Szabo. Therefore, with a wrong model of the composition of the exosphere, one can not draw any exact conclusions about the surface rock.
By far the largest share of the solar wind is formed by protons, and so far it has been thought that they have the greatest influence on rocks. But, as it turns out, helium plays the main role, because unlike the protons, it can be charged twice as positively. The contribution of heavier ions with even greater electrical charge must not be neglected.