On the Moon and Mercury, the uppermost layer of rock is gradually being eroded by the impact of the sun’s particles carried in the solar winds.
An interesting study out of Tu Wein University in Vienna, Austria shows that the effects of solar wind bombardment are in some cases much more drastic than previously thought. The findings were published in the planetology journal Icarus.
The findings by the research team are important for the European Space Agency’s (ESA) BepiColombo mission, a joint venture with the Japan Aerospace Exploration Agency (JAXA).
BepiColombo is scheduled to launch in October 2018, with an arrival at Mercury planned for December 2025, after a flyby of Earth, two flybys of Venus, and six flybys of Mercury.
Rock particles in the exosphere
“The solar wind consists of charged particles—mainly hydrogen and helium ions, but heavier atoms up to iron also play a role,” explains Prof. Friedrich Aumayr from the Institute of Applied Physics at TU Wien.
The thing is – when these particles hit surface rocks at a speed of 400 to 800 kilometers per second (248 to 497 miles per second), the impact can eject numerous other atoms. The particles rise high before falling back to the surface, creating an “exosphere” around Mercury and the Moon.
This exosphere is an extremely thin atmosphere of atoms sputtered from the surface rocks by solar wind bombardment. The ESA’s BepiColombo mission to Mercury will obtain information about the geological and chemical properties of Mercury from the composition of the exosphere.
While the ESA mission may seem to be simple, a precise understanding of the effects of the solar wind on the rock surfaces is necessary, and this is where gaps in that knowledge exist. This is where Tu Wien scientists stepped in – investigating the effect of ion bombardment on wollastonite, a typical moon rock.
Wollastonite (CaSiO3) can contain small amounts of iron, magnesium, and manganese substituting for calcium. It is usually white. It is also found on Earth and is used for a variety of products, including ceramics, friction products (brakes and clutches), metal making, paint filler, and plastics.
Fixing the modeling
“Up to now it was assumed that the kinetic energy of the fast particles is primarily responsible for atomization of the rock surface,” says Paul Szabo, a Ph.D. student in Friedrich Aumayr’s team and first author of the current publication.
“But this is only half the truth: we were able to show that the high electrical charge of the particles plays a decisive role. It is the reason that the particles on the surface can do much more damage than previously thought.”
Szabo explains, saying that when solar wind particles are multiply charged, they end up lacking several electrons. They can then carry a large amount of energy which is released in a flash on impact.
“If this is not taken into account, the effects of the solar wind on various rocks are misjudged,” says Paul Szabo. “Therefore, it is not possible to draw exact conclusions about the surface rocks with an incorrect model from the composition of the exosphere.”
Because the solar wind is made up of a large number of protons, it has long been thought they had the strongest influence on the rock. However, the research team discovered that helium actually plays the main role. This is because, unlike protons, helium can be doubly charged positively.
And we must not forget the heavier ions, which have an even greater electrical charge, the researchers write. To reach their conclusions, the scientists required the cooperation of a number of institutions and high-tech instruments, like the specifically developed microbalance at the Institute of Applied Physics.
At the Vienna Scientific Cluster VSC-3, complex computer simulations with codes developed for nuclear fusion research were carried out in order to be able to interpret the results correctly. The Analytical Instrumentation Center and the Institute for Chemical Technologies and Analytics of the TU Vienna also made important contributions, reports the university.
