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There is no time in space in our sense of the word – however, the ground can also “hold” in the vacuum of space if it is constantly bombarded with high-energy particles, such as those emitted by the sun.
The Martian moon Phobos is affected by a particular situation: it is so close to Mars that not only the solar wind but also the irradiation of particles from Mars plays a decisive role there. A research group from TU Wien has now been able to measure it in laboratory experiments. In a few years, a Japanese space mission will take soil samples from Phobos and return them to Earth.
Billions of years of particle irradiation
“There are several theories as to how the Mars moon Phobos might have formed,” says Paul Szabo, who is working on his doctoral thesis in the research group of Prof. Friedrich Aumayr at the Institute of Applied Physics of TU Wien. “It is possible that Phobos was originally an asteroid that was later captured by Mars, but it could also have been created by a collision of Mars with another large object.”
When studying these celestial bodies, it must always be borne in mind that their surfaces have been completely changed over the course of billions of years by the bombardment of cosmic particles. The surface of the Earth remains unaffected, because our atmosphere protects the particles. However, the geology of celestial bodies without an atmosphere, such as our Moon or Phobos, can only be understood if “spatial alteration” can be correctly assessed.
Therefore, elaborate experiments were conducted at TU Wien: “We used a mineral as found on Phobos and bombarded it in vacuum chambers with different charged particles,” explains Paul Szabo. “Using a highly accurate balance, we can measure how much material is removed in the process and how much each particle affects the surface.
The special properties of the moon Phobos must be taken into account: its distance from the surface of Mars is less than 6000 km, not even two percent of the distance between our Moon and the Earth. Just like our Moon, it is in a tidal locked rotation around its planet: the same side is always facing Mars.
“Due to the extremely short distance between Mars and Phobos, not only the particles emitted by the Sun play a role on the surface of Phobos, but also the particles of Mars,” says Paul Szabo. The Martian atmosphere is made up mostly of carbon dioxide. But there are also greater amounts of oxygen in the outer regions of the atmosphere. When solar wind particles penetrate there, oxygen ions can be created, which then hit Phobos at high speed and change the surface material.
Data for the 2024 space mission
“With our measurement methods we were able to estimate the Phobos erosion much more accurately than was previously possible,” says Friedrich Aumayr. “Our results show that the effect of oxygen ions from the Martian atmosphere cannot be overlooked. It is also important to distinguish between the two sides of Phobos: while the solar wind causes the weather on the opposite side of Mars, the bombardment from the The Martian atmosphere dominates on the other side, when the Sun is protected by Mars.
These considerations could soon play an important role in the evaluation of real Phobos samples: as early as 2024, a spacecraft should reach Phobos as part of the Japanese MMX (Martian Moon eXploration) space mission and bring soil samples back to Earth.
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