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Mars with moon Deimos
Photo: Leemage / imago images
The moons Phobos and Deimos on Mars are a mystery to astronomers.
How exactly they came about is still unclear.
Now researchers from Switzerland and the USA are presenting a new explanation: According to this, they originally formed a single, large moon that broke apart one to three billion years ago.
For the first time, the research team evaluated data from the Mars mission “Insight” for the analysis, they report in the journal “Nature Astronomy”.
The “Insight” probe successfully landed on Mars in November 2018.
Also on board is the »HP3« measuring device, which was developed in Germany and can dig into the ground and is therefore also known as the Mars mole.
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Moon Phobos moves in front of the sun.
From the perspective, the moon looks huge compared to the sun, in reality it only measures a few kilometers
Photo: JPL-Caltech / MSSS / NASA
"The origin of the Martian moons Phobos and Deimos is still unclear," report Amirhossein Bagheiri from ETH Zurich and his colleagues.
The two small celestial bodies are irregularly shaped and have a maximum extension of 27 and 16 kilometers - so they are very small compared to the earthly moon.
The earth's moon is more than 3,400 kilometers in diameter.
So far, planetary researchers have assumed that the moons of Mars consisted of several asteroids that the red planet had captured due to its gravity and that gradually formed the moons.
The new study contradicts the theory.
In addition, the current calculations show that the moon Phobos will break in about 39 million years and form a ring of debris around the red planet.
Last common route billions of years ago
"Their shape and their crater-strewn surface point to an origin in the asteroid belt," the researchers continued about the moons of Mars.
Although it is possible in principle to capture asteroids, the two moons orbit Mars on almost circular orbits in the equatorial plane - and that is difficult to explain in such a scenario.
An alternative explanation would be that the moons were formed in the early stages of the solar system, like those of the earth.
The Earth's moon was probably formed billions of years ago when a Mars-sized celestial body hit the earth.
However, the moons of Mars would then have to be much larger.
Bagheri and his colleagues have now used computer models to calculate the orbits of Phobos and Deimos back in time - and for the first time have taken into account detailed information about the internal structure of the planet that was provided by the "InSight" probe.
Because the exact distribution of matter inside Mars influences its gravitational field and thus the orbital movement of its satellites.
Moon will fall on Mars
The researchers' calculations show that the orbits of Phobos and Deimos - depending on the exact assumptions about the composition of the moons - coincided a billion to 2.7 billion years ago.
At that time, the researchers conclude, a larger moon on the Red Planet broke, and Phobos and Deimos are remaining debris from that catastrophe.
This explains both the small size of today's moons and their age: the original, larger moon could have formed much earlier together with Mars.
"Many other fragments could have fallen on Mars and contributed to the craters visible there today," the researchers speculate.
While Deimos is slowly moving away from Mars, Phobos faces a different fate: In about 39 million years the moon will reach the "Roche Limit", beyond which the tidal forces of Mars are stronger than the internal cohesion of Phobos.
Then the moon disintegrates, forming a ring of debris that eventually falls onto Mars.
Icon: The mirror
koe / dpa