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It may be that the moons of Mars do not seem to have been made with the same materials as the planet. But they are, a new study postulates, based on a new analysis of old data.
Phobos and Deimos, strangely dark and deformed moon moons, were created by capture of asteroids at the passage of gravity. This is not the case, says a new hypothesis, in contradiction with accepted wisdom: they were formed as a result of a catastrophic collision.
At the beginning of the formation of the solar system, a celestial object passing over Mars and lifting pieces of volcanic crust in space, suggest scientists from the American Geophysical Union, in an article published Monday in the prestigious Journal of Geophysical Research- Planets.
One of the reasons why science did not think that Phobos and Deimos were growing out of nothing, like the main planets, is that this spherical shape is not so familiar to other planets and moons (like ours). But science did not think that the two moonlets had formed from the same rock as the mother planet: in visible light, Phobos and Deimos seem much darker than Mars. Hence the assumption that they were captured asteroids. Mark this key phrase: in visible light.
Someone is wrong
Although they were named for the twin sons of Ares, the Roman god of war – Deimos means theft and Phobos means scared – neither one nor the other seems terribly impressive. . Phobos looks nothing like a potato shot with a BB gun (creating the craters). Discovered for the first time in 1877, once scientists saw the thing, they decided that it had to act as a captured asteroid.
The same opinion was expressed about Deimos, who also looks like a pocket spud, but with a missing bite.
Both are tiny, according to celestial standards. The diameter of the Earth is about 12,750 kilometers. Mars is nearly 6,800 kilometers. Phobos has a little more than 11 kilometers in diameter and Deimos 12 kilometers.
But the orbital momentum of the moons does not conform to the capture scenario, neither in shape nor in angles.
"If you talk to people who are really good at orbital dynamics and figure out why some bodies orbit like they do, they say that given the inclination and the details of Phobos' orbit, it's almost impossible that he is captured. So the spectroscopists say one thing and the dynamics say something else, "says lead author Timothy Glotch, a geoscientist at Stony Brook University.
"We have two competing hypotheses that can not both be true," said NASA's Johnson Space Center planetarium and space conservationist Marc Fries, who had not been involved in the work. "I would not consider this a final solution to the mystery of the origin of the moons, but it would help advance the discussion."
Dark as the day
Some asteroids ("class D") are almost black because they contain a lot of carbon, like coal. The quality of the two ebony moons suggested that they were simply captive asteroids. In visible light and near infrared, the Phobos and D class asteroids are virtually devoid of features – no spectrum as they are too dark.
Mid-infrared is another story. In 1998, the thermal signature of Phobos was measured by Mars Global Surveyor. Now, Gloch and the students have compared samples of Phobos heat signals from a meteorite found in Tagish Lake, Canada, which some scientists believe to be a class D asteroid chunk.
The bottom line was that the Phobos heat signal was nothing like the Tagish meteorite, but much like the ground basalt signal. And that's what the Martian crust is made of, mainly.
AGU
"This leads us to think that maybe Phobos could be a remnant of an impact that occurred at the beginning of Martian history," Glotch said. Ta da!
Or maybe Phobos and Deimos were made from a Martian crust sunk into space by a violent impact that collided with celestial objects – and the rest is the story Martian.
A warning, courtesy of Fries again: The Tagish Lake meteorite is unusual, so may not be a big object of comparison. In addition, who knows how the existence of billions of years in space has affected the spectrum of reflection of Phobos. The moonlet could have captured the cosmic equivalent of coal dust.
Thus, are Deimos and Phobos at the origin of violent celestial accidents that pushed the basaltic Martian crust into space? We should all know soon thanks to the probes sent to asteroids. Glotch points out that Japan is developing a mission called MMX, aimed at collecting samples from Phobos, no less.
Why is it useful to know the origin of the moons of Mars? This is not But it's nice to know anyway.
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