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The strange shapes and colors of the tiny Maroon moons Phobos and Deimos have inspired a long-standing debate about their origins.
The dark faces of the moons resemble the primitive asteroids of the outer solar system, suggesting that moons could be asteroids captured long ago in the gravitational pull of Mars. But the shapes and the angles of the orbits of the moons do not correspond to this scenario of capture.
According to a new study conducted as part of a Journal of Geophysical Research: Planets, a publication of the American Geophysical Union.
According to the authors of the study, the dataset contained unchanging clues about the composition of Phobos, which could be closer to the crust of the red planet than it appears.
"The most fun part for me was to take a look at some of the existing ideas using an old data set that was underutilized," said Tim Glotch, a geoscientist at Stony Brook University in New York City. lead author of the new study.
Marc Fries, a planet scientist and conservator of cosmic dust at NASA's Johnson Space Center, who was not involved in the new study, said that the inability to explain the genesis of two moons around A neighboring planet formation. Clarification will help to interpret how other moons and planets have formed in our solar system and beyond. The new study does not reach the mystery, but it's a step in the right direction.
"The question of the origins of Phobos and Deimos is a kind of fun mystery because we have two competing hypotheses that can not both be true," said Fries. "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 objects
The debate over the origin of the moons of Mars has been dividing scientists for decades, since the beginnings of global science. In visible light, Phobos and Deimos seem much darker than Mars, giving weight to the assumption of adoption.
Scientists study the mineral composition of objects by breaking the light that they reflect into components with a spectrophotometer, thus creating distinctive visual 'fingerprints'. By comparing the spectral imprints of planetary surfaces with a library of spectra for known materials, they can deduce the composition of these distant objects.
Most research on the composition of asteroids has examined their spectrum in visible light and near-infrared light, far exceeding the human view of the red side of the visible spectrum.
In visible light and near infrared, Phobos as well as Class D asteroids look very similar: their two spectra are almost devoid of detail because they are very dark. Class D asteroids are almost black like coal because, like coal, they contain carbon. This dark aspect of Phobos led to the hypothesis that the moon is a captive asteroid that flew a little too close to Mars.
But scientists observing the orbits of the moons of Mars said they could not have been captured. These scientists believe that the moons must have formed at the same time as Mars or be the result of massive impact on the planet during its millennia of training.
"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, "said Glotch.
Heat footprints
Glotch decided to look at the problem from a different angle: the middle infrared, which is in the same range as the body temperature. He examined the thermal signature of Phobos captured in 1998 by an instrument he described as a fancy thermometer worn by the Mars Global Surveyor. The robotic spaceship spent most of its life watching Mars, but glanced at Phobos as he passed the moon before settling into an orbit closer to the planet.
Thermal energy, like visible light, can be divided into a spectrum of "colors". Even objects that appear black in visible light can emit a distinct infrared spectrum. Although Phobos is very cold, its thermal spectrum has a discernible signature.
Glotch and his students compared the average infrared spectra of Phobos glimpsed by Mars Global Explorer to samples of a meteorite that fell on Earth near Tagish Lake, British Columbia, which some scientists have suggested as a class D asteroid fragment. other types of rocks.
In the laboratory, they subjected their samples to vacuum conditions in the form of Phobos, heating them up and down to simulate extreme temperature changes from sunny sides to shaded sides of airless objects in the space .
"We found at these wavelengths that the Tagish Lake meteorite is not like Phobos at all, and in fact, what best fits Phobos, or at least one of the characteristics of the spectrum, is the ground basalt. common volcanic rock, and that's what most of the Martian crust is made of, "Glotch said. "This leads us to think that perhaps Phobos could be a remnant of an impact that occurred at the beginning of Martian history."
Is the planetary crust cooked?
The new study does not argue that Phobos is entirely made up of materials from Mars, but the new findings are consistent with the moon containing part of the planet's crust, perhaps as an amalgam of debris from the planet and remains of the object.
Fries said the Tagish Lake meteorite is unusual, and may not be the best example of a class D asteroid available for a compelling comparison with Phobos. It is unlikely that the new study will produce a definitive answer because Phobos is subject to alteration of space, which affects its reflection spectrum and is difficult to reproduce in the laboratory. Fries said that it was interesting that a mixture of basalt and carbon-rich materials matched Phobos perfectly. Another possibility is that the carbon-rich space dust in the vicinity of Mars has gathered on satellites in close orbit, darkening their surfaces.
Scientists could have their answer to Phobos' origins over the next two years, if the Martian Moon eXploration spacecraft and OSIRIS-Rex and Hayabusa2 osteoid explorers complete their missions to collect samples and return them to Earth for analysis. Hyabusa2 landed two mini robots on the asteroid known as Ryugu on September 21st.
"What's really great is that this assumption is verifiable, because the Japanese are developing a mission called MMX that will go to Phobos, collect a sample and bring it back to Earth," Glotch said.
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