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Phobos, the largest of Mars' two tiny satellites, photographed near the Mars limb by the Mars Express space robot in 2010. Source: G. Neukum (FU Berlin) and others, Mars Express, DLR, ESA; Acknowledgments: Peter Masek
The strange shapes and colors of the tiny Maroon moons Phobos and Deimos have inspired a long-standing debate about their origins.
The dark sides of the moons resemble the primitive asteroids of the outer solar system, suggesting that the 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.
A new review of 20-year-old Mars Global Surveyor data confirms the idea that Mars moons formed after a major impact on the planet have thrown a lot of rocks into orbit, according to a new study from Journal of Geophysical Research: Planets, a publication of the American Geophysical Union.
According to the authors of the study, the dataset contained unsupported clues to the composition of Phobos, which could be closer to the crust of the red planet.
"The fun part for me was to take a look at some of the existing ideas using an old dataset that was underutilized," said Tim Glotch, a geoscientist at Stony Brook University in New York City and 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, he said.
"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 definitive solution to the mystery of the moons, but it would help advance the discussion."
The new study compared the average infrared spectra of a whole chip of Tagish Lake meteorite (right) and earth (left) to the spectra collected at Phobos by the Mars Global Explorer probe in 1998. Credit: AGU
Dark objects
The debate over the origin of the moons of Mars has divided scientists for decades, since the beginnings of planetary 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 as they reflect in component colors with the help of 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, the Phobos and D class asteroids look very similar, that is, their two spectra are almost devoid of detail because they are too 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 have argued that 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 determine why some bodies orbit as they do, they say that, given the tilt and the details of Phobos' orbit, it It's almost impossible to capture it, 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.
Phobos sets on Mount Sharp, March, in a mosaic of three images captured by the Curiosity rover in 2014. Credit: NASA / JPL / MSSS / Justin Cowart CC-BY-3.0
Thermal energy, like visible light, can be divided into a spectrum of "colors". Even objects that appear black in visible light can shine in a distinctive 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 have found at these wavelengths that the Tagish Lake meteorite is not like Phobos at all, and in fact, what best corresponds to Phobos, or at least one of the characteristics of the spectrum, is the Ground basalt This is common volcanic rocks, and that's what makes most of the Martian crust, "said Glotch." This leads us to think that maybe Phobos could be a remnant of the earth's crust. 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 material from Mars, but the new findings are consistent with the moon containing part of the planet's crust, perhaps an amalgam of planet debris and remains of the object.
Fries, the scientist who did not participate in the new study, said the Tagish Lake meteorite was unusual and may not be the best example of a class D asteroid available for comparison convincing with Phobos. Fries added that the new study was unlikely to produce a definitive answer because Phobos is subject to alteration of space, which affects its reflectance spectrum and is difficult to reproduce in the laboratory.
But Fries said it was interesting that a mixture of basalt and carbon-rich materials matched Phobos perfectly. Another possibility is that carbon-rich space dusts in the vicinity of Mars have gathered on satellites close to orbit, darkening their surfaces, he added.
Scientists could have their answer to Phobos' origins over the next two years, if the Martian Moon eXploration spacecraft and the OSIRIS-Rex and Hayabusa2 asteroid 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 is a verifiable hypothesis, because the Japanese are developing a mission called MMX that will go to Phobos, collect a sample and bring it back to Earth for us to analyze." said Glotch.
Explore more:
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More information:
Timothy D. Glotch et al, the MGS-TES spectra suggest a basaltic component in the Phobos regolith, Journal of Geophysical Research: Planets (2018). DOI: 10.1029 / 2018JE005647
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