The moon of Mars Phobos could have formed from a giant impact

Where do the moons of Mars come from? It is difficult to answer this question with two competing theories: either the moons were captured or they came from Mars. Although the obvious answer remains elusive, new "old" evidence discovered from 20-year-old data indicates that the Mars moon, Phobos, may have formed as a result of an impact on the red planet.

The evidence, published on 24 September in the Journal of Geophysical Research: Planets, comes from the Thermal Emission Spectrometer (MGS-TES) made by Mars Global Surveyor in the infrared (heat) part of the spectrum, taken in 1998 as the spacecraft passed the little moon in orbit around the planet. While the spectrum of Phobos – the light that it reflects, scattered by wavelength to reveal features such as its composition – looks very much like that of a class D asteroid in visible light and even near infrared, in the middle infrared light (slightly longer wavelengths still), it does not look like an asteroid. Instead, his compositional signature shows basalt, a rock supposed to form the majority of Mars' earth crust.

Where is the match?

Compared to the planet itself, the moons of Mars appear dark in visible light. This discordance initially led planetary scientists to believe that satellites had been captured by the gravity of the planet in the past. (Unlike the Moon of the Earth, which is too big compared to the planet for being captured, the moons of Mars are tiny compared to the planet). But researchers studying orbital dynamics and retracing the movement of Phobos from its orbit exclude this possibility.

Now, to solve the mystery of the origin of the moon, scientists have turned to spectral analysis, which allows them to determine whether the composition of the moon corresponds more closely to that of the planet or the asteroid . Visible light observations indicate that Phobos contains carbon, like an asteroid. But what about longer wavelengths?

To determine the characteristics of Phobos in the mid-infrared, Tim Glotch of Stony Brook University in New York and lead author of the new study compared MGS-TES observations with mean infrared observations of the Tagish Lake meteorite and other rocks. some samples. The Tagish Lake meteorite is arguably the best preserved meteorite specimen in the world, and astronomers believe it is a class D asteroid, one of the possible origins of Phobos. To ensure a more accurate comparison, the Glotch team subjected their samples to conditions similar to those of the moon, including exposure to cold and heat in the vacuum.

"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. These are common volcanic rocks, and that's what makes most of the Martian crust, "Glotch said in a press release. "This leads us to think that perhaps Phobos could be a remnant of an impact that occurred at the beginning of Martian history."