Space and Physics

One Of Mars' Moons May Actually Be A Chunk Of The Red Planet


Dr. Alfredo Carpineti

Senior Staff Writer & Space Correspondent

clockSep 25 2018, 12:28 UTC

Phobos snapped with Mars in the background. G. Neukum (FU Berlin) et al./ Mars Express/ DLR/ ESA; Acknowledgement: Peter Masek

Mars has two moons called Phobos and Deimos, named after the Greek mythological personifications of dread and fear. They are odd moons. They are small and a bit misshapen. Their size, grayish color, and strange shape make them look like asteroids, so astronomers have assumed them to be exactly that, captured by Mars. Now, a new study provides fresh evidence against this idea.


Researchers have re-analyzed previous observations of Phobos and found evidence suggesting that its composition is very similar to the Martian crust. For this reason, they suggest that Phobos and Deimos are the results of a serious collision on the Red Planet that threw into orbit enough material to form the two moons. The results are published in the Journal of Geophysical Research: Planets.

The team investigated Phobos using a 20-year-old dataset. They produced a chemical fingerprint of the moon’s regolith, the layer of loose terrain on its surface. Data on Phobos have always matched up with observations of asteroids, but the team wanted to conduct a more detailed analysis. They compared the data with a meteorite that fell to Earth in the year 2000. It’s supposed to be a good representative of the asteroids that Phobos looks like and the fact it landed on our planet means we can get some detailed information about its composition. It turns out that Phobos and the meteorite are quite unlike each other.

“We found, at these wavelength ranges, the Tagish Lake meteorite doesn’t look anything like Phobos, and in fact what matches Phobos most closely, or at least one of the features in the spectrum, is ground-up basalt, which is a common volcanic rock, and it’s what most of the Martian crust is made out of,” lead author Tim Glotch, from Stony Brook University, said in a statement. “That leads us to believe that perhaps Phobos might be a remnant of an impact that occurred early on in Martian history.”

This is not the first time this idea has been proposed. Other clues suggest that the origin of the two moons is closer to home than thought. Phobos’ orbit, for example, has certain features such as its inclinations that cast doubt on the theory that it was captured.


“The issue of the origins of Phobos and Deimos is a fun sort of mystery, because we have two competing hypotheses that cannot both be true,” added Marc Fries, a planetary scientist from NASA’s Johnson Space Center who was not involved in the study. “I would not consider this to be a final solution to the mystery of the moons’ origin, but it will help keep the discussion moving forward.”

It might not be a final solution, but the captured scenario now seems even more unlikely thanks to this new work.

Space and Physics
  • asteroid,

  • Mars,

  • moons,

  • Phobos,

  • Deimos