Astronomers suggest whole new origin of Mars’ weird moons

25 Sep 2018

Phobos, the larger of Mars’ two tiny satellites, pictured near the limb of Mars by the robot spacecraft Mars Express in 2010. Image: G Neukum (FU Berlin) et al, Mars Express, DLR, ESA; Peter Masek

New research looking into the origin of Mars’ two weird moons suggests they are not cosmic visitors after all, but mostly homegrown.

Our understanding of our own moon shows likely evidence for its formation from the debris left from a massive impact between Earth and a Mars-like planet billions of years ago.

But when it came to the possible origin of two of Mars’ weirdest moons – Phobos and Deimos – it was believed that they were actually just asteroids captured by the planet’s gravity.

This was because its dark faces resemble those of primitive asteroids found in the outer solar system. After all, when observed in visible light, both moons appear noticeably darker than Mars. However, what didn’t make sense was the fact that the shapes and angles of their orbits didn’t fit in with asteroid theory.

Now, research published to the Journal of Geophysical Research: Planets may have found a way to explain a Martian origin story.

Taking a different approach, lead author of the study, Tim Glotch, and the rest of his team decided to observe the moons in mid-infrared within the same range as body temperature.

Totally different

Despite being incredibly cold, Phobos has a discernible heat signature at mid-infrared, and when compared with an meteorite sample that fell to Earth, called the Tagish Lake meteorite.

“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,” Glotch said. “That leads us to believe that perhaps Phobos might be a remnant of an impact that occurred early on in Martian history.”

Furthermore, the team did not say Phobos is made entirely of Martian material, suggesting it is a combination of the planet’s crust and debris from an impacting object.

“The really cool thing is that this is a testable hypothesis, because the Japanese are developing a mission called MMX that is going to go to Phobos, collect a sample and bring it back to Earth for us to analyse,” Glotch said.

Colm Gorey was a senior journalist with Silicon Republic