On May 21st, 2015 we learned about

Are Martian moons the product of capture or collision?

Mars currently has two moons, Phobos and Deimos. They are both small satellites at no more than 14 miles in diameter, and as such were some of the last moons discovered in our solar system. They also keep a tight orbit around the Red Planet, most likely spiraling in for an eventual impact in around 40 million years. Beyond that, the two moons are the source of many questions, most notably about their their composition, their origins, and their missing siblings.


There are two theories for where Phobos and Deimos came from. The predominant theory was that they were both asteroids that had drifted away from the neighboring asteroid belt and been captured by Mar’s gravity. Supporters of this theory point to the moons’ dark color and apparent density as evidence they arrived near Mars fully formed.

Martian offspring

The competing theory is that these two moons were formed much like our own. A large object hit the planet, kicking up enormous quantities of debris that then formed a dense ring around Mars. That ring then congealed into the two moons in question, as well as a possible sibling, estimated to have been around 186 miles across. Alternatively, instead of a larger sibling, a group of sibling moons may have been formed, but in both versions, these missing moons have since slowed and crashed back into Mars itself. On top of all that, the initial impact kicking all this off is thought to have increased the rotational speed of the planet, getting a Martian day down to the current 24 hours and 37 minutes.

Supporters of this theory recently got a boost from independent simulations done at the Southwest Research Institute in Boulder, Colorado, and the University of California, Berkeley. Key indicators include the small, circular orbits of the two remaining moons, rather than longer, elliptical orbits you’d expect from an object being captured on a fly-by. Giant impact craters on Mars may also be the relic of the initial impact that would have started this Martian family of satellites.

All of this may be settled if we can successfully get a physical sample of one of the moons to see what it is composed of. Water would have been vaporized in an impact, so large amounts of ice inside Phobos or Deimos would indicate that it hadn’t been born from Mars’ surface.

My kindergartner asked: How could we get samples of the moons’ back to Earth safely? Wouldn’t they be wrecked if they fell into the ocean (as with recovered booster rockets)? It would be very difficult to land a rover on Deimos or Phobos and then have it propel itself back to Earth. Fortunately, that’s not necessary. As with the Curiosity rover, any lander would basically be a small “lab” where samples could be heated and released gasses analyzed right on the spot. Only the resulting data would need to be transmitted back to Earth.


Source: Are Mars's moons homegrown—or snatched from the asteroid belt? by Ken Croswell, Science

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