The Origin of Martian Moons

As you may or may not be aware, Mars has two moons, Phobos and Deimos. They are not large, and are more potato-shaped than spherical. For a while, the going theory was that these moons were asteroids captured into orbit by the pull of Mars’ gravitational field. More recently, it was suggested that these moons formed the same way our Moon did: by the accretion of material from a giant impact that formed a ring around the planet. The material would attract other material due to the attraction between objects with mass, and would eventually build to form a body in orbit. Small bodies don’t have enough gravity within themselves to form a spherical body, thus the potato shape of Mars’ two moons.
Taken by Spirit, a now-silent Mars Exploration Rover, this image shows Deimos on the left and Phobos on the right. The dimmer dots above the two moons are stars in the constellation Sagittarius. Courtesy NASA/JPL/Cornell/ Texas A&M.
Taken by Spirit, a now-silent Mars Exploration Rover, this image shows Deimos on the left and Phobos on the right. The dimmer dots above the two moons are stars in the constellation Sagittarius. Courtesy NASA/JPL/Cornell/ Texas A&M.
The evidence of moons formed by re-accretion includes:
  • Phobos and Deimos are not made of materials usually found in asteroids, as discovered by the Mars Global Surveyor of the National Aeronautics and Space Administration (NASA) and the Mars Express mission of the European Space Agency (ESA)
  • Phobos contains some of the same minerals as those found on the surface of Mars
  • Mars Express found that Phobos has a density and porosity (measure of how easily liquids pass through) that is unusual for asteroids
  • Captured asteroids would likely have ovular orbits at random angles, but Phobos and Deimos both have circular orbits near the equator of Mars. While the orbit of Phobos could have been forced to be more circular due to the gravitational effects of Mars, Deimos is too far away from its host planet to really be affected by this.
All in all, this is a pretty good indication that Mars’ moons were created by the re-accretion of a disk of material created by a giant impact. The remaining question: which impact was it?
The largest impact scar on Mars is probably the Borealis Basin. While it is not entirely proven yet that was created by an impact, it is possible, and that would make it the largest impact crater in the solar system. It takes up most of Mars’ northern hemisphere.
That brings us to today’s scientific article, “Formation of Phobos and Deimos via a Giant Impact” by Robert I. Citron, Hidenori Genda, and Shigeru Ida and published in March of 2015 in the journal Icarus. These three authors set out to see that if the Borealis Basin was indeed caused by an impact, could that impact have thrown enough material into orbit around Mars to create the current moons?
The short answer is yes. They simulated an impact on the surface of Mars that fits the conditions needed to create the Borealis Basin (as found in other works). Changing the angle of impact made little difference in the amount of mass in the disk. They found that there was enough material orbiting Mars thirty hours after impact to account for Phobos, and Deimos, and other moons that have since ‘de-orbited’ and crashed into Mars. Phobos will eventually be one of those moons: is getting close and closer to Mars, and will eventually impact the surface or be torn apart by gravitational forces to create a ring. That will be in about fifty million years though, so I don’t suggest waiting around to see it.
The authors call for more research to be done. We still have a lot to figure out about the origins of the Borealis Basin, Phobos and Deimos. As many things are answered in science as there are new questions that arise. There’s always something more to learn, much to the benefit of scientists, those who are interested in new scientific discoveries, and the societies that gain from these discoveries.

 

Bibliography
Sources used for this writing.
Citron, Robert I., Hidenori Genda, and Shigeru Ida. “Formation of Phobos and Deimos via a Giant impact.” Icarus. 252 (2015): 334-338.
The main article for today.
National Aeronautics and Space Administration. “Mars: Moons” N.p., n.d. Web.
Information about Phobos and Deimos.
National Aeronautics and Space Administration. “NASA Spacecraft Reveal Largest Crater in Solar System.” N.p., 25 June 2008. Web.
Information about the Borealis Basin.
National Aeronautics and Space Administration, Jet Propulsion Laboratory, California Institute of Technology. “Two Moons Passing in the Night.” N.p., 9 September 2005. Web.
Press release images for the Spirit rover. This is where I took the image from. A captioned version of the image, and other images of Phobos and Deimos from the view of Spirit are also found on this page.
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