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Mars Once Had A Third Moon Accompanying Phobos And Deimos

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Mars has two moons today - Phobos and Deimos - but new research suggests the Red Planet was once home to a much larger satellite in the distant past.

Astronomers believe the moon orbiting our own planet was formed when Earth was struck by a planet roughly the size of Mars. A new international team of researchers now theorize that Phobos and Deimos may have formed in a similar fashion, when Mars collided with another body roughly 1 billion years ago. Together with this pair of satellites, the cataclysmic event may have also formed a third, much larger, moon that coalesced in orbit around the planet.

Computer simulations of a collision between Mars and another body suggest that conditions that produce two smaller moons, similar to those seen at Phobos and Deimos, would have also created a third, massive satellite, as well as a ring of debris. Such a moon would have created tremendous tidal forces on Mars, which would have pulled the satellite out of orbit, sending it crashing to the surface of the Red Planet.

If this theory is true, than roughly half the material seen on the moons of Mars today would have come from Mars, together with material from the impactor. The Japanese space agency, JAXA, has recently announced plans to send a sample return mission to a Martian satellite.

"Phobos and Deimos are orbiting on the Martian equatorial plane with very circular orbits. They are very small satellites and their masses are less than 10-7 of Mars mass. Because of their irregular shapes and spectral features, many researchers have thought that they were captured objects by Mars coming from the asteroid belt," researchers wrote in a press release announcing their findings.

This tremendous moon would have been 1,000 times more massive than Phobos, and its gravity would have helped form the moons we see today.

The earlier theory would not explain the highly circular orbits, however, suggesting a different origin for the Martian moons. Satellites captured from the asteroid belt, now orbiting Jupiter, are in highly eliptical orbits, far different from those seen around Mars.

It is possible that a giant impact crater on the face of Mars known as Borealis is the remnant of this ancient collision that formed the moons of Mars. Computer simulations designed to mimic conditions that would produce this massive feature show such an impact would have sent vast amounts of debris into orbit over the planet. Such an event could, theoretically, have resulted in the formation of the moons we see today, as well as a more massive satellite.

Analysis of the formation of Phobos and Deimos was published in the journal Nature Geoscience.

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