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An astronomer at the U.S. Naval Observatory has developed a hypothesis to explain the unique shape and terrain of Mars. He suggests both are due to a long-lost moon. Michael Efroimsky has published a outlining the hypothesis on the arXiv preprint server.

Prior research has shown that Mars, unlike the other planets in the solar system, has a triaxial shape, which means it has size differences along three axes. It also has impressive geography, with both the tallest mountains in the solar system and the biggest canyon. It also has a unique highland, called the Tharsis bulge because it bulges oddly out from the surface.

The bulge, which has an unknown origin, is approximately 5,000 kilometers across, situated near the planet's equator. The planet has another highland, also near the equator, directly across from the Tharsis bulge.

In this new study, Efroimsky suggests that a long-lost moon, which he calls Nerio, could explain all of Mars's unique characteristics.

He suggests that if Nerio were large enough, perhaps a third the size of our moon, it would have exerted an enormous pull on the surface. And during the early days of the planet when the surface was still soft, it would have pulled surface material upward due to a synchronous orbit. When the planet cooled, the material remained in place as a permanent bulge.

The creation of the bulge would have added to equatorial ellipticity, resulting in the planet's triaxial shape. It also would have made the planet more prone to volcanic activity, leading to the development of highlands on the opposite side of the Tharsis bulge.

Addressing the fate of Nerio, Efroimsky suggests it could have simply wandered away due to the pull of some other body, or more likely, it might have been hit and destroyed by another large body, leaving behind nothing but the two tiny moons Phobos and Deimos. Efroimsky concludes that more research is required to confirm this hypothesis.