Mars may look like a sphere, but it’s actually a triaxial ellipsoid. Unlike any other rocky planet in the Solar System, it’s shaped rather like a rugby ball, but with different sizes along all three axes.
This is most evident as a prominent bulge in elevation in the Tharsis Rise region and in an almost diametrically opposite region called Syrtis Major.
Astronomer Dr Michael Efroimsky of the US Naval Observatory recently proposed that this weird shape may be explained by a missing, ancient moon of Mars.
This moon, named Nerio – after a Roman war goddess who was Mars’s partner – altered the shape of Mars with tidal forces, much as Earth’s Moon does with this planet’s oceans.
But, as Mars cooled, its distorted shape was eventually permanently frozen in place.
The stress of Nerio’s tidal forces also made the raised regions of Mars weaker and more prone to warping geological forces, such as internal convection, tectonic shifts and volcanic eruptions. These all contributed to Mars’s asymmetry.
Researchers suggest that a moon of less than a third of the mass of our own, in a synchronous orbit around Mars, could easily create the planet’s initial triaxial shape. The equatorial bulge would have been even bigger if Nerio had existed while Mars still had magma oceans.
Of course, Mars no longer has such a moon, just tiny Deimos and Phobos. So, at some point, Nerio was either destroyed or gravitationally removed from the Martian system by another large body.
This article is an answer to the question (asked by Otto Sykes, via email) 'Why does Mars have such a weird shape?'
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