Nerio: Mars May Be Missing A Third, Far Larger Moon

New research has suggested that Mars had – and then lost – a third, far larger moon. If correct and supported by further evidence, it could explain a number of unusual features on the planet's surface.

Mars, rubbing it in the face of moonless Venus and Mercury , has two moons to its name. The moons – Phobos and Deimos – are a little different from the other moons of the Solar System. They are tiny compared to their host planet, are covered in craters, and are surprisingly low-density.

There are a few theories about how these moons formed, though there are a few problems with both. According to one idea, the moons were created when a large object collided with Mars, creating a debris disk around Mars that slowly coalesced into the objects. Another suggests that the moons are captured D-type asteroids, usually found in the outer asteroid belt, or with Jupiter's trojan asteroids . A third and more recent hypothesis is that Phobos could be a captured comet .

Mars itself has some unusual features that require some explaining. For a start, Mars is triaxial , or it is an asymmetrical spheroid with three distinct axes.

"Mars’ triaxiality makes itself most evident through the equatorial ellipticity produced by the Tharsis Rise and by a less prominent elevation located almost diametrically opposite to Tharsis and constituted by Syrtis Major Planum and an adjacent part of Terra Sabaea," Michael Efroimsky of the US Naval Observatory explains in a new paper, which has been submitted to the Journal of Geophysical Research: Planets but has not yet been peer-reviewed.

Tharsis Rise is the largest highland province on Mars or, in fact, the Solar System. But even if this region is discounted, as it was in a separate study looking at Mars's gravitational field, Mars remains apparently triaxial.

According to Efroimsky, these features could be explained by a third moon, lost by the planet long ago.

"An initial, ‘seed’ triaxiality was created by a massive moon orbiting a young and still plastic Mars on a synchronous orbit. Showing the same face to the moon, Mars assumed a shape close but not identical to a triaxial ellipsoid, its longest axis aligned with the moon," Efroimsky proposes.

"After the moon produced the seed triaxiality and asymmetry of Mars, the tidally elevated provinces became, more than others, prone to convection-generated uplifts and tectonic and volcanic activity. These processes began to gradually add to the equatorial ellipticity. Owing to the degree-3 initial asymmetry of the shape, they were not acting in a symmetrical way; hence the resulting height disparity between Tharsis and Syrtis Major."

He suggests that a moon less than a third of Mars's, created during early formation of the Solar System or captured slightly later on, could have shaped the planet before it was fully solidified. The hypothetical moon, which they named Nerio after the ancient Roman war goddess, is now gone, if it existed in the first place. Efroimsky suggests that Nerio could have been destroyed in a collision –  though so far no telltale craters along Mars's equator have shown signs of such an event –  or sent out of orbit during the tumultuous early days of the Solar System by another massive body.

While an intriguing idea and a possible avenue for further investigation, we will need to observe Mars and its confirmed moons a lot further before Nerio could be confirmed or ruled out.

The study has been submitted to the Journal of Geophysical Research: Planets and is available as a pre-print on arXiv .