We are all familiar with our one Moon but other planets have different numbers of moons; Mercury has none, Jupiter has 95 and Mars has two. A new paper proposes that Mars may actually have had a third larger moon. Why? The red planet has a triaxial shape which means it bulges just like Earth does but along a third axis. The paper suggests a massive moon could have distorted Mars into this shape. 

Celestial bodies that orbit planets or dwarf planets are known as moons. They vary significantly in size from just a few kilometres to several thousand kilometres. Earth’s Moon (notice capital ‘M’) is the moon everyone is familiar with but there are many fascinating moons in the outer Solar System from the largest moon Ganymede to the icy ocean world Europa or Titan with its methane lakes. Even Mars has two moons; Phobos and Deimos.

In a paper published by Michael Efroimsky from the US Naval Observatory in Washington the shape of Mars is explored with a view to assessing the liklihood of a third moon of Mars. Efroimsky explains that the triaxial nature of Mars is noticeable through the equatorial ellipticity which is produced by the Tharsis Rise. Another less noticeable bulge is located almost opposite to the Tharsis rise and is in the Syrtis Major Planum region.

The paper proposes the peculiar bulge shape of Mars has been caused by two different elements. The initial shape was caused by a massive moon in orbit around the young and pliable Mars. It was in a synchronous or captured orbit so the same face of Mars was always pointing toward the moon. Under the constant tug of gravity, a triaxial ellipsoid shape evolved. A triaxial ellipsoid is shaped like a rugby ball but the three axes are of different lengths. The longest axis was aligned to the Moon while the others were forged by other tidal effects. 

The second element of the development of the shape of Mars relates to the convection processes under its surface. After the triaxial ellipsoid shape developed, the tidally raised regions became more prone to uplift driven by convection, tectonic and volcanic activity. The activity slowly enhanced the triaxial ellipticity seen today. 

Efroimsky demonstrates that a moon of less than a third of the mass of the Moon, in a synchronous orbit around Mars was capable of creating the initial triaxiality (this is my new favourite word!) The research also put showed that the asymmetry of the equator was significant if the synchronous moon existed while Mars still have magma oceans, and was weaker if the moon showed up at the solidification stage.

In order for the second element to be evidenced, further research is required. However Efroimsky believes the tidal deformations could very easily oscillate and generate heat. A moon in an elliptical but synchronous orbit would appear to oscillate east/west around the same region of sky. This would enhance the tidal deformation and internal heating of the system giving credence to Efroimsky’s theory that Mars did indeed once have a third larger moon. 

Source : A synchronous moon as a possible cause of Mars’ initial triaxiality