Mars used to have a massive 'ghost moon' that permanently deformed the planet's shape before being violently destroyed.
April 14, 2026
Original Paper
The demise of the synchronous moon that gave Mars its triaxiality. The role of solar tides and a palaeo ocean
arXiv · 2604.10781
The Takeaway
The paper proposes that a lost moon named Nerio created a tidal bulge in Mars' ancient oceans that eventually froze into the planet's current lumpy shape. This provides a single historical event to explain Mars' lopsided geology and its mysterious orbital history.
From the abstract
Mars' asymmetric figure -- with two opposing equatorial elevations -- stemmed from a frozen tidal bulge raised by a primordial synchronous moon Nerio. Nerio's emergence, through in situ formation or by capture in the disk's remnants, and its synchronisation with Mars' rotation preceded or coincided with crust formation. The submoon and antimoon regions hypothetically developed thinner crusts, intensifying tectonics that amplified Mars' triaxiality. We investigate Nerio's orbit stability and demi