The demise of the synchronous moon that gave Mars its triaxiality. The role of solar tides and a palaeo ocean

TL;DR

This paper explores how the ancient moon Nerio influenced Mars' shape, rotation, and tectonics, and investigates its orbital stability and eventual demise due to solar tides and the planet's ocean formation.

Contribution

It presents a new analysis of Nerio's orbital evolution, its impact on Mars' triaxiality, and the implications for Mars' rotational history and lunar stability.

Findings

Nerio's orbit was transiently stable but became unstable after the LHB.

Mars' rotation rate at Nerio’s desynchronization matches current observations.

Nerio likely perished during the LHB, affecting Mars' rotational evolution.

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 demise, and its impact on Mars' rotation. The synchronous orbit is stable transiently: solar tides adiabatically shrink it, accelerating Mars' rotation. This evolution proceeds gradually, so Mars' tidal bulge freezes. Following the LHB water delivery and ocean formation, solar tides intensify, making Nerio's synchronous orbit unstable. Nerio departs synchronism and spirals down, accelerating Mars' spin. Mars' angular velocity at the desynchronisation moment matches its present-day value to the first decimal place. This coincidence should not be overinterpreted, as post-desynchronisation evolution included Mars' continued spin-up during Nerio's descent (till Nerio's destruction amid the LHB), followed by Mars' despinning by solar tides. Nerio's reaching the Roche limit intact is questionable. Beyond LHB hazards, it would imply Mars' larger spin-up, necessitating k2/Q ~ 7.3 to allow subsequent despinning to the present-day rate. Such values may be high even for shallow oceans. Absent future evidence supporting such elevated k2/Q values, Nerio likely perished during the LHB. This viewpoint may be reconsidered should new data on Mars' palaeo ocean show up.