Michael Efroimsky
Mars' asymmetric figure, with two opposing equatorial elevations, originated from a frozen tidal bulge raised by a primordial synchronous moon Nerio. Nerio's emergence, through in situ accretion or by capture in the disk's remnants, and its synchronisation with Mars' rotation preceded or was coeval with crust formation. The sub-moon and anti-moon regions hypothetically developed thinner crusts, intensifying the tectonics that further amplified Mars' triaxiality. We investigated Nerio's orbit stability and demise, and its impact on Mars' rotation. Nerio may also have been the cause of the tidal rhythmites discovered in Martian sediments . We derive a relation governing the orbit evolution of a planet-moon pair under the influence of stellar tides in the planet. These tides gradually shrink the orbit of the planet--moon pair, so the synchronous orbit is stable only transiently. On reaching a critical separation from the planet, the moon departs synchronism and spirals down, accelerating the planet's spin. The application of this development to Mars and Nerio shows that the rotation rate of Mars at the desynchronisation moment matches the present-day rate to the first decimal place. This coincidence should not be overinterpreted, as post-desynchronisation evolution included Mars' continued spin-up during Nerio's descent, until Nerio's destruction amid the Late Heavy Bombardment (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 k_2/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 k_2/Q values, Nerio more likely perished during the LHB than ever reached the Roche limit. This viewpoint may be reconsidered should new data on Mars' palaeo-ocean show up. The existence and subsequent demise of Nerio are indirectly confirmed by the presence of rhythmites in Vastitas Borealis and Gale Crater. In the course of its post-desynchronisation descent, Nerio was capable of generating tidalites within the palaeo-ocean. After Nerio's demise, this generation may have been continued by its massive remnant(s).