Tidal Demise: The Evolution and Fate of a Hypothetical Venus Moon

Abstract Venus possesses no natural satellite, raising the question of whether a formed moon could have survived. We explore the tidal evolution of a Venus-moon system, coupling Venus’s spin to the satellite’s orbit under tides from the moon and Sun. We survey spin period ( P 0 = 5–100 hr), moon mass ( M m = 0.01–10 M Moon ), eccentricity, quality factor, and initial semimajor axis under both constant- Q and constant time lag models. Survival depends on competition between outward migration (∝ M m ) and synchronous radius expansion ( ∝ M m 2 ): for circular orbits around a rapidly spinning Venus ( P 0 ≲ 12 hr), a lunar-mass satellite survives the age of the solar system in both models. For P 0 ≲ 10 hr, eccentricity pumping can destabilize low-mass satellites, while for P 0 ≳ 15 hr or M m ≳ 2 M Moon , the synchronous radius overtakes the orbit and drives Roche destruction within ∼0.03–1.7 Gyr in the constant- Q model. The constant time lag model instead permits quasi-synchronous survival for massive moons at fast spin. Explaining Venus’s present state requires satisfying two constraints simultaneously: loss of the satellite and despinning of an initially rapid rotator. Both are met only within a restricted region of parameter space, favoring moderate postimpact spin periods and lunar-to-super-lunar masses. Giant impact simulations predict spin periods ≳12 hr for Venus’s present rotation, placing a lunar-mass satellite at the survival boundary. For last-impact conditions within this region, the present absence of a Venusian satellite arises through tidal evolution alone; a subsequent catastrophic stripping event, while capable of removing a moon, is not required.