Obliquity Evolution of an Early Venus

Present-day Venus is a slow rotator whose spin is controlled by solid-body and atmospheric thermal tides. However, conditions may have been far different 4 Gyr ago, when the Sun was fainter and most of the carbon within Venus may have been in solid form, implying a low-mass atmosphere. We investigate how a obliquity would have evolved on a hypothetical rapidly-rotating Early Venus. As with our previous investigation of a Moonless Earth, prograde rotation of our hypothesized Early Venus generally leads to larger variations in obliquity than does retrograde rotation. But this trend is far weaker for Venus than for a Moonless Earth, and doesn't hold for all initial obliquities and spin periods. The possible mechanisms for these variations and implications for early habitability will also be discussed.