A RETROGRADE CAPTURE MODEL FOR EXPLAINING SOME OF THE ENVIRONMENTAL CONDITIONS OF OUR "SISTER" PLANET VENUS

Singer (1970, Science, 170:1196-1198) proposed that the environmental conditions of Venus might be explained via a retrograde gravitational capture model. He proposed that the energy for capture could be dissipated in the planet but he did not demonstrate the feasibility of that energy-dissipation mechanism. McCord (1968, JGR, 73:1497-1500) and Counselman (1973, Astrophys. J., 180:307-314) also considered the generalities of both prograde and retrograde capture models but did not speculate on the mechanism(s) of capture. Malcuit et al. (1989, Proc. 19^th LPSC:581-591) were the first to demonstrate that sufficient energy for capture could be stored and subsequently dissipated in a lunar-like planetoid during the initial close encounter of a capture episode. Another important feature for gravitational capture is the geometry of stable capture zones (SCZ) (Malcuit, Springer, Ch. 6:253). For planet Venus the prograde SCZs are very limited making prograde capture very improbable. But the retrograde SCZs are much larger so that retrograde capture has a reasonable probability if the encountering planetoid is in a venus-like heliocentric orbit of low eccentricity (0.3 to 1.0 %).

A problem with all capture models is the place or origin of the planetoid. Malcuit (2015, Springer, Ch. 4:65) proposed that lunar-like planetoids could form between the orbit of Mercury and the Sun at about 0.1 to 0.2 AU, a region characterized by very stable heliocentric orbits (Evans and Tabachnik, 1999, Nature, 399:41-43). Such “Vulcanoid” planetoids would then eventually be perturbed by planet Mercury into Venus- and Earth-crossing orbits from which they could be captured. A generalized sequence of events following the retrograde capture event for planet Venus is: (1) circularization of the orbit to ~18 venus radii by dissipation of orbital energy in both the planetoid and planet, (2) a lengthy period, about 3 billion years, of circular orbit evolution in which the orbital radius of the captured planetoid decreases from 18 venus radii to ~1.6 venus radii (the Roche limit for a solid body), and (3) a period of intense tidal energy dissipation during the latter part of the circular orbit evolution which causes the crust of the planet to be subducted into the Venusian mantle, massive outgassing of the mantle, and a global resurfacing event.