A RETROGRADE PLANETOID CAPTURE MODEL FOR PLANET VENUS: IMPLICATIONS FOR THE VENUS OCEANS PROBLEM, AN ERA OF HABITABILITY FOR VENUS, AND A GLOBAL RESURFACING EVENT ABOUT 1.0-0.5 GA AGO

A retrograde planetoid capture scenario for planet Venus was proposed by Singer (1970, Science, 170, p. 1196). His model featured a lunar mass planetoid and was developed to explain (1) the slow retrograde rotation of Venus and (2) the massive carbon dioxide atmosphere of the planet. In this paper a 0.5 moon-mass planetoid is used and the resulting time-scale of events is much longer.

Using the calculation procedures in Malcuit et al. (1989, Proc., 19^th LPSC, p. 581), it is demonstrated that retrograde capture of a 0.5 moon-mass body is possible for a planetoid with a displacement Love number near 0.3 and a specific dissipation factor (Q) near 1. A stable post-capture orbit has a major axis of about 140 venus radii and and eccentricity of about 0.86. A two-body simulation program suggests a timescale for orbit circularization of about 70 Ma to a circular orbit of 18 venus radii. Much outgassing of carbon dioxide, water vapor, and other gasses as well as crustal disruption and subduction would be associated with this orbit circularization scenario.

Evolution of the circular orbit from 18 to 10 venus radii is very slow and could be as long as 3.0 billion years. Outgassed water would condense to form shallow oceans and the equilibrium rock tidal amplitudes would gradually increase from 4.5 meters to 32.0 meters throughout this time. The number of months per venus year would increase from 49 to 119 as the prograde rotation rate of Venus decreases from 21 to 33 hr/day. There is a good prospect for HABITABILITY during this orbital era especially if some of the carbon dioxide is precipitated as carbonate rock in the shallow Venus oceans.

The era of potential habitability would gradually come to an end as the circular orbit decreases in radius fairly rapidly from 8 venus radii to 2 venus radii. The equilibrium rock tidal amplitudes would increase from 62.5 meters to 4000 meters as the number of months increases from 166 to1327 per venus year. The rock tides would eventually cause major crustal subduction and outgassing in the equatorial zone as the ocean water evaporates due to a combination of the tidal energy flux and the intense greenhouse effect. Eventually most, or all, of the earlier surface rocks of Venus would be subducted and a new basaltic surface formed (A GLOBAL RESURFACING EVENT). The final rotation rate for Venus would be very slow in the retrograde direction.