The Venus Hypothesis

It is known that if plate tectonics was active early on a habitable Venus, then it must have ceased by the time the planet developed into the stagnant lid phase (10) with violent resurfacing occurring by about 600-400 My BP (11)(12)(13)(14). These age estimates have an uncertainty factor of about 200%, as they are obtained from crater counting models (14). All of the Venusian crust is thought to have melted and then resolidified at that time. Any carbonate rocks existing before this happened would have been reprocessed in the mantle of Venus, releasing their CO2 just as happens on Earth, but on an enormous scale, leading to a runaway greenhouse effect and the evaporation of water, with the concomitant negative implications for life.

The stagnant lid period would have seen increasing pressure beneath the crust (10) as heat transfer by conduction through the Venusian crust was no longer efficient enough to balance the radioactive heating within the planet’s interior. Occasional volcanic eruptions and impacting asteroids would therefore have resulted in more explosive ejecta launch. This would be a perfect scenario to increase the amount of material being launched from Venus’ crust to escape velocity and even to Earth-crossing orbit. The escape velocity for Venus is around 10km/s and an additional impulse of 22km/s would be required for an object to just reach Earth orbit (Hohman Transfer orbit calculation) with a transfer time of only 150 days.

Recent models (8) suggest that the resurfacing event would have taken approximately 100 My to complete, indicating that it was taking place between 700My and 400 My BP.

Complex life appeared suddenly in Earth’s fossil record with the Cambrian Explosion(15, 16) of 540My BP, when all but one of the known phyla appeared on Earth. This was followed by the Great Ordovician Biodiversity Event (GOBE) in the period up to 460My BP, when diversity greatly increased (17). Land plants also appeared around 470My BP (18). The timing is suggestively close to the resurfacing event on Venus.

The nature and rapidity of the Cambrian explosion represent one of the major challenges in the study of evolution on Earth (19). Any model of the Cambrian explosion, and the GOBE, must explain the timing of this period -why did it start, and why did it stop? Why is there a lack of early fossil precursors of species which can be shown by genetic analysis to have evolved 1 billion years ago (20)? In particular, what could have caused all 34 known phyla to appear during the Cambrian or just shortly afterwards, with no new phyla ever evolving subsequently? Why was this period unique? The GOBE presents the additional challenge that timings of significant changes are ‘diachronous across groups, environments and regions’ (17).

A systematic review of all of these problems and possible solutions (21) showed that none of the current environmental, developmental or ecological theories convincingly answer all of the questions. Much work is currently being focused on ocean chemistry and the increase in oxygen levels (22), acting as a trigger to sudden development and the rise of predation, but this is not conclusive at present. The sporadic arrival of complex life from Venus, during the lead up to and early stages of its violent resurfacing, is suggested here to provide an elegant solution to these outstanding questions.

The timing of the arrival of material from Venus was dictated by the slowing and cessation of plate tectonics on that planet, leading to the stagnant lid state and eventual violent resurfacing (8,10), possibly exacerbated by the break up of the L-chondrite parent asteroid (23). The lack of precursor fossils arises because the precursors were left behind on Venus. The geographical inconsistency of the GOBE, with some species suddenly flourishing in one area while elsewhere dying out, would be expected, as some meteorites might deliver new lifeforms, but others might have very destructive effects on established communities.

The fantastic genetic diversity, much of which died out never to evolve again, would be predicted to arise on Venus due to its proximity to the Sun. Venus is subjected to twice the intensity of solar radiation and solar wind experienced on Earth, while the tidal derotation has led to very long day lengths (24), and hence a negligible magnetic field. Without the protection of a magnetosphere, life on Venus would have been bombarded by the Solar wind. Exposure to radiation leads to accelerated mutation rates (25) . If life evolved on Venus, therefore, we would predict that it would be wildly more exotic and further advanced than that evolving on Earth. It would be plausible for Venusian and Earth life to share common ancestry, through earlier periods of lithopanspermia (1,2).

Another key difference between Venus and Earth, leading to more genetic variety, would be the smaller Venusian water volume, estimated at only 10% of Earth’s (7). If Venus therefore had more land an

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