Other
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.u42b..03o&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #U42B-03
Other
1645 Solid Earth (1225), 8124 Earth'S Interior: Composition And State (1212, 7207, 7208, 8105), 8125 Evolution Of The Earth (0325)
Scientific paper
Despite their superficial similarities, Venus and Earth's atmospheric evolution have diverged significantly. Without significant CO2 sinks, ongoing volcanism has resulted in the build up of extremely high CO2 concentrations, which have contributed to the dehydration of Venus's surface, and perhaps also the cessation of plate tectonics on Venus, either due to dry faults or surface temperatures. Its degassing history is to some extant recorded in its atmospheric argon signatures. Nonradiogenic Argon-36 is ~80 times that off Earth. Given most 36-Ar is primordial, this suggests very different initial atmospheric conditions for the two planets, with Venus retaining most of its initial atmosphere due to its fortuitous impact history. On the other hand, the deficit of radiogenic Argon-40 (~24 percent escaped from the mantle, compared with ~52 percent for Earth), hints at a very different volcanic and tectonic history, particularly in its deepest past. Recent convection modelling has shown that plate tectonic regimes break down under hot mantle conditions, due to a partial decoupling of stress from the less viscous mantle to the plates, resulting in insufficient stress for plate boundary deformation. This sends the system into an "episodic overturn" regime - similar to that which has been proposed for Venus today - where long periods of stagnant lid convection are interrupted by periods of massive lid recycling and overturn. Conversely, a planet in an episodic regime may transit into a stagnant lid regime for hotter mantle conditions. We couple a model for production and degassing of radiogenic Ar-40 from the mantle, and couple it with evolutionary models for stagnant, episodic and mobile lid tectonics. Earth's deficit in radiogenic Ar-40 may in some part be due to the different degassing efficiency of episodic convection in a hot early Earth. If Venus was stagnant for a large portion of its early evolution, the cumulative degassing efficiency of Ar-40 would be much lower than Earth, providing an explanation for Venus's very low atmospheric Ar-40 concentrations.
Lenardic Adrian
O'Neill Craig
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