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Catastrophe Not Required: A New Steady-State Model of Venus

Introduction: New cosmochemical models of Venus indicate that its lower mean density is a consequence of a smaller core, which results in fewer whole mantle plumes (volcanic rises) than on Earth. The correspondingly thicker mantle generates a surface heat flow about the same as Earth's but Venus maintains a steady-state in which subcrustal plate tectonics efficiently cools the mantle. Geologically this manifests itself in a high rate of rifting, coronae production and associated volcanism, but decoupling of the crust ensures an otherwise low rate of tectonic deformation at the surface and little or no crustal recycling. In the past, higher heat production resulted in much higher rates of surface deformation and, more than ~1·5 – 2·0 Ga ago, crustal recycling. These model predictions are consistent with both the geological record of Venus and its present distribution of impact craters. New Composition Model: Estimates of the bulk chemical composition of Venus are more than 30 years old [1] and most authors now assume an Earthlike composition. Is this assumption realistic? The bulk chemical composition of the Earth is rather well constrained and approximates to a mix of ~85% local condensate with ~15% chondritic material, at least some of which was a relatively late-stage addition from a collision with a Mars-sized object. Venus most likely consists of a similar mixture of local and exotic material. To determine the likely Venus bulk composition a numerical geochemical model of Venus was constructed which crudely estimates core composition, mantle composition and a differentiated MORB-like crust. This model was combined with a geophysical model of the internal compression, including internal phase changes, temperature profile and a range of other data. The mix of local condensate to exotic material was the only variable required to match the mass and radius of Venus. To constrain the uncertainty in the results, two different models of local condensate (Equilibrium and Weidenschilling) and four chondritic compositions (CI, CM, CO and CV) were used. Although large differences in the relative proportions were found (from 4 to 21%), the differences between the model results is within a few %, lending confidence to the method. As an attempt at an independent test, the predicted MORB-like crustal compositions were compared with Venera/Vega XRF data but with inconclusive results. Implications: The most immediate conclusion is that the core of Venus is much smaller than Earth's (~25% versus ~32%) such that the mantle of Venus is actually significantly thicker than Earth's, at 3280 km (Earth is 2880 km). Thus the moment of inertia factor is also much higher (I/MR is 0·375) than expected. The smaller core generates fewer whole mantle plumes and consequently sustains fewer volcanic rises. The models predict ~5·5 plumes equivalent in size to the Hawaii, consistent with the 6 or 7 large volcanic rises observed on Venus. With a mantle as massive as Earth's, the surface heat flow is similar (90 mW m) and the internal temperature rather higher (the potential temperature is ~1640 K) and the viscosity correspondingly lower. Plate tectonics, in the usual sense, does not operate on Venus, so it is generally accepted that Venus is in a stagnant-lid mode of convection and has at some time experienced a global overturn [2] and resurfacing event. Evidence of plate-like behaviour implying a hindered form of plate tectonics [3] also results in a global meltdown and resurfacing event. The implication is therefore that Venus is not in a steady-state. Steady-State Heat Loss: However, it has been known for some time [4] that the high surface and internal temperature results in a weak zone in the yield stress envelope (YSE) of the crust and mantle. YSEs calculated for each of the Venus models indicate that this weak zone starts at a depth of ~3 km (Fig 1) and extends to the bottom of the crust (at ~8·5 km), effectively decoupling the upper crust from the mantle below. Fig 1. Venus Yield Stress Envelope