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[58.06] Could Plate Tectonics Have Operated on Early Mars?

F. Nimmo, D.J. Stevenson (Caltech)

We consider the likelihood of plate tectonics operating on early Mars and Earth. Crust produced by mid-ocean ridge spreading at the same potential temperature is ~~3 times thicker on Mars than on Earth; the density contrast between crust and mantle is probably similar on the two planets. Assuming that the radiogenic heat flux is balanced by plate tectonic heat loss, the maximum mechanical boundary layer (MBL) thickness on Mars is twice as great, and plate velocities ~~4 times slower, for similar size plates. For both planets, subduction is favoured at potential temperatures less than \rm 1500-1600^\circ~C, where crustal thicknesses are low.

The maximum MBL thickness on the present-day Earth is limited to about 100~km by the dropoff of the thermal boundary layer. The viscosity of this layer is about \rm 10¹⁹~Pa~s, probably because of the presence of melt. For a similar layer on Mars, the maximum MBL thickness would be ~150~km, reducing the potential temperature at which subduction could have occurred to less than \rm ~1400^\circ~C.

Plate velocities are probably limited by viscous drag on the descending slab, which depends on the density contrast, slab length, MBL thickness and mantle viscosity. For a slab length of 300~km, the ancient terrestrial mantle viscosity needs to be < \rm ~10²⁰~Pa~s to allow the drag-limited plate velocity to remove all the heat. On early Mars, the limiting viscosity is a factor of 4 larger.

Whether plate tectonics could have operated on early Mars therefore depends mainly on the viscosity structure of the mantle, which will be strongly affected by the presence or absence of water. If early Mars possessed a low viscosity zone similar to the present-day Earth's, subduction is unlikely to have occurred. In the absence of such a zone, subduction was probably {\em easier} to sustain on Mars than on Earth.