[M-IS14] Aqua planetology

[MIS14-13] Degassing history and the structural evolution of Martian mantle

★Invited Papers

*Masaki Ogawa¹ (1.Division of General Systems Studies, Graduate School of Arts and Sciences, University of Tokyo)

Keywords:Mars, mantle evolution, degassing

The surface environment of Mars was clement in the early Hesperian, about 3.7 Gyr ago, when active magmatism took place in the Tharsis and the surrounding area. The earlier less clement surface environment suggested in the literatures is, however, difficult to explain with the traditional thermal history models of Mars where its magmatic activity is suggested to have been maximum at 4.5 Ga and have monotonously declined with time since then. To resolve this difficulty, I propose a four-stage evolution model of Mars caused by the structural evolution of the mantle based on a numerical model of coupled magmatism-mantle convection system: at 4.5 Ga, extensive magmatism formed the crust and compositionally stratified the mantle (Stage I); the stratification suppressed mantle convection and magmatism for the next several hundred million years (Stage II); then, hot plumes grew from the deep mantle to cause episodic magmatism and degassing to make the surface environment clement (Stage III); eventually, magmatism waned, as heat producing elements are extracted from the deep mantle (Stage IV). The plume magmatism of Stage III is caused by a positive feedback called the magmatism-mantle upwelling feedback, where mantle upwelling flow generates magma whose buoyancy boosts the upwelling flow itself (the MMUb feedback). I extensively carried out numerical experiments at various parameter values to see how robust the four-stage evolution model is. I found that the four stages arise, when the permeability of magma through the coexisting matrix is moderate. When the permeability is too high, the MMUb feedback does not operate, the mantle stratification formed on Stage I suppresses magmatism throughout the 4.5 Gyr history, and Stage III does not arise. When the permeability is too low, the MMUb feedback strongly stirs the mantle on Stage I to suppress mantle stratification, the mantle remains homogeneous throughout the 4.5 Gyr history, and the mantle evolves as traditional thermal history models predict. The permeability is probably “moderate” for Mars, and Mars is likely to have evolved in the four stages.