Thermal and crustal evolution of Mars

Abstract

[1] We present a coupled thermal-magmatic model for the evolution of Mars' mantle and crust that may be consistent with estimates of the average crustal thickness and crustal growth rate. By coupling a simple parameterized model of mantle convection to a batchmelting model for peridotite, we can investigate potential conditions and evolutionary paths of the crust and mantle in a coupled thermal-magmatic system. On the basis of recent geophysical and geochemical studies, we constrain our models to have average crustal thicknesses between 50 and 100 km that were mostly formed by 4 Ga. Our nominal model is an attempt to satisfy these constraints with a relatively simple set of conditions. Key elements of this model are the inclusion of the energetics of melting, a wet (weak) mantle rheology, self-consistent fractionation of heat-producing elements to the crust, and a nearchondritic abundance of those elements. The latent heat of melting mantle material is a small (percent level) contributor to the total planetary energy budget over 4.5 Gyr but is crucial for constraining the thermal and magmatic history of Mars. Our nominal model predicts an average crustal thickness of $62 km that was 73% emplaced by 4 Ga. However, if Mars had a primary crust enriched in heat-producing elements, consistent with SNC meteorite geochemistry, then our models predict a considerably diminished amount of post 4 Ga crustal emplacement relative to the nominal model. The importance of a wet mantle in satisfying the basic constraints of Mars' thermal and crustal evolution suggests (independently from traditional geomorphology or meteorite geochemistry arguments) that early Mars had a wet environment. Extraction of water from the mantle of a one-plate planet such as Mars is found to be extremely inefficient, such that 90-95% of all water present in the mantle after the initial degassing event should still reside there currently. Yet extraction of even 5% of a modestly wet mantle ($36 ppm water) would result in a significant amount (6.4 m equivalent global layer) of water available to influence the early surface and climate evolution of the planet.

Constraints on the radiogenic heat production rate in the Martian interior from viscous relaxation of crustal thickness variations

“…The heating rates derived from the radioactive abundances of Lodders and Fegley [1997] lead to unreasonable crustal thickness values, which are almost four times larger [Hauck and Phillips, 2002] than those derived from gravity and topography data [Neumann et al, 2004;Wieczorek and Zuber, 2004]. Crustal thicknesses calculated using the radioactive abundances of Dreibus and Wänke [1984] are, however, consistent with this data.…”

“…We consider T m = 1700-1800 K, a range for which crustal production rates are consistent with the observations [Hauck and Phillips, 2002]. Furthermore, the crust will be cooler if radioactive elements are concentrated there and we assume a crustal enrichment factor of 10 with respect to the undepleted mantle [Taylor et al, 2006].…”

Implications of large elastic thicknesses for the composition and current thermal state of Mars

“…The properties of nonNewtonian rheologies can be approximated by Newtonian flow with reduced activation enthalpy (Christensen, 1984), resulting in higher average viscosities, consistent with the larger observed crustal production rates for nonNewtonian flow laws (Hauck and Phillips, 2002). Scaling laws derived from non-Newtonian mantle convection simulations (Solomatov and Moresi, 2002) indicate that time-dependent convection models using dislocation creep have mantle temperatures which are on average about 100 K higher than models using diffusion creep for similar stagnant lid thicknesses and surface heat flows.…”

“…Instead, we assume 6 A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPT that the bulk of the crust is primordial and although there is evidence for late crustal production even after 4 Gyr (Hartmann et al, 1999;Hartmann and Berman, 2000;Neukum et al, 2004;Grott, 2005), its volumetric contribution is probably minor on a global scale (Nimmo and Tanaka, 2005). Note that the latent heat of melting associated with crustal production can have a large influence on the total amount of melt produced during the evolution (Hauck and Phillips, 2002), but does not significantly affect the current thermal state of Mars as it is expected to change the mantle energy balance by less than one percent. Details of the model used here may be found in Grott and Breuer (2008a).…”

“…Different models for the bulk composition of Mars have been proposed (Morgan and Anders, 1979;Treiman et al, 1986;Wänke and Dreibus, 1994;Lodders and Fegley, 1997), but it seems unlikely that compositional models which generate heat at rates larger than the chondritic heat production rate (e.g., Lodders and Fegley (1997)) are representative of the planet's bulk composition (Hauck and Phillips, 2002;Grott and Breuer, 2008b). The model by Wänke and Dreibus (1994) is the one best compatible with the amount of K and Th in the near surface layers as observed by gamma ray spectroscopy (Taylor et al, 2006) and we will use this model as a reference.…”