Icarus 150, 195–205 (2001) doi:10.1006/icar.2000.6572, available online at http://www.idealibrary.com on Modeling the Volcanism on Mars A. Weizman Department ofGeophysics and Planetary Sciences, Tel Aviv University, Tel Aviv, Israel D. J. Stevenson Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125 and D. Prialnik and M. Podolak Department ofGeophysics and Planetary Sciences, Tel Aviv University, Tel Aviv, Israel E-mail: dina@planet.tau.ac.il Received September 24, 1999; revised September 28, 2000 The total amount of melt produced in Mars during its evolution is estimated by means of a parameterized, one-dimensional, analytic mantle convection model that assumes a stagnant lid and whole mantle convection. The fertility of the mantle—defined as the po- tential to create basalt—and its variation with time are taken into account. The model is composed of core, mantle, and lithosphere, with two boundary layers separating them. The contributions to volcanism by pressure release melting (PRM), and by plumes from the core–mantle boundary layer, are compared and discussed. We show that such models tend to produce considerable melting during the early evolution of the planet, and that the amount of melting de- pends strongly on the abundances of radioactive elements. Although the model’s assumptions may not be valid for the early evolution of the planet, the model is relevant to the laterhistory, which is insensi- tive to initial conditions. We find that PRM volcanism should have ceased between 1 and 2.5 Byr ago and any recent volcanic activity must have originated in plumes. c  2001 Academic Press Key Words: Mars; Mars, interior; Volcanism. 1. INTRODUCTION Mars shows abundant evidence of volcanic activity. Recent evidence from the Mars Global Surveyor (MGS) suggests activ- ity at the Tharsis area to be no older than 40–100 Myr (Hartmann et al. 1999). There is also evidence of early volcanism (after the heavy bombardment period), suggesting that it was probably more voluminous than previously thought (McEwen et al. 1999). Volcanic features are divided into two main types: (a) central vol- canos, such as those of the Elysium and the Tharsis provinces, and (b) volcanic plains (Basaltic Volcanism Study Project 1981). There are examples both of relatively old and of relatively young plains and shields, indicating long-term stability of the volcanic processes (e.g., Neukum and Hiller 1981). Most of the volcanic activity on Mars appears to have involved eruption of mafic to ultramafic lavas—most probably basaltic (Greeley and Spudis 1981). Greeley (1987) has estimated the volume of volcanic ma- terial based on Mariner 9 data at 2 × 10 8 km 3 (see also Tanaka et al. (1988)); this volume is equivalent to a global layer of vol- canic material 1.5 km thick, and should be accompanied by a much larger volume of igneous intrusions (on Earth it is larger by a factor of 10 in continental regions (Crisp 1984)). Therefore, a significant fraction of the martian crust may have been added by igneous activity since the end of the heavy bombardment era (Schubert et al. 1992). Volcanism due to mantle partial melting may arise in two main ways: (1) Pressure release (adiabatic decompression) melting (PRM), when hot mantle rock ascends to fill gaps in the litho- sphere. The material is not anomalously hot but its pressure decreases in upwelling. In the case of Earth, this arises mainly through plate tectonics. Mars may have had plate tectonics early on (Sleep 1994), but most of its evolution was probably as a “one-plate” planet (Nimmo and Stevenson 2000). In this paper we use a one-dimensional model and hence we must assume a stagnant lid regime for the whole evolution time. In this case, pressure release melting may occur mainly when passively up- welling material replaces cold material that peels away from a boundary layer at the base of the lithosphere (Agnon and Liachovsky 1995). This form of volcanism is related to the prin- cipal mode of convection in an internally heated planet (Scott and Stevenson 1989). (2) Plumes that peel away from a lower boundary layer, just above the core–mantle boundary, delivering heat from the core. Although the heat delivery is expected to be small, after the 195 0019-1035/01 $35.00 Copyright c  2001 by Academic Press All rights of reproduction in any form reserved.