Sulfur - Related Greenhouse Warming on Early Mars

Introduction: A building body of evidence from recent Mars exploration missions suggests that sulfur chemistry may have played a prominent role in the planet’s early evolution. Although the Martian atmosphere contains virtually no sulfur species at present, both soils and outcrop observed by landed missions have high sulfate abundances [1,2]. The OMEGA hyperspectral imager has now also identified sulfates from orbit on localized layered terrains that extend far beyond these landing sites [3]. Outgassed sulfur volatiles can act as powerful greenhouse gases and may have been important during periods of enhanced volcanic activity on Mars [4,5]. Isotopic analyses in Martian meteorites further support this view, reflecting deposition of sulfur species created by atmospheric chemical reactions [6]. Here we present a preliminary model for sulfur delivery to the early Martian atmosphere and its potential warming effects. Degassed sulfur species, and the subsequent formation of sulfuric acid aerosols in the atmosphere, may have been responsible for: 1) producing ubiquitous sulfur-rich dust and widely dispersed sulfate platforms, 2) creating low enough pH levels to allow for jarosite deposition at Meridiani and the prevention of surface carbonate formation on a global scale, and 3) generating shortlived warmer climate conditions that allow for the presence of fluvial features on the Martian surface. Sulfur Solubility Calculations: A batch melting model, in which decompression melting of the mantle takes place with the solid residue staying with the melt during most of its ascent, is used to assess the sulfur solubility in Martian silicate melts in equilibrium with metal sulfide. Because of the unique negative pressure dependence for sulfur solubility that dominates the positive temperature dependence in systems that contain FeO, magma from mantle source regions in this model will arrive at the base of the lithospheric lid undersaturated in sulfur regardless of ascent velocity and melt fraction volume [7]. When decompression melting commences, sulfur from immiscible metal sulfide blebs will begin and continue to dissolve directly into the silicate melt. At the base of the lithospheric lid, a final equilibration will take place before the liquid melt is advected to the planet’s surface, at which point the remaining metal sulfide blebs will largely be left behind. While significant cooling in passage through the crust could affect the Sulfur Solubility Limit (SSL), here we assume that chemical and thermal halo effects insulate the magma along cracks and in magma chambers. As surface temperature and pressure conditions are well beneath the vapor saturation pressures for both H2S and SO2 [8], we assume soluble sulfur can be released to the atmosphere. In calculating the SSL in liquid silicate conditions, we use the formula [7]: