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Project Progress

In 2007, Halevy et al. suggested in Science that SO₂ may have been the key to warming early Mars. They suggested a feedback loop similar to that proposed for CO₂ cycling on Earth: If the Martian climate was too cold, volcanic SO₂ would have built up in the atmosphere, creating additional greenhouse effect, and thereby warming the surface. They did not, however, perform any calculations to test this prediction.

We first performed 1-D climate model simulations where we added mixtures of SO₂ and CO₂ into the early Martian atmosphere. We found that SO₂ mixing ratios in excess of 10 ppmv, combined with multiple bars of CO₂ could theoretically warm up early Mars above the freezing point. However, these calculations did not take into account what would happen to SO₂ photochemically in the early Martian atmosphere.

SO₂ in Earth’s atmosphere is rapidly oxidized to H₂SO₄, sulfuric acid, which then condenses out to form sulfate aerosols, which both reflect and scatter light. VPL showed that the same thing happens, albeit not quite as fast, in a reduced early Martian atmosphere. Results for a representative 3-bar CO₂-SO₂-H₂O atmosphere (calculated with our updated 1-D photochemical model) are shown in the figure. Panel 'A’ shows aerosol extinction optical depth as a function of SO₂ mixing ratio. Optical depths become large, i.e., >1, for SO₂ concentrations exceeding a few ppmv. Panel 'B’ shows the effect of the aerosols on planetary albedo. The albedo increases rapidly at SO₂ concentrations exceeding a few ppmv. Panel 'C’ shows calculated mean surface temperatures for a 'no aerosol’ case compared with two cases in which aerosols are included. The ‘no aerosol’ case warms monotonically with increasing SO₂, whereas the two 'aerosol included’ cases both decrease with increasing SO₂ because the increase in planetary albedo outweighs the increase in the greenhouse effect. Panel 'D’ shows the calculated total sulfur (SO₂ + H₂S) outgassing rates in our model compared to the outgassing rate assumed by Halevy et al. We conclude that, if early Mars was indeed warm, it must have been a result of factors other than SO₂ (Tian et al 2010).

In addition, VPL ran energy balance models to simulate the effects of surface temperature on putative Martian rainout. This can serve as a stabilizing feedback to maintain “cool” temperatures at the boundary between evaporation-driven and sublimation-driven hydrological cycles. This work in progress was presented by undergraduate Jonathan Briener at the AGU Fall meeting. (AGU Fall Meeting p51D-1161, 2009)

NASA’s announcement of the discovery of perchlorate on Mars came fortuitously with the announcement of our DDF award. As it became clear very quickly that the SO₂ was an unambiguous null result, we were able to repurpose the time of senior personnel from their detailed follow-on work to address perchlorate. Co-I’s Catling, Claire, and Zahnle enhanced their photochemical model and provided the first explanation of gas-phase atmospheric production of perchlorate over Earth’s Atacama Desert (Catling et al. 2010). This “value-added” DDF project enabled VPL Co-I’s to successfully propose to the Mars Fundamental Research program to model the formation of perchlorate on Mars. In a second re-purposing, Co-I Marion added perchlorate species into the FREZCHEM model, instead of sulfides as originally proposed. We show that calcium perchlorate would not have precipitated from a mixture of water added to Phoenix soil, and that different ratios of sodium and magnesium perchlorate would result from long-scale slow evaporative process versus a more rapid freezing pathway. These results suggest that future sample return or in-sity X-ray diffraction may help discriminate between evaporation and freezing processes in the Martian soil (Marion et al. 2010).