Climatic Consequences of Episodic Eruptions on Early Mars

Introduction: An abundance of geomorphological, mineralogical and geochemical evidence suggests widespread aqueous activity on the surface or early Mars [13]. However, recent studies of Mars’ early climate, using sophisticated three-dimensional climate models, find average surface temperatures too low to explain the observations [4,5]. It seems likely, therefore, that Mars’ early climate was, on average, cold and dry, implying that winds easily lifted fine-grained particles and that the atmosphere was dusty, much like the present atmosphere in which the average visible dust optical depth is approximately 0.5 [6]. Volcanic sulfur-bearing gases have been suggested as a possible solution to the early climate problem [7,8]. However, scattering of incoming solar radiation by sulfuric acid (H2SO4) aerosols has been suggested to result in net cooling as SO2 levels increase [9,10]. In addition, we argue here that even in the absence of H2SO4 aerosols, reasonable long-term average volcanic outgassing rates are unable to sustain climatically important atmospheric concentrations of SO2. This is a consequence of the reactivity of SO2 and its sensitivity to ultraviolet photodissociation [9,11]. Here, on the basis of morphological similarity to terrestrial flood basalts, we suggest that early martian eruptions were highly episodic, and characterized by exceptionally rapid emission rates during short bursts of activity, separated by long quiescent periods. We show that volcanic emission rates during brief and strong (“punctuated”) eruptions were enough to sustain climatically important SO2 levels, and that the net effect of injection of SO2 into a dusty atmosphere is warming, despite the formation of H2SO4-bearing aerosols. We discuss probable climate feedbacks associated with the above scenario, and the possible existence of multiple climate states. Methods: A flood basalt analog to plains volcanism. A long maximum in volcanic activity during the transition between the Noachian and Hesperian [12] appears coeval with widespread evidence for aqueous activity [1-3, 13]. The Hesperian Ridged Plains (Hr) on Mars appear to have effused rapidly from wide fissures, rather than central cones or calderas [14,15]. This, in addition to their occurrence as laterally extensive plains, suggests an analogy with terrestrial flood basalts. Individual flows in the Columbia River Flood Basalts and the Deccan Traps [16,17] imply instantaneous sulfur outgassing rates up to several hundred times the total global rate. Considering the higher sulfur content of martian magmas [18], the sulfur outgassing rate during similar eruptions on Mars could be thousands of times the terrestrial rate. Finally, plains basalts on Mars are larger in volume and area than any known terrestrial flood basalts, suggesting that Hr emplacement involved a greater number of eruptions and lasted for a longer time than is typical on Earth. A coupled aerosol microphysics-radiative transfer model. We developed a model of aerosol microphysics, which treats the formation, growth and transport of pure H2SO4 and mixed dust-H2SO4 aerosols. Model inputs are a size distribution of newly lofted dust [6], and a volcanic emission rate. Sulfur photochemistry is parameterized using full photochemical model results [9,10]. Model output is a time-dependent size distribution of aerosols composed of a dust core and a H2SO4 coating, which we use in a line-by-line radiative transfer model [19] to calculate radiation fluxes. Results and Discussion: The background climate state. With a solar constant 75% its present value, we calculated global mean annual surface temperatures (MAST) of 207 and 206 K for a 0.5 and 1 bar clear-sky CO2 atmosphere, at 80% relative humidity (Figure 1). Dust decreases MAST by ~10 and ~12 K, respectively in these cases. Tropical