Formation and deposition of volcanic sulfate aerosols on Mars

Abstract

Sulfurous gases released at the surface of Mars during episodes of volcanic activity would be naturally transported into the upper levels of the Martian atmosphere (20-to 30-km altitude) owing to the synoptic convergence of lower-level winds in the Tharsis region of the planet. The circulation of the upper atmosphere would globally disperse such gases over time scales of 25 earth days. The rate limiting step in sulfate aerosol formation on Mars is inferred to be the gas phase oxidation of SO2 which is accomplished via chemical reactions with O, OH, and HOe. Under present-day atmospheric conditions aerosol formation would occur over periods of several thousand earth days. However atmospheric concentrations of odd hydrogen species (OH, HO2) should increase significantly during periods of persistent surface volcanism. Under these circumstances sulfate aerosol formation could occur on time scales of several hundred earth days. Submicron-sized aerosol particles would circuit Mars several dozen times before they are removed from the atmosphere by gravitational forces. Such aerosols would be globally dispersed and would be deposited over a wide range of equatorial and mid-latitudes. Volcanic sulfate aerosols on Mars are inferred to consist of liquid droplets and liquid-solid slurries containing aqueous solutions of sulfuric acid. These acidic solutions would be chemically reactive and would trigger leaching processes within surface materials upon deposition. Sulfate aerosol deposition is a viable mechanism for transporting sulfur to the Viking lander sites. Atmospheric circulation models indicate that time-averaged vertical winds over Chryse Planitia and Utopia Planitia are directed toward the surface during most Martian seasons. Aerosol particles settling out of the atmosphere would be expected to be preferentially deposited in these regions. Aerosol deposition on a global or hemispheric scale could account for the similar concentrations of sulfur within surficial soils at the two Viking lander sites. VOLCANIC DEGASSING ON MARS Volcanic activity at the surface of Mars has produced enormous shield volcanoes and areally extensive lava plains. The morphology and morphometry of these landforms suggest that Martian eruptions have been largely extrusive in nature [Carr, 1973; Carr et al., 1977; $chaber et al., 1978]. Individual lava flows observed in the Tharsis region commonly extend over distances of several hundred kilometers. Detailed examination of flow lobe thicknesses, channel structure, and Paper number 9C 1310.

Marine‐target craters on Mars? An assessment study

“…However, there may be explanations for this other than their absence, such as sparse spectral data of sufficient resolution to see in detail the diversity of rock materials that may exist on the surface of Mars; depletion of carbonates in the uppermost martian crust due to secondary chemical alteration (Huguenin 1974;Settle 1979;Craddock and Howard 2002); hiding under younger materials (Banin et al 1997;Clark 1999;Blaney 1999); or, maybe the putative transient ocean environments were simply not adequate for the formation of carbonates.…”

The Potential for Biologically Catalyzed Anaerobic Methane Oxidation on Ancient Mars

“…Moreover, it has been proposed that groundwater systems and hydrothermal subsurface aqueous alteration of basalt were globally widespread during the Noachian period and formed the hydrated silicate mineral assemblages found in thousands of exposures of Noachian crust on Mars (Mustard et al, 2008;Ehlmann et al, 2011). In the most potentially widespread scenario, sulfate-bearing groundwater is generated from the dissolution of oxidized sulfur aerosols, which may have precipitated from the atmosphere following volcanic expulsion (Settle, 1979). Iron oxidation, mediated by atmospheric molecular oxygen or solar radiation, and the precipitation of schwertmannite (an iron-oxyhydroxysulfate mineral) would have generated acidic geochemical conditions (Hurowitz et al, 2010).…”

Polygenic eruptions on Alba Patera, Mars

“…Since the shapes of plinian eruption clouds are such that the width at the top is approximately equal to the height (Wilson and Walker 1987), eruption clouds with heights of this The dashed line indicates a particle diameter of 50 u,m, which we take to be a liberal estimate of the size found to be dominant in the pyroclastic deposits on the flanks of Alba Patera, based on Viking Orbiter IRTM data order would produce deposits with widths of only a few tens of kilometers; thus the observed deposits, which occur in all azimuthal directions from the summit calderas, would have to be the accumulated result of very large numbers of eruptions taking place with the mean wind vector changing over a very wide range. It is not clear that such a pattern would be expected given what little is known about the distribution of winds on Mars as a function of altitude (Webster 1977;Settle 1979;Thomas and Veverka 1979). The deduced eruption cloud heights change, roughly in inverse proportion, if the assumed mean wind speed is changed; but since wind speeds are very unlikely to be as much as a factor of 2 higher than the 20 m/s used [Settle (1979) gives a mean value of 10 m/s], a plausible lower limit cloud height can be set at about 15 km.…”

“…It is not clear that such a pattern would be expected given what little is known about the distribution of winds on Mars as a function of altitude (Webster 1977;Settle 1979;Thomas and Veverka 1979). The deduced eruption cloud heights change, roughly in inverse proportion, if the assumed mean wind speed is changed; but since wind speeds are very unlikely to be as much as a factor of 2 higher than the 20 m/s used [Settle (1979) gives a mean value of 10 m/s], a plausible lower limit cloud height can be set at about 15 km. The plausible upper limit is harder to set, since the wind speed could conceivable have been negligible at the time(s) of ash emplacement.…”

“…The largest clast size decreases steadily with height in the cloud since both the mean upward gas speed and the local gas density decrease upward. We have used a plausible martian wind speed of 20 m/s for the time-averaged intensity of zonal upper-level winds [values are very likely to lie within a factor of 2 of this speed: Settle (1979)] to compute the lateral transport distance of clasts falling from their respective release heights in a standard martian atmosphere using the numerical methods of Wilson (1972).…”