Mud Volcanoes - a New Class of Sites for Geological and Astrobiological

Introduction: Mud volcanoes provide a unique low-temperature window into the Earth’s subsurface – including the deep biosphere – and may prove to be significant sources of atmospheric methane. The identification of analogous features on Mars would provide an important new class of sites for geological and astrobiological exploration. We report new work suggesting that features in Acidalia Planitia are most consistent with their being mud volcanoes. Previous Research: Terrestrial mud volcanoes are low shields or domes, meters to over one kilometer in diameter, composed of mud and rock. They have been documented at over 40 sites onshore and over 20 subsea locations [1]. The largest concentration of mud volcanoes is in Azerbaijan and the adjacent Caspian Sea. Mud volcanoes are formed when overpressured gases and liquids from as deep as several kilometers breach the surface, carrying slurries of fluid, mud, and rocks [2]. These conditions often occur in regions of rapid sedimentation and hydrocarbon generation, and many mud volcanoes emit copious quantities of volatiles, primarily water and methane. Mud volcanoes have been cited as possible analogs to martian features from a wide range of locations. These include the dichotomy boundary regions near Isidis [3] and Utopia [4], the Borealis “back basin” [5], and Chryse Planitia [6,7]. Farrand et al. [8] hypothesized that numerous domes and cones in Acidalia are either mud volcanoes or spring mounds. We have reexamined these features, using the full range of datasets now available from Mars orbit. Acidalia: Domes and cones, hundreds of meters to kilometers in diameter, are scattered across much of Acidalia Planitia. These features overlie the Vastitas Borealis plains, mapped as Hesperian to Amazonian sedimentary units of basaltic composition [9,10]. Farrand et al. [8] based their analysis on images from the MOC (Mars Orbiter Camera; 1.5 to 12 m/pixel) and THEMIS (Thermal Emission Imaging System; 19 m/pixel visible; 100 m/pixel infrared) instruments. We conducted high-resolution morphologic and spectral analysis of specific Acidalia features employing HiRISE (High Resolution Imaging Science Experiment; 0.3 m/pixel) panchromatic and color images, as well as multi-spectral images and point spectra obtained by the CRISM (Compact Reconnaissance Imaging Spectrometer for Mars; 16 to 20 m/pixel; 0.362 to 3.920 μm spectral range) instrument. Farrand et al. [8] reported that the domes and cones appear darker than the adjacent plains in THEMIS nighttime infrared images. This observation demonstrates that the domes and cones are composed of material with lower thermal inertia than the plains. HiRISE images show isolated and overlapping high-albedo domes and cones, many with central depressions. The centers (inferred high points) of the features often show concentric layering, while the outer portions are finely furrowed and generally rock-free at sub-meter resolution (Fig. 1). The dome material clearly overlies the plains. Meter-scale texture characteristic of the underlying plains is locally detectable at the margins of the domes, indicating that this material can thin to a feather edge. Impact craters that pierce several domes indicate that the bright material is irregularly layered and tens of meters thick. Small dunes of bright material, possibly eroded from the domes, are observable within nearby troughs.