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Mounds on Mars in Acidalia Planitia, Mars. From
Oehler and Allen, Evidence for pervasive mud volcanism
in Acidalia Planitia, Mars, Icarus, 208(2),pp. 636-657, 2010.
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Since the days of Viking exploration, there has been speculation that mud volcanism occurs on Mars. Candidate features populate the Northern Plains (see review in the paper referenced in the figure caption to the left). On Earth, mud volcanism is triggered by tectonic compression or generation of hydrocarbons (esp. methane, CH4). Neither process appears to be prominent on Mars. Mechanisms that have been proposed include: (1) dewatering of debris flows; impact-related, seismic shaking causing liquefaction; sedimentation with compaction and degassing; and sublimation of CH4 or CO2 clathrates. The mounds in Acidalia are on a very large scale, and Oehler and Allen favor an explanation that includes the basin's unique geologic setting. Acidalia Planitia sits where large quantities of sediments were deposited from outflow channels. It was a "depocenter" for accumulation of mud and fluids from this sedimentation. The mounds may be attributable to large overpressure developed in response to the rapid outflow deposition, "perhaps aided by regional triggers for fluid expulsion related to events such as tectonic or hydrothermal pulses, destabilization of clathrates, or sublimation of a frozen body of water." They could account for a significant release of gas, and the process may have created long-lived conduits for upwelling groundwaters.
In a recent article in Earth and Planetary Science Letters (v. 304, pp. 511-519, 2011), Pondrelli et al. reported on possible mud volcanoes within Firsoff impact crater. The mounts are on the crater floor, and appear as isolated or composit cones 100-500 m in diameter, and tens of meters high. More than 1/3 have subcircular depressions on their apices, 5-39 m deep, interpreted as vents. The mounds themselves are meter-sized boulders embedded in a finer-grained matrix, a mud breccia. The mounds are located on or near faults and are aligned with fractures, suggesting larger pathways for fluid migration along faults related to the impact that produced the crater. The authors speculate that methane was involved in the process of forming the mounds.
