MARTIAN CENTRAL MOUNDS AS VOLCANICLASTIC SEDIMENT?

Central mounds are stacks of sediment located within craters on Mars. They can be up to several kilometers high and typically exhibit layers. Their hypothesized formation mechanism includes aeolian, lacustrine, fluvial, volcanic, and ice-related processes. The Mars Science Laboratory rover, Curiosity, is currently investigating one of these mounds, Mt. Sharp, within Gale crater. Here we investigate whether mound sediment could have been deposited as ignimbrites.

The Medusae Fossae Formation (MFF) on Mars has been hypothesized to be an explosive volcanic deposit. Several central mounds are located near the MFF, which could suggest that these mounds, including the upper portion of Mt. Sharp, were deposited as a part of the previously more extensive MFF unit. Similarities between ignimbrite deposits in the Andes and the MFF support the idea that the MFF is an explosive volcanic deposit (de Silva et al. 2010). One important observation made by Burr et al. (2013) was that dark-toned sand grains weather out of the light-toned ignimbrite deposits in the Andes to form dark-toned dunes. Burr et al. (2013) compared this to locations within the MFF, but dark-toned sand eroding out of light-toned units may also occur at central mounds.

In this study we test the hypothesis that dark sand is eroding out of central mounds on Mars, and discuss the implications of this scenario. At least 31 mound hosting craters also contain dunes or sand patches on the crater floor, including Gale crater. In some instances, including at Mt. Sharp, dark toned sand is also observed on the mound. We map out the location of the sand on Mt. Sharp and at other central mounds using visible images (THEMIS-VIS, CTX, HiRISE) and near-infrared spectra (CRISM). We test if sand on the mound is constrained to a particular layer or if the sand is distributed across the entire mound.

If dark toned sand is eroding out of layers in central mounds, these deposits are consistent with ignimbrites and some mounds could have been deposited by explosive volcanism. However, this would not definitively prove that mound materials are ignimbrite deposits. Coarse grained material eroding out of layers in central mounds would prove that unit does not consist entirely of dust, which would eliminate several aeolian and ice-related mound formation mechanisms.