Cryogenic Evaporite Formation at Lewis Cliff, Antarctica: a Mars Analog Study

Introduction: The discovery of substantial evaporite deposits at the Opportunity rover landing site in Meridiani Planum on Mars argues for significant interactions between liquid water and local rocks during some stage of the Martian past. However, evaporites can form under a dazzling variety of water/rock ratios, from long-term evaporation of expansive reservoirs to periodic recycling of water on the surface of individual rocks. On the warm and high water/rock ratio end, evaporites can form from evaporating pools of brine as large as oceans; on the cold and low water/rock ratio end, evaporites can form through extremely short-term but periodic recycling of water percolating across grain and rock surfaces. Between these two extremes is a broad continuum of processes that can produce evaporites-however, the cold, low water/rock ratio processes that may be most relevant to current day Mars are among the least understood. Exploring the full range of possible mechanisms is thus crucial for interpreting MER results and those from future exploration of Mars, and terrestrial analog studies offer an important approach. This abstract describes the initial results of fieldwork now being conducted during December 2005 and January of 2006 in a moraine surrounding the Lewis Cliff Ice Tongue in the Transantarctic Mountains, Antarctica. Exploration of the region by meteorite recovery teams revealed that the moraine contains significant amounts of evaporite minerals in a variety of formats. The morainal regolith consists of a mixture of finely ground " glacial flour " and larger cobbles of locally exposed rock types. These include Permian-Triassic sedimentary sequences (the Beacon Supergroup) and later Jurassic intrusives and volcanics (the Ferrar Group). These sequences contain a wide variety of rocks, from igneous dolerites to cyclothemic coal and volcaniclastic sequences. Weathering efflorescences are common on rock surfaces, and holes dug into the morainal surface often reveal stratified layers of evaporitic materials. Some of these layers are dramatic in scope, reaching several meters in maximum dimension (Figure 1). Studies of a few small returned samples show that these blocks consist of masses of euhedral nahcolite (NaHCO 3), trona (Na 3 H(CO 3) 2 • 2H 2 O), borax (Na 2 B 4 O 7 • 10H 2 O) and an unknown hydrous sulfate species [1]. Jarosite, halite, calcite and phosphates have also been identified in recovered samples but have not yet been described in the literature. Formation mechanisms: The proposed cryogenic formation mechanism is relevant to understanding similar evaporite …