Preservation In Acidic Environments

Late Noachian to Early Hesperian surface environments on Mars were probably ruled by global acidic conditions as inferred through the sulfate and oxide materials occurring in different areas such as Meridiani (Fig. 4A), Gusev, Valles Marineris, and North Polar regions. However, acidic environments have long been considered incompatible to life and related to highly contaminated areas (Blowes et al., 2005) given that some acidic solutions are sourced in mine tailings resulting from mining operations (Davis et al., 2000; Blowes et al., 2005). However, recent studies on the Earth geological record of ancient and modern acidic environments have identified natural systems (Fernández-Remolar et al., 2005; Benison, 2006).

Preservation in modern and ancient acidic environments have been reported from the Tyrrel Lake at nothwestern Victoria and other lacustrine areas of Western Australia (Benison and LaClair, 2003), acidic mine drainage of Indiana (Brake et al., 2002) and Río Tinto (Fernández-Remolar et al., 2007; Fernández-Remolar and Knoll, 2008). Combination of studies on modern and ancient environments are essential to understand long-term preservation in acidic environments. Interestingly, in Rio Tinto modern and ancient sediments dating back to more

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Figure 4. Burns Formation at the Burns Cliff in the Endurance Crater of Meridiani (A), Pan cam image PIA03241 resulted of a false color composite mosaic (courtesy of NASA/JPL-Caltech). Whattanga and Wellington sedimentary boundaries (Grotzinger et al., 2005), showed as 1 and 2, respectively; grained-sulfates and the hematites in concretional structures (blueberries). Both mineralogies have been identified as mineralogies giving some clues concerning to the acidic conditions of o rira

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Figure 4. Burns Formation at the Burns Cliff in the Endurance Crater of Meridiani (A), Pan cam image PIA03241 resulted of a false color composite mosaic (courtesy of NASA/JPL-Caltech). Whattanga and Wellington sedimentary boundaries (Grotzinger et al., 2005), showed as 1 and 2, respectively; grained-sulfates and the hematites in concretional structures (blueberries). Both mineralogies have been identified as mineralogies giving some clues concerning to the acidic conditions of than 6 million years coexist in the same area (Moreno et al., 2003; Fernández-Remolar et al., 2005), allowing an intregrative study concerning the syngenetical and postsedimentary taphonomic processes over time (Figs. 4B-D). Moreover, subsurface sampling from the Río Tinto underground regions has allowed sampling of the aquifers sourcing the surface acidic solutions (Fernández-Remolar et al., 2008a).

Physichochemical and geobiological analysis of the Río Tinto subsurface areas have determined very different environmental conditions when compared to the surface acidic environment, which are characterized by neutral and reducing conditions with lower concentration in ions (Fernández-Remolar et al., 2008b). Under these new environmental circumstances geopolymers and specifically geolipids of different origin (Fig. 4D) are preserved. On the contrary, the organics detected in the Río Tinto acidic sulfate deposits should have a long-term preservation if protected against the meteoric and diagenetic solutions as demonstrated by Aubrey et al. (2006), who reported organics from jarosite mineralogies in the Panoche Valley (California) with an age of 40 million of years.

Preservation of sulfates on some Mars deposits (Fig. 4A) like jarosite (a very soluble mineral phase) suggests that late meteoric and diagenetic solutions only partially remobilized the sulfate and the iron to form hematite concretions. If this interpreation is true, and assuming that life emerged sometimes on Mars, the Burns Formation could be expected to bear organic and paleobiological structures in the sulfate rich units (Fernández-Remolar et al., 2008c).

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