Abstract
The Mars rover Spirit encountered outcrops and regolith composed of opaline silica (amorphous SiO2·nH2O) in an ancient volcanic hydrothermal setting in Gusev crater. An origin via either fumarole-related acid-sulfate leaching or precipitation from hot spring fluids was suggested previously. However, the potential significance of the characteristic nodular and mm-scale digitate opaline silica structures was not recognized. Here we report remarkably similar features within active hot spring/geyser discharge channels at El Tatio in northern Chile, where halite-encrusted silica yields infrared spectra that are the best match yet to spectra from Spirit. Furthermore, we show that the nodular and digitate silica structures at El Tatio that most closely resemble those on Mars include complex sedimentary structures produced by a combination of biotic and abiotic processes. Although fully abiotic processes are not ruled out for the Martian silica structures, they satisfy an a priori definition of potential biosignatures.
Highlights
The Mars rover Spirit encountered outcrops and regolith composed of opaline silica in an ancient volcanic hydrothermal setting in Gusev crater
Given the volcanic hydrothermal setting and presence of opaline silica at both sites, the qualitative similarities in size and shape of the silica nodules and their digitate structures leads to the hypothesis that they may have formed through similar processes
Many El Tatio nodules are silica coated and cemented breccias composed of reworked pebbles of older, locally derived volcanic rocks and fragments of silica sinter
Summary
The Mars rover Spirit encountered outcrops and regolith composed of opaline silica (amorphous SiO2nH2O) in an ancient volcanic hydrothermal setting in Gusev crater. Acid-sulfate steam condensates produced by fumaroles have the capacity to leach metal cations from basaltic rocks, leaving behind a residue of opaline silica This process was hypothesized for the origin of Home Plate silica in the work of Squyres et al.[3], and favoured over the alternative hypothesis of silica precipitation from neutral-to-alkaline hot spring fluids. A subsequent study by Ruff et al.[4] presented observations of the silica outcrops that support a hot spring and/or geyser origin, including: their typical overlying stratiform relationship with a local rock unit (Halley Subclass) with no apparent crosscutting or fracture controlled occurrences; and their unique morphology and textures that cannot be tied to any of the potential precursor lithotypes in the exposed stratigraphic section. Previous work demonstrated that this feature varies as a function of viewing geometry[6] and that opaline silica measured at high emission angles (445°) results in a feature that in some cases has a depth sufficient to match that in some of the Mini-TES spectra[3,4]
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