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Home , Life on Mars, McMurdo (crater), Puchuldiza

51st Lunar and Planetary Conference (2020) 2929.pdf

LINKING HYDRATED SILICA TO HABITABILITY AND PRESERVATION POTENTIAL ON : LESSONS LEARNED. Steven W. Ruff1, 1Arizona State University, School of and Space Exploration, Tem- pe, AZ, 85287-6305, [email protected].

Introduction: On Earth, volcanic hydrothermal tent residue on silica sinter produced in arid environ- systems are known to produce habitable environments ments on Earth and is evident among the silica deposits that typically become inhabited by diverse microbial in the [5]. Thus, the co-occurrence of communities. Hot springs and are manifesta- hydrated silica and halite is a possible indicator of sili- tions of such hydrothermal systems that, through pre- ca sinter. cipitation of opaline silica (hydrated amorphous SiO2), produce deposits known as sinter that commonly pre- serve in the rock record textural, morphological, and geochemical evidence of microbial activity. For these reasons, sinter deposits on Mars have long been an exploration target in the search for ancient there[1]. Hydrated silica occurrences have been identified in situ by rovers on Mars, and by orbiting spacecraft in many locations across the planet. However, the pres- ence of hydrated silica does not uniquely represent a habitable environment with preservation potential. There are multiple processes known to produce hydrat- ed silica on Earth, but on Mars the most likely one relevant to is sinter deposition from hot springs/geysers. Therefore, making the connection between hydrated silica and sinter deposits is key in the search for ancient . Given that obvious geomorphic features of ancient hot springs/geysers may not be preserved (holes in the ground tend to get filled over time), I highlight the following observations to help inform current and fu-

ture exploration of hydrated silica on Mars. Some of Figure 1. Co-located silica outcrops and -rich these are detailed in a recent publication [2]. materials in the Columbia Hills (top, “McMurdo” Pan- Composition: Volcanic hydrothermal systems on cam mosaic) and Roosevelt Hot Springs, Utah (bot- Earth that manifest hot springs and/or geysers typically tom) indicative of a volcanic hydrothermal system. include other manifestations, like , which Infrared spectra of hydrated silica contain multiple emit magmatically derived sulfur-rich gases (SO2 and identifying features that have been recognized on Mars H2S) leading to S precipitates and sulfuric acid con- in both orbital and in situ observations. However, a densates with consequent acid-sulfate leaching. There- feature near 1 µm in multi-spectral Pancam observa- fore, the co-occurrence of sulfur-rich materials and tions of opaline silica materials in the Columbia Hills hydrated silica rocks is a hallmark of volcanic hydro- has remained enigmatic. It does not appear in a range thermal systems. Although this combination was doc- of natural hydrated silica materials but is present in umented by the rover in the Columbia Hills (Fig. some synthetic forms [6]. I now have observed this 1) and attributed to volcanic hydrothermal activity [3], feature in aged silica sinter (~1900 years old; [7]) that the likelihood of both fumarolic and activity has experienced incipient diagenesis (Fig. 2). On was not fully appreciated [2]. In crater, S-rich Earth, opaline silica transitions from its amorphous, materials were not observed among the silica-rich hydrated form (-A), through paracrystalline forms rocks identified by the rover, diminishing the (Opal-C/CT), to a fully crystalline, anhydrous quartz likelihood of an origin in a volcanic hydrothermal sys- via extended exposure to water. Opal-A persists on tem [4]. Mars presumably due to its extreme aridity, but per- Silica sinter deposits are produced following cool- haps the ~ 1 µm feature is evidence for incipient dia- ing and evaporation of hydrothermal silica-rich fluids, genesis. At this point, it has only been observed in which commonly include chlorine compounds like aged sinter, so perhaps is another identifying feature. halite (NaCl). Consequently, halite occurs as a persis- 51st Lunar and Conference (2020) 2929.pdf

Figure 2. Near/shortwave infrared laboratory spectra (ASU Mars Space Flight Facility) of ~1900 year old silica sinter from Opal Mound, Utah and fresh silica Figure 3. “Pioneer Mound” in the Columbia Hills sinter from , . Incipient diagenesis results (top) resembles an extinct hot spring vent mound (bot- in a strong feature near 1 µm in the aged sinter that is tom) at Puchuldiza, Chile. Scale bar indicates 2 m. absent in fresh sinter and other hydrated silica spectra, Crater: NASA’s rover mission but is evident in Spirit Pancam spectra [6]. will focus exploration efforts on the delta in Jezero Morphology and Texture: Sinter deposits on crater, but spectral evidence from orbital observations Earth display a wide range of morphologic and textural of hydrated silica adjacent to the delta adds another characteristics. One of the most common and areally potential exploration target [9]. At this time, the origin extensive sinter textures is a , produced from of this hydrated silica material is ambiguous, with hy- disaggregated and re-cemented earlier generations of drated volcanic glass as one of several possible expla- sinter. Evidence for sinter breccia is found among the nations. silica outcrops in the Columbia Hills [2], but has not Conclusions: Hydrated silica occurrences on Mars been documented among silica-rich rocks at other rov- represent another of its water related features. Howev- er sites. Nodular silica outcrops are a morphologic er, the most astrobiologically relevant occurrences are feature of sinter deposits on Earth, and also have been those associated with volcanic hydrothermal systems found in the Columbia Hills [2]. Likewise for digitate hosting hot spring/ sinter deposits. A suite of (finger-like) silica structures, which on Earth arise observables, as partially outlined above, can be used to from passive biomediation [5]. This suite of features disambiguate among the different possible origins for has only been recognized in silica sinter deposits, and hydrated silica on Mars. The opaline silica occurrence is evident in the Columbia Hills. in the Columbia Hills displays all of these features, Landscape Expression: Extensive, layered sinter which is most consistent with a sinter deposit there. aprons are features of large, long-lived volcanic hydro- The hydrated silica occurrence in Jezero crater repre- thermal systems like in Yellowstone National Park sents the first time such an orbital identification has (USA), but much smaller and unlayered examples are been made in advance of a mission. This offers the more common. However, in all cases the sinter con- to investigate it using a well informed ex- forms to the local pre-hot spring topography, produc- ploration model. ing a stratiform expression of outcrops, which may or References: [1] Farmer, J. D., and D. J. Des may not be continuous depending on the of Marais (1999), J. Geophys. Res. Planets, 104, E11, deposition and erosion. The discontinuous nodular 26977-26995. [2] Ruff, S. W., et al. (2019), silica outcrops identified in the Columbia Hills display Astrobiology, 19, 12. [3] Squyres, S. W., et al. (2008), a stratiform expression consistent with sinter deposits Science, 320, 1063-1067. [4] Frydenvang, J., et al. (Fig. 1)[2]. Although hot spring pools and geyser vents (2017), Geophys. Res. Lett., 44, 10, 4716-4724. [5] are rarely preserved due to infill (example in Fig. 1, Ruff, S. W., and J. D. Farmer (2016), Nat. Commun., bottom), in some cases they build mounds of sinter that 7, 13554. [6] Rice, M. S., et al. (2013), Icarus, 223, can be preserved. A possible vent mound has been 499-533. [7] Lynne, B. Y., et al. (2005), Sediment. identified in the Columbia Hills (Fig. 3)[2] and, based Geol., 179, 249-278. [8] Skok, J. R., et al. (2010), Nat. on orbital images, perhaps in Nili Patera [8]. Geosci., 3, 838-841. [9] Tarnas, J. D., et al. (2019), Geophys. Res. Lett., 46, 22, 12771-12782.