Siliceous Hot Spring Deposits: Why They Remain Key Astrobiological Targets
Sherry L. Cady; John R. Skok; Virginia G. Gulick; Jeff A. Berger; Nancy W. Hinman
Abstract
For nearly 40 years, hydrothermal deposits have been recognized as potential paleobiological repositories for astrobiological exploration of Mars. Here, we summarize the motivation for this astrobiological search strategy as it pertains to our current understanding of silica-depositing hot spring ecosystems and terrestrial siliceous hot spring deposits. We also discuss the rover and orbital observations of recently discovered hydrothermal opaline silica deposits on Mars—interpreted as evidence of hot spring activity. The opaline silica digitate sinters near Columbia Hills represent the strongest evidence to date for potential fossilized biosignatures on Mars. The high habitability and preservation potentials of hot spring deposits on Earth, along with their ability to reveal insight into the metabolic evolution of life, strengthen the rationale for targeting siliceous hot spring deposits as high-priority astrobiology sites for future Mars missions.
Keywords
Hot springs; Siliceous sinter; Hydrothermal; Opal; Silica; Columbia Hills; Nili Patera
Acknowledgments
SLC, VG, and NWH thank and acknowledge financial support from the NASA Astrobiology Institute under the SETI Institute NAI Team’s grant NNX15BB01A and NASA Interagency NNA16BB06I. Additional financial support for SLC was provided by EMSL, a DOE Office of Science User Facility sponsored by the Office of Biological and Environmental Research. JRS thanks and acknowledges financial support from the NASA PSTAR Program under grant NNX15AJ38G. JB thanks the University of Guelph. Research by SCL within Yellowstone National Park has been supported by the National Park Service permit number 1994. The authors thank the referees for comments and suggestions that improved the chapter.
For nearly 40 years, hydrothermal deposits have been recognized as potential paleobiological repositories for astrobiological exploration of Mars. Here, we summarize the motivation for this astrobiological search strategy as it pertains to our current understanding of silica-depositing hot spring ecosystems and terrestrial siliceous hot spring deposits. We also discuss the rover and orbital observations of recently discovered hydrothermal opaline silica deposits on Mars—interpreted as evidence of hot spring activity. The opaline silica digitate sinters near Columbia Hills represent the strongest evidence to date for potential fossilized biosignatures on Mars. The high habitability and preservation potentials of hot spring deposits on Earth, along with their ability to reveal insight into the metabolic evolution of life, strengthen the rationale for targeting siliceous hot spring deposits as high-priority astrobiology sites for future Mars missions.
7.1 Introduction
If microbial life ever emerged on Mars, it is likely that it would have thrived in hot springs, given that hydrothermal systems would have been widespread throughout the planet’s history (Walter, 1996; Farmer, 1996; Schultze-Makuch et al., 2007). In general, hydrothermal systems develop when subsurface fluid (meteoric, magmatic, connate), heated by rising or impact-generated magma, circulates and interacts with subterranean rock during its ascent to the surface. Hydrothermal fluid reaches the surface via subterranean fractures and passes through hydrothermal spring and geyser effluents where it redeposits dissolved minerals and aqueous precipitates as it cools (Fournier, 1989; Henley and Ellis, 1983; Sillitoe, 2015). Hydrothermal systems that developed along midocean ridges and terrestrial hot springs, whether they were initiated by volcanic, impact, or tectonic activity, represent some of the most ancestral niches for microorganisms on the early Earth (Walter, 1996; Henley, 1996; Nisbet and Sleep, 2001).
Relevant to an astrobiology search strategy for Mars is the high habitability and preservation potentials of hydrothermal ecosystems. Silica-depositing terrestrial hot springs, in particular, have the ability to serve as paleontological repositories on Earth, as well as on Mars or any other rocky planet that could have experienced sustained periods of hydrothermal activity and hosted life as we know it. Silica sinter1 deposits can preserve a variety of microbial biosignatures that include body fossils (morphologically and chemically recognizable cellular remains), biofilm and microbial mat fabrics, and biosedimentary structures (stromatolites, microbialites, and microbially induced sedimentary structures), along with chemical fossils (biosynthetic molecules, biologically fractionated stable-isotope signatures, biominerals, and anomalous concentrations and combinations of elements and minerals) (e.g., Walter, 1976b; Cady and Farmer, 1996; Jones and Renaut, 1996; Jones et al., 2001; McKenzie et al., 2001; Hinman and Walter, 2005; Georgiou and Deamer, 2014; Campbell et al., 2015a,b; Siljeström et al., 2017). Recent discoveries of massive primary opaline silica deposits in two locations on Mars, hypothesized for different reasons to be hydrothermal in origin (Squyres et al., 2008; Ruff et al., 2011; Skok et al., 2010; Ruff and Farmer, 2016), strengthen the relevance of the astrobiology search strategy for hydrothermal spring deposits on Mars that was originally proposed nearly four decades ago (Walter and Des Marais, 1993), and has been pursued and subsequently reported in hundreds of publications.
The key attributes that make siliceous sinters a compelling astrobiological target on Mars are reviewed here. In the sections that follow, we discuss a number of topics that highlight the potential astrobiological importance of the recently discovered hydrothermal opaline silica deposits on the red planet.
7.2 Hot Spring Deposits as Astrobiology Targets
Silica-depositing hot spring ecosystems host a wide range of metabolic strategies that are concentrated in sequential, concentrically arranged zones around hot spring effluents and pools. When hot spring fluids flow out and away from near-boiling pools and geysers, the metabolism of the primary producers in the microbial communities changes from chemotrophic to anoxygenic phototrophic to oxygenic phototrophic. As shown in Fig. 7.1, the zonal distribution of distinct phototrophic microbial communities—visible because of differences in the colors of their dominant photosynthetic pigments—produces a distinctive bullseye pattern when viewed from above. The major controls on the occurrence of biofilm- and mat-forming communities in silica-depositing hot springs include temperature, pH, and H2S and O2 concentrations (e.g., Peary and Castenholz, 1964; Brock, 1978; Castenholz and Pierson, 1995; Ward et al., 1998; Bryant et al., 2007; Boomer et al., 2009; Inskeep et al., 2010). In general, the steeper the thermal and geochemical gradients along the pool rims and edges of the outflow channels, the more abrupt the transition from one microbial community to the next and the more concentrated the biodiversity at such transition zones.
Hyperthermophilic chemotrophs live, by definition, in fluids that range in temperature from near-boiling to 80°C (Brock, 1978; Stetter, 1996). In terrestrial silica-depositing hot springs, like the one shown in Fig. 7.2, hyperthermophilic communities are dominated by filamentous bacteria (Blank et al., 2002; Inskeep et al., 2013). When filamentous hyperthermophilic biofilms colonize geyserite2 surfaces, they form very thin biofilms only a few cell-layers thick, which are typically not visible with the naked eye in the field (Cady and Farmer, 1996). In rapidly flowing outflow channels of near-boiling hot spring pools, filamentous hyperthermophiles can form long (a few to a few tens of centimeters long) filamentous “streamers” that flow freely from continuous (e.g., as on a stick or rock) or isolated attachment points (e.g., from a topographical high point a millimeter or two in diameter on a flat mat surface or on a loose piece of sinter) (Reysenbach et al., 1994). Cady and Farmer (1996) were the first to demonstrate that communities of hyperthermophilic biofilms colonize nearly any surface within and along the rims of near-boiling silica-depositing hot springs and that their presence influences the fabrics and the macrostructural characteristics of geyserites. The paleobiological relevance of geyserites was recently reviewed by Campbell et al. (2015a). Hyperthermophiles are considered by many to be the closest living relatives of microbes that occupied ancient ancestral hydrothermal niches at the ocean floor and on land (e.g., Shock, 1996; Farmer, 1998; Doolittle, 1999; Nisbet, 2000; Nisbet and Sleep, 2001; Rothschild and Mancinelli, 2001; Reysenbach and Cady, 2001; Schwartzman and Lineweaver, 2004; Stetter, 2006; Glansdorff et al., 2008; Deamer and Szostak, 2010; Deamer, 2012; Weiss et al., 2016; Soto et al., 2016; Cavalazzi et al., 2018; Strazzulli et al., 2017; Dai, 2017; Price et al., 2017). Recent chemical, geological, and biochemical computational evidence has reinforced the hypothesis that life could have originated in terrestrial hot springs (cf., Van Kranendonk et al., 2018; Westall et al., 2018, and references therein).
The transition from hyperthermophilic chemotrophic biofilms to filamentous thermophilic anoxygenic phototrophic communities occurs downstream, or in vertical microniches (Jones et al., 1997), where fluid temperatures drop below 80°C. Their visible light-harvesting bacterial chlorophyll pigments reveal that these microbial populations occupy the upper layers of stratified thermophilic mats that can reach several millimeters in thickness (Castenholz and Pierson, 1995; Boomer et al., 2009). Filamentous “streamers” of anoxygenic phototrophs can also develop in rapidly flowing outflow channels in this region of a hot spring system when they become intertwined with one another and with the highest-temperature oxygenic phototrophs (cf., Meyer-Dombard et al., 2011; Siljeström et al., 2017).
Different populations of cyanobacteria, which are oxygenic thermophilic phototrophs (Castenholz, 1969; Brock, 1978), produce an even wider variety of microbial mat types located downstream from the anoxygenic phototrophic communities at lower-to-ambient fluid temperatures. Conical, pinnacle, tufted, bubble, and pustular mats develop in hot spring outflow channels and across the discharge aprons of silica-depositing hot springs. Massive low-relief, yet laterally continuous, “carpets” of sheathed cyanobacteria often form where fluid depths thin to only a few millimeters thick (i.e., “sheet” flow) and temperatures drop to ambient. The combination of evaporation and cooling of hydrothermal fluids at the distal ends of silica-depositing hot spring ecosystems typically encapsulates sheathed cyanobacteria, which enhance their preservation potential (Farmer, 1999; Guido and Campbell, 2017).
The sequential distribution of distinct thermophilic communities and their corresponding lithified remains in silica-depositing thermal springs in Yellowstone National Park led Walter (1976b) to propose a correlative set of biological and lithological facies3 for such systems. When mineralized, the different mat biofacies can be correlated with morphologically similar silica sinter deposits that Walter (1976b) described as sinter lithofacies (cf., Fig. 7.1, Cady and Farmer, 1996). This biofacies-lithofacies model provides a robust framework for reconstructing the paleoecology of these types of hot spring deposits, even after the primary opaline silica of the sinters transforms diagnetically to more thermodynamically stable silica phases (Rodgers et al., 2004; Lynne, 2012).
The biofacies-lithofacies model is highly relevant to astrobiology search strategies on Mars for three main reasons: (1) Lithofacies of silica hot spring/geyser deposits preserve a variety of biosignatures indicative of the primary microbial inhabitants of the ecosystem across multiple spatial scales. Multiple lines of evidence increase the probability of the biogenic origin of possible biosignatures (Mustard et al., 2013; Westall et al., 2015a; Hays, 2015; Horneck et al., 2016). For silica-depositing hot springs, the biosignatures with the highest morphological fidelity include microbial mat fabrics and fossilized microbial remains (Walter, 1976b; Walter et al., 1996; Hinman and Walter, 2005; Campbell et al., 2015b); (2) Recently discovered primary opaline silica deposits on Mars, interpreted as silica sinters (Squyres et al., 2008; Skok et al., 2010; Ruff et al., 2011), do not appear to have undergone even the earliest stage(s) of diagenesis, which implies that the deleterious effects of silica phase transformations, which cause subsequent, incremental loss of paleobiological information, would not have impacted microbial biosignatures if they were preserved in such deposits (Walter et al., 1996; Hinman and Walter, 2005; Guido and Campbell, 2017); (3) Lithofacies of hot springs and geysers preserve a paleobiological record of the biodiversity of its inhabitants. If the paleoecology of a hot spring ecosystem was decipherable in siliceous sinter deposits on Mars, the discovery of different lithofacies could reveal whether anoxygenic and oxygenic phototrophy ever evolved as key microbial metabolic strategies on another world. In addition to having implications for astrobiology, such a discovery would impact the selection and/or optimization of life-detection instruments on future Mars missions and the design of exploration strategies for other possible habitats (Parnell et al., 2007; Worms et al., 2009; Hays, 2015; Horneck et al., 2016; Vago et al., 2017).
The ubiquitous occurrence on Earth of laminated microbial mat fabrics in silica-depositing hot spring systems, enhanced by pervasive in situ mineralization of biofilms and mats, strengthens the proposition for astrobiological exploration of sites where hydrothermal silica deposits have been located on Mars. The morphological and cellular fidelity of biosignature preservation in hot springs on Earth depends upon the intrinsic cellular characteristics of heat-loving organisms (e.g., Jones et al., 2001; Konhauser et al., 2001; Yee et al., 2003; Benning et al., 2004a,b; Amores and Warren, 2007, 2009; Hugo et al., 2010; Campbell et al., 2015b) and the extrinsic geochemical, hydrodynamic, and seasonal factors that influence silica sinter precipitation and accumulation (e.g., Hinman and Lindstrom, 1996; Braunstein and Lowe, 2001; Amores and Warren, 2007; Yee et al., 2003; Orange et al., 2013; Alleon et al., 2016). An example of how the intrinsic characteristics of particular cells can enhance their preservation is illustrated by the sheathed cyanobacterium Calothrix, the dominant cyanobacterial population in distal low-temperature regions of silica-depositing hot springs. These organisms tend to be preferentially preserved in the geological record due to their dynamic response to silicification (i.e., their sheath thickens (Benning et al., 2004b) and mineral precipitation can be localized on specific ultracellular structures in their sheathes (Hugo et al., 2010)) and the preferential preservation of their sheaths compared to other cellular components in siliceous sinter deposits (Farmer, 1999; Hinman and Walter, 2005; Guido and Campbell, 2011; Campbell et al., 2015a,b). An example of the impact of extrinsic factors on preservation is illustrated via a comparison of the effects of different modes of fossilization (cf., Fig. 9 in Cady and Farmer, 1996): cells rapidly and completely replaced by opaline silica can retain their morphological fidelity; cells completely entombed but only partially replaced by opaline silica can be permineralized (cf., Cady, 2002); and cells incompletely entombed and cemented while still viable in opaline silica typically lyse after death and their cellular contents are rapidly destroyed through oxidation. When the carbonaceous remains of cells and extracellular polymeric substances (EPS) are preserved in opaline silica, they can be characterized by multiple chemical biosignatures (e.g., Siljeström et al., 2017). The degree to which the effects of high UV and ionizing radiation and post-preservational oxidation affect the structural and chemical fidelity of microbial remains has only recently been explored. Theoretical considerations and experimental studies indicate that cosmic rays can destroy amino acids mixed with hydrated silica phases like opaline silica in < 100 million years (Summons et al., 2011; Pavlov et al., 2012).
Comparative studies of modern and ancient thermal spring deposits have shown that the highest morphological fidelity of preservation is skewed toward sinter biofabrics and stromatolite structures (Walter, 1976b; Hinman and Walter, 2005; Handley et al., 2008; Campbell et al., 2015b; Westall et al., 2015b; Guido and Campbell, 2017). The ability to recognize macroscale stromatolite structures and especially the microscale biofabrics of the majority of hot spring and geyser lithofacies in older sinter deposits is remarkable given that, on Earth, the primary opal-A of sinter ultimately transforms to quartz (Rice et al., 1995, 2002; Walter et al., 1996, 1998; Herdianita et al., 2000; Campbell et al., 2001, 2015b; Trewin et al., 2003; Lynne and Campbell, 2003, 2004; Lynne et al., 2005, 2007; Guidry and Chafetz, 2003; Hinman and Walter, 2005; Guido and Campbell, 2011; Barbieri et al., 2014; Westall et al., 2015b; Djokic et al., 2017). The fine-scale (millimeter) laminations of macrostromatolites (columns, digitate structures) are preserved in the oldest known siliceous sinters, which were recently discovered in the Archean Dresser Formation of the Pilbara Craton in Western Australia (Djokic et al., 2017). On Earth, the long residence time of quartz at the Earth’s surface is imperative for preservation of such morphological details in the geological record (Farmer and Des Marais, 1999). On Mars, the persistence of massive monomineralic opaline silica deposits that could have originated in hot springs suggests that the more delicate morphological features of microbial fossils and stromatolites can be expected to still exist in such deposits.
Relevant to interpretations of the source of the opaline silica deposits on Mars is the fact that economic geologists and geochemists have known for decades that both alkali-chloride and acid-sulfate-chloride fluids can source surficial hot springs that deposit silica sinters in terrestrial hydrothermal environments (cf., Ellis and Mahon, 1977; Henley and Ellis, 1983; Nicholson, 1993; Renaut and Jones, 2011a). In addition to sourcing hot springs, subsurface hydrothermal fluids contribute to the production of a variety of different types of silica deposits that have only recently been recognized as being distinct from sinters (Sillitoe, 2015). These may include silica residue and pseudosinter (e.g., silicified water table deposits, silicified travertines, silicified volcanics, and silicified volcaniclastics (Rodgers et al., 2004; Guido and Campbell, 2011, 2017; Sillitoe, 2015). The association of such deposits with hydrothermal processes can lead to their misidentification as sinter because they may display laminated fabrics and have a primary opaline silica mineralogy. The discovery of fossil evidence of filamentous fabrics in epithermal samples collected from hydrothermal deposits worldwide, archived in independent museum and researcher collections, suggests that mineralization of filamentous chemotrophic microbiota is a common process in low-temperature (< boiling) subterranean environments where silica-saturated fluids precipitated primary opaline silica on Earth (Hofmann and Farmer, 2000).
Walter and Des Marais (1993) emphasized that remote-sensing techniques could be used to distinguish surface sinters produced by hydrothermal activity due to their distinctive geomorphic features (typically mounds flanked by terraces, channels, and broad discharge aprons) and the geochemical and mineralogical differences between the nearly monomineralic sinters and their surrounding country rock. Hydrothermally altered ground in large hydrothermal fields, caused by fumarolic activity and hydrothermal fluid-rock interactions, can produce a range of geochemically predictable mineral assemblages (Buchanan, 1981; Tosdal et al., 2009). Another consideration relevant to remote detection of hydrothermal deposits is the possibility that surficial hot spring deposits may have been partly/completely removed by erosion, weathering, or impacts, leaving the exposed epithermal paleosurface of the more voluminous subsurface component of a hydrothermal system. This evidence for hydrothermal activity could also be detected remotely as local to regional-scale mineral alteration haloes, the geochemical nature of which could reveal insight with regard to the composition of the hydrothermal fluid if the composition of unaltered country rock can be determined (Henley, 1996; Sillitoe, 1993, 2015).
A final consideration relevant to an astrobiology search strategy for hydrothermal systems on Mars is the geological context of hydrothermal systems, which require a localized subsurface heat source, typically volcanic or impact-related, and subsurface water or ice (cf., Newsom, 1980; Gulick, 1998). Such features have been used to prospect for large-scale epithermal mineral deposits for decades (e.g., Huntington, 1996). Even on small rocky planets like Mars that never developed appreciable plate tectonic activity, hot spring deposits likely formed whenever impact formation, volcanism, magmatic intrusions, or tectonic events resulted in prolonged hydrothermal activity (Gulick, 1998; Osinski et al., 2013; Westall et al., 2015b).
Some of the earliest convincing evidence for hydrothermal activity on Mars were remote images of simple channel systems along the margins of impact craters that were found in data sets of images generated from mapping Mars globally at relatively low spatial resolution (~ 200 m per pixel) during the Viking Orbital Missions. These investigations revealed that several major martian volcanic provinces contained possible hydrothermal features evidenced by the superposition of fluvial geomorphological features associated with extended volcanic activity (e.g., locations that include Apollinaris Mons, Alba Patera, Hadriaca Patera, Tyrrhena Patera, Hecates Tholus) (Gulick and Baker, 1989, 1990; Gulick, 1998, 2001; Schultze-Makuch et al., 2007; Osinski et al., 2013). In spite of the promising findings from remote imaging data sets, the remote and standoff acquisition of elemental and mineral spectra from the surface of Mars proved instrumental in the discovery of geochemical evidence of hydrothermal activity on Mars.
7.3 Detection of Siliceous Hydrothermal Hot Spring Deposits on Mars
Once the technologically more advanced imaging spectrometers were flown on the recent Mars Global Surveyor (MGS) and Mars Reconnaissance Orbiter (MRO) missions, the discovery of possible hydrothermal features on Mars that were associated with fluvial morphology and a volcanic geological context steadily increased (Crumpler, 2003; Farrand et al., 2005; Crumpler et al., 2007; Rossi et al., 2007, 2008a,b; Raitala et al., 2008; Allen and Oehler, 2008). MGS returned high-resolution (~ 6 m/pixel) narrow-angle camera coverage over targeted regions for nearly 10 years (nearly 5 Mars years) after its arrival in 1997, and the MRO HiRISE camera imaged the surface with ~ 0.25 m/pixel resolution starting in late 2006. CRISM visible and near-infrared (VNIR) spectrometers, also onboard the MRO mission, acquired mineralogical data (~ 20 m/pixel highest resolution) that resolved surface and geochemical features on spatial dimensions of ~ 80 m or larger.4
Even with the combination of higher resolution imaging and spectroscopic data sets, there were still complications in making reliable identifications of hydrothermal fields from orbit, let alone identifying compositionally distinct individual hot spring deposits. Many areas that display the geomorphological evidence consistent with the presence of hydrothermal activity on the surface of Mars failed to produce the needed geological evidence in terms of mineralogical and surface landforms at higher resolution. This is largely because much of the hydrothermal activity associated with the formation of large volcanoes or impacts on Mars is ancient and took place between several hundred million and 3.5 billion years ago. Numerous subsequent resurfacing events and erosional and depositional processes would have erased much of the meters-to-tens-of-meters-scale landforms associated with hydrothermal activity. Large volcanoes, like the Tharsis Montes and the Elysium and Apollinaris Mons, are situated at high elevations where global dust storms can deposit material but where the atmosphere is too thin for subsequent winds to remove it. This creates recent, thick dust layers at high elevations that obscure spectral and geomorphic observations of small-scale hydrothermal features. The presence of a persistent dust cover is a major hindrance for reliable remote mineralogical identification. For example, in CRISM VNIR spectra, the mineralogy of only the top few tens of micrometers of the surface can be detected.
Fortunately, CRISM VNIR data from the volcanic complex of Syrtis Major—one of the lowest elevation volcanoes and most dust-free regions on Mars—revealed robust evidence of regional hydrothermal activity in a volcanic geological setting. Multiple distinct opaline silica mound structures identified at this site are similar in mineralogy and morphology to those associated with terrestrial hot springs on Earth (Skok et al., 2010).
7.4 Mars Hot Spring Deposits at Nili Patera
At the center of Syrtis Major is a series of nested caldera depressions, the best defined of which is a 50-km-wide depression in Nili Patera (Fig. 7.3). The caldera is unique among martian volcanic terrains in that it hosts evidence of both effusive and explosive volcanism, nearly monomineralic silica deposits, and compositional diversity that ranges from olivine-rich basalts to silica-enriched units (Fawdon et al., 2015).
Northeast of the caldera is the 300-m-high volcanic cone Nili Tholus. White-toned deposits were reported by Skok et al. (2010) as opaline silica, which was detected in CRISM spectral data on and around the cone (Fig. 7.4).
As shown in Fig. 7.5, CRISM data also revealed that—when normalized to the surrounding materials—the proximal white-tone deposits have a strong, asymmetric 2.21-μm VNIR absorption feature, which is caused by a combination vibrational absorption (OH stretch and SiOH bend) (Skok et al., 2010). These deposits also have weak (even for Mars) hydration absorptions at 1.4 and 1.9 μm (cf., Skok et al., 2010; Sun et al., 2016a,b). In combination, these spectral absorptions are consistent with an opal-A sinter composition, an interpretation strengthened by the geological context and distribution of the deposits on, and adjacent to, the Nili Tholus volcanic cone within the Nili Patera caldera. Since the initial report of the discovery of these silica sinter deposits around Nili Tholus, additional Si-OH spectral signatures have been identified in several areas within the volcanic flows. Deposits with similar absorptions were also detected in regions to the west and southwest of the cone, all lying on an evolved silica-enriched unit identified by thermal infrared observations with the use of THEMIS (Christensen et al., 2005).
In contrast to the discrete and point source nature of the near-cone mound deposits, opaline silica deposits to the west tend to be laterally more continuous (e.g., inset, Fig. 7.5). Their distribution may represent the remains of a transient hydrothermal system driven by volcanic heat flow and the release of subsurface volatiles. Alternatively, the laterally continuous nature of the westernmost silica deposits, in contrast to the distinct mound structures of the proximal deposits, could be the result of enhanced hydrothermal venting and subsequent opaline silica deposition along shallow subterranean fractures.
Hydrothermal silica deposits to the southwest of Nili Tholus lie within the circumference of a region that consists of ejecta of two ~ 100-m-wide craters. Such features are consistent with opaline silica deposition during active volcanism. Syrtis Major is estimated to have been active for about 100 million years and experienced multiple caldera-forming events (Hiesinger and Head III, 2004). While hydrothermal systems may have been active throughout this entire period in this region of Mars, only the events younger than any local volcanic resurfacing or dust burial would be visible from orbit. Such a long-lived, stable subterranean hydrothermal system could have generated numerous surface springs, each of which had the potential to support life and preserve it in paleobiological sinter repositories if Mars was ever inhabited by extremophilic bacteria.
7.5 Opaline Silica Deposits at Columbia Hills
Robust evidence for hot spring sinter deposits was discovered near Gusev crater during ground exploration by the Spirit rover during the twin Mars Exploration Rover (MER) missions. The Spirit rover landed in 2004 on relatively young, Hesperian, lava plains. In addition to other sedimentary rock types, hydrothermal deposits were predicted to occur within Gusev crater, likely sourced from exposed deposits that rimmed and surrounded smaller impacts (Cabrol et al., 2003). After exploring Gusev crater via multiple ground traverses, the Spirit rover traversed eastward for 6 months toward the Columbia Hills.
The terrain at the Columbia Hills was found to be remarkably different from the volcanic terrain located at the landing site, consisting of older Noachian terrains that lie topographically higher than the surrounding, younger volcanics. From the top of Husband Hill, a panoramic survey photographed by Spirit revealed a pentagon-shaped light-toned feature in the valley below. As shown in Fig. 7.6, this ~ 80-m-wide feature, named Home Plate, became a prime mission objective. The rover reached Home Plate on sol 744 and began a multiyear study of the surrounding terrain.
In the Eastern Valley, located between Home Plate and Mitcheltree Ridge, the results of multiple in situ instruments showed that the light-toned nodules found in association with hydrated ferric sulfates were enriched in Si. Alpha Particle X-Ray Spectrometer (APXS) analysis revealed their highly Si-enriched nature relative to the hydrated ferric sulfates located in nearby rocks and soils (Ming et al., 2008). Miniature Thermal Emission Spectrometer (Mini-TES) analysis of outcrop nodules known as the “Tyrone-nodules” (sols 1100 and 1101), named for its proximity to the Tyrone sulfate-rich soil deposit (Yen et al., 2008; Wang et al., 2008), revealed that the light-toned materials produced an opaline silica spectral signature comparable with recent hydrothermal sinter deposits on Earth (e.g., Fig. 3 of Squyres et al., 2008; Ruff et al., 2011). On sol 1148, a nearby underlying patch of light-toned sediment (named Gertrude Wiese (e.g., Fig. 2 in Squyres et al., 2008 Navcam frame 2N233253342) was exposed after the Spirit rover drove over an encrusted, poorly consolidated unit that crumbled due to the resistance created when the broken wheel of the rover dragged across its surface. Subsequent APXS elemental analysis of this white-toned sediment indicated that it ranged in composition from ~ 65% to 92% wt% SiO2 (Squyres et al., 2008), the latter concentration representing the highest silica content found to date on Mars. An example of the nodular opaline silica in outcrop is shown in Fig. 7.7.
Additional analytical measurements made via the Spirit rover on soils and outcrops around Home Plate over the next several months revealed opaline silica deposits in a variety of locations in the region. By sol 1220, 17 Mini-TES outcrop measurements that were consistent with the presence of opaline silica, though contaminated to varying degrees by silica-poor soil and dust, were acquired before analysis had to be stopped due to interferences created by the accumulation of airborne dust on the Mini-TES optics. Collectively, the silica-rich materials were interpreted by Squyres et al. (2008) as having formed under low-pH hydrothermal conditions, either as fumarole-related acid-sulfate leaching of basalts or precipitated as low-pH silica sinter. Low-pH hydrothermal conditions were favored because of the trends in major element enrichment (Si and Ti) and depletion (Fe, Na, Al) relative to other Gusev crater volcanic materials and the proximity of the opaline materials and the ferric sulfates, the latter of which were also interpreted as having a probable low-pH hydrothermal origin (Yen et al., 2008).
A critical examination of the characteristics and distribution of opaline silica led Ruff et al. (2011) to conclude that the silica-rich deposits around Home Plate originated as the erosion of a laterally persistent, stratigraphically restricted interval of silica sinter. They found no spectroscopic evidence to indicate that the opaline silica had been diagenetically matured beyond opal-A and no geochemical evidence in the deposits that ruled out the precipitation of the opaline silica by near-neutral pH thermal spring fluids.
Morphological comparisons of several features imaged in silica deposits at Home Plate with those of silica sinters associated with hot springs at El Tatio, Chile, strengthened the interpretation that the martian silica-rich deposits originated in hot spring discharge outflow channels (Ruff, 2015; Ruff and Farmer, 2016). The presence of centimeter-size pieces of nodular masses of opaline silica with different types of digitate protrusions of various length, shape, and orientation, along with their distribution in stratiform outcrops on the floors of local topographic lows, are remarkably consistent with the morphology and distribution of nodular sinters produced in outflow channels and debris aprons covered with water of various depth around some silica-depositing hot springs at El Tatio (Fig. 7.8). Spectroscopic analysis of halite-encrusted nodular and digitate silica structures at El Tatio also produces infrared spectra that are most similar to the spectra obtained from morphologically similar structures in the siliceous sinter deposits at Home Plate (cf., Fig. 5a, Ruff and Farmer, 2016).
7.6 The Likelihood of Finding More Hot Spring Deposits on Mars
The question arises as to whether additional hydrothermal deposits will be found with further exploration, given that these two detections are the basis of our understanding of spring deposits on Mars. After the caldera spring deposits were found in Nili Patera, a significant effort was made to search the other martian calderas for orbital evidence of hot spring-like deposits characterized by similar morphology and composition. However, the concerted effort resulted in no new findings, and hence the question is whether Nili Patera is fundamentally unique, either in its geological setting or because the region is relatively dust free.
A comparative analysis of fundamental differences in the geological setting of the Nili Patera caldera and Syrtis Major volcanic region indicates that their geological setting is unique. The Nili Patera caldera surface lies in an ~ 2-km depression that likely formed as a result of the crustal relaxation of the Isidis basin (Fig. 7.9). This setting contrasts that of most of the main Hesperian and Amazonian volcanics, which lie topographically higher than the volcanic terrain that surrounds Syrtis Major (estimated to be ~ 500-m thick, Hiesinger and Head III, 2004). Regional crustal relaxation would also have created fractures that could have enhanced volcanism and caldera collapse. The latter would be expected to produce a longer-lived source of high-silica hydrothermal fluid and more voluminous spring sinter deposits at surface effluents (e.g., generated from a deeper part of the volcano closer to differentiating high-Si magma bodies). Syrtis Major is surrounded by hydrated Noachian crust, unlike all other calderas on Mars that are surrounded by their own volcanics. Such high concentrations of dissolved silica in crustal hydrothermal fluid could have sustained large-scale hydrothermal systems and made spring deposits common in this region.
The discovery of siliceous hot spring deposits at Home Plate was fortuitous, even though hydrothermal deposits were predicted as potential sedimentary rock in Gusev crater (Cabrol et al., 2003; Schwenzer et al., 2012). If widespread volcanism during this period on Mars drove hydrothermal systems that fed abundant spring systems across the surface of the planet (including possible deep-sea hydrothermal deposits, cf., Michalski et al., 2017), hot spring deposits like those found at Home Plate could be more common in Noachian terrains. They may not, however, be obvious; extensive erosion would have erased much of the volcanic context and global dust layers would have covered local deposits. Such factors may inhibit the remote detection of locally restricted outcrops of sinter from orbit, as they did at Home Plate. To test this hypothesis, further exploration of Noachian-age terrains is required.
7.7 Geochemical Considerations
Given the necessary geochemical context (i.e., some type of heat source and subsurface water), it might be considered strange not to find silica-rich hydrothermal deposits distributed across Mars. The average martian crust is basaltic (Taylor and McLennan, 2010) with a relatively narrow range of silica content (45–52 wt%), inferred primarily on evidence from martian meteorites, orbital spectroscopic observations, and in situ rover analyses of rocks interpreted as igneous (including volcaniclastics). Orbital spectral signatures indicate that basalt and basaltic minerals (i.e., pyroxene, plagioclase, and olivine) dominate much of the martian surface. Analytical comparison of the geochemical composition of unconsolidated regolith located thousands of kilometers apart by the Spirit and Opportunity rovers and the Mars Science Laboratory (MSL) mission rover Curiosity indicates regional basaltic sources (Yen et al., 2008; O’Connell-Cooper et al., 2017). Dust measured by the three rovers is also basaltic, with enrichments in volatile and moderately volatile elements (S, Cl, and Zn) (Yen et al., 2008; Berger et al., 2016). Because global dust storms have been observed regularly (about every three martian years), and the dust has a uniform composition (like at these three rover sites), it is considered a global geological unit that represents an average sampling of the Mars surface. Indications of magmatic diversity have been discovered (e.g., Papike et al., 2009; Thompson et al., 2016; Treiman et al., 2016); however, evidence of high-silica lithologies that formed by igneous fractionation processes (e.g., andesite, trachyte, rhyolite, granite) is limited and often ambiguous, suggesting that this pathway to silica enrichment is not widespread on Mars.
On Earth, aquifers with abundant mafic minerals (pyroxene and olivine) usually have the highest silica concentrations. Pyroxene, olivine, and glass in basalt weathers readily, releasing silica into solution. Silica concentrations can be even higher in volcaniclastic deposits, where small irregular grain shape, small grain size, and increased porosity increase the surface area of the sediments to promote chemical and physical weathering processes. Another consequence of weathering is that groundwater in pyroxene- and olivine-rich volcaniclastic units can have ~ 5 times more dissolved silica than in average groundwater (Langmuir, 1997). Increasing temperature can increase silica solubility by up to two orders of magnitude, leading to concentrations ~ 5–30 times higher in hydrothermal systems on Earth. Thus, a hydrothermal system in a basaltic regime has a high potential for mobilizing and concentrating silica.
The buffering capacity of basalt must be considered when predicting and modeling silica solubility in hydrothermal fluids in a basaltic setting such as the martian crust. Silica solubility generally does not change with pH in circumneutral to acidic waters; however, silica solubility increases significantly in alkaline fluids (pH > 10). Thus, buffering of weathering reactions influences dissolved silica concentrations. Weathering of basalt by mildly acidic fluids will lead to the release of cations into solution, which buffers the pH to circumneutral or mildly alkaline levels. In rare ultramafic settings on Earth, this process can lead to a pH of 12 in Ca2 +-OH− waters (Barnes et al., 1978). In open weathering systems on Earth, atmospheric CO2 provides acidity via carbonic acid formation at pH < 6, which promotes the formation of carbonates in basaltic settings. Atmospheric CO2 is a plausible source of acidity for ancient Mars, but evidence of sulfate deposits greatly exceeds that of carbonates, an indication that sulfuric acid may have inhibited massive carbonate formation and dominated weathering. Alternative hypotheses suggest that carbonate deposits—in addition to those already detected (Ehlmann et al., 2008)—are deeply buried, or that they formed early and were dissolved by acidic fluids later in Mars’ history. Nevertheless, at low water-to-rock ratios and in highly concentrated hydrothermal brines, the buffering capacity of basalt can be exceeded and acidic or alkaline solutions can evolve.
7.8 Competing Hypotheses for the Origin of Silica-Rich Deposits on Mars
The traverses of both MER Spirit and MSL Curiosity rovers led to the discovery of silica-rich materials that illustrate the diversity of silica-rich systems on Mars. In Gusev crater, the rover Spirit encountered highly localized silica enrichments in soils and rocks; whereas, at the base of Mt. Sharp in Gale crater, the rover Curiosity encountered localized high-silica rocks as well as a major unit with moderately elevated silica. A comparison of the occurrence, distribution, and potential source mechanisms of high-silica concentrations in rocks and soils analyzed at the two sites provides insights about silica-depositing systems on Mars.
Home Plate. The association of the silica-rich unconsolidated sediment (soil) and nodular outcrop (up to 92 wt% SiO2) with volcanic tephra comprising Home Plate is consistent with formation under hydrothermal conditions. Two interpretations for the silica enrichment have been proposed: (1) the high silica is residue after the leaching of a basaltic precursor by acid-sulfate solutions (Squyres et al., 2008; Milliken et al., 2008) and (2) silica-saturated hydrothermal fluids precipitated opaline silica as sinter (Ruff et al., 2011).
Low pH solution leaching of the basaltic precursors in the Home Plate area is suggested by the compositional diversity of the silica-rich rocks (Squyres et al., 2008). Additionally, the apparent enrichment of Ti with the silica, both of which have low solubility in low pH fluids, indicates the Ti may be a residue derived from primary titanomagnetite in the basalt. Acid-sulfate solution weathering processes are also evident in the sulfate-rich soils of Gusev crater. These soils contain 4–33 wt% silica, and thus the sulfate and silica enrichments could have a common origin.
Precipitation of silica from high pH alkaline hydrothermal fluids can account for the nodular morphology and unique textures in the silica-rich rocks near Home Plate, which would be less likely to occur by way of alteration of basaltic rocks in the area (Ruff et al., 2011). Low pH acid-sulfate alteration commonly results in the addition of sulfur, but the silica-rich rocks and soils in Gusev crater have low sulfur content. In addition, Ti can be mobilized in alkali-chloride brines and precipitate as anatase (e.g., Kinsinger et al., 2010; Campbell et al., 2015b); thus, the Ti enrichment in opaline silica in Gusev crater is not necessarily an indicator of low pH conditions.
Gale crater. Silica-rich materials discovered within Gale crater have four different occurrences, two of which are grouped as altered sedimentary bedrock units (Rampe et al., 2017), and the third is in fracture-associated haloes (Yen et al., 2017). A fourth occurrence is a group of high-alkali mugearitic rocks (a class named Jake M). Silica enrichment in the Jake M rocks has been interpreted to be due to metasomatic and igneous processes (Stolper et al., 2013) and is less relevant to our discussion of the origin of primary hydrothermal silica deposits.
The altered sedimentary bedrock units have elevated silica concentrations (i.e., most in the 45–55 wt% range), though one tridymite-bearing unit named Buckskin has up to 75 wt% silica (Morris et al., 2016). The mineralogy of these units varies in detail, but crystalline SiO2 and high-Si amorphous material comprise about 25–60 wt% of the samples analyzed by the Chemistry and Mineralogy (CheMin) X-ray diffractometer (XRD) instrument (Rampe et al., 2017). The crystalline silica phases in these units include, in variable fractions, cristobalite and tridymite, and microcrystalline opal-CT. Quartz is present at ~ 1 wt% or less. The fracture-associated haloes have silica concentrations up to ~ 70 wt%. In contrast with the microcrystalline-to-crystalline silica bedrock units, silica in the haloes is largely X-ray amorphous (Yen et al., 2017).
Conflicting interpretations have been proposed to account for silica enrichment in Gale crater. Several models suggest that acidic diagenetic fluids leached basaltic sedimentary bedrock of cations (Al, Ca, Mg, Fe, Mn, Ni, and Zn), while Si and Ti were retained as residue (Rampe et al., 2017; Yen et al., 2017; Berger et al., 2017). Jarosite, a sulfate that forms at low pH, was detected in CheMin XRD data, which support the acid-sulfate fluid alteration scenario (Rampe et al., 2017). Both low (diagenetic < 150°C) and high (hydrothermal > 150°C) temperature fluids have been invoked in geochemical leaching models. One interpretation is that silica enrichment, via precipitation from either circumneutral or alkaline diagenetic fluids, occurred via chemical weathering during erosion and transport of sediment into the crater (Frydenvang et al., 2017; Hurowitz et al., 2017). The addition of high-silica phases to the sediments analyzed in the crater, which includes detrital material such as tridymite in the Buckskin unit (a likely product of silicic volcanism), would enhance silica enrichment in Gale crater (Morris et al., 2016). Reconciling these different hypotheses is challenging, in part, because the provenance of the Gale crater sedimentary rocks is poorly constrained.
At present, the same conundrum faced in interpreting the origin and potential importance of the silica deposits at Home Plate near the Gusev crater is plaguing interpretation of the opaline silica detected at Gale crater. Are the deposits leached basalts, primary aqueous hydrothermal precipitates, or diagenetic precipitates, and what was the temperature and pH of the fluid from which the opaline silica precipitated?
7.9 Site Selection Considerations Relevant to the Return to Mars
On March 22, 2010, the Spirit rover stopped operations while still investigating the Home Plate region, leaving many unanswered questions about the formation of the hydrothermal opaline sinters and their potential to host biosignatures. These questions and the detailed exploration by the Spirit rover led to selection of the Home Plate target and Columbia Hills, considered representative of hydrothermal spring environments, as one of the final three options for the Mars 2020 rover-landing site. The other two landing site options include the river delta in Jezero crater (Goudge et al., 2017, 2018), considered representative of fluvial and lacustrine environments, and Northeast Syrtis (Bramble et al., 2017), considered representative of deep crustal environments and cold springs.
The Mars 2020 mission is designed to identify ancient environments capable of supporting microbial life, and seek signs of possible past microbial life in those habitable environments, particularly in specific rocks known to preserve signs of life over time. The rover will explore sites that could have supported life as we know it on Earth and then collect and cache samples that may be returned to Earth as part of a subsequent Mars sample return mission. Returned samples can be analyzed in greater detail with higher-resolution instruments in the laboratory to determine whether life established itself on Mars and if any unique biosignatures indicative of life are preserved in such materials.
As a landing-site candidate, Columbia Hills has had mixed support. Its strength as a sample return target is often seen as a weakness to many in the scientific community, i.e., that is, the site has already been explored. As one of the only three locations on Mars to have been explored by a long duration rover, extensive details are known about the target rocks and intended samples, including the presence of multiple sedimentary facies in the hydrothermal opaline silica deposits and the morphological similarities of macrostructures to hot spring sinters. Thus, the strength of this target is that more detailed exploration and analysis of a known site with potential biosignatures is likely to be critical for astrobiology missions. Earth-based studies have shown that it can take years to decades to understand the interplay of geochemical, hydrological, and biological processes that characterize specific habitats. Having ground-truthed data for a known site on Mars would result in significantly more strategic exploration of a potential ecosystem (Cabrol, 2018). For a mission designed to quickly sample and cache the rocks of interest, knowing the rocks and their location and context would be a significant advantage. For scientists eager to explore the diversity of ancient martian geology, there would be a missed opportunity if such a major rover and return mission was dedicated to incrementally advancing a previously explored location when so much about the planet is still unknown.
The case for returning to Columbia Hills is centered on the realization that opaline silica sinter at the site exhibits potential morphological biosignatures—millimeter-scale digitate features—that are texturally similar to those found in terrestrial hot spring systems, such as the El Tatio, Chile, geothermal field (Ruff, 2015; Ruff and Farmer, 2016). The high elevation, correspondingly lower pressure, high UV, and high evaporation rates at El Tatio (Nicolau et al., 2014) make it arguably the most Mars-like of the known terrestrial spring deposits. Geomorphic mapping has demonstrated macroscale and microscale similarities in the digitate deposits at El Tatio and those on Mars. This mapped, crossplanetary feature would be the primary target of a Mars 2020, or future, mission to Columbia Hills.
On Earth, digitate sinter features preserve abundant microscale evidence of the microbes that colonized their surfaces when the opaline silica structures formed (Cady and Farmer, 1996; Braunstein and Lowe, 2001; Jones and Renaut, 2003; Lowe and Braunstein, 2003; Handley et al., 2005, 2008). Digitate sinters on Earth are associated with hot spring features in which microbial life thrived throughout our planet’s history. Whether the macroscale and microscale biogenic characteristics of digitate sinters are a requirement for their formation or just the result of having formed on the biologically ubiquitous Earth is still an active area of research.
The opaline silica digitate sinters on Mars represent some of the strongest evidence to date for potential biosignatures on the planet (Ruff and Farmer, 2016). Until a future mission returns to the Columbia Hills, or a rover traverses to Nili Patera, or encounters siliceous hot spring deposits at some other site on Mars, a number of key outstanding questions with regard to the potential for life to have gained a foothold and evolve on Mars remain unanswered. As discussed in this chapter, until sinter deposits on Mars are proven to be void of biosignatures, silica hot springs deposits on Mars remain one of the more compelling astrobiology targets known in our Solar System.