Field observations

Hot spring and geyser discharge channels at El Tatio commonly contain nodular masses of opaline silica sinter (Fig. 2). Many of these silica nodules display mm-scale digitate structures that are strikingly similar in overall form to those adjacent to Home Plate (Fig. 3). 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. Breccia clasts become coated by laminated opaline silica via silica precipitation during transport along outflow channels, and are then subject to further fragmentation during cycles of transport and cementation. This produces pebble to cobble-sized breccias containing a complex association of volcaniclastic and silica sinter materials with diverse internal textures and compositions that reflect local sediment sources. Breccias that line channel floors and margins provide the substrate upon which digitate structures form (Supplementary Fig. 1).

Figure 2: A portion of the volcanic hydrothermal system at El Tatio in Chile. Discharge channels emanating from small (∼1–3 m) steaming hot springs behind stone barricades (arrows) deposit silica sinter and efflorescent salts amidst volcanic detritus. Rock hammer is 33 cm long. Full size image

Figure 3: Comparison of opaline silica structures adjacent to Home Plate with those of hot spring discharge channels at El Tatio. (a) Home Plate opaline silica occurs in nodular masses with digitate structures that resemble those at El Tatio (b), at the same scale. Mars scene is cropped from Fig. 1b. White scale bar in a and b represents 10 cm. (c) Home Plate opaline silica digitate structures resemble those at El Tatio (d) at the same scale. The white scale bar in c,d represents 5 cm. Insets highlight notably similar structures. Mars scene is a Pancam ATC image (‘Elizabeth Mahon’, sol 1160, P2582). Reddish hues in Mars scenes are due to thin airfall dust accumulation. (e) Grayscale Microscopic Imager mosaic (sol 1157) of a portion of the ‘Elizabeth Mahon’ silica outcrop on Mars shown in c has similar structures as those on sample ET1-1A from a hot spring discharge channel at El Tatio (f). The white scale bar in e,f represents 1 cm. Full size image

El Tatio discharge channels that host nodular, digitate sinter typically have shallow (<5 cm depth) flowing water that supports microbial biofilms and mats containing a diverse assemblage of diatoms and filamentous cyanobacteria, where water temperature is <40 °C (ref. 10). Water pH is circum-neutral (∼6.5–7.5) throughout El Tatio11. The aspect ratio, shape, and spatial density of the nodular and digitate structures vary among different channels and even along flow paths within a given channel (Supplementary Fig. 2), likely due to differences in depth, flow direction and velocity and the microenvironmental conditions created by microbial communities12. Morphologic variations also are evident among the Home Plate silica structures (Figs 1 and 3a,c,e), perhaps indicative of similar variations in depositional conditions.

Some of the mm-scale textural features of El Tatio silica sinters appear to have counterparts among Home Plate nodular silica outcrops. One of the Home Plate outcrops was intentionally disturbed with the rover’s wheel (Fig. 1d), producing broken and/or overturned fragments with exposed interior and/or underside surfaces that likely are relatively pristine. These fragments were investigated using the Microscopic Imager (MI), a camera mounted on the rover’s arm capable of grayscale imaging at ∼30 μm per pixel (ref. 13). As shown in previous work, one of the fragments, dubbed Norma Luker, displays a texture suggestive of a sinter breccia4, which is common among terrestrial sinter deposits including those at El Tatio (Fig. 4a,b). A second fragment, dubbed Innocent Bystander, displays a pervasive microporous surface texture with coated grains4 that together resemble a sample of El Tatio nodular silica sinter collected from a discharge channel (Fig. 4c,d). At El Tatio, this precipitated texture is less common than breccia textures and appears limited to the underside of sinter nodules there. This may be consistent with Innocent Bystander representing the now exposed underside of an overturned fragment of the outcrop.

Figure 4: Comparison of mm-scale textural features of home plate opaline silica rocks and El Tatio silica sinter samples. (a) Fragments dubbed Norma Luker (MI image, sol 1291) from the disturbed outcrop seen in Fig. 1d display a texture like that of El Tatio sinter breccia in b. (c) Another disturbed outcrop fragment, dubbed Innocent Bystander (MI image, sol 1251), displays microporosity and possible coated grains like that of the underside of El Tatio silica nodule in d. (e) The variably textured surfaces of Elizabeth Mahon (MI image, cropped from Fig. 3e) are similar to those of El Tatio silica nodule in f, which is the topside of the one shown in d. White scale bar represents 1 cm and applies to all images. Full size image

A porous, sponge-like texture was described previously for the Home Plate nodular silica outcrop dubbed Elizabeth Mahon, along with its smoother digitate protrusions4 (Fig. 4e). The smoother portions were suggested to be the result of aeolian abrasion, but the porous texture was not interpreted. Close inspection raises the possibility that the appearance of porosity may be due in part to fine basaltic sand, evident elsewhere in the scene, trapped within roughness elements on the surface of the outcrop. We now recognize a candidate for the variably textured surfaces of Elizabeth Mahon among samples of El Tatio sinter, in which sub-mm roughness creates an irregular pattern that mimics the appearance of mm-scale porosity (Fig. 4e,f). The protruding features of the El Tatio sample are smoother, akin to those on Elizabeth Mahon. The variable texture of the El Tatio sample is actually a surficial fabric rather than a manifestation of porosity, as readily demonstrated by comparing this topside surface to the underside of the same sample, which displays unambiguous microporosity (Fig. 4d). Apparently silica accumulated on the top surface of the El Tatio sample via evaporative precipitation in a manner that obscures the bulk porosity and created variable roughness. Although this candidate textural analog does not preclude the possibility that aeolian abrasion is responsible for the variable texture of Elizabeth Mahon, it demonstrates that silica deposition alone can lead to a similar texture.

It is important to recognize that independent of whether we have identified the correct textural analog for this or other Home Plate silica outcrops, the presence of mm-scale textural variations seen among them is a characteristic consistent with what is seen in terrestrial silica sinter deposits. The varied textures of terrestrial sinters reflect the diverse depositional environments of hot spring/geyser systems over a range of spatial scales14,15,16. This also is true of the microscale internal textures of silica sinters (<100 μm), including El Tatio samples for which we present scanning electron microscopy (SEM) and petrographic thin section views in a subsequent section. Unfortunately, the resolution of Spirit’s microscopic imaging capability precludes our ability to observe any microscale features among the Home Plate silica structures.

Spectroscopy

We have found that laboratory thermal infrared emission spectra of some silica sinter samples from El Tatio have a strong ∼1,260 cm−1 feature independent of emission angle. The presence of this feature in some Mini-TES spectra of Home Plate silica was assumed to result from high emission angle viewing geometry3,4 (Supplementary Fig. 3), but some El Tatio samples produce this feature at 0° emission angle, providing a good fit to Mini-TES spectra of some Home Plate silica outcrops (Fig. 5a). We attribute this spectral behavior to a thin (tens of micrometers) patchy crust of halite (NaCl) that coats sinter surfaces (Fig. 5b). The spectral contribution of halite is apparent by measuring the same sample before and after gentle scrubbing with a toothbrush and deionized water. This action effectively removed halite from the surface without disturbing the silica, which was confirmed by SEM (Fig. 5c), elemental analysis using energy dispersive spectroscopy (EDS; Supplementary Table 1), and taste. Samples measured after halite removal display a feature shifted to ∼1,250 cm−1 and substantially reduced in contrast, resulting in spectra notably similar to halite-free silica sinter, for example, from a hot spring in Yellowstone National Park (Fig. 5a).

Figure 5: Spectral effect of halite on silica. (a) Mini-TES spectrum of an opaline silica nodular outcrop adjacent to Home Plate (black, scaled by 2 × ; target Clara Zaph4, sol 1168, P3968) displays a strong feature at ∼1,260 cm−1 (vertical line) also found in halite encrusted silica sinter from El Tatio (sample ET3-3A) measured at 0° emission angle (blue; vertically offset). This feature is diminished substantially and slightly shifted after halite is removed (purple; vertically offset) and also in sinter that was never halite encrusted, like that from Yellowstone National Park (magenta; vertically offset). (b) SEM view of El Tatio sample with blue spectrum in a displays a patchy halite crust (lighter areas). (c) Same view as in b but with halite mostly removed by dissolution and scrubbing, yielding the purple spectrum in a. White scale bar represents 1 mm and applies to both b,c. Full size image

Some Mini-TES spectra of Home Plate silica outcrops display a ∼1,260 cm−1 feature with a depth and position not achievable from the viewing geometry effect alone but evident among halite encrusted El Tatio sinter samples independent of emission angle (Supplementary Fig. 4). Given that halite has no absorption features in this spectral range17, the appearance of a strong ∼1,260 cm−1 feature in halite-encrusted sinter samples is enigmatic and apparently has not been documented previously. Halite has an index of refraction of ∼1.5 near 1,260 cm−1 (ref. 18) versus ∼0.5 for amorphous silica, which perhaps accentuates the known geometric effect.

The ∼1,260 cm−1 feature observed in Mini-TES spectra of Home Plate silica outcrops ranges from strong to absent4. The presence or absence of a thin, patchy halite crust akin to that of El Tatio sinter could explain this variability. The detectability of the Na and Cl in such a crust by Spirit’s Alpha Particle X-ray Spectrometer (APXS) is unknown, but would be dependent on its thickness and coverage. Unfortunately, none of the outcrop targets displaying a strong ∼1,260 cm−1 feature was measured by the APXS, precluding a direct comparison between the two instruments.

Microscopy

Our investigation of the nodular and digitate silica structures from El Tatio using high vacuum and environmental scanning electron microscopy (SEM/ESEM) revealed internal microlaminations with fenestral porosity and silica encrusted microbial biofilms with filaments, sheaths, and exopolymeric substances (EPS) on both internal and external surfaces (Fig. 6a,b). EDS showed C enrichment consistent with the presence of organic matter (Supplementary Table 1). In cross section, laminae alternate between non-porous silica, filamentous sinter, and open fenestrae comparable to microstromatolitic sinter from Yellowstone19, New Zealand20,21 and Iceland22. The role of microbial biofilms and their EPS in contributing to these microtextural features was demonstrated previously for some New Zealand siliceous microstromatolites23. Among El Tatio digitate silica structures, we have documented at least one example where unsilicified EPS film is present in an especially large (∼100 × 1,000 μm) fenestra (Supplementary Fig. 5).

Figure 6: Microscopic views of El Tatio digitate silica structures. (a) SEM image from a digitate structure broken off sample ET1-1A (Fig. 3f). Alternating non-porous and filamentous concentric laminae with fenestral porosity are evident. Inset highlights webs of silica-encrusted filaments within fenestral cavities. White scale bar represents 1 mm. (b) SEM image of a surface biofilm community showing silica-encrusted microbial filaments and sheaths, and spindle-shaped diatoms (arrows) occupying the outer surface of another digitate structure from the same sample. White scale bar represents 20 μm. (c) Photomicrograph of a transverse petrographic thin section through a digitate structure from a second El Tatio sample (ET1-1C). Microtextures include nonporous and fine-scale laminae, porous laminae with irregular to flattened fenestral cavities, and tufted palisade fabrics formed by silicified populations of filamentous cyanobacteria resembling Calothrix (family Rivulariaceae). White scale bar represents 500 μm. (d) Enlarged view from boxed area in c showing silicified Calothrix sheaths oriented roughly perpendicular to laminae. Calothrix sheaths (some containing cellular trichomes) have been heavily permeated by silica and are overlain by laminae containing silica encrusted fine filaments with orientations roughly parallel to laminae. White scale bar represents 50 μm. Full size image

Petrographic thin sections of El Tatio silica structures reveal textural and compositional complexity, reflecting a range of microenvironmental conditions during their formation. Finely laminated internal textures are evident, including both flat laminated and columnar forms of stromatolitic opaline silica (Fig. 6c) sometimes containing coated grains and pisoliths formed where silica laminae accreted onto angular clasts of porphyritic volcanics during transport (Supplementary Fig. 1). Fine laminae of clear opaline silica cement (tens of micrometers thick) typically lack identifiable microfossils. However, they alternate with thicker laminae displaying fenestral cavities that contain fine, silica encrusted filamentous microfossils and empty sheaths. Finally, columnar forms include discrete laminae that contain a wide variety of unidentified filamentous and coccoidal biomorphs, diatom frustrules, and occasionally, local populations of heavily ensheathed fossil cyanobacteria (Fig. 6d) resembling Calothrix (family Rivulariaceae)24.

Thin sections of El Tatio sinters commonly display laterally persistent, lenticular to wavy laminae dominated by distinctive palisade microtextures oriented roughly perpendicular to laminae (Fig. 6c,d). The palisades are dominated by heavily ensheathed, Calothrix-like filamentous cyanobacteria that sometimes alternate with thinly laminated intervals containing finely filamentous microfossils with recumbent orientations, parallel to laminations. The fossiliferous intervals are interpreted to be surface biofilm communities (Fig. 6b) that were entombed by opaline silica and incorporated into stromatolite profiles contributing to their accretion. In modern siliceous hot springs, such palisade microtextures have been reported widely from lower temperature, distal apron environments below ∼35 °C (ref. 25), as well as ancient analogs from the Devonian of Australia26. Palisade microtextures also were documented previously among El Tatio silica oncoids and crusts10,27. Based on the suite of textural and microbial features apparent in thin sections and SEM images, we infer that El Tatio digitate silica structures are microbially mediated microstromatolites.