3.2. Earth field sites and samples

Interpretations of Mars images and compositional information benefit from terrestrial field sites that provide analogous characteristics. Here, we include observations and samples from hydrothermal settings in the United States and Chile. Samples from field sites were collected with permission where required.

These two sites, separated by ∼4 km, are surface manifestations of the same active hydrothermal system driven by a young magmatic intrusion beneath the Mineral Mountain Range, northeast of Milford in southwestern Utah (Lynne et al., 2005). Opal Mound is the largest sinter deposit evident at the surface and was emplaced from ∼1900 to 1600 years ago from now-extinct hot spring vents on the Opal Mound fault (Lynne et al., 2005). Smaller sinter deposits are present on the Hot Springs fault to the north and are part of the Roosevelt Hot Springs that were active until 1957 (Parry et al., 1980). Currently, active surface manifestations are limited to fumaroles, steaming ground, and small seeps. Hot spring fluids were alkali-chloride in nature with a near neutral pH toward the end of their activity in the 1950s (Capuano and Cole, 1982).

This is the third largest hot spring/geyser field on the Earth, and at ∼4300 m elevation, is one of the highest. It is located in the Altiplano of northern Chile, ∼90 km north of San Pedro de Atacama. This location is noteworthy for its high evaporation and low precipitation rates, high solar ultraviolet flux, and diurnal freeze-thaw conditions (Nicolau et al., 2014), which provide more Mars-like conditions than most hydrothermal settings on the Earth (Ruff and Farmer, 2016). More than 100 geysers and other manifestations, including flowing hot springs, hot pools, mud pots, and fumaroles, have been documented, spanning an area of ∼10 km² (Glennon and Pfaff, 2003). Discharging waters are of alkali-chloride composition at near neutral pH (Nicolau et al., 2014). Sinter deposits are abundant and include nodular forms with biomediated digitate structures (stromatolites) (e.g., Jones and Renaut, 1997; Barbieri et al., 2014; Ruff and Farmer, 2016).

Similar to El Tatio, these two hydrothermal fields are part of the Central Andean Volcanic Zone and occur at high elevation (4100–4200 m) in the Altiplano. Both are adjacent to the active Isluga volcano and display hot springs and geysers among the various manifestations. Puchuldiza spans ∼1 km² and Tuja, located ∼6 km northwest, spans ∼0.15 km² (Tassi et al., 2010). More than 100 active manifestations are present at Puchuldiza, with most having near neutral pH waters of alkali-chloride composition and abundant sinter deposits, some with nodular forms (Sanchez-Yanez et al., 2017). Discharging waters at Tuja also are alkali-chloride, with pH spanning ∼5–6.5, although an extreme value of 1.76 has been documented (Tassi et al., 2010). Abundant sinter deposits are present, and they include both nodular and digitate forms as presented in Section 4.