Mars Extant Life: What's Next? Conference Report

Abstract

On November 5-8, 2019, the "Mars Extant Life: What's Next?" conference was convened in Carlsbad, New Mexico. The conference gathered a community of actively publishing experts in disciplines related to habitability and astrobiology. Primary conclusions are as follows: A significant subset of conference attendees concluded that there is a realistic possibility that Mars hosts indigenous microbial life. A powerful theme that permeated the conference is that the key to the search for martian extant life lies in identifying and exploring refugia ("oases"), where conditions are either permanently or episodically significantly more hospitable than average. Based on our existing knowledge of Mars, conference participants highlighted four potential martian refugium (not listed in priority order): Caves, Deep Subsurface, Ices, and Salts. The conference group did not attempt to reach a consensus prioritization of these candidate environments, but instead felt that a defensible prioritization would require a future competitive process. Within the context of these candidate environments, we identified a variety of geological search strategies that could narrow the search space. Additionally, we summarized a number of measurement techniques that could be used to detect evidence of extant life (if present). Again, it was not within the scope of the conference to prioritize these measurement techniques-that is best left for the competitive process. We specifically note that the number and sensitivity of detection methods that could be implemented if samples were returned to Earth greatly exceed the methodologies that could be used at Mars. Finally, important lessons to guide extant life search processes can be derived both from experiments carried out in terrestrial laboratories and analog field sites and from theoretical modeling.

Keywords: Astrobiology; Biosignatures; Life detection; Life in extreme environments; Mars extant life.

FIG. 1.

Over 1000 pits and potential lava cave skylights have been documented on Mars in the Mars Global Cave Candidate Catalog (Cushing & Okubo, ; Cushing, 2017). These possible cave entrances provide numerous targets for the search for life in the martian subsurface. Left: Map of potential cave entrances in the Tharsis Region, where over 300 lava tube skylights and atypical pit craters are concentrated. Map from the Mars Global Cave Candidate Catalog (Cushing, 2017). Right: Three types of candidate caves were cataloged on Mars. Upper right: lava tube skylights; middle right: deep fracture systems; and lower right: atypical pit craters. Image from Cushing (2012). Slide shown by Spilde et al. ( abstract 5036) at the conference.

FIG. 2.

The NASA Ames K-Rex testing robot, renamed CaveR (“Cave Rover”) for its deployment in a volcanic cave at Lava Beds National Monument in northern California. Here, CaveR is shown moving in the down-flow direction inside Valentine Cave, scanning one side of the cave wall. Attached to the rover is a rectangular instrument pod, housing lamps, cameras, and spectrometers that were used to interrogate the cave wall during a Mars mission simulation that was a component of NASA's BRAILLE project. In the future a system like this could be used to explore caves on Mars (shown by Blank, abstract 5108).

FIG. 3.

Distribution of terrestrial subsurface biomass, from Onstott et al. (2019a). (A) Cell concentrations versus depth for ice sheets, subglacial sediments, and permafrost. Open squares = Tibetan glacial ice sheets; brown-filled diamonds = Siberian permafrost; blue-filled diamond = Siberian cryopeg; light gray–filled triangles = Antarctic ice sheets and lakes; brown-filled triangles = Antarctic subglacial sediments; brown crosses = Antarctic permafrost and subglacial sediment in New Zealand; orange crosses = Canadian High Arctic and Svalbard permafrost; light blue–filled circles = Greenland ice sheet; orange-filled circle = Greenland subglacial sediment. (B) Cell concentrations versus depth for rock and soil cores from nonpolar regions. Orange-filled circles = water-saturated sediments or sedimentary rock; orange open circles = vadose zone sediments or sedimentary rock; brown squares = Chesapeake Bay Impact sediments; pink squares = Chesapeake Bay Impact impactite; black-filled orange circles = oil-gas-coal-bearing sediment or sedimentary rock; gray-filled gray diamonds = water saturated rhyolitic ash; open gray diamonds = deep vadose zone rhyolitic ash; open black diamond = vadose zone basaltic rock; gray-filled black diamonds = water-saturated basaltic rock, which includes recent Deccan Trap data from Dutta et al. (2018); red-filled diamonds = Deccan Trap granite data from Dutta et al. (2018); purple squares = metamorphic rock. The rest of the data are from Magnabosco et al. (2018). Blue open circles = Atacama desert soil from Connon et al. (2007) and Lester et al. (2007). Solid and dashed lines represent the best-fit power law for subseafloor sediments proposed by Parkes et al. (2014).

FIG. 4.

Schematic cross-section showing the “deep” subsurface exploration target on Mars, with the range of depths of interest. The concept illustrated here is VALKYRIE, which would sound for liquid groundwater and constrain geochemical gradients with depth to establish the first reconnaissance of the martian subsurface habitability trades with depth. Liquid water is expected at depths of kilometers (illustrated with blue shading), but salty brines could possibly exist at shallower depths. Orbiting radar suggest that large-scale brine bodies are not common at depths shallower than ∼200 m (illustrated by the purple shading; see Stamenković et al., 2019b). Shown by Stamenković et al. ( abstract 5045) at the conference.

FIG. 5.

Elements supporting the potential habitability of high-latitude ground ice on Mars. 5-1 shows color-coded elevation of Mars. The lowest-lying regions are in the northern hemisphere where atmospheric pressure is high enough to allow pure liquid water to form. 5-2 shows water abundance in the upper meter of the subsurface. Red areas are regions of surface and near subsurface ice. 5-3 shows current Mars axial tilt of 25°, and 5-4 shows Mars at axial tilt of 45° that occurred from 5–10 Myr ago. 5-5 shows how orbital tilt and summer solstice insolation changed over the last 5 Myr at the Phoenix landing site. At high orbital tilt the increased insolation results in summer temperatures that cause melting of ground ice up to 1 m depth. 5-6 shows “segregated” nearly pure ice just below the surface seen by the Phoenix mission. Shown by Stoker ( abstract 5107).

FIG. 6.

Schematic of rovers operating a drill on Mars to access pockets of ice in the near subsurface. The critical depth marks the boundary in which conditions favored recent habitability (within 10 Myr at a time of high obliquity) in terms of both water activity and radiation tolerance. Current models place the critical depth at 0.5–2.5 m depending on location. Schematic modified from an original produced by NASA/JPL.

FIG. 7.

Examples of terrestrial microbes that can be found in salt deposits. Left: Pigmented halophilic microorganisms such as haloarchaea may survive entombed in halite crystals and are detectable from spectroscopic properties of carotenoids. Adapted from Microbe, Vol 5, no. 3, cover image, courtesy of Priya DasSarma and Christopher Jacob, UMB ©ASM, and shown at the conference by DasSarma (DasSarma and DasSarma, abstract 5092). Right: Stratified endolithic halophilic microbial communities are common below the gypsum layer, such as in White Sands National Monument, New Mexico, and display characteristic red and green layers. Courtesy of Benjamin Brunner and Jie Xu, UTEP. Shown at the conference by Xu (LaJoie et al., abstract 5051).

FIG. 8.

Schematic diagrams depicting attributes of habitable environments, types of biosignatures, and issues surrounding the preservation of environmental indicators and biosignatures which can be used to inform the search for evidence of life (from Des Marais, abstract 5023).

FIG. 9.

Some examples of measurements that could be made in support of Mars extant life detection. There are a range of potential biosignatures that can be detected which range from materials that could be either biotic or abiotic in origin (left) to materials that are very unlikely to be produced in the absence of life (right). Detection of multiple “life signatures” would provide strong evidence for the presence of life, while abiotic signatures could provide important context. Shown by Mahaffy et al. ( abstract 5022; from Getty, 2018) at the conference.

FIG. 10.

Summary of methods commonly used in terrestrial samples for detecting live, dead, and dormant cells—all of these could be applied to the problem of Mars extant life detection. Note that some of the key measurements on this figure are not currently possible at Mars and would require Mars Sample Return. Shown by Mackelprang et al. ( abstract 5015) at the conference.

FIG. 11.

Mars simulators can be relatively simple with low-cost controllers (A) or connected to polycarbonate desiccators (B) (see Schwendner and Schuerger, abstract 5006). In contrast, more complicated Mars simulators (C; from Schuerger and Britt, abstract 5004; Schuerger et al., 2008) are generally required to simulate additional environmental parameters (e.g., UV flux on Mars) while concomitantly holding low temperature, low pressure, and CO₂-enriched hypoxic conditions. All simulators have caveats on what and how to recreate surface conditions on Mars, but simulation experiments are essential to close the knowledge gaps among analog research, planetary missions, and habitability models.