The SR-SAG2 report described various phenomena observed on Mars that might be indicative of Special Regions and discussed possible mechanisms for their formation. Examples include recurring slope lineae (RSL) (Figure 3.1), slope streaks (Figure 3.2), polar dark dune streaks, gullies (Figure 3.3), craters, and caves. The detection of deliquescent minerals and the presence of water in the form of subsurface ice, snow, and liquid brines were considered in view of ambient martian temperature and pressure conditions and the potential bioavailability of water. In general, the review committee agrees with the relevant findings and conclusions in the SR-SAG2 report relating to these topics. However, in some cases the committee has different opinions. For example, the possible detection of methane in the martian atmosphere was seen by the review committee as an important new factor suggesting that methane source regions be designated as an Uncertain Regions. The sections below expand on these topics. Modified text in the findings is shown in italic font.
The SR-SAG2 report concludes in Finding 2-4 that the detection of indigenous organic compounds on Mars at very low concentrations (e.g., Freissinet et al. 2015) should not be used to distinguish Special Regions. However, it is appropriate that special consideration be given to methane, recently detected near the surface of Mars (Webster et al. 2015).1 The review committee asserts that the lack of knowledge about the source(s) and sink(s) that control the possible episodic release of methane requires that it be considered a special class of organic compound and that its source region(s), once identified, be designated as an Uncertain Region. The abiotic processes most likely to produce methane in the subsurface to account for its intermittency include serpentinization and hydrothermal processes. The dissociation of methane clathrates and the production of biogenic methane by contaminants of polyextremophile terrestrial methanogens delivered to Mars on spacecraft or by potentially existing martian methanogens could also release methane (Figure 3.4).
If unambiguously confirmed, methane is the first indigenous organic compound discovered on Mars. The presence of possible intermittent plumes of methane at various latitudes, in different geographical settings and different seasons, has made it difficult to attribute the source of methane to a single process or release mechanism
1 The presence or absence of methane in the martian atmosphere remains highly controversial. For a contrary view see, for example, Zahnle et al. (2011) and Zahnle (2015).
FIGURE 3.1 Recurring slope lineae (RSL) in a crater on the floor of central Valles Marinaris. RSL are narrow, dark markings on steep, rocky slopes in the equatorial and southern mid-latitude regions of Mars. They appear to incrementally lengthen during warm seasons and fade in cold seasons, which is best explained as a result of seasonal water seepage by terrestrial analogy, although the origin of water is unknown. This image shows an area approximately 200 m wide. SOURCE: Portion of HiRISE image ESP_031059_1685; courtesy of NASA/JPL/University of Arizona.
FIGURE 3.2 Many dark- and light-toned slope streaks on a dust-covered slope in the Acheron Fossae region of Mars (37.32°N, 229.11°E). Although the mechanism of slope streak formation and triggering is debated, slope streaks are commonly believed to be dark subsurface material exposed by the downward movement of very dry sand or fine-grained dust in a fluid-like manner, analogous to a terrestrial snow avalanche. The darkest slope streaks are the youngest, appearing to cross-cut and overlay older, lighter streaks, which are believed to be dark streaks that are lightening with the deposition of new dust on their surface. SOURCE: HiRISE image PSP_001656_2175 image; courtesy of NASA/JPL/University of Arizona.
(Komatsu et al. 2011). The time-transient nature of atmospheric methane in trace concentrations lasting from months to years (e.g., Webster et al. 2015) is consistent with the presence of active sources and multiple sinks for methane on Mars (Atreya et al. 2006). Locating the source(s) of methane and identifying the process(es) by which it is produced and/or released must remain key mission priorities because of the high potential that such processes operate at temperatures and water activity values that define Special Regions.
The likely processes that could individually, or together, produce or release enough methane from the subsurface to account for the atmospheric concentration observed on Mars include serpentinization, hydrothermal alteration, and the dissociation of methane clathrates (Sleep et al. 2004; Oze and Sharma 2005; Fonti and Marzo 2010; Osinski et al. 2013; Kargel 2004; Wray and Ehlmann 2011; Herri and Chassefière 2012). All three of these processes would likely involve and/or release liquid water (i.e., would imply a Special Region), albeit within the subsurface of Mars.
Given the relatively short lifetime inferred for the methane possibly detected on Mars, which is inconsistent with its anticipated photochemical lifetime (300-600 years; Wong et al. 2003; Lefèvre and Forget 2009), it is also important to consider the likelihood that multiple types of methane sinks might exist on Mars. Potential sinks of methane include oxidants produced in global dust storms and local dust devils (Delory et al. 2006) as well as highly reactive mineral surfaces produced by wind-driven erosion (Jensen et al. 2014).
The possibility cannot be excluded that Mars’ subsurface could host indigenous anaerobic microbial communities dominated by lithoautotrophs that could be similar to methanogenic Archaea isolated from Siberian permafrost (Wagner et al. 2013). The general question of the habitability of Mars to lithoautotrophs has been examined by numerical modelling (Jepsen et al. 2007) and empirically by microbial growth experiments with autolithotrophs under simulated martian conditions. A putative food web on Mars was formulated on the basis of the metabolic capabilities iron-sulfur bacteria and on the minerals present on Mars (Bauermeister et al. 2014). Even if such putative microorganisms are not active today, they could become activated in spacecraft-induced Special Regions and survive present-day martian conditions. For example, Siberian permafrost methanogens have survived for 3 weeks
FIGURE 3.3 Mid-latitude martian gullies at 37.46°S, 222.95°E, exhibiting erosional alcoves, channels, and depositional aprons; all geological features that appear to be actively evolving and resemble landforms that on Earth are formed by water. Observations of gullies over the last decade reveal occasional mass wasting and show that they are currently active. However, present-day activity occurs when it is too cold for liquid water and is likely driven by dry granular processes involving CO2 frost. This image shows an area approximately 1.5 km from top to bottom. SOURCE: HiRISE image ESP_033290_1420; courtesy of NASA/JPL/University of Arizona.
under simulated martian thermophysical conditions (Morozova et al. 2007). Such organisms are tolerant to multiple stresses (i.e., low temperatures, high salinity, and prolonged starvation), can grow on different martian regolith analogs when supplied with carbon dioxide and molecular hydrogen (Schirmack et al. 2015), and can tolerate periodic desiccation such that they can endure aw shifts between 0.1 and 0.9 (Morozova and Wagner, 2007). Some permafrost isolates also show an extreme resistance to the effects of ultraviolet radiation (F37 = 14-15 kJ m–2) and ionizing radiation (D37 = 6-7 kGy; Wagner, unpublished data) comparable to the most radiation-resistant bacteria Deinococcus radiodurans (Ito et al. 1983). Such microorganisms could live in potentially habitable environments on Mars, such as in subsurface caves (see the section “Caves and Subsurface Cavities” below); beneath or in the lower boundary layer of subsurface clathrates and the cryosphere; and in cryopegs, the lenses of ground that contain over-cooled (–9°C to –11°C) water brines that could periodically source so-called recurring slope lineae (see the next section of this chapter).
Section 3.1 of the SR-SAG2 report discussed the possibility that martian methane could be indicative of a biosphere on Mars. But, SR-SAG2 did not address what the review committee considers to be a central issue related to methane’s occurrence in trace concentrations in the atmosphere.
If the methane on Mars is of biological origin, the planet either has an active biosphere that includes methanogens or the methane produced by an extinct biosphere could have been sequestered in methane clathrates in the
FIGURE 3.4 A schematic illustration of the known ways that methane (CH4) could be added to or removed from the atmosphere, processes known, respectively, as methane sources and sinks. NASA’s Curiosity Mars rover is searching for methane traces as a potential sign of life (a biomarker), as well as to gain an understanding of modern surface and subsurface organic processes on Mars. Curiosity has indeed detected fluctuations in methane concentration in the atmosphere, implying that both methane sources and sinks are currently at work in the martian environment. Detecting methane does not necessarily mean the presence of life. Methane can be generated by microbes as well as by non-biological processes, such as geochemical reactions, sunlight-induced reactions (photochemistry), or delayed release from subsurface methane stores. Reactions between water and olivine (or pyroxene) can generate methane. Ultraviolet (UV) radiation can induce photochemical reactions that produce methane from other organic compounds that are themselves formed by biological or non-biological means, such as comet dust falling on Mars. Recent or ancient subsurface methane may be stored within lattice-structured methane hydrates called clathrates and released over time, a source of modern atmospheric methane that may have formed in the past. Concentrations of atmospheric methane can drop due to redistribution or photochemical sinks. Wind on Mars can quickly reduce localized methane concentrations from an individual source. Just as methane can be generated through photochemistry, it can be broken down in the same way; sunlight-induced reactions oxidizing the methane through intermediary chemicals like formaldehyde and methanol into carbon dioxide, the predominant component of the martian atmosphere. SOURCE: NASA Jet Propulsion Laboratory/California Institute of Technology, “Possible Methane Sources and Sinks,” image PIA19088, December 16, 2014, http://www.jpl.nasa.gov/spaceimages/details.php?id=pia19088; courtesy of NASA/JPL-Caltech/SAM-GSFC/University of Michigan.
Mars subsurface (Atreya et al. 2007, Chassefière 2009, Mousis et al. 2013). In either case, methane gas could be released today either because methane-producing organisms are still alive in the subsurface (this would imply a Special Region that hosts extant Mars life), or changes in the physical conditions have resulted in the dissociation of methane clathrates and the release of the gas to the surface where it has been measured. In the latter case, if episodic clathrate dissociation in the subsurface environment was accompanied by the production of liquid water at those locations, the region would be a Special Region.
Even if the methane production is abiotic, liquid water will likely be involved and, thus, the location where the methane is generated is best considered as an Uncertain Region, to be treated as a Special Region until proven otherwise. Moreover, abiotic methane could, potentially, be used as a source of carbon and energy by putative martian methanotrophs. It is imperative that further research be conducted to discriminate the origin of methane on Mars. Therefore, the review committee proposes a revision of the Finding 2-4.
SR-SAG2 Finding 2-4: Organic compounds are present on Mars (or in the martian subsurface); although in very low concentrations in samples studied to date. Such detections are not used to distinguish Special Regions on Mars.
Revised Finding 2-4: Organic compounds are present on Mars (or in the martian subsurface); although in very low concentrations in samples studied to date. Abiotic or potentially biotic processes can explain the detection of episodic plumes of methane at various latitudes. In both cases, liquid water solutions would be involved. Therefore, the source regions of methane are considered as Uncertain Regions, even if the methane production is abiotic.
Suggestions for future research directions relating to the issues discussed in this section can be found in Appendix A.
One prerequisite of life as we know it is the availability, at least temporarily, of liquid water and hence, the centrality of water in the definition of Special Regions on Mars. Finding 3-14 of the SR-SAG2 report states that pure2 liquid water is currently possible on some areas of Mars for short periods of time. In particular, the report states that “. . . snow, however, has been detected on Mars (see Section 4.11). If snow melting yields liquid water on the surface of Mars, even periodically for only a short time, that water could be available for microbial use and define (for however short a time) a Special Region on Mars.”3 The review committee disagrees with this statement and, therefore, Finding 3-14. The fact that the surface pressure can be above that of the triple point simply indicates that water would not boil, not that liquid water would be stable. The review committee asserts that pure liquid water simply cannot exist on Mars because the atmosphere is too dry to allow it. The partial pressure of atmospheric water vapor is typically less than 1 Pa near the surface of Mars, whereas the partial pressure of water vapor at the triple point of water is about 600 Pa. Thus, even at the lowest temperature at which it could exist (the triple point), pure liquid water would evaporate and be cooled quickly, and therefore freeze quickly when exposed to the dry martian air, making pure liquid water near the triple point of water (above 0°C and below about 7°C; see in Figure 10 of the SR-SAG2 report).
Liquid brines (liquid water solutions), however, are possible on Mars (e.g., Fischer et al. 2014; Martín-Torres et al. 2015) because at low temperatures the saturation vapor pressure above them can be as low as the partial pressure of water vapor in the martian atmosphere. Therefore, the committee does not support Finding 3-14.
2 The committee uses the terminology pure water to draw a clear distinction between it (i.e., liquid H2O) and aqueous solutions of mineral salts (i.e., liquid brines).
3 SR-SAG2 report, pp. 907-908.
SR-SAG2 Finding 3-14: Mars average atmospheric pressure allows for liquid water when it exceeds that of the triple point of water, and at lower altitudes (e.g., Hellas and Argyre Basins) that is commonly the case. Higher temperatures and/or insolation may allow melting or condensation over limited areas for short time periods.
Not supported by the review committee.
The committee also proposes the following corrections to Figures 9 and 10 of SR-SAG2 report:
- In Figure 9, p/po at the top be replaced by aw for the figure to be consistent with the text.
- There is a typo in the caption for Figure 9. The committee suggests that it read “as water is lost from the system between aw = 1.0 (saturation) and aw = 0.9 . . .”
- Figure 10 is missing the vertical line corresponding to 7°C and the horizontal line corresponding the maximum Mars surface atmospheric pressure referred to in Section 3.8.7 as “the narrow window above 608 Pa (0.006 atm) where pure liquid water can be stable when temperatures are above 0°C and below about 7°C.” However, the committee does not agree with this statement because pure liquid water would be stable in this case only if the air were saturated. Stability implies that the water vapor partial pressure would have to be 608 Pa.
In addition, the committee proposes a revision to Finding 5-4, in order to expand the finding and include liquid aqueous solutions.
SR-SAG2 Finding 5-4: The mid-latitude mantle is thought to be desiccated, with low potential for the possibility of modern transient liquid water.
Revised Finding 5-4: The mid-latitude mantle is thought to be desiccated, with low potential for the possibility of modern transient liquid aqueous solutions. However, a local detailed analysis for a particular area is necessary to determine if it could be a Special Region.
Dark Slope Streaks
The recent discovery of “recurring slope lineae” (RSL) (McEwen et al. 2011) prompted the SR-SAG2 report to devote considerable attention to these surface features, which are found on steep, warm, rocky slopes. RSL extend and contract or fade in appearance on a seasonal cycle, suggestive of possible wetting and chemical precipitation. The review committee can accept SR-SAG2’s Finding 4-1, as modified below, in that currently RSL may be caused by an aqueous process and, if true, may meet the criteria for an Uncertain Region, to be treated as a Special Region until proven otherwise. However, the committee disagrees with the statement “There are other features on Mars with characteristics similar to RSL, but their relationship to possible liquid water is much less likely” because the SR-SAG2 report does not indicate what is meant by “characteristics similar to RSL” and does not justify why “their relationship with possible liquid water (pure or saline solutions) is much less likely.” Ongoing research suggests that RSL differ from at least some phenomena classed as “slope streaks” only because of their smaller size and shorter fading time (Mushkin et al. 2014a).
The SR-SAG2 report devoted minimal discussion to slope streaks, treating them as general phenomenon distinct from RSL, and did not cite recent research results that may suggest a continuum between the phenomena (Mushkin et al. 2014a). For example, Mushkin et al. (2014b) documented observations of some slope streaks with shorter formation and fading timescales than indicated in the SR-SAG2 report. They report seasonal change and incremental growth of slope streaks near Olympus Mons and Arabia Terra, in direct contrast to the SR-SAG2 report’s generalization for the slope streaks as a phenomenon distinct from RSL. Moreover, recent analyses of the slopes on which slope streaks form suggest they do not have significant inertia that would be expected for
dry granular flow (Brusnikin et al. 2015). Although Brusnikin et al. (2015) consider slope streaks to be different from RSL (in agreement with SR-SAG2), their results suggest that the formation of slope streaks is far from being understood. These results are sufficient to indicate that more attention needs to be devoted to understanding the relationships between the now intensely studied RSL and at least some of the much less well studied features that have been grouped into the general category of “slope streaks.” Like RSL, it is advisable that these phenomena be documented on a case-by-case basis for the planned landing ellipse of specific missions, to demonstrate that they really are “dry dust avalanches” and not caused by aqueous processes. This review committee therefore suggests a slightly revised Finding 4-8.
SR-SAG2 Finding 4-8: The 2006 Special Regions analysis did not consider dark/light slope streaks to be definitive evidence for water. Recent results have strengthened that conclusion for non-RSL slope streaks.
Revised Finding 4-8: The 2006 Special Regions analysis did not consider dark/light slope streaks to be definitive evidence for liquid (saline) water. Although some recent results have strengthened that conclusion for non-RSL slope streaks, other recent reports suggest that there are problems explaining all dark slope streaks by dry granular flow, and therefore aqueous processes cannot be definitely excluded for all dark slope streaks.
Suggestions for future research directions relating to the issues discussed in this section can be found in Appendix A.
Specific Physical Conditions of Recurring Slope Lineae
Recurring slope lineae are narrow (<5 m wide), dark features that occur on steep (25°C to 40°C) slopes during warm seasons on low albedo surfaces (McEwen et al. 2011, 2014; Ojha et al. 2014). What is special about RSL is that they grow incrementally, can be more than 1 km long, and recur over several years. All confirmed RSL locations have warm daily peak temperatures (typically >273 K at the surface) during the seasons in which RSL are active (McEwen et al. 2011). These are not the characteristics that one would expect from flows of pure liquid water or liquid brines. Surface temperatures above 273 K would produce water vapor pressures above ~600 Pa, causing rapid evaporation and cooling of any pure liquid water or liquid brines exposed to the extremely dry martian air (atmospheric water vapor pressure <1 Pa). In fact, it follows from order-of-magnitude calculations that pure liquid water at 273 K exposed to the dry martian atmosphere would be subject to cooling rates of the order of 200 W/m2 (of the order of the peak midday heating of the ground in the tropics) and evaporation rates of about 5 mm/Sol. The presence of liquid brines, instead of pure liquid water, would reduce evaporation and sublimation by no more than 50 percent, and it would be difficult to reconcile large evaporation rates with the long and narrow “wet” features of the RSL (e.g., Martínez and Rennó 2013). In addition, RSL originate near the top of steep slopes where subsurface water reservoirs are unlikely to exist. Moreover, RSL have the low thermal-inertial characteristics of loose regolith (compared to the surrounding terrain), not the higher thermal inertia expected from a wet regolith (Edwards and Piqueux 2015). The committee suggests rewording Finding 4-1.
SR-SAG2 Finding 4-1: Although no single model currently proposed for the origin of RSL adequately explains all observations, they are currently best interpreted as being due to the seepage of water at >250 K, with aw unknown, and perhaps variable. As such they meet the criteria for Uncertain Regions, to be treated as Special Regions. There are other features on Mars with characteristics similar to RSL, but their relationship to possible liquid water is much less likely.
Revised Finding 4-1: No single model currently proposed for the origin of RSL adequately explains all observations. However, there are suggestions that they are due to the seepage of liquid water (in some form) at >250 K. As such, they meet the criteria for Uncertain Regions and, together with slope streaks, be considered as Special Regions. However, a local detailed analysis for a particular area,
based on the latest scientific information, is necessary to determine if it is to continue to be treated as a Special Region.
Observations of gullies (Figure 3.3) over the past decade reveal occasional mass wasting and show that they are currently active. Because this present activity occurs when it is too cold for liquid water, it is likely that gullies can be reactivated, and possibly even be formed, by dry granular processes involving CO2 frost as opposed to liquid water. Nevertheless, the exact origin of gullies is still unknown, and the debate whether liquid water is involved in certain stages of gully evolution continues. It may also be possible that gullies evolved during variable climate conditions and that both aqueous and dry CO2-driven processes contributed to their present morphology. Although CO2 is very likely responsible for the present-day activity seen in gullies, this does not preclude a former type of activity involving liquid water from being involved in their formation. Therefore, the review committee proposes a slight revision to Finding 4-2 to allow for the possibility that the formation of gullies and their present-day activity are driven by different processes.
SR-SAG2 Finding 4-2: Some martian gullies (Gully Type/Taxon 1) have been observed to be currently active, but at a temperature far too low to be compatible with the involvement of liquid water—a CO2-related mechanism is implied in their formation.
Revised Finding 4-2: Some martian gullies (Gully Type/Taxon 1) have been observed to be currently active, but at a temperature far too low to be compatible with the involvement of liquid water—a CO2-related mechanism is implied in this current activity.
Polar dark dune streaks are a distinct class of active martian slope features that occur on dunes in both the north and south polar regions (Kereszturi et al. 2009, 2010). Möhlmann and Kereszturi (2010) argued that the streak morphologies and growth rates are consistent with viscous liquid flows, hypothesized to be concentrated brine. These features appear to develop as the regional temperatures slowly rise from their wintertime low at the CO2 frost point (150 K). The SR-SAG2 report argues that this indicates that it is unlikely that these polar dunes streaks are brine flow because known brines are not liquid at temperatures below 200 K. However, Möhlmann (2010) shows that solid-state greenhouse effects can easily increase the upper subsurface temperature of snow and ice packs well above the eutectic temperature of salts known to exist on Mars. This could explain the formation of liquid brines in Mars’ polar regions (Martinez et al. 2012; Martinez and Rennó 2013).
The presence of ice, likely in contact with salt particles, and the possibility of solid-state greenhouse effect (i.e., the warming of ice covered surfaces by the absorption of solar radiation and re-emission of infrared radiation) indicate that the formation of polar dune streaks could potentially involve liquid brines. Thus, to be conservative, the committee suggests revising Finding 4-9.
SR-SAG2 Finding 4-9: Polar dark dune streaks are considered extremely unlikely to involve liquid water warmer than 253K (–20°C), and most likely do not involve liquid water at all, given the low surface temperatures present when they are active.
Revised Finding 4-9: A conservative interpretation of the evidence suggests that polar dark dune streaks could potentially involve liquid brines but only in the presence of heating mechanisms such as solid-state greenhouse effects.
The discovery of soft, segregated ice in the shallow subsurface of Mars by NASA’s Phoenix lander (Rennó et al. 2009; Cull et al. 2010) was unexpected and is not well understood yet. However, liquid brines could easily
form in the polar region when perchlorate salts come into contact with ice, frost, or snow (Fischer et al. 2014). Moreover, liquid brines are far more likely to occur on Mars than pure liquid water. Thus, the committee suggests revising Finding 5-7.
SR-SAG2 Finding 5-7: We do not have accepted models or tested hypotheses to explain the phenomenon of “excess” ice on Mars. It is not known whether this ice was produced in the past by a process involving liquid water, or whether it is an ongoing process. The age of that ice and its implications for the next 500 years are unknown.
Revised Finding 5-7: We do not have accepted models to explain the presence of segregated ice on Mars yet. However, a conservative interpretation of the evidence suggests that processes involving liquid brines (likely at temperatures below –25ºC) could have produced the segregated ice. The age of that ice and its implications for the next 500 years are unknown.
Suggestions for future research directions relating to the issues discussed in this section can be found in Appendix A.
The committee suggests combining the SR-SAG2 report’s Findings 4-6 and 4-7 into the Revised Finding 4-6/7 to eliminate ambiguities.
SR-SAG2 Finding 4-6: Within the bounds of several limitations of the MARSIS and SHARAD radar surveys (including attenuation, location-specific surface clutter, relatively low spatial resolution, saturated porosity, and areal coverage), groundwater has not been detected anywhere on Mars within ~200-300 m of the surface. This does not preclude the existence of groundwater at greater depths, which should be considered as an Uncertain Region (and a potential Special Region) until further geophysical investigation proves otherwise.
SR-SAG2 Finding 4-7: We cannot rule out the possibility of near-surface water that may be present at a vertical and/or horizontal scale finer than that detectable by MARSIS and SHARAD.
Revised Finding 4-6/7: Within the bounds of several limitations of the MARSIS and SHARAD radar surveys (including attenuation, location-specific surface clutter, relatively low spatial resolution, saturated porosity, and areal coverage), groundwater has not been detected anywhere on Mars within 200-300 m of the surface. This does not preclude the existence of groundwater at greater depths, or near-surface groundwater at a vertical and/or horizontal scale finer than that detectable by MARSIS and SHARAD.
In the review committee’s opinion the statement in Section 4.9, the second paragraph of the SR-SAG2 report: “Ground-ice stability occurs when the annual mean vapor density over ice in the soil pore space, integrated over these seasonal cycles, equals that of the atmosphere (Mellon and Jakosky, 1993)” is not strictly correct. It represents only a first order approximation.
The statement in paragraph four of the same section, “The low amount of water in the atmosphere of Mars results in a very low relative humidity at the site when the temperatures approach the lower temperature limit for microbial cell division (255 K),” is not correct because over ice, in the shallow subsurface, the air in the soil pore space would be saturated (e.g., Rennó et al. 2009). Thus, even if the amount of water in the atmosphere is low, it could be high in the shallow subsurface where ground ice exists.
Section 4.10 of the SR-SAG2 report states that “in order to understand if, when, and where deliquescence may be occurring on Mars and under what conditions the resulting aqueous solutions may persist, we need to understand . . . the kinetic factors that may affect aqueous-phase formation and disappearance.” This is an excellent point that needs to be emphasized because deliquescence is strongly limited by kinetics. Fischer et al. (2014) report that when water vapor is the only source of water, bulk deliquescence of the salts that have been discovered on Mars is not rapid enough to occur during the short periods of the day during which the ground temperatures are above the salts’ eutectic temperatures. Only when the salts are in direct contact with water ice can bulk deliquescence occur in Mars environmental conditions. Thus, liquid aqueous solutions could form temporarily during diurnal cycles only where salts and ground ice co-exist in the shallow martian subsurface and on the surface when frost or snow are deposited on saline soils (Fischer et al. 2014).
The review committee suggests rewording Finding 4-14.
SR-SAG2 Finding 4-14: Natural deliquescence of calcium perchlorate, the mineral with the lowest eutectic temperature relevant to Mars, is predicted for short periods of time each day at each of the three landing sites for Viking 1, Phoenix, and MSL (where we have measurements) and presumably at many other places on Mars.
Revised Finding 4-14: Liquid solutions of calcium perchlorate, the mineral with the lowest eutectic temperature relevant to Mars, could form for short periods of time each day at each of the three landing sites for Viking 1, Phoenix, and Mars Science Laboratory (where we have measurements) and presumably at many other places on Mars when water ice gets in contact with salt.
A stable aqueous solution might form (not “will form” as stated in the second paragraph of Section 4.10 of the SR-SAG2 report, because on Mars deliquescence is strongly limited by kinetics) via deliquescence when the atmospheric relative humidity at the surface of a given salt is greater than or equal to the deliquescence relative humidity of that salt.
Figure 27 of the SR-SAG2 report shows a non-standard stability diagram plotted by relating the concentration of the solution with the relative humidity of the air just above it. Some of the labels in the diagram are ambiguous, and the figure caption and text are inconsistent with what are expected based on standard stability diagrams. According to these standard diagrams, aqueous salt solutions are possible whenever the temperature is above the eutectic value. Therefore, liquid brines are not rare. They are expected to form when ice gets into contact with salt whenever the temperature exceeds the eutectic value.
Fischer et al. (2014) report that when salts are in direct contact with water ice, bulk deliquescence occurs within minutes when the temperature exceeds the eutectic temperature of calcium perchlorate. The atmospheric water vapor content is not relevant in this case because aw = 1.0 over the ice. This indicates that liquid aqueous solutions are likely to form temporarily when snow is deposited on saline soils. Thus, the review committee suggests revising Finding 4-16.
SR-SAG2 Finding 4-16: Snow may be deposited in polar or equatorial regions and elsewhere, although its volume is thought to be negligible. It is expected to fall during the coldest part of the night and may disappear (by sublimation or melting/evaporation/boiling) soon after the day begins on Mars. It is unknown whether this process could create a Special Region on Mars.
Revised Finding 4-16: Snow is deposited in the polar region and might also be deposited in small amounts elsewhere. Snow is expected to fall during the coldest part of the night (when the ground temperature is
below –25°C) and may sublimate shortly after sunrise. However, snow could melt if deposited on salts with eutectic temperature lower than that at which sublimation occurs, possibly creating temporary Special Regions.
There appears to be substantial subsurface ice on Mars, even in equatorial regions (e.g., Vincendon et al. 2010; Scanlon et al. 2015). For example, work by Vincendon et al. (2010) as well as theoretical modelling (see references in Vincendon et al. 2010) demonstrates that on pole-facing slopes at mid-latitudes and in the tropics ice can be at a depth of less than 5 m. The committee therefore suggest that Finding 5-3 be revised.
SR-SAG2 Finding 5-3: Depths to buried ice deposits in the tropics and mid-latitudes are considered to be >5 m.
Revised Finding 5-3: In general, depths to buried ice deposits in the tropics are considered to be >5 m. However, there is evidence that water ice is present at depths of <1 m on pole-facing slopes in the tropics and mid latitudes. Thus, a local detailed analysis for a particular area is necessary to determine if it could be a Special Region.
Finally, the committee proposes a small revision to Finding 5-9.
SR-SAG2 Finding 5-9: Mineral deliquescence on Mars may be triggered by the presence of a nearby spacecraft, or by the actions of a spacecraft.
Revised Finding 5-9: Mineral deliquescence and the melting of ice on Mars may be triggered by the presence of a nearby spacecraft, or by the actions of a spacecraft.
SR-SAG2 Finding 4-11: On Earth, special geomorphic regions such as caves can provide radically different environments from the immediately overlying surface environments providing enhanced levels of environmental protection for potential contaminating organisms. The extent of such geomorphic regions on Mars and their enhancement (if any) of habitability are currently unknown.”
The committee generally concurs with Finding 4-11 which is related to caves on Mars. Although their number and sizes are largely unknown, caves and other subsurface cavities on Mars would represent environments with ambient conditions (e.g., temperature, humidity, exposure to radiation) that are very different from those at the surface, and most probably, those conditions are likely to be favorable for microbial colonization. Consideration of caves and subsurface cavities is paramount for two reasons. First, they provide a protected environment (e.g., from extremely low temperatures and radiation). Second, they can provide a means by which terrestrial contamination can access martian subsurface environments. In addition to drained lava tubes, voids resulting from tension fracturing, and possible caves in evaporites (e.g., gypsum karst), there are types of subsurface cavities on Mars not mentioned in the SR-SAG2 report that may have been produced by subsurface erosion by water (analogous to piping; e.g., Higgins and Coates 1990) or by expulsion of material through hydrothermalism (Rodríguez et al. 2005) or mud volcanism (Rodríguez et al. 2012). However, to the best of the review committee’s knowledge, there is no data on the availability of water in martian caves.
The committee also concurs with the report’s identification of specific knowledge gaps related to caves. Specifically, the actual number and location of potential caves on Mars is difficult to assess. Because current subsurface information (e.g., from radar) is insufficient to detect caves, their identification is only possible through a combination of high-resolution imaging and thermal data (Cushing 2012; Cushing et al. 2015; Jung et al. 2014; Lopez et al. 2012). Ground-based thermal observations by rovers may enable detection of accessible subsurface cavities that are too small to be detected from orbit (Groemer et al. 2014). ESA’s Mars Trace Gas Orbiter may be able to identify point sources of potential anomalous gases released from the subsurface, as discussed earlier in this chapter (see “Methane: Potential Abiotic and Biotic Sources”).
An important factor that could limit microbial metabolisms in any subsurface environment is adequate energy sources. Hydrogen can be produced by hydrolysis of olivine in basalts at relatively low temperatures (343 K) by carbonate-containing solutions (Neubeck et al. 2014). On Earth, many thermophiles, mainly archaea, live in this temperature range; hyperthermophiles live even at temperatures of up to 386 K (113°C). Nevertheless, in oligotrophic subsurface sediments, microbial enhancement of H2 production from the alteration of minerals has been detected (Parkes et al. 2011). Thus, if H2 could be liberated, corresponding electron acceptors might be Fe3+ (from mafic minerals in basalts); CO2 from the atmosphere for metabolisms, such as hydrogenoclastic methanogenesis or the Wood-Ljungdahl pathway; or SO42– for sulphate reduction, O2, or halogens for dehalorespiration. Independently, all of these possible redox couples exist on Mars. Methane could be a source of electrons coupled to either O2 or SO42–—if clathrates have been breached. Organics, if present, could be coupled with Fe3+, SO42– or halogens. Fermentation reactions could take place with amino acids, organic acids and/or alcohols, or halogens. Thus, the review committee proposes a revision to Finding 2-1.
SR-SAG2 Finding 2-1: Modern Mars environments may contain molecular fuels and oxidants that are known to support metabolism and cell division of chemolithoautotrophic microbes on Earth.
Revised Finding 2-1: Modern Mars environments may contain chemical compounds that are used as electron donors and electron acceptors by chemolithoautotrophic microbes. If organic compounds are also present on Mars, then heterotrophic microbes may also find a home there.
In conclusion, there could be a number of possible primary sources of the necessary ingredients for life inside caves and subsurface cavities on Mars, and therefore, they are best classified as Uncertain Regions and treated as Special Regions until proven otherwise.