Introduction
Michael Faraday observed in 1850 ‘that a particle of water which could retain the liquid state whilst touching ice on only one side, could retain the liquid if it were touched by ice on both sides’ (Faraday Reference Faraday1850). Hence, Michael Faraday was the first to suggest that the ice–air interface consists of a thin wet film (Faraday Reference Faraday1850), variously called the quasi-liquid layer (QLL), premelting layer, liquid-like layer, surface premelting layer, or surface melting layer. This key observation introduced the concept of interfacial melting, and the presence of a liquid water film on the surface of ice at subfreezing temperatures. The ice–air interface of solids is an area that exhibits characteristics different from those of the bulk material. This is primarily due to the fact that atoms (or molecules) at the surface only encounter bonding forces with other molecules from one side: simultaneously, there is a similar imbalance at other interfaces. Furthermore, this behaviour causes the dislocation of atoms from their original locations, alterations in their associated force and energy constants, and effects on layers below the ice–air demarcation. The disordered region at the ice surface is a quasi-liquid: when it is microscopically thin, it retains the structure of the underlying solid, but when it thickens, the outer layers become identical to the bulk liquid.
The wetting of the boundary between the solid and vapour phase by a thin film causes the free energy of the boundary to be lower than it would be if the thin liquid film were absent (Boxe Reference Boxe2005; Dash et al. Reference Dash, Rempel and Wettlaufer2006; Huthwelker et al. Reference Huthwelker, Ammann and Peter2006; Boxe & Saiz-Lopez Reference Boxe and Saiz-Lopez2008, Reference Boxe and Saiz-Lopez2009). As a result, if the surface of ice were initially dry, then it would reduce its interfacial free energy by converting a layer (e.g. the surface) of the solid to liquid. Hence, a liquid-like layer should exist over some measurable and quantifiable temperature range on the surface of ice, below its bulk normal melting temperature. The existence of the QLL is made possible due to its thinness and closeness to the normal melting temperature of the ice and is present at a state where the free energy of the ice system, which is at a minimum and is governed by the competition between the free energy of the ice surface and the energy required to melt a solid layer.
The presence of thin liquid films is a well-established phenomenon in earth science and detailed reviews for ice are available in the literature (Boxe Reference Boxe2005; Dash et al. Reference Dash, Rempel and Wettlaufer2006; Huthwelker et al. Reference Huthwelker, Ammann and Peter2006; Boxe & Saiz-Lopez Reference Boxe and Saiz-Lopez2008, Reference Boxe and Saiz-Lopez2009). Among ice, a variety of other substances, such as solid rare gases, soil media, molecular solids, metals, complex organic molecules and semiconductors contain a thin liquid film on their surfaces and subsurface. With the single exception of Elbaum et al. (Reference Elbaum, Lipson and Dash1993), whose experiments were carried out on exposed horizontal facets in the prismatic orientation 
these studies have shown that the thickness of thin films on ice surfaces increases with increasing temperature. These measured thicknesses range from a few to 103 nm, tens of degrees below and close to the melting point of pure water ice. The onset for surface disorder for pure water ice, which ultimately leads to the formation of thin liquid films, has been detected down to 180 K (Suter et al. Reference Suter, Anderson and Pettersson2006). Furthermore, the presence of an impurity (or impurities), such as sodium chloride (NaCl), increases the thin liquid film thickness from a few tens of nanometres to 105 nm (100 μm), a few degrees below and close to the melting point of a NaCl-doped water ice sample (Boxe Reference Boxe2005; Dash et al. Reference Dash, Rempel and Wettlaufer2006; Huthwelker et al. Reference Huthwelker, Ammann and Peter2006; Boxe & Saiz-Lopez Reference Boxe and Saiz-Lopez2008, Reference Boxe and Saiz-Lopez2009; Wettlaufer Reference Wettlaufer1999). Laboratory and theoretical simulations have also established that the contact between ice and soil surfaces and soil surfaces alone consists of an interfacial unfrozen zone of water reaching a maximum of 25 molecular layers thick or 7.5 nm (one monolayer of liquid water=0.3 nm) with liquid-like properties down to 77 K (Price Reference Price2007; Sutter et al. Reference Sutter, Sriwatanapongse, Quinn, Klug and Zent2002; Jakosky et al. Reference Jakosky, Nealson, Bakermans, Ley and Mellon2003; Mohlmann Reference Mohlmann2003, Reference Mohlmann2004, Reference Mohlmann2008, Reference Mohlmann2009, Reference Mohlmann2010a, Reference Mohlmannb).
Mainly focused on ice and snow environments, recent evidence has been mounting, indicating that thin liquid films play a pivotal role in physico-chemical and biochemical life processes on Earth, particularly in extreme environments, such as the polar regions (Boxe Reference Boxe2005; Dash et al. Reference Dash, Rempel and Wettlaufer2006; Huthwelker et al. Reference Huthwelker, Ammann and Peter2006; Price Reference Price2007; Boxe & Saiz-Lopez Reference Boxe and Saiz-Lopez2008, Reference Boxe and Saiz-Lopez2009; Dubowski et al. Reference Dubowski, Colussi, Boxe and Hoffmann2002; Boxe et al. Reference Boxe2003, Reference Boxe2005, Reference Boxe2006; Saiz-Lopez & Boxe Reference Boxe and Saiz-Lopez2008). Thin liquid films play an important function in environmental phenomena, such as: (i) controlling the friction of ice and snow; (ii) soil freezing, permafrost formation and frost heave; (iii) sintering and sliding in glaciers, sea-ice and snow fields; and (iv) behaviour of atmospheric ice (Dash et al. Reference Dash, Rempel and Wettlaufer2006; Boxe & Saiz-Lopez Reference Boxe and Saiz-Lopez2008, Reference Boxe and Saiz-Lopez2009). Thin liquid films have also been suggested to contribute to the electrification of thunderclouds via charge transfer at the liquid–ice interface (Dash et al. Reference Dash, Rempel and Wettlaufer2006; Boxe & Saiz-Lopez Reference Boxe and Saiz-Lopez2009). Abbatt et al. (Reference Abbatt, Beyer, Fucaloro, Mcmahon, Wooldridge, Zhang and Molina1992) and Molina (Reference Molina and Calvert1994) even proposed that polar stratospheric clouds are able to accommodate HCl by dissolution in multilayer thin liquid films on ice grains, where they can efficiently release Cl. The latter participate in ozone destruction during winter and spring months in the Arctic and Antarctica. These hypotheses were later confirmed by seminal laboratory analysis, under polar stratospheric conditions, showing that HCl efficiently accommodates snow/ice surfaces through trace-gas-induced thin liquid film formation of 100 nm (Molina Reference Molina and Calvert1994). More importantly, McNeill et al. (Reference McNeill, Loerting, Geiger, Trout and Molina2006) showed that the solubility of HCl in thin liquid films exhibits a solubility that is intermediate between that in bulk ice and liquid water, rather than that of the solubility expected for a true-liquid matrix. In Earth's extreme environments, micro-organisms are known to metabolize in thin liquid water veins or films with thicknesses as small as a few monolayers (e.g. in desert environments) to as much as a few microns contained on and within soil and ice matrices (Nersesova Reference Nersesova1950; Anderson Reference Anderson1968; Anderson & Tice Reference Anderson and Tice1979; Thiel & Madey Reference Thiel and Madey1987; Williams & Smith Reference Williams and Smith1989; Chuvilian et al. Reference Chuvilian, Ershov and Smirnova1998; Henderson Reference Henderson2002; Price Reference Price2007). Given the association between water and terrestrial life, notably for micro-organisms that live in extreme environments (Price Reference Price2007), the search for liquid water elsewhere within our Solar System has received substantial attention, with Mars being of considerable interest. It has long been argued that liquid water once flowed on Mars (Paige Reference Paige2005), but the contemporary mean surface temperature and pressure are 220 K and 5.6 mbar, respectively, and such conditions do not allow for the stability of bulk pure water (i.e. large volumes of water) as a stable phase. The maximum surface pressure on Mars does not exceed 12 mbar, and surface temperatures vary from 135 to 295 K; the maximum boiling point on Mars is, therefore, ∼283 K. This is in stark contrast to a surface pressure of 1000 mbar and 373 K boiling temperature for Earth. On an annual average, the Martian atmosphere contains a global water vapour content of 2.0×1011–2.3×1012 kg H2O with substantial seasonal variations. These values are 104 orders of magnitude less than Earth's atmospheric water content (Tokano Reference Tokano and Tetsuya2005). Figure 1 shows 1D model simulations to illustrate this point by displaying the fraction of atmospheric water vapour on Mars compared to Earth. Analogous to Earth, thin liquid films on the Martian surface and subsurface could temporarily exist even if it is not stable according to the phase diagram of pure water.
Fig. 1. 1D model simulation of altitude (km) versus [H2O(v)]Mars/[H2O(v)]Earth.
Given that the availability of water is one of the crucial parameters that limit biochemical activity, we focus on the quantity of thin films that may exist on the surface of Mars (i.e. on the surface of the Martian regolith) and the amount that may exist between the contact of the regolith and the ice table. We also focus on regions on Mars that would produce the maximum thin film thickness on soil and ice at the subsurface, which would be defined by the cold limit of water stability, 273 K. We support these results with Mars Exploration Rover (MER) Spirit and Opportunity's Alpha Proton X-ray Spectrometer (APXS) data of increased levels of bromine (Br) beneath the immediate surface (Yen et al. Reference Yen2005).
Results and discussion: quantification of upper- and lower-limits of thin liquid film thicknesses
The presence and dimensions of thin liquid films at Mars’ surface and/or subsurface will be limited by: (i) atmospheric water vapour (Fig. 1); (ii) surface and subsurface temperature; (iii) surface and subsurface pressure; (iv) surface frost; (v) surface and subsurface ice; (vi) slopes and surface topography; (vii) surface evaporation rate; and (viii) the presence and depth of the regolith (Paige Reference Paige2005). This pertains to both bulk water and water in contact with soil media. In addition, the pressure–temperature phase diagram of water pertains to the equilibrium of bulk water (curvature is neglected), without any impurities (i.e. zero solute concentration). For energetic reasons, both evaporation and freezing of liquid water may be sluggish on Mars, and once ice melts, liquid water may persist for a longer period than would be expected (Hecht Reference Hecht2002; Kuznetz & Gan Reference Kuznetz and Gan2002; Chevrier et al. Reference Chevrier2007). Whether melting can actually occur depends on the surface heat balance, which significantly varies from place to place due to differences in meteorology, topography, or geology, but it may occur on slopes nearly anywhere on Mars; therefore, thin liquid films may form, due to melting, on ice and/or frost surfaces that are located on slopes illuminated by normal-incidence insolation (Hecht Reference Hecht2002). Ice/frost is the only phase of water that has been detected on the surface of Mars, and it is predominantly covered by layers of regolith, except at the polar caps. Given the present-day obliquity of Mars (i.e. 25.2°) and peak summer-time polar-ice temperature of ∼205 K (Kieffer & Zent Reference Kieffer, Zent, Kieffer, Jakosky, Snyder and Matthews2002), the thickness of a thin liquid film at this maximum temperature on water ice is uncertain as it has only been quantified down to 220 K for pure water ice (Boxe Reference Boxe2005; Dash et al. Reference Dash, Rempel and Wettlaufer2006; Huthwelker et al. Reference Huthwelker, Ammann and Peter2006; Price Reference Price2007; Boxe & Saiz-Lopez Reference Boxe and Saiz-Lopez2009). This is independent of the fact that the detection of the onset of surface mobility for pure water ice has been detected in the laboratory at 180 K (Suter et al. Reference Suter, Anderson and Pettersson2006).
Utilizing calculated water adsorption isotherms for soil and an atmospheric water vapour content of 0.1–10 pr μm, Anderson et al. (Reference Anderson, Gaffney and Low1967) provide the first quantification of the maximum adsorbed or unfrozen water content in soil under Martian conditions. Their study concluded that soil exposed to the atmosphere of Mars cannot be expected to attract and retain more than a monolayer (or 0.3 nm) of adsorbed water. In addition, they show that at 10 pr μm at 273 K, 0.06 nm of adsorbed water will exist on soil surfaces exposed to the Martian atmosphere. The retrieved column abundance of water vapour varies according to the seasonal date and latitude, ranging from ∼5 to 100 pr μm with a mean low-to-mid-latitude value of 10 pr μm (Tokano Reference Tokano and Tetsuya2005; Smith et al. Reference Smith2009). The global annual average Martian atmospheric water content ranges from 2.0×1011 to 2.3×1012 kg. Therefore, 0.06 nm of adsorbed water represents a minute fraction, a 10 pr μm water vapour column (i.e. 6×10−4%). Given that ∼30% of Mars's surface area is at or above 273 K at some point during the year (Haberle et al. Reference Haberle2001), 0.06 nm of adsorbed water represents 3×107 litres of water (i.e. volume (l)=(0.30)(4/3π[(R (equatorial)+0.06 nm)3−(R (equatorial))3])) which is 0.001–0.02% of the Martian atmospheric water vapour inventory. Note, this amount of unfrozen water represents a lower-limit of the amount that could exist on a bare Martian surface.
These above calculations do not accurately represent the total amount of adsorbed water contained on the surface of Mars as the calculations do not take into account the specific surface area (SSA) of porous soil in the upper depths of the Martian regolith. Therefore, the total amount of water at Mars’ surface can be quantified via the relationship between the equilibrium moisture content (EMC) and equilibrium relative humidity (ERH) (Sun Reference Sun1998). The derived relation (equation 1) gives the mass of the adsorbed water layer 
as a function of the thickness (d) of the adsorbed water layer, the SSA (S M), density of liquid water (ρ water) and the dry mass (m dry) of a porous material.
The EMC/ERH relation, i.e. the dependence of the EMC (a m) on atmospheric temperature and water activity a w is then given by
where A represents the Hamaker constant, T m is the melting point temperature, T is the soil temperature under consideration, q is the latent heat per mass and ρ s is the density of water ice at temperature T. Within the following context, take into account the fact that the most uncertain parameters in equation (2) for Mars is S M (the SSA) and the Hamaker constant A. Given these uncertainties, we estimate the amount of liquid water in a surface regolith, 1 mm thick, covered by 0.06 nm of adsorbed water for 30% of the geometrical surface of Mars. For S M=17×103 m2 kg−1 (Ballou et al. Reference Ballou, Wood, Wydeven, Lehwalt and Mack1978), 
=1000 kg m−3, and d=0.06 nm, which yields a m=1.02×10−3. The thickness of the regolith that participates in the soil–atmosphere diurnal exchange of water is ∼1 mm (Mohlmann Reference Mohlmann2008), which for 30% of Mars geometrical surface, yields a total volume of 1 mm of regolith at Mars’ surface of 4.34×1010 m3, which in turn, using an assumed density of 3000 kg m−3 (Mohlmann Reference Mohlmann2008), gives a dry mass of 1.3×1014 kg. Given an adsorbed water thickness of 0.06 nm, this amounts to 1.33×1011 litres of waters; this represents 6–66%, of the upper- and lower-limit atmospheric water vapour inventory, respectively (Tokano Reference Tokano and Tetsuya2005). Using S M=105 m2 kg−1 (Mohlmann Reference Mohlmann2008), yields a m=6×10−3 and a dry mass of 1.3×1014 kg, yields 7.80×1011 litres of interfacial water of, almost four times and 33%, the lower- and upper-limit Martian atmospheric water vapour inventory, respectively. The 0.06 nm of adsorbed water also represents a minute fraction a 10 pr μm water vapour column, 5.0×10−4%. We do acknowledge here that 0.06 nm of adsorbed water does not have a physical meaning since the thickness of one monolayer of water is ∼0.3 nm; in other words, a 0.06 nm thickness of adsorbed water would mean that the Martian surface regolith is relatively dry.
The calculations above, however, do not consider the effect of impurities, such as deliquescent salts, though recent spacecraft measurements support the presence of such salts in soil and ice (Gellert et al. Reference Gellert2006). Salts greatly increase the water vapour sorption capacity of any soil media and decrease the pressure of water vapour in equilibrium with a liquid solution at a given temperature. The presence of salts would also lower the freezing temperature of the soil water and increase the thickness of thin liquid films surrounding soil particles (Boxe Reference Boxe2005; Dash et al. Reference Dash, Rempel and Wettlaufer2006; Huthwelker et al. Reference Huthwelker, Ammann and Peter2006; Price Reference Price2007; Boxe & Saiz-Lopez Reference Boxe and Saiz-Lopez2009; Döppenschmidt & Butt Reference Döppenschmidt and Butt2000).
Laboratory measurements reveal much higher film thicknesses for the QLL. Data summarized by Dash et al. (Reference Dash, Rempel and Wettlaufer2006), which show that the QLL thickness ranges from 1 to 1000 nm, where, again, the thicknesses are directly dependent on temperature, solute concentration and technique used to measure the QLL. Theoretical and experimental data are lacking in relation to the effect of solute concentration on the state and dimension of thin liquid films surrounding water ice and soil particles. Döppenschmidt & Butt (Reference Döppenschmidt and Butt2000) conducted the only laboratory investigation of the effect of impurities on thin liquid film thickness on water ice. Utilizing atomic force microscopy, they showed that the addition of 10 mM KCl resulted in a maximum enhancement of 2 (e.g. at 263 K, the thickness of the QLL was ∼28 nm, compared to ∼13 nm on pure water ice). Wettlaufer (Reference Wettlaufer1999) and Benatov & Wettlaufer (Reference Benatov and Wettlaufer2004) conducted the only theoretical investigation of the effect of impurities on the thickness of the QLL within the context of Derjaguin–Landau–Verewey–Overbeek theory. Wettlaufer (Reference Wettlaufer1999) and Benatov & Wettlaufer (Reference Benatov and Wettlaufer2004) model H2O(ice)–NaCl mixtures, ranging from 6 mM to 6 M NaCl and measure QLL thicknesses between 1 and 1000 nm. For reference, the NaCl concentration in the Earth's oceans is 0.55 M. We do not know the salt content of the surface of Mars, but recent measurements from MER Alpha Particle X-ray Spectrometer suggest that salts are abundant on Mars (Yen et al. Reference Yen2005; Gellert et al. Reference Gellert2006; Boynton et al. Reference Boynton2009; Hecht et al. Reference Hecht2009). Hecht et al. (Reference Hecht2009) recently showed, by way of the Phoenix spacecraft Wet Chemistry Laboratory, that Martian soil solutions contained ∼10 mM of dissolved salts, where the Cl− concentration ranged from 0.24 to 0.6 mM.
Given the unknown and limited data with regard to the chemical nature of the Martian regolith and the concentration of species within it, we estimate an upper-limit for the effect of impurities on the thickness of water at the Martian surface. This estimation is calculated by taking the mean of all measured QLL thicknesses (Sadtchenko & Ewing Reference Sadtchenko and Ewing2002) close to the melting point of pure water ice, which yields 11 nm as a baseline QLL thickness for pure water ice. Döppenschmidt & Butt (Reference Döppenschmidt and Butt2000) measure a QLL thickness of 28 nm, which yields a factor of ∼3 for the minimum enhancement due to impurities. Wettlaufer (Reference Wettlaufer1999) and Benatov & Wettlaufer (Reference Benatov and Wettlaufer2004) measure a maximum thickness of 1000 nm; this yields scaling the maximum enhancement factor of ∼90. The use of these scaling factors is supported by the fact that the adsorption enthalpy (adsorption bond) of water molecules is about as twice as strong on mineral surfaces (1.24 eV (120 kJ mole−1)) compared with water ice (0.53 eV (51 kJ mole−1)) and bulk water (0.47 eV (45 kJ mole−1)); therefore, evaporation of thin liquid films or adsorbed water in and on the Martian regolith exceeds those of sublimation of water ice by orders of magnitude (Mohlmann Reference Mohlmann2003, Reference Mohlmann2004, Reference Mohlmann2008). Close to the cold limit of water stability (273 K), the range in enhancement factor of 3–90, due to impurities yields an upper limit ranging from 0.2 to 5 nm for the thickness of thin liquid films on the surface of Mars. This upper limit corresponds to 2.3×104–1.5×106 litres of water, which equates to 6.0×10−7–4.0×10−4%, of a 10 pr μm water vapour column, respectively, and 3.0×10−6–4.0×10−4% and 6.0×10−6–8.0×10−4%, respectively, of the Martian atmospheric water vapour inventory. Taking into account S M=17×103 m2 kg−1 (Ballou et al. Reference Ballou, Wood, Wydeven, Lehwalt and Mack1978) and S M=105 m2 kg−1 (Mohlmann Reference Mohlmann2008) at 5 nm, yields 1.10×1013 and 6.50×1013 litres of waters, respectively, 55–325 times larger than Mars’ atmospheric water vapour inventory.
The lower and maximum upper limits of adsorbed water or thin liquid film thicknesses on the Martian surface are expressed in Fig. 2 (a, without impurities) and (b, with impurities), which display the amount of water on Martian analogue soil (i.e. sodium montmorillonite, Wyoming bentonite, etc.) at partial pressures of water vapour in the Martian atmosphere as a function of temperature. Figure 2 was formulated based on calculated adsorption isotherms, at sub-zero temperatures, from Mooney et al. (Reference Mooney, Keenan and Wood1952) data for water vapour adsorption isotherms between 273 and 293 K for sodium montmorillonite. Given thermodynamic equation, representing the change in relative vapour pressure with temperature, at constant water content (i.e. surface coverage),
where p is the vapour pressure of the sorbed water; p o is the vapour pressure of ice at the temperature in question, T is the absolute temperature and 
is the difference in the partial molar enthalpy of the sorbed water and that of ice. The only assumption involved in the derivation of equation (3) is that does 
exhibits a small change with temperature. When utilizing equation (3), 
is equivalent to 
where is the enthalpy of pure water at 0°C. The first of these two terms is the differential heat of wetting; the second term is the heat of freezing pure water (−1435.5 cal mole−1 at 0°C). Since Mooney et al. (Reference Mooney, Keenan and Wood1952) provide data for 
and p/p o at 0°C for a number of water contents, which allow for estimation p/p o of for specified water contents at sub-zero temperatures. Figure 2 represents results that are consistent and complement similar Anderson et al. (Reference Anderson, Gaffney and Low1967) adsorption isotherm calculations for Mars soil analogues as for sodium-saturated Wyoming bentonite.
Fig. 2. (a) Adsorption isotherm for pure water in contact with Wyoming bentonite. (b) Adsorption isotherm for water containing impurities in contact with Wyoming bentonite, exhibiting a maximum thin liquid film enhancement of 90.
Similarly, lower and upper limits can be calculated for the thickness of thin liquid films on/in subsurface ice on Mars. Haberle et al. (Reference Haberle2001), Lobitz et al. (Reference Lobitz, Wood, Averner and McKay2001) and Richardson & Mischna (Reference Richardson and Mischna2005) focused on regions on Mars where the surface temperature and pressure were conducive for the melting of ice. In this scenario – we denote here as Scenario 1, where it is assumed that the diurnal thermal wave is equal to the temperature of ice tens of centimetres below the surface, allowing for such ice to experience temperatures close to 273 K. A minimum thin liquid film thickness of 11 nm is expected for pure water ice, correlating to 4.0×106 litres of water. Accounting for impurities yields a maximum thin liquid film thickness of 90 nm, which correlates to 3.0×107 litres of water.
Theoretical models, observations and laboratory experiments indicate that water ice is stable (i.e. at 1 m thick layer of ice 1 m below Mars’ surface resembling JSC Mars-1 at 235 K would last ∼800 years) in the shallow subsurface of Mars (Chevrier et al. Reference Chevrier2007; Kieffer & Zent Reference Kieffer, Zent, Kieffer, Jakosky, Snyder and Matthews2002; Jakosky et al. Reference Jakosky2005), and these results also indicate that the water-ice table steadily increases with depth in decreasing latitude. More detailed modelling has predicted that the depth at which water ice is stable can be highly variable, owing to local surface heterogeneities, such as rocks and slopes, and the thermal inertia of the ground cover (Bandfield Reference Bandfield2007). Jakosky et al. (Reference Jakosky2005) and Mellon et al. (Reference Mellon, Feldman and Prettyman2004) show that between 30°N and 30°S Mars maximum subsurface temperatures range from 220 to 235 K. Sadtchenko & Ewing (Reference Sadtchenko and Ewing2002) conducted experiments on pure water ice that quantified the thickness of interfacial thin liquid films and showed that they are negligible below ∼−30°C. Scenario 2 (pure water-ice context) then suggests that the amount of water below this temperature is less than a few molecular layers thick – i.e. <1.2 nm, the limit of detection for the experimental technique used in Sadtchenko & Ewing (Reference Sadtchenko and Ewing2002). Concomitantly, the effect of impurities at 235 K allows for a thin liquid film thickness on/within subsurface ice of 0.5 nm, corresponding to 6.0×104 litres of water (Wettlaufer Reference Wettlaufer1999; Benatov & Wettlaufer Reference Benatov and Wettlaufer2004).
The existence of thin liquid films on Mars is supported by elementary Br and chlorine (Cl) measurements from the MERs Spirit and Opportunity's Alpha Proton X-ray Spectrometer, showing elevated levels of Br compared to Cl. Figure 3 displays Br enhancements in soils at Gusev Crater, where Br enhancements increase as a function of depth in its subsurface soils. Br and Cl are highly soluble species, and their presence is indicative of past water activity. At present Martian conditions, these elevated levels of Br indicate the possible presence of transient thin liquid films at the surface and transport of such films to the subsurface (Yen et al. Reference Yen2005). The implications of existence of thin liquid films on Mars on its surface and subsurface is illustrated in Fig. 4, where uptake (and release), heterogeneous photochemical processing and downward transport may occur. The diffusion of H2O vapour into and out of the regolith during diurnal and seasonal cycles may produce unfrozen films of water around soil particles (Zent Reference Zent2008). However, most recently, the Thermal and Electrical Conductivity Probe (TECP) electrical conductivity measurements onboard Phoenix (at 68.22°N, 234.25°E) were consistent with a fully open circuit, implying that there was no effective transport of charge carriers on the scale of 15 mm (Smith et al. Reference Smith2009). Given our lower- and upper-limit nanometre-scale calculations for the thin liquid films on the surface of Mars, these dimensions are below the detection limit of TECP. Still, night-time increases in regolith dielectric permittivity, observed during the latter half of the Phoenix mission (i.e. mid to late summer), imply an overnight accumulation of H2O molecules (Smith et al. Reference Smith2009). In addition, a small amount of adsorbed or volumetrically bound water in the surface layer is implied by near-infrared data from the OMEGA (Observatoire pour la Minéralogie, l'Eau, les Glaces, et l'Activité imaging spectrometer on Mars Express) and CRISM (Compact Reconnaissance Imaging Spectrometer for Mars on Mars Reconnaissance Orbiter) at the Phoenix site (Smith et al. Reference Smith2009).
Fig. 3. Mole percent Cl versus Br detected in soils by the MER Alpha Particle X-ray Spectrometer at the Gusev Crater landing site (47–48). Surface measurements (red) have lower concentrations of Br than subsurface soils that provide transient, diurnal/seasonal cold traps for water (blue).
Fig. 4. Simplified schematic illustrating the deposition of atmospheric species A(g) and absorption in thin liquid films on the Martian surface (i.e. regolith (a) and ice (b)), followed by heterogeneous photochemical processing and transport of species A, B, C downwards or back to the gas phase.
Conclusions
Bulk liquid water on the surface and subsurface of Mars does not exist due to pressure and temperature constraints. Given that biological activity is limited by the availability of water and that thin liquid films are known to exist on and in soil and ice media at temperatures well below 273 K, we quantified lower- and upper-limits for the thickness of thin films at on the Martian surface and in subsurface ice, which are supported by spacecraft data – i.e. MER's Spirit and Opportunity APXS measurements. Despite the discovery of the smallest life (i.e. Nanoarchaeum equitans) (Huber et al. Reference Huber2002), these findings indicate that biological activity at Mars’ surface and subsurface is not limited by the nanometre dimensions of such films (Rivkina et al. Reference Rivkina, Friedmann and McKay2000; Price Reference Price2007). Given the extremely limited laboratory and theoretical data quantifying the thickness of interfacial films for subsurface soil-pure-water-ice and soil-water-ice systems containing impurities and the uncertainties for the equatorial Martian subsurface temperature profile, the possibility that biological activity may be limited at Mars’ subsurface is not definitive. This notion is supported by a recent laboratory study of a simulated Martian-like shallow subsurface layer at very low atmospheric pressures (i.e. 0.01–0.1 mbar) and cold average temperatures (Pavlov et al. Reference Pavlov, Shelegedin, Vdovina and Pavlov2010), which show results indicative of metabolic activity and the reproduction of terrestrial non-extremophile bacteria (Vibrio sp.). To better constrain the limits for potential subsurface life on Mars, under Martian conditions, additional rigorous and careful laboratory and theoretical analyses of the influence of impurity type and concentration on the dimension of thin liquid films on and within ice and soil media are required.
