The question of whether liquid water can still exist today in Mars’ hostile climate has been around for decades, perhaps in underground aquifers or even for brief periods of time on the surface as salty brines. Debate still continues about the droplet-like features seen on one of the legs of the Phoenix lander, suggested by some members of the science team as possibly just such drops of liquid or partially liquid brines. Two recent papers from the 42nd Lunar and Planetary Science Conference this year further address the possibility of current liquid water.
The first looks at possible evidence for small seasonal pond-like features of saline (salty) water in the polar regions.
From the paper:
“Here we report the discovery of the spectral evidence for liquid water in flow-like and pond- like features of Mars polar region. This suggests that liquid saline water forms sporadically on the surface and should be common in the shallow subsurface.”
“More interestingly, measurements of the reflectance in different directions and therefore the albedo at the blue green (400-600 nm) and near infrared (780-1060 nm) portions of the spectrum show that ice, snow and soil have reflectances and albedo approximately constant with wavelength and that the reflectances and albedo of brines (melt ponds in sea ice) peaks at approximately 400-500 nm and decreases by almost an order of magnitude beyond 700 nm (Fig. 1). This happens because the absorption coefficient of liquid water is more than two orders of magnitude larger at near infrared than at blue green. In contrast, the reflectances and albedo of typical Mars soil is larger at near infrared, because it is rich in iron. Thus, the reflectances in a given direction or albedo at various portions of the spectrum, can be used to fingerprint liquid brines because it distinguishes them from frost, ice, snow, and soil.”
“HiRISE images of flow-like and pond-like features on polar dunes are analyzed by visually finding the brightest 25-50 pixels indicating frost/snow, the 25-50 darkest pixels in the darkest shadows, and adjusting the reflectance of each pixel with the aid of Eqn (7). Fig. 2 show that this analysis unveil the fingerprint of liquid water on Mars. Deliquescence occur in frost-covered areas that the temperature exceeds the eutectic temperature of salts in contact with it, before frost sublimates. This implies the existence of an optimum zone in the polar region where deliquescence occurs seasonally and produce surface flows and puddles of liquid saline water.”
The second paper suggests that salty brines may also still flow on the surface during the summer creating what are called “transient slope lineae.”
From the paper:
“In a companion abstract we describe HiRISE ob- servations of features we call transient slope lineae (TSL) . TSL are defined as narrow (up to a few m wide) albedo markings on steep (>20°) slopes that are transient—present in some HiRISE images but not others. They extend downslope, typically from bedrock outcrops or from rocky areas, and are often associated with and may form small channels (Figure 1). TSL are very different from slope streaks that form on dust-mantled slopes, as summarized in Table 1.”
“Our favored model at present for TSL formation is that shallow brines mobilize thin flows, as first proposed for slope streaks . To produce flows, there must be sufficient liquid to fill the pore space between particles; interfacial water  is not sufficient. The TSL mechanism may resemble that of , again for slope streaks, except that no runaway process is needed as TSL form incrementally. These flows may advance a little near the warmest time of each day, or they may advance by greater amounts on some days but not others.”
There is also a good blog post about these proposed TSLs on the Sky & Telescope web site (March 11). What’s interesting is that they seem to occur only on sun-facing slopes during mid-summer. Mars Reconnaissance Orbiter has observed them form and then change over time, starting out darker then gradually lightening in appearance.
Water? Only further observations will answer that question definitively.