The Dry Valleys, located about 100 km west of the U.S. McMurdo Research Station (Figure 1 -Click to enlarge) and typically known as the McMurdo Dry Valleys (MDV), comprise the largest ice-free region of Antarctica and are aptly characterized as a cold desert. In the lower valleys (e.g., Taylor Valley) precipitation occurs only as snow. The mean annual temperatures are about -20ºC with the summers frequently > 0ºC. In the upper valleys (e.g., Beacon or University Valleys) the air temperature rarely rises above -15ºC in the summer. The soils in the MDV are unlike any other soils on Earth, including the thoes in the northern artic region or in other deserts. The upper layer of the soil is typically loose material and extends to below the depth at which temperatures never exceed the freezing point. Below the dry permafrost there is ice-cemented permafrost and the transition between them is abrupt. The combination of ice-cemented ground, under dry permafrost, with a thin active surface layer above (warmed above freezing in the summer) in an environment without rain, make the dry valley soils unique on Earth and the most similar analog to expected conditions in the polar regions of Mars.

The following hypotheses were addressed by our research: 

• The presence of a shallow ice-cemented layer provides an impermeable barrier that affects the motion of soluble materials and influences the physical properties and habitability of the soil.
  

• Climate variability in the dry valleys results in the lowering and raising of the level of the
  ice-cemented layer and that this process leaves a characteristic signature in the distribution of salts.
  

• Variation in the physical properties, chemistry, and history of soils and ices in the Antarctic dry valleys vary with elevation and parent materials (e.g., granitic vs. basaltic).
  

• The ADV soils are the most analogous on Earth to those expected on Mars. The physical and chemical models that we derive will be directly applicable to understanding martian processes.

In preparation for the Phoenix Mars mission, an expedition during the 2007-08 field season to the ADV tested several of the Phoenix instruments, conducted a systematic study of the biological, mineralogical, chemical, and physical properties of the upper and lower valleys, and collected samples for more detailed studies. We performed both on and off-site analyses from both University and Taylor Valleys (Figures 2 & 3) [1]. Our most surprising and significant finding was the presence of perchlorate (ClO₄^-) in all soils, from the pavement to the ice-cemented soil, in all the samples from University, Taylor, and Beacon Valleys with a concentration of 31-630 μg/kg in University Valley and 167-575 μg/kg in Beacon Valley [2].

Figure 2. University Valley (~ 1,700 meters) (Click image to enlarge)	Figure 3. Taylor Valley (~ 0-800 meters) (Click image to enlarge)

Figure 4 shows the ClO₄^- versus depth profile for the University and Beacon Valley soils. Since the ClO₄^- ion is highly mobile in the presence of thin H₂O films, its distribution in the soil column is indicative of site aridity.  The ADV profile difference between Taylor and University Valleys confirm that ClO₄^- gradients are sensitive indicators for the presence and form of liquid H₂O on both Earth and Mars. Our results are consistent with the hypothesis that ClO₄^- is produced photochemically and continuously in Earth's atmosphere, or perhaps on mineral surfaces, and accumulates in hyperarid regions where the rate of deposition is greater than the rate of depletion. Even though atmospheric production mechanisms may be possible on Mars [3], the lack of ozone makes it highly unlikely with production on Cl-bearing mineral surfaces being more likely [4,5].

Figure 4. Concentration Profiles for perchlorate in University & Beacon Valleys [2]. (Click to enlarge)

Using its estimated global flux of 4×10^-3 kg/km2/yr, the oceans would accumulate ClO₄^- at a rate of 1.8 pg/L/yr.  Given the seawater concentration of between 0.09 to 0.1 μg/L, all oceanic ClO₄^- must be lost on a time scale of ~ 5×10⁴ years.  Since aqueous ClO₄^- is highly inert, we hypothesize that its lack of accumulation in the ocean or ground waters may be attributed to microbial conversion to chloride in areas of low nitrate or anaerobic zones.  The perchlorate ion provides an ideal electron acceptor for perchlorate reducing bacteria that have been isolated from a variety of environments.  The discovery of ubiquitous perchlorate in Antarctica may have significant implications for its origin, and global biogeochemistry. One aspect of our research is aimed at advancing our understanding of the ADV, their formation, history, and habitability, and the techniques that will allow for on-site analyses.  Further sampling and analysis of perchlorate is needed to help in determining its extent on the ice caps, global distribution, interactions with terrestrial ecosystems, and isotope systematics for tracing its atmospheric formation pathways.   This research will not only enhance the value of future Mars mission results, but will also increase our understanding of the Earth’s polar regions and extreme habitable environments.  

We have also compared the MDV soils with thoes of the Phoenix landing site [6] by averaging the samples at analogous 0-5 cm depths and also all the samples from the pavement to the ice-table. Results from each analysis reveal similar ion concentrations ranging plus or minus one order-of-magnitude for all ions except perchlorate, which was three orders-of-magnitude greater in the Phoenix soil.  The pH and solution electrical conductivity were also found to be similar for the ADV and Mars soils. The ADV profiles confirm that ClO₄^- gradients are sensitive indicators for the presence and form of liquid H2O on both Earth and Mars. The ADV results imply that the Phoenix site is significantly more arid than University Valley, and has been for a greater period of time, as evidenced by the lack of salt gradients and the age of the soils. Overall, comparison of these samples of soil indicates that the soils from the ADV are similar to the Phoenix landing site thus strengthening the argument for the MDV as a suitable terrestrial Mars analog environment.