Don Juan Pond
The Don Juan Pond in western Antarctica is the most saline water body on Earth, at approximately twelve to thirteen times the salinity level of other typical seas of the world. This hypersaline lake is located east of Ross Island in Wright Valley in the region of the McMurdo Dry Valleys, in a cold desert biome. Don Juan Pond has not been observed to form any ice, in spite of reaching temperatures as low as minus 40 degrees Celsius.
Due to the extreme conditions of intense cold, aridity, hypersalinity and extremely high ultraviolet radiation, a number of scientists have argued that San Juan Pond is Earth's best possible proxy location in which to simulate conditions where life on Mars might exist. The most important discovery from the Mars research is an abiotic phenomenon, in which brine derived nitrates react with basaltic rock to produce nitrous oxide, a powerful greenhouse gas. There is conflicting data on the presence of biota in San Juan Pond, but, to the extent life exists, it is likely restricted to microbial forms, chiefly consisting of certain bacteria and fungi.
Don Juan Pond, is nestled in Wright Valley between the Asgard Mountain Range and the Olympus Mountain Range, the latter of which rises quite steeply from the valley floor. This lake is approximately 50 kilometers west of the Ross Sea. This hypersaline body lies nine kilometers west of Lake Vanda and has a surface elevation of 162 meters above mean sea level.
The shape of Don Juan Pond, often called Lake Don Juan, is roughly oval, with the long axis parallel to the valley centerline. Areal size has varied markedly over the last 50 years since initial discovery. Its maximal dimensions have been approximately 1000x400 meters, with a greatest historic water volume of no more than 90,000 cubic meters, although 2011 lake size is substantially smaller (at about 3000 cubic meters) than the historic maximum, and involves a depth of only about ten centimeters.
Hydrology and Chemistry
The chief influent flow to Don Juan Pond occurs through groundwater flow seeping down from the bases of the Olympus and Asgard Ranges. These waters dissolve minerals from subsurface rock strata and thus contribute a richly mineralized input to the lake. Evaporation further enhances the salinity of Don Juan Pond. The chief ions within Don Juan Pond are calcium and chloride, with an effective level of calcium chloride (CaCl2) of approximately 400 grams per kilogram. Sodium ions are also present and can be considered to represent an effective mass concentration of approximately 30 grams per kilogram, as sodium chloride.
The surface area and depth of Lake Don Juan change appreciably by season and from year to year, depending on snowfall in the basin. The slopes above the lake are snow covered for the austral winter and melt entirely in the austral summer, producing chiefly a subsurface influent to the lake.
Sampling of San Juan Pond in western Antarctica has yielded DNA signatures; however, it is not clear that these lifeforms are indigenous as opposed to presence from aeolian deposition. If there are native organisms present here, they likely consist of heterotrophs (species that cannot fix atmospheric carbon); these taxa are likely restricted to bacteria and fungi. By definition most of these species are likely to be extremophiles, e.g. lifeforms that can thrive in extraordinary abiotic conditions such as the extreme cold, aridity, hypersalinity and intense ultraviolet radiation regime present at San Juan Pond.
The limiting factor for the sustenance of life may actually be water activity (i.e. the affinity for water to combine with solid material), which has been measured to be very low in San Juan Pond. Generally a water activity of at least 0.9 is required to sustain most bacteria, and a level of 0.7 and a level of 0.9 to sustain fungal growth. The water activity of Don Juan Pond has been measured at about 0.45, a value generally viewed as too low to support most life forms. These simplistic conclusions do not reflect the ability of extremophiles to adapt to extraordinary condtions, such as those present in San Juan Pond. The history of life exploration in Don Juan Pond has been controversial, with some reported data not being readily duplicated in later years. Efforts have been hampered by the difficulty and seasonality of access. One of the early studies reported an elaborate shoreline microbial mat, which contained filamentous structures containing fungi, diatoms, unicellular cyanobacteria and other organisms.
Analogy to Life on Mars
sunlight. Like the McMurdo Dry Valleys, vast portions of Mars are effectively classified as cold dry deserts, with very high salinities.A number of scientists have alluded to Antarctica's San Juan Pond as a laboratory in which to study the possiblity of life on Mars due to its topographic depression, extreme temperature regime, high salinity and high ultraviolet irradiation from
Correspondingly NASA scientists recently discovered an arsenic based lifeform in the hypersaline Mono Lake, demonstrating that present knowledge of lifeforms may be in its infancy, particularly in relation to extremophile habitats.
As far as data from Mars itself, Doran et al. have pointed out that the NASA Viking Lander observed high sulfur and iron levels, moderate magnesium levels and unexpected levels of chloride, the combination of which can be best be explained not by weathering of known rock types but by actions such as the leaching by volcanic gases. Knauth and Burt (2002) first advanced the eutectic brine hypothesis for Mars, whereby a NaCl rich hydrosphere became evapoconcentrated, with pore fluids evolving into CaCl2 enriched brine via chemical reaction with the regolith; the ongoing freeze-down of Mars created pore fluids from the eutectic brines.
From a longer time perspective, it is worth noting that early microbial lifeforms on Earth in the Archaean Period involved a setting of hot and brine-like seas, supporting mostly thermophile and halophile microbial biota. Mars has some of these abiotic conditions of at least localized high salinity environments coupled with extremes in hot and cold. Thus Mars has precedent condtions not dissimilar to Earth of the distant past, with the quandary of absence of water being the chief mystery.
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