The
world’s saltiest body of water is tucked away in a valley in one of the most
extreme environments on Earth. It rarely snows and never rains in the McMurdo
Dry Valleys. Winter temperatures can drop to -50 degrees Celsius (-58 degrees
Fahrenheit), and the few ponds and lakes in the valleys are capped by ice that
is several meters thick.
Then
there’s Don Juan Pond. The ankle-deep pond in the lowest part of Upper Wright
Valley is so salty that its calcium-chloride rich waters rarely freeze. Salt
particles lower the freezing point of water by moving between water molecules
and impeding the formation of the crystal lattice structure of ice.
With
a salinity level over 40 percent, Don Juan is significantly saltier than most
of the other hypersaline lakes around the world. The Dead Sea has a salinity of
34 percent; the Great Salt Lake varies between 5 and 27 percent. Earth’s oceans
have an average salinity of 3.5 percent.
The
Advanced Land Imager (ALI) on NASA’s Earth Observing-1 (EO-1) satellite
captured this image on January 3, 2014. The ellipse-shaped lake is situated at
the bottom of a basin between the Dais plateau and the Asgard Range to the
south. It has a slightly darker hue than the salt-encrusted lake bottom around
it.
The
lower image shows a broader view of Wright Valley. Notice the network of
channels just east of Wright Upper Glacier, an intricate feature eroded into
dolerite bedrock known as the Labyrinth. Frozen Lake Vanda is visible to the
northeast of Don Juan Pond.
While
hydrologists have long thought that groundwater bubbled up from below to feed
the pond, recent research by Brown University geologists Jay Dickson and James
Head has shown that the water most likely comes from the atmosphere. By setting
up cameras that took thousands of time-lapse photographs of the lake, the
scientists observed that salts in the soil suck available moisture from the air
through a process called deliquescence. These water-rich salts then trickle
down slopes toward the pond, often mixing with small amounts of melt water from
snow and ice. The process creates dark water tracks on the surface, some of
which are visible in the ALI image.
For
astrobiologists, one of the most tantalizing aspects of Don Juan Pond is the
possibility that its salty water contains microscopic life. If life can survive
in such an extreme environment, it would lend credence to the idea that life
exists—or once existed—in hypersaline features on Mars. “There is certainly
biology in the vicinity of the pond and some evidence for biologic activity in
the pond itself, but this activity could be explained by abiotic processes,”
said Dickson. ”Mars has a lot of salt and used to have a lot of water.”
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