Don Juan Pond: Antarctica's salty, syrupy lake that never freezes, even when it's minus 58 F

Don Juan Pond is a lake in Antarctica's extremely cold McMurdo Dry Valleys region that has a salinity above 40%, making it one of the saltiest bodies of water on Earth. The pond contains so much salt, it has a syrupy consistency that never freezes, even though temperatures in the Dry Valleys often fall to minus 58 degrees Fahrenheit (minus 50 degrees Celsius).

Don Juan Pond is 4 inches (10 centimeters) deep and slightly smaller than the total area of six football fields. It is named after two U.S. Navy helicopter pilots, Donald Roe and John Hickey, who spotted the pond during a reconnaissance expedition in 1961. There are other lakes in the Dry Valleys ‪—‬ but none are as salty as Don Juan Pond, so they stay frozen solid for most of the year.

Scientists are fascinated by Don Juan Pond because the harsh conditions in the McMurdo Dry Valleys resemble those on Mars, according to NASA. The Red Planet is bitterly cold and dry, rich in a variety of salts, and potentially hosts water. There is evidence for microbial life near Don Juan Pond, and it's possible that some organisms survive in the brine. If life can persist in the Dry Valleys, then life may exist, or may have once existed, in hypersaline features on Mars.

Don Juan Pond has elevated levels of calcium chloride, which is a highly soluble salt that lowers the pond's freezing temperature by keeping apart water molecules that would otherwise bond to make ice crystals. At 40% salinity, Don Juan Pond is much saltier than the Dead Sea, which has a salinity of 34%, and it is 12 times saltier than Earth's oceans, which have a salinity of 3.5%.

Scientists have been studying Don Juan Pond for more than 60 years, but they still aren't sure where the water and calcium chloride come from. Since the lake's discovery, the dominant hypothesis has been that Don Juan Pond is fed by groundwater bubbling to the surface. But in a 2013 study, Brown University geologists reported that the water is moisture from the atmosphere that is trapped by salts in the soil and eventually trickles down into the Upper Wright Valley, where Don Juan Pond is located.

The researchers took thousands of time-lapse photographs of the pond and the Upper Wright Valley's steep slopes. The images showed dark streaks near Don Juan Pond that the team argued were wet, salt-rich sediments funneling water toward the pond. Similar streaks, known as recurring slope linear features, occur on Mars' surface, hinting that a corresponding process may have once taken place on the Red Planet.

However, this hypothesis was contradicted by a 2017 study that revived the groundwater theory. That research, which was based on model simulations, indicated that the only way Don Juan Pond could have acquired its unique chemical makeup is if the water source were a deep groundwater system.

"If we accept that the deep groundwater theory is true, then what we're seeing could be part of a bigger process that involves quite an extensive aquifer," first author Jonathan Toner, an astrobiologist and research assistant professor at the University of Washington, said in a statement in 2017. "When thinking about the implications for a similar environment on Mars, that's much more exciting than just a localized surface phenomenon."

However, the source of water at Don Juan Pond is still debated.

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