Don Juan Pond

Liquid at minus 50, and the best model anywhere for water on Mars

Don Juan Pond lies at the lowest point of the South Fork of Upper Wright Valley, in the McMurdo Dry Valleys of Antarctica, wedged between the Asgard Range and the Dais, about 9 km west of Lake Vanda. It is barely a pond at all: ankle-deep, usually about 10 centimetres, ellipse-shaped, darker than the crust of salt around it. But it is the saltiest natural body of water on Earth, more than 40 percent salt by mass, against 34 percent for the Dead Sea and 3.5 for the oceans, and that salt keeps it liquid when winter drops the surface temperature to minus 50.

Its salt is strange too. About 90 percent of it is calcium chloride, a composition unlike any other body of water in the world, and the white flats around the shore are crusted with calcium and sodium chloride left behind as the water evaporated. Unlike the big ice-covered lakes of the Dry Valleys it gets no fresh glacial runoff, boxed in by steep slopes of granite and Jurassic dolerite, a debris-covered rock glacier to the west and a gentle slope of loose soil over permafrost to the east. It was discovered in 1961 and named for the two helicopter pilots who flew the first field party in, Lieutenants Don Roe and John Hickey.

For decades the accepted explanation was groundwater welling up from a weathered dolerite aquifer some 60 metres down. Then the Brown University geologists Jay Dickson and James Head set cameras on the Dais, 750 metres above the pond, and took 16,280 time-lapse pictures through the summer of 2009 and 2010. They saw no groundwater coming in. Instead, the salts in the soil to the east were pulling moisture straight out of the air, a process called deliquescence, and the brines that formed trickled downslope along dark water tracks atop the permafrost, flushed toward the pond by snowmelt and seeping out around boulders on its eastern shore. Each pulse came with the midday warmth, while the sun stood above the Asgard Range. Other researchers still argue for a deep source, and the question remains open.

The stakes reach beyond Antarctica. Mars has dark streaks called recurring slope lineae that appear seasonally on steep slopes and are thought to be brines, and the water tracks above Don Juan Pond are the closest analogue on Earth. A sparse microflora of four species of bacteria and a yeast has been found in its water, and, as Dickson put it, Mars has a lot of salt and used to have a lot of water.