The short answer is salt. The more interesting answer is that Don Juan Pond is barely a pond in the ordinary sense of the word.
It sits in Antarctica’s Upper Wright Valley, one of the McMurdo Dry Valleys, where winter temperatures can fall to minus 50 degrees Celsius. Nearby lakes carry ice several metres thick. Don Juan Pond, meanwhile, can remain liquid through conditions that would turn freshwater solid almost immediately.
That contrast is real, although the popular version needs two corrections. The pond is not held continuously at minus 50 degrees, and it does not possess some absolute immunity to freezing. It is a shallow, shifting body of calcium-chloride brine whose freezing point has been pushed almost as low as the coldest temperatures it normally encounters.
Living in Singapore, I find minus 50 difficult to picture. What caught my attention was the idea that temperature alone does not decide whether water freezes. Composition matters just as much.
This is closer to brine than ordinary pond water
Don Juan Pond is only about ankle deep. Its exact area and depth change, and at times much of the visible water can disappear. The liquid that remains is among the most saline natural surface waters known on Earth.
NASA’s Earth Observatory describes a salinity above 40 per cent. Ocean water, by comparison, averages about 3.5 per cent. More unusually, the dissolved material in Don Juan Pond is dominated by calcium chloride rather than the sodium chloride most of us mean when we say salt.
That chemical difference matters. Calcium chloride dissolves readily and is strongly hygroscopic, meaning it attracts water. It is also a particularly effective freezing-point depressant. This is one reason calcium-chloride products are used to de-ice roads in conditions where ordinary rock salt becomes less useful.
The pond is therefore not a pocket of normal water somehow resisting the Antarctic winter. It is a dense chemical solution with physical properties very different from those of the ice around it.
Why the freezing point falls to about minus 52 degrees
Pure water freezes when its molecules settle into the ordered crystal structure of ice. Dissolved ions get in the way of that transition and lower the temperature at which the solid phase becomes favourable.
In a school chemistry calculation, this is called freezing-point depression. Don Juan Pond is far too concentrated to behave like a simple dilute solution, but the basic intuition still holds. Water molecules surrounded by large quantities of calcium and chloride ions cannot organise into ice under the same conditions as freshwater.
The useful number is the eutectic temperature. For a calcium-chloride brine at the right concentration, the liquid can remain stable down to roughly minus 52 degrees Celsius. A 2017 paper in Earth and Planetary Science Letters by Jonathan Toner, David Catling and Ronald Sletten gives that figure while describing pond water containing up to 40 per cent salt by weight.
“Eutectic” does not mean the brine can never freeze. It marks the lowest temperature at which that liquid mixture can remain in equilibrium. Below it, solid phases form. Depending on the exact temperature and composition, the products can include ice and hydrated calcium-chloride minerals such as antarcticite.
So Don Juan Pond is not breaking the freezing rule. Its chemistry has moved the rule from zero degrees to approximately minus 52.
The same pond does not necessarily sit there unchanged
The phrase “year after year” can give the impression of one stable pool surviving intact through every winter. Don Juan Pond is more dynamic than that.
It is extremely shallow and exposed to one of the driest environments on Earth. Its outline changes as water arrives and evaporates. Some accounts describe the surface pond disappearing almost entirely at times, leaving salt-rich sediment behind.
The 2017 geochemical study reached an especially interesting conclusion. Its model suggested that even the most concentrated brines in the pond had undergone closed-basin evaporation for less than a year. The authors proposed that brine rises from a regional groundwater system, becomes concentrated at the surface, then returns to the subsurface and is recycled over roughly annual timescales.
On that reading, Don Juan Pond persists as a hydrological and chemical system, not necessarily as the same unmoving parcel of liquid.
That distinction also explains why evaporation does not simply eliminate it forever. The salts remain, and new water entering the basin can dissolve them again. Each concentrated batch inherits a very low freezing point from the material already accumulated there.
Where the water comes from is still contested
For decades, researchers proposed that deep groundwater fed the pond from beneath. Then a team led by James Dickson of Brown University used time-lapse photography and meteorological measurements to watch dark water tracks form on nearby slopes.
Their 2013 study in Scientific Reports concluded that deliquescence was active in the watershed. During this process, calcium-chloride salts absorb moisture from the air until they dissolve into their own brine. Small amounts of seasonal snowmelt could then flush that brine downslope over the shallow permafrost and into the pond. The team found that this near-surface process could control summer water levels, and it did not observe groundwater entering during the study period.
Four years later, Toner and his colleagues argued that shallow inputs could not adequately reproduce the pond’s unusual ratios of calcium, magnesium, potassium and other ions. Their geochemical model fit deep groundwater better.
The two ideas are not necessarily mutually exclusive. Near-surface deliquescence and meltwater can affect when the pond fills, while a deeper system may help explain where its distinctive salt inventory came from. On the available evidence, the balance between those contributions remains unsettled.
I like this part of the story because the headline question has such a clean answer, but the pond itself does not. We understand why the brine stays liquid more confidently than we understand how the whole system is supplied.
Liquid water does not automatically mean a good home for life
Don Juan Pond attracts planetary scientists because Mars is also cold, dry and salty. If brines can form and remain liquid in Antarctica under conditions close to the edge of terrestrial water stability, similar chemistry might help explain transient moisture or ancient salt deposits on Mars.
But liquid is not the same thing as habitable.
In a 2016 Astrobiology paper on the limits of Martian brines, Mark Fox-Powell and colleagues argued that Don Juan Pond’s water activity falls below the accepted limit for cellular division. So much salt is dissolved that very little water is chemically available for organisms to use. Calcium-rich brines can also destabilise cellular structures.
A recent genomic report adds a careful wrinkle. Researchers published draft genomes from three bacterial isolates obtained from brine-saturated Don Juan Pond sediments collected in 2018. That demonstrates that microbes can be recovered from the pond’s sediment system. It does not, by itself, show that an active community is multiplying in the most concentrated surface brine at minus 50 degrees.
The pond is useful for thinking about Mars partly because it separates two questions that are often bundled together. Can liquid exist? Can life use it? Chemistry can answer yes to the first and still make the second extremely difficult.
Why the pond stays liquid
Don Juan Pond remains unfrozen during much of the Antarctic winter because its water carries an enormous load of calcium chloride. Those dissolved ions lower the freezing point from zero degrees Celsius to roughly minus 52. As long as the brine stays above that lower boundary, liquid water is physically allowed.
The pond can shrink, evaporate, gain new water and be replenished from sources that are still debated. It may occasionally form ice, and extreme cold below the eutectic limit would freeze the remaining liquid.
It does not refuse to freeze. It is water whose chemistry makes minus 50 degrees just warm enough.
