What Earth’s saltiest pond says about Mars

An Antarctic pond that’s the saltiest natural body of water on Earth stays wet in part by pulling moisture out of the air, scientists have discovered. And that has implications for possible brine seeps and reservoirs on Mars.

DRIP, DRIP, DRIP. Water from snowmelt does most of the job keeping Antarctica's Don Juan Pond full of water. But some additional water comes from humidity in the air, thanks to a nearby source of calcium chloride salts. (South is up in the image, which is taken from Figure 1 in the paper.)

Don Juan Pond lies in Antarctica’s McMurdo Dry Valleys, one of the best terrestrial analogs for Mars. The pond has a salinity of 40 percent (the Dead Sea is “only” 34 percent). This allows it to remain almost always liquid, despite local temperatures dropping to -50°C (-58°F) during southern winter.

A paper published in Nature‘s open-access science journal Scientific Reports by a group of geologists reports on studies that identified where the pond’s moisture comes from. The team, led by James Dickson (Brown University), also sought to identify the source for the pond’s salt, which is 90 percent calcium chloride, an unusual chemical composition for natural saline ponds.

To determine the pond’s water cycle, the scientists used time-lapse photos (video link) taken over two months during austral summer. These revealed the appearance of dark “water tracks” running down local slopes as temperatures warmed. The pond’s water level also rose in step with the daily temperatures, suggesting that meltwater from snow was feeding the pond.

But the source for the high salt content was harder to pinpoint. It turned out that loose sediment rich in calcium chloride salt lay to the west of the pond. As the humidity in the air rose, the salt in the soil absorbed the moisture (video link) in a kind of inverse-evaporation called deliquescence. As it turned liquid, the salty water trickled down through the soil until it hit the impenetrable permanently frozen layer below. Then, whenever snowmelt flowed, it washed the salty water down into Don Juan Pond.

The link to Mars lies in regard to the “recurring slope lineae” (RSL). These are dark streaks that flow down canyon and crater walls in local springtime. Scientists have hypothesized that the lineae may be caused by seeps of brine or salty water. Deliquescence working with salts and traces of moisture in the Martian atmosphere might possibly create brief flows of brine.

“Broadly speaking,” says lead author Dickson, “all the ingredients are there for a Don Juan Pond-type hydrology on Mars.” While Mars appears too cold and dry today to have such ponds, they could have been abundant in the Martian past.

“Don Juan Pond is a closed basin pond and we just documented a couple hundred closed basins on Mars,” says co-author James Head. “So what we found in Antarctica may be a key to how lakes worked on early Mars and also how moisture may flow on the surface today.”

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