Billions of years ago, Mars lost its atmosphere, and with it went Mars's once prolific assemblage of liquid water lakes and streams. Yet even without an atmosphere, the Red Planet hasn't always been as bone dry as it is today. A new discovery suggests that as recently as a mere 500,000 years ago, certain nooks on Mars had far more active, flowing water than scientists had imagined.

A research team led by Tjalling de Haas, a physical geographer at Utrecht University in the Netherlands, has just completed their scientific study on a one-million-year-old Martian crater called Istok, which appears today in Nature Communications. To their surprise, the scientists found that the gullies and sediment deposits on the now-dry Istok crater's slopes must have been caused by periodic flows of an incredible amount of muddy ice water. These flows, caused by melting snowpack, would have been several inches deep. To make them would have required 10 times more liquid water (and snow) than predicted by our best models of Mars's historical climate.

About the time Homo erectus was learning how to control fire, water was still flowing on Mars

The last of these wet periods for the Istok crater may have been only half a million years ago. That means that about the time our evolutionary predecessor Homo erectus was learning how to control fire, water was flowing on parts of Mars.

"I was actually really quite surprised to discover the volume and activity of water required to explain the [gullies] and debris in the crater," de Hass says. "The takeaway is that on a relatively recent timescale, there was quite some fluvial activity at certain locations [like the Istok crater] on Mars, at least for short periods."

Spying on craters

Istok crater, in the southern highland planes of Mars's Aonia Terra, has not been studied by NASA's Curiosity or Opportunity rovers because both are much farther north. For their study, de Haas and his colleagues used images from the 10-year-old Mars Reconnaissance Orbiter's HiRISE camera.

"The pictures from HiRISE give us a great amount of detail," says de Haas. It's about "25 centimeters of spacial resolution on the ground," he says which was plenty of resolution for his research. While scientists have had access to high-res photos of Istok for years now, only recently has the crater, with its immaculately preserved gullies and fan-shaped deposits of sediments on the northern slope—been singled out to study.

High-res photos in hand, de Haas's team found Istok's gullies and sediment deposits look almost identical to those that that occasionally occur in Mars-like arid regions here on Earth, such as the Atacama desert in Chile. When they modeled how much water would be required to create the gullies and sediment deposits found in Istok, they were struck with the surprising volume.

"I was actually really quite surprised to discover the volume and activity of water."

So where did this snow come from? De Haas says that like on Earth, Mars experiences periodic ice ages. These ice ages are caused by the planet's slowly-shifting rotational tilt. "On Earth, we have a 2 to 3-degree tilt [change] which causes ice ages and warm ages. On Mars that difference is between twenty and thirty degrees," de Haas says. As Mars's tilt moves into its extreme range, the ice at its polar region migrates downward, with snowfall occurring at lower latitudes.

A much different Mars

NASA

De Haas's team estimates that there have been five separate periods in the last million years where Mars' tilt was severe enough to see frequent muddy water flows at the Istock crater (frequent meaning somewhere between every 10 and 100 Earth years) . The most recent was around half a million years ago.

Still, de Haas says nobody is sure why so much snow and water might have collected at the Istok crater. It's a fact that seemingly flies in the face of the snowfall projections in our best historical climate models. De Haas has a pet theory, though, which is very much yet to be proven.

"Well, there's a lot of wind on Mars, and it seems possible that wind could have been blowing an accumulation of snow could into sheltered areas around the crater," he says, giving rise to an impressive buildup and the subsequent water flows.

For now, it's one more watery mystery on the Red Planet.

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