Among the many mysteries of Mars, perhaps the most challenging is what happened to the Red Planet’s water. It once was wet and fertile. “It’s now bone dry,” said Cosmos host, Neil deGrasse Tyson, “Something bad happened on Mars. I want to know what happened on Mars so that we may prevent it from happening here on Earth.”
Mars is laced with distinctive signs of big, long-dead rivers but scientists still don’t know what kind of weather fed them, because their understanding of the Martian climate billions of years ago remains incomplete. Early in the planet’s history, Mars only received a third of the sunlight of present-day Earth, which shouldn’t be enough heat to maintain liquid water. That is but one of the long list of its unsolved mysteries.
No Apparent Evidence of a Water Source
Adding to the list of enigmas is a strange, as-yet unnamed Mars crater being studied by researchers at Brown University. It differs dramatically from typical Martian crater lakes, like those found at the Gale and Jezero crater sites where NASA Curiosity and Perseverance rovers are currently exploring for signs of ancient microbial life. The new-found crater’s floor has unmistakable geologic evidence of ancient stream beds and ponds, yet there’s no evidence of inlet channels where water could have entered the crater from outside, and no evidence of groundwater activity where it could have bubbled up from below.
New research, led by Brown Ph.D. student Ben Boatwright concluded that the ancient crater lake in the southern highlands appears to have been fed by glacial runoff, bolstering the idea that the Red Planet had a cold and icy past. “That the system was likely fed by runoff from a long-lost Martian glacier,” the researchers report. “Water flowed into the crater atop the glacier, which meant it didn’t leave behind a valley as it would have had it flowed directly on the ground. The water eventually emptied into the low-lying crater floor, where it left its geological mark on the bare Martian soil.”
Unrecognized Type of Hydrological System
“This is a previously unrecognized type of hydrological system on Mars,” Boatwright said. “In lake systems characterized so far, we see evidence of drainage coming from outside the crater, breaching the crater wall and in some cases flowing out the other side. But that’s not what is happening here. Everything is happening inside the crater, and that’s very different than what’s been characterized before.”
Clues About the Ancient Climate
Importantly, Boatwright says, the crater provides key clues about the early climate of Mars. There’s little doubt that the Martian climate was once warmer and wetter than the frozen desert the planet is today. What’s less clear, reports the Brown study, however, is whether Mars had an Earthlike climate with continually flowing water for millennia, or whether it was mostly cold and icy with fleeting periods of warmth and melting. Climate simulations for early Mars suggest temperatures rarely peaking above freezing, but geological evidence for cold and icy conditions has been sparse, Boatwright says. This new evidence of ancient glaciation could change that.
“The cold and icy scenario has been largely theoretical — something that arises from climate models,” Boatwright said. “But the evidence for glaciation we see here helps to bridge the gap between theory and observation. I think that’s really the big takeaway here.”
Boatwright was able to map out the details of the crater’s lake system using high-resolution images taken by NASA’s Mars Reconnaissance Orbiter. The image shown at the top of the page revealed a telltale signature of ancient streambeds — features called inverted fluvial channels. When water flows across a rocky surface, it can leave behind course-grained sediment inside the valley it erodes. When these sediments interact with water, they can form minerals that are harder than the surrounding rock. As further erosion over millions of years whittles the surrounding rock away, the mineralized channels are left behind as raised ridges spidering across the landscape. These features, along with sediment deposits and shoreline features, clearly show where water flowed and ponded on the crater floor.
But without any sign of an inlet channel where water entered the crater, “the question becomes ‘how did these get here?”‘ Boatwright said.
The Glacial Clues
To figure it out, Boatwright worked with Jim Head, his advisor and a research professor at Brown. They ruled out groundwater activity, as the crater lacked telltale sapping channels that form in groundwater systems. These channels usually appear as short, stubby channels that lack tributaries — completely opposite from the dense, branching networks of inverted channels observed in the crater. A careful examination of the crater wall also revealed a distinct set of ridges that face upward toward the crater wall. The features are consistent with ridges formed where a glacier terminates and deposits mounds of rocky debris. Taken together, the evidence points to a glacier-fed system, the researchers concluded.
Age of the Crater Lake –”It’s Ancient”
“The age of crater lake we observed,” Boatwright wrote in an email to The Daily Galaxy, “comes from global geologic maps that place the terrain in which it formed in the Early Noachian period, shortly after the formation of the Hellas basin about 4.0 billion years ago. Global climate models predict glaciation somewhat later near the Noachian–Hesperian boundary, 3.7 billion years ago, and that’s our best estimate for when the channels and lakes formed inside the crater due to glacial melting.”
Subsequent research has shown that this crater isn’t the only one of its kind. At this month’s Lunar and Planetary Science Conference, Boatwright presented research revealing more than 40 additional craters that appear to have related features.
“We have these models telling us that early Mars would have been cold and icy, and now we have some really compelling geological evidence to go with it,” Head said. “Not only that, but this crater provides the criteria we need to start looking for even more evidence to test this hypothesis, which is really exciting.”
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