June 13 (UPI) -- Today, Mars is a freezing cold desert and has been for at least 3 billion years. And yet, the Red Planet features a network of water-carved valleys that scientists believe formed between 1.5 and 3 million years ago.
The network branches outward from an impact crater called Lyot. Until now, scientists weren't sure how the shallow valleys were formed.
New research suggests the region was covered by a thick slab of ice at the time of the Lyot impact. The collision projected hot rocks on the surrounding ice, creating flows of meltwater that carved the valleys.
"Based on the likely location of ice deposits during this period of Mars' history, and the amount of meltwater that could have been produced by Lyot ejecta landing on an ice sheet, we think this is the most plausible scenario for the formation of these valleys," lead researcher David Weiss, who recently earned his PhD from Brown University, said in a news release.
Evidence of water activity on the surface of Mars is plentiful, but the canyon systems and remnants of ancient lake systems are all older than 3 million years -- before the Amazonian epoch ushered in a cold, dry climate.
The Lyot valleys represent a relatively recent episode of water activity -- an anomaly.
Scientists have previously suggested the Lyot impact released a reservoir of groundwater. But the latest analysis suggests such an explanation doesn't fit the geologic evidence.
"If these were formed by deep groundwater discharge, that water would have also flowed into the crater itself," Weiss said. "We don't see any evidence that there was water present inside the crater."
Scientists have also hypothesized that ejected material briefly warmed the region's atmosphere, encouraging rain. But erosion from rainwater would have carved the rubble ejected by the impact. There is no evidence of eroded ejecta.
"The vast majority of the valleys seem to emerge from beneath the ejecta on its outer periphery, which casts serious doubt on the rainwater scenario," said Brown grad student Ashley Palumbo.
The findings -- detailed in the journal Geophysical Research Letters -- left glacial meltwater as the most plausible explanation for the shallow valleys.
The fact that Lyot is surrounded by so few secondary craters suggested to scientists the region was covered by ice at the time of the impact. When giant rocks are expelled during an impact, they often create a series of smaller craters where they land. However, a layer of ice would have prevented the formation of secondary craters.
Models showed the Lyot impact likely blasted several tons of rock onto the surrounding ice. Some of the boulders were likely heated to temperatures of 250 degrees Fahrenheit or more. The layer of hot rocks would have been sufficient to produce several thousand cubic kilometers of meltwater.
"What this shows is a way to get large amounts of liquid water on Mars without the need for a warming of the atmosphere and any liquid groundwater," grad student James Cassanelli said. "So we think this is a good explanation for how you get these channels forming in the Amazonian."