Last week, NASA announced they’d spotted occasional signs of flowing water on Mars. These briny flows, discerned from orbit, originated on the steep slopes of valleys or craters at four widely scattered sites in the planet’s southern hemisphere. Now, a comprehensive analysis of images gathered by NASA’s Curiosity rover provides the strongest evidence yet that Mars once was warm and wet enough to have lakes and flowing water year-round and for extended periods of time—possibly for millions of years. The findings hint that the Red Planet once had a climate hospitable enough for microbial life to develop and evolve.
The lander that carried the Curiosity rover to Mars dropped into the northwestern portion of Gale crater in August 2012. That immense pockmark, which measures about 150 kilometers wide and is the lowest spot on Mars for more than 1000 kilometers in any direction, was blasted by an impact that occurred somewhere between 3.6 billion and 3.8 billion years ago, says John Grotzinger, a planetary geologist at the California Institute of Technology in Pasadena. Scientists came up with that figure by analyzing the size and number of craters that now blemish the blanket of material tossed out of the crater when the impact occurred. Using similar techniques, he notes, they estimate erosion of the sediments that accumulated inside Gale crater largely ceased sometime between 3.1 billion and 3.3 billion years ago.
The tale chronicled in the crater’s sediments is one of abundant flowing water and a substantial lake that lasted for tens of thousands, if not millions, of years, Grotzinger and his colleagues report online today in Science.
The team’s analyses are based on the same tried-and-true, well-established methods used to infer the geological processes that sculpted Earth’s rocks. “You don’t need magic new science to understand the geology of Mars,” notes Janok Bhattacharya, a sedimentary geologist at McMaster University in Hamilton, Canada.
Since its landing, the Curiosity rover has been gathering data as it makes its way up a long, gentle slope. So far, the rover has climbed across—and thoroughly scrutinized—a 75-meter-thick layer of material that seems to have accumulated under a variety of conditions.
“For the first time, researchers have a reasonably thick section of sediments that provide a long-term picture of what was going on on Mars at the time,” says Kevin Bohacs, a sedimentary geologist at ExxonMobil in Houston, Texas.
The lowest layers of rocks, and therefore the first to accumulate, are chaotically layered sandstones that include pebbles ranging up to 22 millimeters across (slightly larger than a nickel). These bits show various degrees of smoothing—evidence, Grotzinger says, that the rocks were fiercely tumbled by moving water as they hopscotched downhill within the crater. “How [these pebbles] are shaped and how they’re arranged in the sediments are consistent with their origin in the crater walls dozens of kilometers away,” says Douglas Jerolmack, a geophysicist at the University of Pennsylvania.
“Wind simply can’t move the types of sediment that water can,” Bhattacharya says. So, the size and arrangement of materials in these rocks strongly suggest they were deposited by running water.
It’s possible to assess the strength of the flows that carried the material by judging the size and roundedness of the largest pebbles, Grotzinger says. These were not catastrophic floods, he notes, but were ankle-deep to waist-high flows “probably akin to a vigorous canoe ride.”
The rocks immediately overlying the streamflow deposits suggest that over time, waters accumulated in that portion of the crater to form a small lake. That inference comes from the sloping deposits whose layers dip southward, away from the crater wall, at angles between 10° and 20°. These sediments—in geological terms, these clinoform sandstones—haven’t shifted to tilt since they were deposited, says Grotzinger; they actually stacked up on an angle as they formed. On Earth, such deposits develop when sediment-laden waters flow into a lake or other standing body of water.
With a sudden drop in current speed, the flow can no longer carry as much sediment, so that material falls to the lake bottom. As more and more sediment accumulates, the deposit grows, with much of the new material added on the sloping, downstream edge of the deposit. (Similar large slugs of sediment have formed at the upstream end of dam-created reservoirs such as Lake Mead and Lake Powell in the southwestern United States, Grotzinger notes.) It’s not likely that such layering resulted from windblown sediments such as volcanic ash or from the slumping of sand dunes or other loose material, he adds.
“I have a lot of faith in their interpretations,” says Alan Howard, a planetary scientist at the University of Virginia in Charlottesville. And from those interpretations and other data, he notes, researchers can tell a lot about martian climate at the time.
For example, Grotzinger and his team found no evidence of freeze-thaw cycles in the Gale crater sediments. That suggests that temperatures in areas down at lake level stayed above freezing, or at worst only dipped below freezing slightly for brief periods. Yet it’s possible that a few kilometers higher, up along the crater rim, snow or sleet accumulated to provide meltwater in the summer months—flows that carried sediment down to the lake.
Although it’s clear that each layer records a single event, it’s not so straightforward to say how often those occurred. Although the layers could represent seasonal deposits that piled up year after year, it’s only possible at this point to say they occurred episodically, Grotzinger says. Yet from the amount of sediment in the layers and presumed rates of accumulation, the team estimates that the Gale crater’s lake—or a series of lakes that dried up during cold arid spells but then formed again in warmer, wetter times—existed for tens of thousands of years. Altogether, the sediments analyzed by Curiosity may have taken millions of years to pile up.
“That’s an awfully thick pile of rocks” that records evidence of both moving and standing water, says Marjorie Chan, a sedimentary geologist at the University of Utah in Salt Lake City. “It shows a diversity of environments—rivers, deltas, lakes—that we simply don’t see on Mars today.”
“There’s clearly a period on Mars when the paleoclimate was remarkably wet and stable,” Bohacs says.
What that means for the possibilities of life on Mars is anyone’s guess at this point. But it’s clear that the key ingredients for microbial life to originate and evolve were once available on the Red Planet, Chan says. The evidence of plentiful and relatively long-lasting water in liquid form dangles the tantalizing possibility that extraterrestrial life might exist or have been preserved, she notes.