The universe is feeling a little less lonely these days.
A year and a half after the Curiosity rover executed its gymnastic dismount onto the surface of Mars, mission scientists on Monday published the strongest evidence yet of an ancient, freshwater lake on the planet that could have been home for life as we know it.
The lake is dry and there are still no signs of little green men—or even microbes. But that report, published in the journal Science, comes just a month after a team of scientists analyzing data from the NASA space-based telescope Kepler announced that one in every five stars like the sun is circled by an Earth-like planet.
For decades, scientists have known there was water on Mars. Its red surface was once carved into canyons and riverbeds by liquid water, and there is ice in its soil and at its poles. What they haven’t known as much about was what that water was like.
An earlier Mars rover, Opportunity, which landed on the planet in 2004, sent back reports of rocks that formed in a “very bizarre, acidic bath-like environment”—harsh conditions in which to imagine life starting, said John Grotzinger, the chief scientist of the Mars Curiosity mission. What the new rover has discovered, as it has drilled, zapped, and baked samples, is something very different—evidence of a freshwater lake that stuck around for hundreds or tens of thousands of years and would have taken up about the same area as the Finger Lakes in upstate New York.
Asked what that lake might have looked like when it existed 3.6 billion years ago, Grotzinger said it may be useful to think of the western U.S. where vast lakes once existed in spots that are dry.
“What you sort of have to imagine is if you take a place like the Great Salt Lake that has a little bit of water today or Las Vegas Valley or Death Valley—if you go back 10,000 years ago … those basins all filled with water,” Grotzinger said. “They were broad lakes, but they were shallow.”
After Curiosity landed in Gale Crater in August 2012, scientists decided to take a temporary detour in the opposite direction of its ultimate destination—the three-mile high Mount Sharp. Before the mission, scientists had spoken mostly about how the rover was going to analyze the mountain layer by layer, reading through the geological history of Mars. But it landed temptingly close to rocks that satellite measurements had indicated might be of interest, so the rover started to explore these rocks in an ancient environment known as Yellowknife Bay, which was clearly shaped by water of some kind.
“When we first got the images back, after we had brushed the dust away from some of these rocks—the dust is the typical red-orange color of Mars ... there was this fantastic grayish, greenish blue color of the rock that was very un-Mars like,” said Ralph Milliken, a participating scientist on the Curiosity team and a geologist at Brown University. “That told us there was definitely going to be something interesting and distinct about the chemistry and mineralogy.”
The rover drilled and analyzed two rocks in detail using instruments on the robot. The scientists couldn’t tell directly from those sedimentary rocks whether life existed in the lake. But the makeup of the clay rocks revealed that they had to form in the presence of fresh water, not the acidic conditions detected elsewhere on the planet. There was also evidence of iron and sulfur in chemical states that indicated they could have been used to fuel the metabolism of a type of microorganisms found on Earth that feed on rocks, called chemolithoautotrophs.
Now, Grotzinger said, as the rover turns back toward Mount Sharp, its primary mission is complete—Curiosity has detected an environment that would have been habitable. The team is turning now to a much more difficult task: finding carbon-based organic molecules, the type necessary for the kind of life found on Earth.
That effort has been complicated by the pervasive presence in the soil of a salt called perchlorate. The instrument designed to detect signs of organic chemicals, called SAM, heats up samples. That causes the perchlorate to undergo a chemical reaction that can burn up organic carbon in the soil, obscuring its source.
Roger Summons, an astrobiologist at the Massachusetts Institute of Technology and a participating scientist on the mission, said that there is hope for overcoming the perchlorate problem—in part because of experiments going on in his laboratory and at Goddard Space Flight Center, where scientists are using a similar instrument to try and figure out how they can use the equipment to detect the source of the carbon.
One possible way of overcoming this problem is already used in labs on Earth and is currently in the planning stages on Curiosity, Summons said. By deliberately adding oxygen to samples in combustion experiments, scientists can compare the amount of carbon dioxide produced. If there is a bigger peak when oxygen is administered than when the sample is simply heated, that may suggest a greater reservoir of organic carbon.
And scientists will have one other powerful new tool in their search for the right rocks to probe on this mission and others: they reported on Monday that they have been able to date a rock on the surface of Mars using instruments on Curiosity. Previously, the primary way scientists could estimate the age of an area was by analyzing the number and size of craters. The older a region, the more it was pocked with craters created by collisions with space debris. But that provided only a rough approximation of age. Now, the Curiosity team has found a new way to refine their search by identifying areas of the planet that are newly exposed and therefore free from damaging radiation that might have destroyed traces of organic molecules.
And Mars isn’t the only spot in the galaxy to look. A group of scientists announced in November that Earth-like planets that sit in their star’s habitable zones, where conditions are right for life, are surprisingly common. Based on observations of one portion of the Milky Way by the space-based telescope Kepler, astronomers estimated that a fifth of all stars like our sun harbor such a planet, and the nearest one is probably just 12 light-years away.