Practically every new rover, lander or orbiter sent to Mars in recent years turns up more evidence of water, both past and present. This vital liquid is necessary for life as we know it, but whether anything has ever lived on Mars is still a mystery. This week scientists reported new findings of water on the Red Planet that bring us closer than ever to finally answering the question.

The Curiosity rover’s landing site on Mars, Gale Crater, used to hold a lake, scientists announced on Monday at the American Geophysical Union (AGU) meeting in San Francisco. And on Tuesday researchers analyzing photographs taken by the Mars Reconnaissance Orbiter (MRO) reported that liquid water appears to be darkening the slopes of mountains near the Martian equator—the first time strong indications of present-day water have been seen in this region. Curiosity's latest results were reported in six papers this week in Science, and the evidence from MRO was published in Nature Geoscience. (Scientific American is part of Nature Publishing Group.)

The findings add to the growing picture that Mars was once habitable—a fact first settled by Curiosity in March. Now scientists are ready to start answering the larger question of whether it was ever actually inhabited. One of the key clues will be the presence or lack of complex organic compounds—molecules featuring carbon that are both ingredients for life and potential by-products of it. "Really what we're doing in the mission is turning the corner from a mission dedicated to the search for a habitable environment to now looking for the subset of habitable environments that also preserves organic carbon," Curiosity's project scientist John Grotzinger of the California Institute of Technology said during a press conference at the AGU meeting on Monday. "That’s the next step we need to take as we explore the possibilities for life on Mars."

Curiosity found evidence for a former lake, about the size of one of New York State's Finger Lakes, in a region called Yellowknife Bay. There, the rover drilled two holes to study subsurface geology and chemistry. By baking these rock samples in its SAM (Sample Analysis at Mars) oven and analyzing the gases they produced, Curiosity found carbon dioxide and nitric oxide. Although some quantities of these molecules could have been contamination from Earth, the large amounts seen by SAM suggest they exist on Mars. "This is really important," said Jennifer Eigenbrode of NASA Goddard Space Flight Center at the press conference. "Carbon and nitrogen are required by all life-forms. Therefore [Yellowknife Bay] could have supported life."

Yet finding more complex organic compounds could be challenging. These molecules, such as the amino acids that build life's proteins, are not very hardy: radiation from the sun easily breaks them apart. "How the organics are actually preserved and what happens after the radiation hits them is something we're going to have to explore further," Eigenbrode said. Even if Mars did have ancient life, fossil evidence might not necessarily survive. The lake might have existed billions of years ago; on Earth, which is teeming with life, it is rare to find microfossils from that far back, Grotzinger said. "The trick is to make sure you have enough of the good minerals and as little as possible of the bad radiation that will vaporize them."

Water was not just a fixture of Mars' past—it could be surprisingly abundant in the present as well. In 2001 researchers announced they had seen what looked like flowing water on mid-latitude Martian slopes. Now the same team, led by Alfred McEwen of the University of Arizona, reports similar dark slope features near the equator based on observations from orbit by MRO. "If our interpretation that this is due to water is correct, then it means the near-surface equatorial region is wetter than we thought," McEwen says. "This is really puzzling. It implies there's a surprisingly active water cycle, given that it hasn’t rained in so long there."

The watery-looking features, called slope lineae, show up in the warmest season and gradually fill out the slope over time. Then when Mars gets colder, the features fade and eventually disappear, recurring again in the subsequent warm season. Scientists first thought this water was melting from surface ice, but that interpretation is less likely for the slopes near the equator, where the surface is probably too warm for ice. Now researchers suggest that perhaps groundwater is rising to the surface or that water vapor from the thin atmosphere on Mars condenses and flows down the slopes. It is also possible that slope lineae are not water at all. They could represent dust streaks or avalanches, although, if so, their growth pattern would be surprising.

If these dark streaks are flowing water, microbes could possibly live there, although life would be unlikely to originate in such a harsh environment, says Peter Smith of the University of Arizona in Tucson, who led NASA's Phoenix lander mission on Mars. "But say four billion years ago, microbes had a rich community on Mars and started to evolve into every niche they could find. If some ended up on these slopes, and over the years maybe they built up a vital community, and then in the dry periods they could go dormant. You could imagine these things."

Until more conclusive evidence of water emerges, Martian life—past or present—will stay in the realm of imagination. "Astronomy has always been this way," Smith says. "You start in a state of incredible ignorance, and you find bits of knowledge here and there and you try to build up a big picture. We need a breakthrough. If Curiosity could just find a good solid piece of organic material and know that it's Martian, that would be a wonderful thing."