Thousands of Earth microbes hitched a ride aboard the Curiosity rover. Can they survive on Mars? PHOTOGRAPH COURTESY NASA / JPL-Caltech / Malin Space Science Systems / Rex

Last week, NASA announced the unravelling of a major Mars mystery. “Now we know there is liquid water on the surface of this cold desert planet,” Michael Meyer, the Mars Exploration Program’s lead scientist, said. Specifically, an orbiting spacecraft has detected what the agency’s scientists are calling recurring slope lineae—cold, salty streaks of damp soil that appear seasonally in several craters and chasms. NASA’s strategy for seeking out extraterrestrial life has traditionally been to follow the water, since all organisms on Earth require it, and Earth organisms are the only kind we know. The announcement therefore raised another persistent question: Does this mean there’s life on Mars?

NASA had an answer for that one, too. “We know there’s life on Mars already because we sent it there,” John Grunsfeld, the associate administrator of the agency’s Science Mission Directorate, said during a press conference on Monday. It is a Faustian condition of space exploration that we cannot search for life on alien planets without bringing along very small amounts of very small Earth life. This process is known as forward contamination, and minimizing, if not preventing, its occurrence is the responsibility of Cassie Conley, a plant biologist who has served as NASA’s planetary-protection officer since 2006. “It’s basic common sense,” Conley told me. “We have to be careful not to blind ourselves with Earth life, the same way you can’t see the stars when the sun is out.”

In an ideal universe, the crewless spacecraft that we send to Mars would be sterile. (Humans are, by definition, contaminants.) In reality, for both technical and economic reasons, they are not. Rather, they are cleaned up enough to satisfy the Committee on Space Research, the international body that sets the ground rules for extraterrestrial exploration. The relevant COSPAR standard was conceived back in the nineteen-fifties, and it relies on an estimate of how likely Earth organisms are to survive on Mars. But, given how little scientists knew about conditions on the red planet at the time, Conley said, it “was pretty much a case of sticking their fingers in the air and saying ‘Hmmm.’ ” Still, the discussions were contentious, and they dragged on for more than a decade. Eventually, the committee settled on what it considered an acceptable level of contamination risk: one in a thousand. In other words, humanity must limit itself to one chance in a thousand of seeding another planet with terrestrial life in the course of exploring it.

The risk was divvied up among the spacefaring nations, with the United States receiving just under half of the total allocation: one chance in forty thousand. Every one of NASA’s subsequent planetary missions—the Viking probes, Pathfinder, and the ill-fated Mars Polar Lander—has used up some tiny fraction of this imaginary figure. Once astronauts get involved, though, all bets will be off. COSPAR’s framework is intended to cover only the short window of time during which a planet remains uncontaminated (and thus alien) enough to be of “biological interest.” Originally, this period was set at an optimistic twenty years. It has since been extended.

The first Martian landers, Vikings 1 and 2, which launched in 1975, were the cleanest things that NASA has ever sent into space. After their final bake-off at Kennedy Space Center (two hundred and thirty-three degrees Fahrenheit for thirty hours), they were determined to have just 0.3 organisms remaining—not an actual tally, of course, but the result of an abstract calculation that took into account the results of pre-assembly surface swabs and average kill rates. When the Viking data came back from Mars, it painted a picture of a much harsher, drier environment than many planetary scientists had hoped or imagined, and subsequent missions were accordingly allowed to carry a higher bioburden. The Mars Science Laboratory—the mission that landed the Curiosity Rover in August, 2012—was estimated to host just under thirty thousand heat-resistant bacterial endospores at launch, many more than Viking, but, as Conley pointed out, “still fewer than the number on your hand.”

As NASA scientists have learned more about both the Martian climate and the extreme hardiness of some Earth microbes, however, they have had to update their risk assessments. “The idea was that we need to be the most vigilant on the early missions, when we don’t have any knowledge of Mars, and we can relax when we know more,” Conley said. “But it turns out maybe we relaxed too much.” When she conducted the planetary-protection review of Curiosity’s proposed landing site, at Gale Crater, she thought, “This is a flat site at the equator. There’s no way it will have water.” Following last week’s announcement, however, it now seems that there may be water only a few miles away from Curiosity’s path. Under current COSPAR rules, the rover is not clean enough to go near it. “Mars continues to surprise us,” Conley said. “This is a good problem to have.” To solve it, she is consulting her advisory committee, an eclectic group of geologists, astrobiologists, planetary scientists, representatives from other government departments and foreign space agencies, a lawyer, a communications expert, and—this spot is currently vacant—an ethicist.

Although Conley will be ultimately responsible for determining how close to the recurring slope lineae Curiosity can get, she will reach that decision using a calculation made by NASA scientists as to how many of the original thirty thousand hitchhikers might still be aboard the rover today. This is where Kasthuri Venkateswaran, a microbiologist at NASA’s Jet Propulsion Laboratory, and Nick Bernardini, the planetary-protection lead on both Curiosity and the Mars 2020 mission, will play an important role. After Curiosity launched, Venkateswaran and his colleagues took the cloths that had been used to wipe down the rover’s exposed surfaces and examined the genetic traces left behind on them. Thanks to these efforts, we now have the first Martian passenger manifesto: lots of Actinobacteria and Alphaproteobacteria; some Nitrososphaeraceae; a few dark-pigmented, double-wall-protected fungal spores of Melanommatales; and various others.

As Conley pointed out, though, it is not enough just to “have name tags for the microbes on Curiosity—we need to know what their capabilities are.” In other words, could they survive a lengthy journey in the vacuum of space and then proliferate on Mars? To answer that question, Venkateswaran’s group has started sending some of the hardiest candidates up for eighteen-month stints aboard the International Space Station. So far, the findings confirm what many scientists suspected—that NASA’s rigorous cleaning and decontamination processes have inadvertently selected for the kinds of microorganisms that don’t mind high radiation, extreme aridity, and low nutrient levels. These bugs tend to hibernate inside tough exterior shells until conditions are right for them to come out. _Bacillus pumilu_s SAFR-032, for instance, a bacterium that was first discovered at the Jet Propulsion Laboratory’s Spacecraft Assembly Facility, was damaged but not killed by its ride through space—and, Venkateswaran added, it “could potentially survive for millions of years once deposited on the Martian surface.”

Some of Earth’s extremophiles are now Martians; that much is evident. Whether they can emerge from dormancy and grow—whether they, as Venkateswaran put it, are capable “of making the red planet green”—is much less well understood. Venkateswaran’s data will soon be entered into Bernadini’s byzantine planetary-protection spreadsheet, which will spit out a number for Conley. She hopes to decide what Curiosity will do within six months, but she’s currently betting against it. “It’s highly unlikely we would take the risk of potentially contaminating a subsurface aquifer of Mars,” she said. “We just don’t know enough about it to allow that level of risk.”