Keeping Mars clean

At the National Aeronautics and Space Administration (NASA), Catharine A Conley has a lofty job title: planetary protection officer. That conjures to mind shades-wearing Will Smith and Tommy Lee Jones in the “Men in Black” movies. Indeed, on her first day on the job, nine years ago, she was presented with a pair of sunglasses. But with no extraterrestrial invasions on the horizon, Catharine’s job is not so much protecting Earth from aliens as protecting other planets from Earth. Mars, in particular. “If we’re going to look for life on Mars, it would be really kind of lame to bring Earth life and find that instead,” Catharine said. Recently, scientists had identified areas of flowing water on the Martian surface — some possibly reachable by NASA’s Curiosity rover — that concern has taken on new urgency.

Thousands, millions, sometimes many times more, bacteria travel across the solar system on spacecraft. Earth has been invading Mars since November 1971, when the Soviet Mars 2 lander crashed. Certainly life exists on Mars today — the microbes that have hitchhiked from Earth. Even in the harsh environs of Mars — cold, dry, bombarded by ultraviolet light — it takes many years for all of them to be killed off.

The concern is that some of them might not only survive but thrive. Because of the residual microbes, NASA’s Opportunity and Curiosity rovers are prohibited from visiting what are known as “special regions” — places that Earth bacteria might happily call home. InSight, NASA’s next Mars lander, to be launched in March, and the next rover, to be launched in 2020, will also not be sterilised. In considering landing sites for the 2020 rover, NASA has crossed off those in special regions.

Travelling microbes
The thinking is that some Earth microbes have been jostled to the surface of the
inhospitable parts of Mars, but they would remain dormant and not proliferate. “So far, Mars is still pretty clean,” Catharine said. Areas treated as special regions
include the periodic dark streaks known as recurrent slope lineae — RSLs for short — spotted on the sides of craters, canyons and mountains. Scientists recently said they were generated by the percolating of liquid water, one of the essentials for life.

The caution brings up a Catch-22. NASA at present cannot explore the places with the greatest potential for life — one that could come into play for Curiosity, which is slowly climbing a mountain in Gale Crater. Gale was selected as Curiosity’s destination in part because it was the “driest and least likely to have special regions,” Catharine said. But some candidate RSLs have been unexpectedly spotted from orbit on the mountain within a couple of miles of Curiosity’s planned path. James L Green, the director of NASA’s planetary science division, did not rule out that Curiosity might go to one of them. If the streaks are confirmed as RSLs, James said, NASA might then evaluate how many Earth microbes are likely to survive on the outside of Curiosity.

“That would tell us, if we can get approval from planetary protection, how close we could go,” James said. Catharine would be the one to make that call. She said that at launch, there were probably 20,000 to 40,000 heat-resistant bacterial spores on Curiosity, and perhaps 100 or 1,000 times more microbes not counted. Many of them would have since perished in the vacuum of space.

Intense ultraviolet radiation on the Martian surface would have killed many more — but not all, and some drop in the soil as Curiosity trundles by and performs its science work. “We are still having ongoing discussions,” Catharine said. “It depends on what the results of the calculations are.” This is not just a fastidious whim of NASA, but an international agreement.

On new grounds
The Outer Space Treaty of 1967 dictates that nations should take care when
exploring other planets “to avoid their harmful contamination.” The Committee on Space Research, part of the International Council of Science, develops planetary protection policies that Catharine is responsible for carrying out. For most missions, like the Cassini orbiter at Saturn, the requirements are fairly simple — do not crash into a body where life might exist, and when done, dispose of the spacecraft. Cassini will be sent on a death dive into Saturn, where heat and pressure will obliterate it and any remaining microbes.

Similar care will be taken studying Europa, a moon of Jupiter, and Enceladus, a moon of Saturn, known to have oceans beneath the surface. But it will be many years before a lander sets down there. For now, the main concern is Mars. “We’re treading new ground,” John M Grunsfeld, NASA’s associate administrator for science, said of the discussions of Curiosity. “The issue of planetary protection has gone very much from one where we’re just trying to be careful to one that has very real, near-term consideration.”

With the two Viking landers in 1976 — NASA’s first and so far only attempts at detecting life on another planet — the agency took extraordinary precautions sterilising the spacecraft, first cleaning it to fewer than 300 heat-resistant bacterial spores per square metre. Then it was packed up and baked for several days, reducing the number of spores by a factor of 10,000. But most of the data from Viking pointed to Mars as a lifeless place. Since then, NASA has still cleaned its Mars spacecraft to the same standards — “better than a surgical suite,” Catharine said — but skipped the baking step. Sterilisation would add perhaps $100 million to the price of a mission.

Thus, NASA has avoided the special regions, which include anywhere with water ice within a metre of the surface. An exception was the Phoenix Mars lander, which dug into ice in the polar region. The arm — but only the arm — was sterilised to Viking standards. The salts known as perchlorates that lower the freezing temperature of water at the RSLs, keeping it liquid, can be consumed by some Earth microbes. “The environment on Mars potentially is basically one giant dinner plate for Earth organisms,” Catharine said.
The first Earth settlers could be one of the lowliest of plants. “I worry about lichen,” Catharine said. “I worry about the stuff that grows on your roof. They basically eat rock and they breathe sunlight. And there’s rocks and sunlight on Mars.”

Catharine inadvertently entered the planetary protection field because of the loss of the space shuttle Columbia, which was carrying an experiment of hers involving nematode worms when it disintegrated during re-entry in 2003. The experiment was recovered amid the wreckage, and the worms were still alive. “Multicellular animals, at least small ones, can survive uncontrolled atmospheric entry, so long as they don’t get too hot,” Catharine said.

For some NASA critics, the concerns of planetary protection are just an expensive ball and chain that slow study of the solar system. Why the extreme caution now if one day in a few decades, astronauts, carrying a slew of Earth microbes, arrive to colonise Mars? With a changing picture of present-day Mars that is not quite as desolate as once thought, “we actually do need to be careful,” Grunsfeld said, adding, “We do need to watch what we’re doing, because there could be life on Mars.”