PARIS • A multi-billion dollar robot dispatched to Mars to search for life must steer clear of promising "hot spots" for fear of spreading microbes from Earth, according to the National Aeronautics and Space Administration (Nasa) project scientists.
The spectre of a missed opportunity was thrown into sharp relief by smoking-gun evidence unveiled this week that liquid water, a prerequisite for life, existed not only in a distant Martian past, but is likely there today.
"Curiosity isn't designed to go to a place that can currently support microbial life," said Mr Michael Meyer, a scientist for Nasa's Mars Exploration Programme, referring to the car-sized mobile laboratory parachuted into the Red Planet's Gale Crater in August 2012.
"For that, we need a higher level of cleanliness," which is more complicated and costly to achieve, he told AFP.
This exasperating reality was the result of a fateful decision years ago to forego Nasa's most stringent microbe-removal standards for hardware visiting the moist environments in which Martian life - if it exists - will probably be found.
The danger of letting Curiosity investigate the newly-found sites is real, space scientists and astrobiologists agree. "We don't want to be remembered as the species that went to another planet and wiped out whatever life was there," explained Mr Jorge Vago, a scientist with the European Space Agency's ExoMars Project, due to send its own Mars orbiter up in 2016 and put down a rover in 2018.
Scientists announced on Monday that they had found tracks formed by hydrated salt crystals - essentially super-salty brine - running down steep slopes on the surface of the Red Planet. Nasa's Curiosity rover is especially well-equipped for microbe-hunting in just such an environment. Its core mission is to gather soil and rock samples, and analyse them "for organic compounds and environmental conditions that could have supported life now or in the past," according to Nasa.
But the streaks, dubbed "recurring slope lineae" (RSL) - possibly the best chance yet of finding Martian life - are off-limits for one simple reason: Curiosity is too dirty.
Adding insult to injury, the first spacecraft Nasa landed on Mars some 40 years ago, dubbed Viking, met the highest cleanliness requirement, even if it never had the same golden opportunity to detect life.
"The missions we have sent since Viking have not been cleaned to the same level - Viking was essentially sterile," said Ms Catharine Conley, who heads Nasa's Office of Planetary Protection, set up to prevent cross-contamination between Earth and the Solar System's other heavenly bodies.
"It would be very nice to have that capability again, to be able to drive right into the RSLs and sample (them)," she added.
Beyond Nasa's even more stringent standards, space agencies are bound by an international treaty to "avoid harmful contamination" in space exploration.
Earth bacteria hitching a ride on spacecraft "could under certain circumstances find conditions (in which) they could prosper. That's a no-no," said Mr Vago.
Ms Conley added: "We also try to kill all the bacteria on these probes so that we don't wind up... embarrassingly discovering life on Mars only to realise that it's our own dirty fingerprints."
Sterilising a space-destined probe to the safest level is expensive. And when Curiosity was designed, well before its 2011 launch, chances for an inter-planetary meeting of species seemed more remote than they do today.
Curiosity can still use remote sensors to gather information on the RSLs from a distance, said Mr Jim Watzin, director of Nasa's Mars Exploration Programme. He added: "It is a question of budget and priorities."