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Viking 2 lander
Even the cleanest spacecraft, like the Viking landers (above), are contaminated with terrestrial microorganisms. (credit: NASA/JPL)

Cleaning up after Martian exploration

What happens if we find life on Mars? Pick up our trash, says one scientist

Mars 2. Mars 3. Mars 6. Viking 1. Viking 2. Mars Pathfinder and Sojourner. Mars Polar Lander. Deep Space 2. All these spacecraft have landed—or crash-landed—on Mars since the early 1970s. In a few months, they will be joined by Spirit, Opportunity, and Beagle 2. All have brought scientific instruments seeking to understand the nature of planet Mars, and if the planet once or currently harbors life. They also all brought with them terrestrial bacteria.

It’s true that at least some of the missions were subjected to varying efforts to sterilize the spacecraft. Some, though, particularly the early Soviet missions, went through few, if any, sterilization procedures; some later NASA missions, notably Mars Pathfinder, also went through less rigorous cleaning processes. Even the spacecraft that passed the strictest “planetary protection” protocols, though, harbored at least some terrestrial life forms when they landed on the surface of Mars. This has raised a number of scientific and ethical questions, such as how future missions would be able to determine if any life forms detected were terrestrial or Martian in origin, and whether humans are needlessly contaminating Mars and subjecting it to bacteria that could destroy the native life forms.

Those questions haven’t been answered yet, largely because no one knows whether life exists on Mars. However, that hasn’t stopped the what-if scenarios if life is found there. Planetary scientist and astrobiologist Chris McKay, speaking at the Sixth International Mars Society Conference in Eugene, Oregon, in mid-August, has his own radical, controversial solution: if Martian life is discovered, we should be prepared to “reverse” our exploration and pack up the trash we have left behind in the form of spacecraft lying across the Martian surface. His proposal, outlined during a plenary session of the conference, triggered a lively debate among conference participants.

Biologically reversible exploration

In his Mars Society talk, McKay outlined three possibilities for past life on Mars. The first is that Mars never had any kind the life, a prospect that would offer “no worries” to future robotic and human exploration of Mars. The second is that life did exist on Mars, but had a common origin to life on Earth. In other words, life evolved first on one planet or the other, and migrated to the other through the exchange of meteorites between the two worlds that exists today and was far more frequent early in the solar system’s history. That prospect, McKay said, offers little concern to future exploration since such Martian life would likely share many characteristics with terrestrial life.

Even the spacecraft that passed the strictest “planetary protection” protocols, though, harbored at least some terrestrial life forms when they landed on the surface of Mars.

The one scenario that would cause the most problems for future exploration, McKay believes, is if past life did exist on Mars and it was a “second genesis”, that is, was completely independent of the formation of life on Earth. “This has profound scientific and philosophical implications,” he said. “It poses novel ethical questions regarding how we treat aliens.”

Right now we don’t know which of the three scenarios is actually true, although McKay said that the second—a common origin to Martian and terrestrial life—is the leading contender. However, McKay argues that if the third scenario turns out to be true, we must be prepared to do what’s necessary to protect that Martian life from terrestrial contamination.

As already noted, the spacecraft that have landed on Mars to date carry terrestrial bacteria, despite the best efforts of some missions to sterilize their spacecraft. (He noted that the sterilization process may have, in fact, made some bacteria more likely to survive the journey to Mars.) However, those that survived the trip are not thriving. Ultraviolet light from the Sun, unchecked by the Martian atmosphere, acts as a sterilizing agent itself, killing any bacteria exposed to the Sun.

Those sheltered from the ultraviolet light, either within the spacecraft or in its perpetual shadow, are most likely in a dormant state. The cold conditions and lack of liquid water will not kill the bacteria, but will also prevent them from growing. “Except for the ultraviolet light, Mars has the perfect conditions for storing organisms,” McKay said. Under those conditions the bacteria could remain dormant but viable for thousands to hundreds of thousands of years, until cosmic rays finally killed the bacteria.

“No matter how extensive our exploration is, we can reverse it,” McKay said. “The Martian environment works with us and makes it easy to reverse our effects.”

This means to McKay that if true Martian life is found, it will be necessary to remove terrestrial contamination before the bacteria were somehow transported, for example, to some more hospitable environment below the surface. However, the fact that any bacteria that survived the trip is dormant, and can be killed by simple exposure to ultraviolet light, makes it straightforward to reverse our exploration and remove the contamination.

“No matter how extensive our exploration is, we can reverse it,” McKay said, while showing an illustration of a proposed human habitat on Mars. “The Martian environment works with us and makes it easy to reverse our effects.”

Such a “biologically reversible exploration” would be neither cheap nor easy, McKay admitted. It would involve, as one example, retrieving the debris from the Mars Polar Lander crash site. He estimates that the cleanup could cost three to four times the exploration itself. It would be simpler, he said, if the concept of reversible exploration was incorporated into missions during their design phases: it’s simpler to do retrieval and sterilization if it is built into the spacecraft to begin with, rather than done after the fact.

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