Fear of Aliens: How to Protect Alien Microbes (and Us)

NASA is concerned about aliens. No, not undocumented immigrants or the scary space invaders loved by Hollywood. The real science questions are related to the search for microbial life in our solar system: on Mars, Europa, Titan, or Enceladus. Each of these has a possibility of supporting life: Mars has a geological and climate history most like Earth; Europa and Enceladus both have extensive oceans of warm liquid water under an ice crust; and Titan has a complex organic chemistry but is too cold for liquid water.

If life exists on any of these worlds, it is likely to have begun there, providing evidence of a second (or third or fourth) genesis within our solar system. Finding any lifeforms, or even life precursors or fossils, would be tremendously exciting to scientists and the public alike.

Mars’s habitability is a central focus of today’s missions to the Red Planet, and the results encourage us to move on to a search for life itself. Robotic landers planned for the 2020s may carry the first life-detection experiments since the Viking mission in 1976, or alternatively include the return of samples from Mars to be analyzed in terrestrial laboratories. The discipline called “planetary protection” establishes guidelines to minimize biological cross-contamination between worlds. We think we know how to handle these issues, although assuring the public that our protocols are safe may be a challenge. But now there is also serious discussion of landing humans on Mars. This would introduce a new set of concerns, since surely any humans who land on Mars would want to return eventually to Earth, and there is no way to sterilize humans or their hardware. By landing there, the astronauts might compromise our hope of ever finding indigenous martian life.

Mars missions today follow international planetary protection agreements to minimize the possibility of the transfer of biota between Mars and Earth that might harm any indigenous life on either world. Hardware landed on Mars is decontaminated and may be sterilized to minimize forward contamination. Alternatively, the missions land in places that are considered inhospitable to terrestrial microorganisms. In fact, it seems unlikely that the martian surface would be able to support any organisms from Earth, due to its extreme dry cold and a lethal dose of ultraviolet light from the Sun. Life might exist below the surface or in protected environments such as lava caves, however, and some of these environments might not be lethal to microbial invaders from our planet.

Return of Mars samples to Earth raises another worry: that our planet might be contaminated. This seems unlikely because any life that survives on Mars is adapted to conditions very different from Earth and may have alien chemical or genetic structure. Furthermore, meteor impacts occasionally eject rock fragments from Mars that can reach Earth. We have found numerous Mars meteorites, which are of great value to scientists. As far as we know these meteorites do not carry dangerous microbes; if they did, our planet would likely have been contaminated many times over its history by this natural process.

A new report from the National Academy of Sciences issued in July 2018 urges NASA to create a broad planetary protection strategic plan. This plan should include how to handle returned samples and how to avoid contamination when human missions are sent to Mars. The Academy also expresses concern about private industry missions to Mars, since there is no agency with the authority to regulate space exploration by nongovernment entities. The report concludes: “Soundly framed and executed planetary protection policies will play a critical role in ensuring that space exploration efforts will deliver unambiguous answers to the possibility of life elsewhere in the solar system.”

The challenge is daunting. How can we search for life without simultaneously contaminating Mars? One approach would be to keep humans away from any places on Mars that might harbor indigenous life, confining them to teleoperations from Mars’s moon Phobus, for example, or from surface sites that have been certified sterile. But would astronauts be willing to travel all the way to Mars and then be denied the opportunity to study interesting parts of the planet in person? There is also the philosophical question of who will advocate for possible Mars life, even microbial life, as opposed to opening the entire planet to human exploration and ultimately perhaps exploitation.

H.G. Wells anticipated some of these issues in his classic 1898 novel War of the Worlds. In this story, the invaders from Mars have superior technology but ultimately succumb to terrestrial biota. What if it had been the other way around? In that case I suppose Wells would have lost his readership.

Copies of the Academy’s Review and Assessment of Planetary Protection Policy Development Processes are available at www.nap.edu.


NASA is concerned about aliens. No, not undocumented immigrants or the scary space invaders loved by Hollywood. The real science questions are related to the search for microbial life in our solar system: on Mars, Europa, Titan, or Enceladus. Each of these has a possibility of supporting life: Mars has a geological and climate history …

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