Recently, I was invited to speak at a symposium organized by the Amsterdam Royal Palace Foundation, who, twice a year, brings in experts to discuss some big topic like the COVID pandemic or the future of work. This summer’s meeting was about the search for extraterrestrial life. While I focused on the search in our own Solar System, Sara Seager of MIT presented her ideas on how to look for life on planets circling other stars.

During our talks and the discussions that followed, I dropped a suggestion that some people surely will find provocative: that we already did find life on Mars nearly 50 years ago—but that we inadvertently killed it.

The Viking Lander Experiments

In the mid-1970s, NASA sent two Viking landers to the surface of Mars equipped with instruments that conducted the only life detection experiments ever conducted on another planet. The results of those tests were very confusing at the time and remain so today. While some of them—particularly the labelled release experiment (which tested for microbial metabolism) and the pyrolytic release experiments (which tested for organic synthesis)—were initially positive for life, the gas exchange experiment was not.

The Viking landers also included an instrument to detect organic compounds. It saw trace amounts of chlorinated organics, which were interpreted at the time to be the result of contamination from Earth. This led Viking project scientist Gerald Soffen to utter his famous words, “No bodies, no life.” In other words, there couldn’t be Martian life without organic compounds. So Soffen concluded, as did most other scientists at the time, that the Viking project was negative as to the presence of life, or at best inconclusive.

In the half century since, the picture has changed a lot. Eight more landers and rovers have explored the Martian surface in greater detail. Thanks to the 2008 Phoenix lander, and to later confirmation by the Curiosity and Perseverance rovers, we know that indigenous organic compounds do, in fact, exist on Mars. They’re in a chlorinated form, however—not what the Viking-era scientists expected—and we don’t know whether they derive from biological processes or from some abiotic chemical reactions that have nothing to do with life. Still, one might wonder how Soffen would react today: Would he still say categorically that the Viking results were negative?

Death by Water

At the time of those landings, scientists had very little understanding of the Martian environment. Since Earth is a water planet, it seemed reasonable that adding water might coax life to show itself in the extremely dry Martian environment. In hindsight, it is possible that approach was too much of a good thing. What I and other researchers have learned in extremely dry places on Earth, such as Chile’s Atacama Desert, is that there is a gradual progression of life forms as the habitat gets more arid.

At the very end of that progression, you find microbes that live entirely wit