I was recently invited to speak at a a symposium organized by the Royal Palace Foundation in Amsterdam, which twice a year brings together experts to discuss a big topic like the COVID pandemic or the future of work. This summer’s meeting was devoted to the search for extraterrestrial life. As I focused on searching our solar system, Sarah Seager from MIT presented his ideas on how to look for life on planets orbiting other stars.
During our conversations and subsequent discussions, I dropped a suggestion that some people will surely find provocative: that we already did we discovered life on Mars more than 50 years ago, but that we inadvertently killed it.
Experiments with the Viking lander
In the mid-1970s, NASA sent two Viking landers to the surface of Mars equipped with instruments that conducted the only experiments to detect life ever conducted on another planet. The results of these tests were very confusing at the time and remain so today. While some of these—notably the labeled release experiment (which tests for microbial metabolism) and the pyrolytic release experiments (which test for organic synthesis)—were initially positive for life, the gas exchange experiment was not.
The Viking landers also include a tool to detect organic compounds. He saw traces of chlorinated organics, which at the time were interpreted as the result of contamination from Earth. This prompted Viking Project scientist Gerald Soffen to utter his famous words, “No bodies, no life.” In other words, there could be no Martian life without organic compounds. So Soffen concluded, as did most other scientists at the time, that the Viking project was negative on the existence of life, or inconclusive at best.
In the half century since then, the picture has changed a lot. Eight more landers and rovers have explored the Martian surface in more detail. Thanks to 2008 Phoenix lander and until later confirmed by Curiosity and Persistence rovers, we know that native organic compounds do exist on Mars. However, they are in a chlorinated form – not what Viking Age scientists expected – and we don’t know if they come from biological processes or some abiotic chemical reactions which have nothing to do with life. Still, one might wonder how Soffen would react today: would he say outright that the Viking results were negative?
Death by water
At the time of these landings, scientists had very little understanding of the Martian environment. Since Earth is a water planet, it seemed reasonable that the addition of water would cause life to emerge in the extremely dry Martian environment. In hindsight, this approach may have been too good. 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 becomes drier.
At the very end of this progression you will find microbes that live entirely in salt rocks. These hardy organisms take advantage of a process we call hygroscopicity, whereby certain salts attract water directly from the relative humidity of the air. (This is the same process that causes table salt to clump when you leave it exposed to air.) For this reason, the microbes living in the Atacama salt rocks don’t need rain at all—just a certain amount of moisture in the atmosphere.
Now let’s ask what would happen if you poured water on these dry-adapted microbes. Can this crush them? In technical terms we would say we are hyperhydrating them, but in simple terms it would be more like drowning them. It would be like an alien spaceship finding you wandering half-dead in the desert and your would-be rescuers deciding, “People need water. Let’s put the man in the middle of the ocean to save him!” That wasn’t going to work either.
Many of Viking’s experiments involved applying water to the soil samples, which may explain the puzzling results. Perhaps the putative Martian microbes collected for the labeled release experiments could not handle this amount of water and died after some time. Most of the experiments for the pyrolytic release experiment were conducted under dry conditions, unlike the other experiments. The first cycle was positive for life compared to a control cycle conducted later, which was designed so that no biologics were involved. Interestingly, the only cycle run under wet conditions had a weaker signal than the control.
Following this line of thought, we have to ask whether the Martian soil tested by Viking actually contains hygroscopic salts and whether the relative humidity at these locations is high enough. The Vikings landed in the equatorial region of Mars, where the salt content of the soil is quite low. But there is a lot of hydrogen peroxide and perchlorates in the soil, and both compounds are very hygroscopic. Also, Viking did observe fog on Mars – meaning 100% humidity. In general, the relative humidity would be high enough in the morning and evening for the microbes to suck up the moisture.
Hope from hydrogen peroxide
More than 15 years ago, my colleague Joop Houtkooper and I raised the level of scientific speculation on this topic looking at Viking’s puzzling results from another perspective. We hypothesized that microbial life on Mars might have hydrogen peroxide in their cells—an evolutionary adaptation that would allow them to draw water directly from the atmosphere. The mixture would also have other advantages, such as keeping water liquid in sub-zero Martian temperatures, preventing the formation of ice crystals that would rupture cells.
While at a high enough concentration hydrogen peroxide is used for cleaning and sterilization, a lot germs in your mouth, e.g Streptococcus and lactobacillus, naturally produce hydrogen peroxide, along with others such as Neisseria sicca and Haemophilus indolent, who use it. The Bomber Beetle sprays a 25% hydrogen peroxide solution on anything that bothers it. I mean hydrogen peroxide is not incompatible with life.
Assuming that native Martian life may have adapted to the environment by incorporating hydrogen peroxide into its cells, this could explain the Viking results. The instrument used to detect organic compounds (called a gas chromatograph-mass spectrometer) heats soil samples before analysis. If the Martian cells contained hydrogen peroxide, it would kill them. Furthermore, this would cause the hydrogen peroxide to react with any organic molecules nearby to form large amounts of carbon dioxide—exactly what the instrument found.
A new mission to Mars
As I have argued before, we need a new mission to Mars devoted primarily to the discovery of life to test this hypothesis and others. It should explore potential habitats on Mars such as the Southern Highlands, where life could exist in salty rocks near the surface. We may even be able to access these rocks without drilling – a huge advantage in terms of engineering complications and costs. I can’t wait for such a mission to begin.