December 2023 – January 2024

The Nation Reviewed

Mars attracts

By Paul Davies
Illustration by Jeff Fisher
Reviving the Viking mission’s experiments may yet find life as we know it on Mars, but the best outcome would be something truly alien

Early in 2023, NASA announced the first observational results of an Earth-sized, rocky planet orbiting a red-dwarf star 41 light-years away in the constellation Octans, made using the new James Webb Space Telescope, the largest and most powerful telescope ever built. Ecstatic scientists remarked that it was just the start. “With this telescope, rocky exoplanets are the new frontier,” said Kevin Stevenson of Johns Hopkins University.

The long-term goal is to use the Webb telescope to analyse the atmospheres of exoplanets for signs of life. Unfortunately, such observations are fiendishly difficult and highly unlikely to produce definitive evidence for life beyond Earth in the near future. Meanwhile, there is an Earth-like rocky planet right here in our neck of the cosmic woods that has long been a favourite candidate for extraterrestrial life: Mars.

In 1976, two NASA spacecraft, Viking 1 and 2, landed on the red planet with the express purpose of looking for evidence of life. Each lander was equipped with several on-board experiments to detect any signs of microbial activity in the Martian dirt. One of them, the “labelled-release experiment”, gave positive results on both landers. The experiment worked by putting scooped-up dirt in a nutrient broth and seeing if anything ate it. Well, something did eat it, giving off radioactively tagged carbon dioxide in the process. When the samples were baked at 160 degrees, the fizzing stopped, exactly as expected if the heat had killed any microbes. The experiments were repeated many times, yielding essentially the same outcome. Downplaying these eye-opening results, NASA quickly pooh-poohed the claim that the Viking mission had detected any biological activity. Yet the designer of the labelled-release experiment, Gilbert Levin, insisted until his death in 2021 that he had in fact found life on Mars. Is it possible he did?

Unfortunately, the Viking landings were the first and last time NASA attempted to detect life on Mars, in spite of the fact that almost every one of NASA’s many subsequent Mars missions was touted as “searching for signs of life”. The deflating truth is that this small armada of orbiters and landers has merely tried to determine whether, billions of years ago, conditions on the planet’s surface could have supported life had it ever arisen there. Indeed, in a recent decadal survey, The National Academies of Sciences, Engineering, and Medicine advised NASA to not look for extant life on Mars.

However, 2023 could go down as the year the pendulum finally swung. Disillusioned by NASA’s caution, several tech entrepreneurs have now begun expressing strong interest in seeking out extant life in the solar system. And a growing band of frustrated astrobiologists is outlining how to go about it, with the benefit of the remarkable technological advances in the half-century since Viking. Last June, in a lecture to the Australian Centre for Astrobiology at UNSW Sydney, the distinguished biochemist Steven Benner unveiled a plan for returning to Mars with a suite of life-detection instruments. Benner runs the US-based Foundation for Applied Molecular Evolution, and he repeated this plea to an October meeting of the Mars Society in Arizona. He pointed out that, “Contrary to statements from NASA expert committees, all of the results of the Viking 1976 life detection experiments are consistent with the Viking soils containing microorganisms.”

Part of Benner’s plan to sniff out Martian microbes is to scan large volumes of water extracted from the topsoil to look for the distinctive patterns of electrical charges associated with evolvable information-bearing molecules – a key signature of life. In known organisms, these are negative charges in DNA, but alien life might have a different molecular basis. Benner’s experiment could still identify it using a fairly simple piece of equipment.

The late Gilbert Levin also had a plan for upgrading his original labelled-release experiment. A shortcoming in its design was the inability to distinguish between mundane soil chemistry and biological activity. Perhaps sterile Mars dirt fizzes anyway when you add water? But terrestrial life has a pervasive chemical asymmetry: DNA winds like a right-handed helix, while amino acid molecules that make up proteins are “left-handed”. This molecular lopsidedness is known as chirality. Almost all organic molecules are chiral: they would not look the same in a mirror. Critical to life is that the molecules it uses should have the same chirality, to avoid an unholy muddle of left and right being juxtaposed. It’s possible to manufacture a nutrient broth from the opposite chirality molecules – mirror soup, if you like. Life as we know it finds mirror soup indigestible. If Mars dirt fizzes equally with soup and mirror soup, simple chemistry would explain the results. But if there was a preference for one or the other, it would be a powerful argument for biology. Levin requested for Viking to have such chiral discrimination, but NASA overruled it.

It’s fair to say that most astrobiologists think the surface of Mars is too harsh an environment even for simple microbes to survive today. But Chile’s arid Atacama Desert and the dry valleys of Antarctica offer plausible Mars analogues, and both are host to some resilient microorganisms that eke out an existence, in some cases beneath a protective layer of salt. Could Mars harbour similar microbes? It’s something of a long shot, but if we don’t look, we will never know.

Thirty years ago, I pointed out that Earth and Mars occasionally take a hit from an asteroid or comet with enough force to blast rocks all around the solar system. Mars rocks fall to Earth and Earth rocks go to Mars. Some of these rocks will be laden with microbes, a fraction of which could survive to make a home on the other planet. At the time my suggestion was met with derision, but today scientists generally accept that Earth and Mars are not quarantined; they have been cross-contaminating each other via impact ejecta for billions of years. Indeed, it is entirely possible that terrestrial life did not emerge here de novo, but was transported to Earth from Mars about four billion years ago.

While this two-way traffic of material strengthens the case for finding extant life on Mars, it also undermines one of the prime motivations for seeking life beyond Earth. Whether or not we are alone in the universe is one of the oldest and deepest questions of existence. Half a century ago, the prevailing view among scientists is that life on Earth is a gigantic fluke, a chemical accident so complex it wouldn’t happen twice. Now it is fashionable to declare that the universe is teeming with life, although we have zero evidence one way or the other.

What would clinch it is the discovery of a second genesis. Just a single sample of life, but not as we know it, would demonstrate that the transition from non-life to life cannot be overwhelmingly hard, and therefore life should be widespread in the universe. While it would be hugely exciting to go to Mars and discover an ancient link between terrestrial and Martian life, the real payoff would be to find a totally novel form of life there, a genuine second genesis. This could be “mirror life” with the opposite chirality to ours, or a form of life that uses an alternative informational molecule from DNA, which Benner’s experiment could isolate. The tantalising possibility of aliens on our doorstep is out there. All it needs is an adventurous space pioneer or visionary space agency to transport a life-detection package there to find out.

Paul Davies

Paul Davies is a physicist and astrobiologist at Arizona State University, where he is Regents’ Professor. His latest book is What’s Eating the Universe? And Other Cosmic Questions.

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