Whoops. Over the past four decades, NASA has launched a long series of exploratory probes to distant worlds in our solar system. It has sent up the Kepler space telescope to search for planets around other stars. All these missions have nurtured a hope, however faint, that astronomers might one day see unambiguous signs for life elsewhere in the universe. So it is only with the deepest sense of irony that one can hear the recent news that 36 years ago, scientists might have seen but disregarded proof of life on our closest planetary neighbor, Mars.
Don't judge the experts too harshly, however. The problem of how to recognize alien life -- life that might be radically unlike anything ever seen on Earth even at the molecular level -- has been tormenting would-be exobiologists since the early days of space exploration.
What may be most surprising, though, is that this is not just a problem facing space scientists. Some biologists suspect that we may be overlooking alien types of life right here on Earth, too, even though they may be in fairly plain sight.
Missed on Mars
The two Viking landers that arrived on Mars in 1976 provided much of the foundation for our knowledge about the planet's geology, weather conditions, and atmospheric and surface chemistry until the rovers Spirit and Opportunity arrived in 2003. The most exciting instruments on the Vikings, however, were the ones designed to carry out three kinds of experiments aimed at detecting biological activity in Martian soil. (It's worth pausing to note that in the history of planetary exploration, those Viking experiments have so far been were the only ones explicitly and specifically designed to detect life.)
Two of the experiments gave disappointing, negative results. The third, called the Labeled Release experiment, did initially seem to detect gas release from soil samples provided with nutrients, which would be in keeping with alien cells growing or reproducing. Yet NASA's scientists eventually concluded that spurious inorganic chemical reactions might be a more likely explanation in view of the other results. The Vikings search for life was written off as unsuccessful.
A new study appearing in the International Journal of Aeronautical and Space Sciences, however, argues that conclusion was premature. The four American and Italian authors analyzed patterns in the collected Labeled Release experiment data. They argue that the high complexity of that Viking data is more suggestive of biological causes than of simple nonliving ones. (Tuan C. Nguyen described this work in more detail for SmartPlanet last week.) [Update (added 4/17): Keith Cowing at SpaceRef also offers some reasons for viewing this announcement with many grains of salt.]
This reanalysis is not immediately persuading all skeptics, and most scientists probably won't start believing in Martian life without new, more sharply convincing proof. If nothing else, though, the work demonstrates how tricky and ambiguous chemical evidence for new kinds of life can be -- which is a bit of a shame, because it may be indispensable in the ambitious quest for life not as we know it.
Defining life without knowing the answer
Say "life not as we know it" and many people immediately think of the Horta from "The Devil in the Dark" episode of Star Trek. That fictional subterranean creature -- picture the offspring of a Galapagos tortoise and a heap of carpet samples -- was presented as a life form based on silicon rather than the carbon in our nucleotides and amino acids. Corrosive juices secreted from its fibrous asbestos tissues allowed it to burrow through solid rock as casually as we move through air, according to the episode.
In imagining a silicon life form, Star Trek's writers were less constrained by fact than modern scientists must be. Still, anyone looking for realistic insights can find plenty of inspiration because the scientific literature speculating on the unusual biochemistries of highly exotic alien life is almost surprisingly big.
Two publications in particular show up prominently in recent discussions of the topic. One is a 2004 paper in Current Opinion in Chemical Biology by Steven A. Benner, Alonso Ricardo and Matthew A. Carrigan, "Is there a common chemical model for life in the universe?" The other is a 2007 review, The Limits of Organic Life in Planetary Systems, issued by the National Research Council. Benner contributed to both, which helps to explain many of the similarities in their arguments.
(As an aside: Anyone who wants to know much more about the foundations of exobiology will probably find both of these works to be enjoyable reads. I especially appreciated the whimsy of the NRC report's dedication page, which notes that it is "Dedicated to Non-Human-Like Life Forms, Wherever They Are.")
A starting point for both papers is the development of a definition for life that does not rely on familiar (and restrictive) biological concepts such as cells and DNA. Even young children in science class learn how tricky it can be to define life uniquely by its characteristics -- after all, fire consumes, crystals grow, and so on. The problem only gets harder when one wants to avoid being too close-minded about what alien life could be.
Both publications posit that life, at its most abstract, involves a thermodynamic disequilibrium. That is, life involves physical structures that can only maintain their integrity with inputs of energy. These physical structures will require covalent bonds between atoms (to allow nontrivial chemical reactions), so the environment in which life appears must allow such chemistry to occur. Some kind of liquid, but not necessarily water, would therefore also be necessary to enable those reactions. Finally, some molecules in the living system would need to be capable of Darwinian evolution for the life to arise. (Take note, creationist doubters of evolution: it is now a useful part of the definition of life!)
From theory and experiments, both papers argue that life with these traits could evolve under a wide (but definitely limited) range of environments. Carbon-based life on worlds with liquid water might represent a particularly versatile and common set of solutions, but biochemistry could go in many directions even on Earthlike worlds. And on planets and moons where terrestrial life would perish instantly, life based on silicon instead of carbon or liquid hydrocarbons instead of water might thrive.
Drink methane, breathe hydrogen
An interesting case in point is Saturn's largest moon, Titan. Its average surface temperature is about -180 degrees C. (-290 degrees F.) and its atmosphere contains almost no water vapor, though a liquid mixture of water and ammonia under pressure seems to exist under thick layers of ice, according to the Cassini probe. Parts of Titan's surface are nonetheless wet with lakes of liquid methane and ethane raining out of the hydrocarbon-rich atmosphere.
Measurements by the Cassini mission's Huygens probe in 2010, however, suggest that the amounts of hydrogen, acetylene, and ethane on Titan's surface seem to be lower than investigators had expected to find. That finding fits almost eerily well with a speculation by Chris P. McKay and Heather D. Smith that appeared in the journal Icarus in 2005. They had theorized that Titan might be home to organisms based on liquid methane rather than liquid water. These "methanogens" would breathe in hydrogen gas instead of oxygen and use it to consume organic compounds like acetylene and ethane.
The existence of such methane-swilling organisms is still no more than a hypothesis, but it's one that any future missions to Titan will surely want to test.
A fourth domain
We might not have to embark for Titan (or even Mars) to find bizarre new forms of life, however. There's at least a possibility that life as we don't know it might be lurking on Earth. Perhaps it's hiding in some very secluded refuges, or perhaps we're simply not recognizing it for what it is. After all, one of the three major domains of terrestrial life -- the unicellular organisms called archaea that often live in extreme environments like volcanic vents -- were lumped in with bacteria until about 50 years ago.
Jonathan A. Eisen of the University of California Davis Genome Center and his colleagues, working with Craig Venter, analyzed DNA in samples of seawater collected from around the world, which contained fragments of genomes from countless unidentified organisms. They then tried to organize the collected sequences for certain families of genes into phylogenetic trees to approximate how they might be related to one another. What they found, as they described in the journal PLoS ONE in 2011, was that not all the genes associated simply into the three recognized domains of life. One explanation for the anomalous ones was that they came from some unknown fourth division of life. (Eisen's own detailed blog description of what he and his team did tells the story behind the paper most eloquently.)
No one knows what organisms could be the bearers of those anomalous genes, but one possibility that Carl Zimmer has discussed in his eye-opening book A Planet of Viruses is that they come from the nucleocytoplasmic large DNA viruses, a group of viruses so massive that they were formerly mistaken for bacteria. Science is only beginning to understand what these viruses represent.
Stalking the shadow biosphere
Nor do those viruses represent the outer edge of how weird life on Earth might be. In recent years, some scientists have begun to wonder about the possibility that our planet harbors a "shadow biosphere" of organisms that differ from conventional live at a profoundly basic level. Their evolution might have branched away long before the recognized domains appeared, or they might have evolved separately and in parallel with the rest of life.
As Carol E. Cleland and Shelley D. Copley wrote in the International Journal of Astrobiology, for example, variant forms of life could use different sets of amino acids in their proteins or different pairs of nucleotides in their DNA. Or they might use versions of familiar molecules that have mirror-image symmetries.
An even more radical possibility that the physicist and astrobiologist Paul Davies has discussed is that some of the RNA-based life that may have predated DNA-based life on Earth could have survived. RNA life would not only lack DNA: it might also have no need for proteins because folded RNA could do many of the same jobs as those molecules. Unencumbered by any need for protein-making organelles, RNA-life cells could be much smaller than conventional cells.
RNA life might therefore be extremely well suited to an existence deep underground. (Shades of the Horta!) Not that RNA life would have super-advanced burrowing skills; rather, RNA-life cells could happily occupy minute spaces inside and between rocks.
What's amazing is that most of these unusual forms of life could easily be overlooked. Science has still not had a chance to characterize more than a fraction of the ordinary types of life believed to exist in nature.
In fact, it might be literally true that one piece of evidence for alien types of life has been right in front of mankind all along. Back into prehistory, human beings in desert areas have been scratching glyphs and drawings into rocks that have dark weathered surfaces. Those desert varnishes coating the rocks, however, have often perplexed geologists: good explanations for what causes these mineralized layers to form have been lacking. Biological activity has always seemed like one possibility but the agents responsible haven't been in evidence.
Maybe we just haven't known what to look for.
Image at top: Artist's conception of Viking lander taking samples for biological examination. (Credit: NASA/JPL-Caltech/University of Arizona)