The problem is, we're biased toward detecting terrestrial life -- living things that have a similar chemistry to the familiar life forms here on Earth. It's understandable; I mean, why would we not be? While there is a great diversity of species here on our planet (something on the order of nine million extant, at an estimate), they all share the same basic biochemistry, including:
- ATP as an energy driver
- some form of sugar-fueled cellular respiration to produce that ATP
- phospholipid bilayers as cell membranes, and (for eukaryotes) for the internal membranes that compartmentalize the cell
- proteins to facilitate structure, movement, and catalysis (the latter are called enzymes)
- nucleic acids such as DNA and RNA for information storage and retrieval
- lipids for long-term energy storage
While there are obviously different twists on how exactly these things work, those features are common to just about all life on Earth. (Interestingly, a 2010 paper claiming a microbe had been discovered in California's Mono Lake that incorporates arsenic into the backbone of its DNA instead of phosphorus was just retracted by the journal Nature -- although the retraction is controversial, and the authors are still defending their work as valid.)
The question remains unsolved, therefore, of the extent to which the genesis and evolution of life are constrained -- by which we mean that the pathways taken by biology might be expected to repeat on other Earth-like worlds. (Or, to use Stephen Jay Gould's pithy phrase, we might find that evolution would produce similar forms again here on Earth if we were somehow able to "replay the tape of life.") Would living things, down to the biochemical level, be at least somewhat like terrestrial life, or would they be entirely different?
To be fair to the science fiction writers, there have been instances where they've made a significant effort to consider what life "not as we know it" might look like. Star Trek's Horta ("The Devil in the Dark"), Crystalline Entity ("Silicon Avatar"), and Tholians ("The Tholian Web") come to mind, as well as Doctor Who's Vashta Nerada ("Silence in the Library"), Boneless ("Flatline"), Not-Things ("Wild Blue Yonder"), and Midnight Entity ("Midnight").
Even Lost in Space gave it a creditable try with the Bubble Creatures in "The Derelict."
The problem is more complex than that, though. Since we can't actually go to other planets and search for living things -- even going to planets and moons in our own Solar System is crazy expensive and fraught with difficulties -- we're stuck with looking for biosignatures, traces (either structural or chemical) that show unequivocal evidence of being created by living things.
The sticking point is that word unequivocal. Two good examples of this came to light in the last couple of weeks, one of them (from the standpoint of people like me who would love nothing better than to find out we're not alone in the universe) bad news, and the other one -- at least tentatively -- good news.
Let's start with the bad news first.
Back in 2005, NASA's Cassini mission spotted something exciting -- the presence of organic molecules in water-rich plumes erupted from Saturn's icy moon Enceladus. The surmise was that these plumes were created by pressure in the moon's interior, where water might be kept liquid by tidal deformation from the huge gas giant's gravitational pull. Now, organic doesn't mean produced by life; it's a chemistry term meaning "containing carbon and hydrogen." (Formaldehyde, for example, is an organic compound, and has been found by its spectral signature in interstellar gas, and no one's claiming that it was made by aliens. At least, no one I'd want to have a conversation with.) On the other hand, lots of organic compounds are made by living things, so their presence in Enceladus's geysers certainly seemed to raise at least the possibility that underneath the ice, there might be a watery ocean that harbored life.
Well, a study led by scientists from Italy's Istituto Nazionale di Astrofisica e Planetologia Spaziale has found another possible explanation. The organic molecules could be formed right there on the surface by radiation funneled in by Saturn's magnetic field, then blasted off the surface -- i.e., they didn't come from an interior ocean at all. "While the identification of complex organic molecules in Enceladus's environment remains an important clue in assessing the moon's habitability, the results demonstrate that radiation-driven chemistry on the surface and in the plumes could also create these molecules," said Grace Richards, who led the study. "Molecules considered prebiotic could plausibly form in situ through radiation processing, rather than necessarily originating from the subsurface ocean. Although this doesn't rule out the possibility that Enceladus's ocean may be habitable, it does mean we need to be cautious in making that assumption just because of the composition of the plumes."
The second paper had more hopeful news. It comes out of Stony Brook University, and is an analysis of some mudstones found in Jezero Crater on Mars by the Perseverance rover. The analysis -- done long-distance, obviously -- found traces of organic material, along with ferrous sulfate and ferric sulfide. Most interestingly, the organic material seems to have undergone post-deposition redox reactions; redox reactions are the mechanism by which all terrestrial life forms harvest energy for their life processes (cellular respiration is, basically, one long string of redox reactions). The minerals, the researchers say, "challenge some aspects of a purely abiotic explanation" -- cautious science-speak for, "Life? Yeah, could be."
Of course, this is not proof, and Sagan's principle that I mentioned in a post a few days ago -- "Extraordinary claims require extraordinary evidence" -- certainly applies here. But it is fascinating, and if further inquiries support the biotic explanation for the odd chemistry of the Jezero mudstones, it'll be somewhere beyond exciting. I've always thought it likely that life is common in the universe, but having a real, honest-to-Gallifrey extraterrestrial example would be amazing.
In any case, keep your eyes on the science news. Despite the ridiculous budget cuts NASA is facing, they're still doing some wonderful science. And hopefully, at some point, we'll actually find proof of aliens. Wouldn't that be cool?
I hope it's not the Vashta Nerada, though. Those mofos are scary.
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