In a new paper published in the journal Life, Janusz Petkowski and co-authors from the Department of Earth, Atmospheric, and Planetary Sciences at MIT tackle a subject familiar to many science-fiction aficionados: Could there be life based on silicon rather than carbon? Who could forget the memorable Horta from the famous Star Trek episode “The Devil in the Dark” or the silicon life forms in Isaac Asimov’s short story “The Talking Stone”?
But what’s the reality?
The authors have put together a very elaborate and detailed treatise on the topic—a must-read if you’re interested in ideas about extreme life. Not only do they provide a comparison of the chemical reactivity of carbon and silicon, they offer many amazing examples of how silicon is used by life on Earth and what type of silicon compounds exist in our own environment.
So what’s their verdict about whether Hortas or other silicon-based beings are possible? To make a long story short: It does not look very good. There aren’t many organic silicon compounds to begin with, and silicon-based life in water, or on an oxygen-rich planet, would be all but impossible as any free silicon would react quickly and furiously to form silicate rock. And that’s pretty much the end of the story.
That is, if you’re only talking about Earth-like planets. If you consider planetary bodies very different from our own, like Saturn’s moon Titan or the hypothesized carbon planets thought to contain more carbon than oxygen, a closer look is warranted. Petkowski and colleagues looked into alternative solvents for silicon, including low-temperature cryosolvents like liquid nitrogen. It’s difficult to dissolve any compounds at these extremely cold temperatures, however. One surprising finding in their analysis is that sulfuric acid, of all things, would support, at least in theory, a quite rich diversity of organosilicon chemistry. Perhaps life would find silicon attractive as a building block in a place with lots of sulfuric acid, say, in the clouds of Venus? Or maybe below the surface on Jupiter’s moon Io?
Louis Irwin and I have found that hydrocarbon solvents are well-suited for silicon. But the liquid methane on Titan’s surface would be super-cold, and cryosolvents are unlikely to work, according to Petkowski. (I still wonder about methanol, which is a polar compound like water and has a liquidity range from -94 to +65oC at Earth-like atmospheric pressures.)
Other crazy thoughts come to mind. I’m still fascinated by Feinberg and Shapiro’s lavobes and magmobes, hypothesized microbes living in silicate rocks above the rock’s melting temperature. But Petkowski doesn’t think that would work chemically—the strong silicon-oxygen bonds would all break at about the same time. The Super-Earth planet CoRoT-7b may be the ideal world for testing this hypothesis, say the authors. But Earth is nearly as good, and no one has ever found any remnants or fossils of these hypothesized microbes in the silicate rocks on our own planet.
Despite the many uses silicon has for biology on Earth, the chances of it being the major building block for life here or elsewhere seem to be low. What about its role in jump-starting life, though? Alexander Graham Cairns-Smith has advocated that clay minerals—a type of silicate—could have played a critical role in the early evolution of life on Earth. I still like that idea, and it should be tested further. And we should always keep in mind that silicon may have an important role in the future of life on Earth—in the form of robots and intelligent machines.