Wednesday, March 24, 2010

Titan, Life Without Water, and "Messing With Old Definitions"

"Can Life on Titan Thrive Without Water?" (March 23, 2010)

"New discoveries have a way of messing with old definitions. Take, for example, the concept of a habitable world.

"The standard definition of a 'habitable world' is a world with liquid water at its surface; the "habitable zone" around a star is defined as that Goldilocks region — not too hot, not too cold — where a watery planet or moon can exist.

"And then there's Titan. Saturn's giant moon Titan lies about as far from the standard definition of habitable as one can get. The temperature at its surface hovers around 94 degrees Kelvin (minus 179 C, or minus 290 F). At that temperature, water is a rock as hard as granite.

"And yet many scientists now believe life may have found a way to take hold on Titan. Water may all be frozen solid, but methane and ethane are liquids. In the past few years, instruments on NASA's Cassini spacecraft and images captured by ESA's Huygens probe have revealed an astonishing world with a complete liquid cycle, much like the hydrologic cycle on Earth, but based on methane and ethane rather than on water...."

Actually, the idea that life didn't necessarily need water isn't particularly new. A former professor (of chemistry, apparently) at Boston University put together a pretty good argument for a half-dozen life chemistries that might plausibly work in temperatures ranging from near red-hot to near absolute zero:
  1. Fluorosilicone in fluorosilicone
  2. Fluorocarbon in sulfur
  3. Nucleic acid/protein (O) in water
  4. Nucleic acid/protein (N) in ammonia
  5. Lipid in methane
  6. Lipid in hydrogen
    "View from a Height" Isaac Asimov (1963), Lancer Books (p. 63)
Isaac Asimov might be shaky on the sciences of ecology and physics - at least in his fiction - but that was in his professional field: chemistry. I'm inclined to take his view seriously, that life-as-we-know-it isn't necessarily the only sort. We're #3 on that list, by the way.

Back to the article:

"...The chance to discover a form of life with a different chemical basis than life on Earth has led some researchers to consider Titan the most important world on which to search for extraterrestrial life. In a recent paper in the journal Astrobiology, Robert Shapiro, a professor of chemistry at New York University, and Dirk Shulze-Makuch of Washington State University rated Titan a higher-priority target for investigation than even Mars...."

Not everybody agrees, of course.

I can see the other point of view. The only sort of life we know exists involves nucleic acids and proteins. Nucleic acids and proteins on earth have more oxygen atoms than nitrogen atoms - indicated by (O) in Dr.1 Asimov's list. He pointed out that (as of the early sixties) it wasn't unreasonable to suppose that nucleic acids and proteins with more nitrogen atoms than oxygen - indicated by (N) - couldn't exist. We don't find them on Earth, but considering how boiling hot it is here, that shouldn't be a surprise. (Ammonia is a gas in our home world's temperature range.)

I'm inclined to take Shapiro and Dirk Shulze-Makuch - and Asimov - seriously. At least, I see their points of view as being a trifle more reasonable than a more familiar 'I never thought of such a thing: therefore it doesn't exist' attitude. (More: Exploding Martians and the Viking Life Experiment (March 5, 2009))

Related posts:
Related posts, at
1 Doctorate in chemistry, 1948. ("Isaac Asimov" Columbia 250)

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