Earth’s Oceans as Analogs for Extraterrestrial Seas

In Alien Ocean: Anthropological Voyages in Microbial Seas, Stefan Helmreich argues that the discovery of life deep in Earth’s oceans provoked increased speculation about the possibility of life in ex­traterrestrial seas. Prolific communities of tubeworms, krill, and other hypothermophiles were discovered near deep-sea hydrother­mal vents in 1977 in the Pacific Ocean.43 These organisms live in complete darkness—no sunlight can penetrate to these depths. Sci­entists speculate whether life in primordial times thrived in the sulfur-rich heat plumes that spew nutrients from the ocean floor. Besides the fact that we believe all life on Earth originated in the sea, we have long imagined, notes Helmreich, that extant primeval life-forms might somehow survive in the deep, even today. Life’s origins may best be represented by extremophiles currently living in extreme environments. He writes, “Some marine microbiolo­gists maintain that vent hyperthermophiles are the most conserved life forms on the planet, direct lines back to the origin of life.”44 What we are learning about extremophiles or methanotrophes— bacteria that metabolize methane—may indicate something of the possibilities for life on other worlds. “Astrobiologists treat unusual environments on Earth, such as methane seeps and hydrothermal vents, as models for extraterrestrial ecologies,” writes Helmreich.45 Cassini has revealed that several of Saturn’s moons have liquid or frozen oceans and briny geysers. Enceladus, with its outgassing plumes of ice water and salt, and Titan, are of particular inter – est.46 As the planetary body in the Solar System most similar to

Earth in atmospheric composition, Titan could reveal how life on Earth emerged, suggests astrobiologist Chris McKay.47 Since Titan and Enceladus are likely heated internally by Saturn’s gravitational squeezing, astrobiologists imagine that extremophiles might hud­dle near hydrothermal vents on the floors of their arctic seas. On Earth, Lake Vida in the Antarctic has been capped under a 50-foot thick ice sheet for 2,800 years and yet researchers found ancient microbial life thriving there.48 Helmreich comments, “For astro – biologists, life, extremophilic or no, will exist in a liquid medium. It is for this reason that extraterrestrial seas—alien oceans—are such objects of fascination.”49 In these discoveries alone, the im­plications of the Cassini mission are as deeply cultural as they are scientific.