Volcanoes erupting in the presence of water -- at the bottoms of oceans -- not only can be an abode of life but may be where life began on Earth. Astronomers know of another celestial body where there likely are volcanoes erupting underwater. It, too, may be an abode of life. The body in question is not Mars but Europa, one of the four moons of Jupiter first discovered by Galileo Galilei in 1610. It seemed no coincidence that as scientists were throwing these ideas around at a Seattle meeting in February, a spacecraft named Galileo was making the closest pass ever above that ice-covered satellite.
The annual meeting of the American Association for the Advancement of Science is where all the sciences, physical and social, come together. A pair of sessions in which oceanographers and astronomers sat in on each other's panels seemed to epitomize everything the association stands for.
They were sessions on the unexpected. Until 1977 textbooks stated that all life on Earth depended on sunlight: plants that metabolize sunlight, animals that eat those plants, and animals that eat those animals. Without sunlight, there would be no web of life. But then we found hydrothermal vents around submarine volcanoes, places where icy bottom water that had seeped into cracks in the underlying rock is heated by the 2,200-degree-Fahrenheit magma and shot back up through the seafloor loaded with minerals. These are the "black smokers" and they are hotbeds of life: giant tube worms, crabs and the like. The whole system exists in the darkness of the deep sea, cut off from sunlight and dependent instead on bacteria that can subsist on the hydrogen sulfide pouring forth. The bacteria are eaten by larger organisms, those in turn by larger, and so on.
There's more to those bacteria than meets the eye. Some are species normally found in ocean water that have simply found a good place to live. But others have been found only around the vents, where the uprushing, superheated water has carried them from their home deep in the rock beneath the sea bottom. John Delaney of the University of Washington pointed out that oceanographers were finding that volcanoes under the sea could support life at about the same time that astronomers were finding volcanoes throughout the Solar System. Some are monumental. On Mars the dead volcano Olympus Mons rises 17 miles above what would be sea level. Even more exciting is the Jovian moon Io, the Galilean moon closest to the giant planet. There, as many as eight or nine volcanoes are erupting at any one time. In fact, January 1980 was the only time Scientific American, National Geographic and this magazine all had the same cover: a NASA image of a volcano on Io shooting dust and gases out into space.
Biologists were just as excited about undersea volcanoes as astronomers were about theirs. The newly discovered bacteria, it turns out, are so different from any living thing that we've had to revise our nomenclature. The simple plant and animal kingdoms of my childhood were long gone, to be sure. I was just learning to think in terms of the eight kingdoms my current biology text lists. Now on top of these, we divide life into three domains: eukarya, in which cells have a nucleus; bacteria, in which cells do not; and now the archaea -- also without a nucleus -- a large fraction of whose genes are not found in the other two groups.
The chosen name "archaea" is meaningful. Some scientists now think that life would have had a hard time starting on the surface of the Earth. For the first billion or so years of its existence, the planet was bombarded with asteroidal material, some objects so large that the ensuing explosions when they struck would have boiled away the top layer of the oceans. Perhaps, instead, it began on the seafloor. Our best information now is that the first creature on Earth, from which all other life evolved, was a microorganism that lived in 200-degree water, presumably around the vents.
In a separate session on new worlds and old, Thomas Gold of Cornell University pointed out that photosynthesis is a complicated process, and it seems reasonable that life started in some simpler way. Bacteria that can live on sulfur in hydrothermal vents would seem promising. Gold said people ask him how microbial life could have gotten down to the seafloor. The right question, he said, is how did life get up here?
The idea that the same thing could have happened on Europa was first proposed back in 1983. Steven Squyres of Cornell was the lead author on that paper. At the Seattle meeting, he hypothesized that Europa once had a liquid ocean, but that it is now largely frozen. Other researchers have been working out the science in the intervening years, however, and believe that an ocean with submarine volcanoes is still possible, even probable.
Much of Earth's internal heat comes from the radioactive decay of heavy elements. There are radioactive elements within the moons of Jupiter, but much of their heating comes from another source. Volcanism on Io results from the flexing of the solid material of the moon itself. Io is very close to Jupiter and moves in an orbit that is slightly elliptical rather than circular. Thus, at some times it is a bit closer to the planet than it is at others. So Jupiter's powerful gravitational field pulls with varying strength. When Io is closest, it is stretched out at the equator. When it is farther away, the stress relaxes. This constant flexing produces heat. On Io, enough heat is produced to melt rock.
Europa is the next moon out, and it, too, moves in a slightly elliptical orbit. Recent calculations show that it may be stressed enough by Jupiter's pull to generate the heat necessary to melt rock and fuel volcanoes. The gravitational fields of the outer Galilean moons, Ganymede and Callisto, also contribute to the flexing.