Phenomena, Comment and Notes
Life not only thrives in the heat and violence of Earth’s submarine volcanoes, it may have started there
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.
We already knew that the surface of Europa is covered with water ice. That this water would be ice was no surprise: the surface temperature is in the neighborhood of -225 degrees (and that's without windchill). But the ice has a story to tell. Eugene Shoemaker of the U.S. Geological Survey says the surface is very young. He bases that assertion on the rarity of craters on Europa. He has catalogued all the known comets that have been captured by Jupiter's gravitational field. Some of those eventually crash into the giant planet (SMITHSONIAN, June 1994) while others hit the Galilean satellites. Knowing what's out there, Shoemaker has calculated the current cratering rate on Europa. The surface should be liberally sprinkled with craters, but it is not. Therefore the surface must be renewing itself. Some parts of its surface show so few craters that Shoemaker estimates they are less than ten million years old.
There's more: the surface is laced with cracks and fissures, some of which look as though they have been pushed apart after forming, as if something -- presumably water, possibly in the form of slush -- were pushing up from below. This sort of activity would account for the obliteration of old craters. So it just may be that there is enough heat moving up and out of the solid moon to melt the ice from below, or in other words, an ocean of liquid water is hiding under that ice. Under that water, submarine volcanoes may be spewing out the minerals, including sulfides, from which life could arise.
If we wanted to look for life elsewhere in the Solar System (and if we had the money), our best bet might be to send an oceanographer the 400 million miles to Europa where she could drill through the ice and, just maybe, shout "Eureka!" Naturally we would not expect to find any forms of life "higher" than bacteria because we know all the reasons why that is improbable. But there are those white worms and crabs around thermal vents here at home. The undisciplined mind envisions a new world of intelligent crustaceans. Think of the movie possibilities.