When Galileo Galilei aimed a telescope at Jupiter one dark night in 1610, he spied four large, bright satellites, lost to the naked eye in the glare of the gassy giant planet. He could hardly have guessed that one of those moons--Europa, second closest to Jupiter--might one day shine light on the origin of life on Earth.
Each of the four Galilean satellites is different. Ganymede, the largest, has its own magnetic field and even an atmosphere, albeit a very thin one. Io, the closest to Jupiter, is so deformed by the mother planet’s gravity and that of the other Galilean moons that it is heated into the most volcanically active object in the solar system. Callisto, the farthest out, is a dead moon pockmarked with craters.
Compared with the other three, Europa is almost boring. It is a little smaller than our own moon. It sports no volcanoes spewing molten rock, has no atmosphere to speak of, few large craters, and little topography. Its density, about three times that of water, indicates that it is made mostly of rock. But the surface is clean ice, with a fluffy topping of frost. From a distance, Europa looks as white and smooth as a giant cue ball, but up close, the cue ball shows cracks--dark linear features, some a thousand miles long, crisscrossing the crust. Researchers don’t know what the dark material is or how the cracks formed, nor are they sure what lies beneath the icy shell. But what they suspect is an ocean.
An ocean some 500 million miles from the sun sounds far-fetched, but the story gets stranger still. If Europa does indeed hold an ocean, those waters might harbor life. In the search for evidence of past life on other worlds, many researchers have put even odds on Europa and Mars. And Europa may in fact boast better odds when it comes to claiming the grand prize of exobiology--organisms that are still alive today.
At least one proposal for a mission to explore that tantalizing possibility has already been submitted to nasa. The plan, devised by engineer Henry Harris of the Jet Propulsion Laboratory in Pasadena, California, involves sending an orbiter to Europa to fling a 22-pound metal sphere at the mysterious dark streaks in its ice. Those streaks may be the result of contaminants in ocean water that has flowed up through cracks in the ice. The orbiter would fly through the resulting plume of debris and capture samples to bring back to Earth, where they would be examined for organic material. Other researchers are suggesting that the possibility of Europan life deserves not just a single probe but an entire series of missions. In that case, the first order of business becomes proving that an ocean really exists beneath the ice shell.
The circumstantial evidence is compelling. It’s extremely likely that Europa had liquid water near the surface at some point, says planetary scientist Steven Squyres of Cornell, who has been speculating on the likelihood of a Europan ocean for more than a decade, ever since the Voyager spacecraft beamed back the first images of the moon’s fractured ice. You are not going to start out with a moon that is pure dry rock and suddenly at the end of its evolution slap a lot of water on the outside. Instead it’s going to begin as rocky material with some water dispersed throughout--maybe as ice, maybe as water captured within minerals. As time passed, radioactive compounds generated heat, thus melting and dehydrating the rock. Eventually the denser rock became concentrated in the center of the satellite, and the less dense stuff--water--moved toward the outer part of the moon.
For Europa to have an ocean now, some of that water had to stay liquid. The water at the surface, where temperatures are estimated at -230 degrees, obviously froze. Once in place, however, the ice shell could have protected water beneath it from the cold and the vacuum of space, and calculations performed by Squyres and others suggest there might have been enough heat to keep that water liquid. The key is tidal heating, the same force that deforms Io into a volcanic frenzy. The gravitational forces of Jupiter and the nearby companion moons tug on Europa like dogs worrying a rubber bone, causing it to bend back and forth. That strain is released as heat, and combined with radioactive heat from the core, Squyres says, it could be enough to maintain an ocean beneath the ice.
The surface of the moon also shows signs of a deep ocean. Europa has very few large craters, though Ganymede and Callisto are littered with them (craters that form on Io are rapidly paved over by molten rock). One good way to erase craters--and all the other topography missing from Europa--involves a process called viscous relaxation: if there is a warm, mobile, deformable layer under the frost (either water or warm ice), the surface features gradually fade away, just as a ball of Silly Putty at room temperature will eventually flatten. According to calculations by astronomer Gene Shoemaker of the Lowell Observatory in Flagstaff, Arizona, viscous relaxation almost certainly erased Europa’s large craters--those more than about six miles across. The ice, Shoemaker also suggests, is probably no more than six miles thick. If there’s an ocean beneath it, it’s probably ten times as deep; and the moon’s rocky interior probably has a diameter of about 1,800 miles.
The most prominent features on Europa, the dark bands that form a mesh across the surface, also support the notion of an ocean. If you sort of rotate them back together, they close up very nicely, Squyres says. It looks as if they have spread apart and dark stuff has welled up from beneath. That suggests that while you have an upper layer that is cold and brittle, you really don’t have to go down very far before there is something much more mobile. And then there’s Europa’s fluffy frost. It looks like what you’d get if you cracked the ice to expose liquid water to a vacuum, which would cause the water to vaporize and condense on the surface, says Squyres.
None of this, of course, proves anything. Scientists had hoped that the Galileo spacecraft, when it zoomed to within 436 miles of Europa last December, would yield some evidence of an ocean, perhaps a geyser gushing through a crack in the ice. That didn’t happen, and it’s unlikely that any of Galileo’s scheduled visits to Europa over the next year will produce the smoking gun. Galileo doesn’t carry the right tools to do the job, Squyres says.
Some researchers advocate sending another Galileo-type remote- sensing orbiter with new tools. It could carry better imaging equipment (that could spot water in a crack, for example) and perhaps long-wavelength radar, which is very good at penetrating ice. Since water reflects radar better than rock does, the signal bounced back would have a distinct look if an ocean lay beneath the ice. Alternatively, the orbiter could bounce laser pulses off the moon to measure the effect of tidal stresses--how much Europa flexes back and forth as its orbit takes it closer to Jupiter, then farther away. A moon with a shell of ice over a layer of water will flex more than one with solid ice on top of rock.
Another option is to drop sensors directly onto the moon. For example, a magnetometer might be able to detect variations from tides, and a seismometer could pick up vibrations from ice quakes (a likely effect if an ocean is underneath the frozen surface). Or, taking a page from polar research on Earth, the orbiter could drop penetrators into the ice. A penetrator is basically a high-tech dart, says jpl engineer Joan Horvath. You eject them from orbit and they wham into the surface, and then you watch to see where they go. That could tell you how much the ice is moving.
Should one or more of these techniques prove that Europa has an ocean, Horvath and her colleagues at jpl, in collaboration with several teams of polar researchers, propose sending a cryobot to melt through the ice. The design, patterned after probes used in Greenland, is simple: a thin metal cylinder, about five feet long and six inches in diameter, with a plutonium-powered thermoelectric generator inside to melt a path for the probe. The cryobot would be connected to the surface by a communications cable so that engineers on Earth could receive data and perhaps even direct its actions. Once the probe finally reaches water--if the ice is only six or so miles thick, that would take about ten months--it would release its payload, a five-inch-long mini-submersible, or hydrobot, to explore the foreign sea.
The hydrobot would contain instruments--precisely what sort hasn’t been decided--to capture images and detect hints of life. If all went well, the results could be in by 2015. Nobody, however, expects a Europan whale to swim up and swallow the probe. In fact, even if Europa does have an ocean, it might not be compatible with life. The three things necessary for life, Squyres says, are liquid water, the right biogenic elements, and a biologically useful source of energy. The first two can be satisfied if Europa has an ocean: besides water, you would have salts, soluble organics--a broth of all the ingredients you’d need to create living material--that leached out of the rock along with the water. The big question is the energy.
On Earth, sunlight provides energy for most living things. On Europa, far from the sun, the most likely source is tidal heating, but only if it’s energetic enough to produce submarine volcanism as well. A little warmth won’t do. For example, if you go to the Earth’s seafloor, Squyres says, there is heat leaking out everywhere. But only at the hydrothermal vents, where you have very high local temperatures, is there enough energy for life to run its metabolisms.
If submarine volcanism is supporting life on Europa, the organisms may resemble the high-temperature-loving microbes that thrive on the effluent spewed out of Earth’s hydrothermal vents. From just such life, it is now believed, all organisms on Earth evolved. And yet many researchers question whether this is where life actually began. The fact that we may all be descended from these guys who lived at hydrothermal vents doesn’t necessarily mean life arose there and not at the surface, Squyres says. If somebody took some seawater and hot basalt in the lab and made living organisms from stuff that wasn’t living before, that would make the case for me. Or, perhaps, if living critters are someday found swimming in a dark Europan sea.