What the Galileo probe found was so exciting that NASA deliberately crashed the spacecraft into Jupiter in 2003 to prevent the craft from contaminating one of its own discoveries: the salty, subsurface ocean on Europa.
Although the probe didn't see the ocean directly, scientists are pretty sure it's there, based on the age, composition, and structure of the moon's icy surface.
For instance, pictures of the moon's bright surface suggest it's relatively young, said the University of Arizona's Greenberg, author of Unmasking Europa: The Search for Life on Jupiter's Ocean Moon.
Europa, like the other planets and moons in our solar system, is more than four billion years old. But a relative lack of impact craters implies that the icy crust is just 50 million years old. "It's an entirely different surface now than it was at the time the dinosaurs went extinct on Earth," Greenberg said.
"Repaving" Sends Oxygen Steadily Downward?
Europa's smooth surface is marred only by dark, crisscrossing ridges that suggest the icy shell is being stretched and compressed by tidal forces.
"We're used to thinking of tides on Earth as something seen on the shore," Greenberg explained. But on a larger scale, gravity from the sun and moon constantly squishes and stretches Earth as a whole.
Europa, which is about as big as our moon, also gets tidally stretched, not by the sun but by the gravity of massive Jupiter.
The friction from all this tidal stretching probably heats Europa enough to maintain liquid water, Greenberg said—even though the Jovian moon is 483 million miles (778 million kilometers) from the sun.
The warmer ocean material may be oozing up through cracks in the ice and freezing on the surface at the same rate that older ice sinks and melts into the liquid interior.
This cycle of "repaving" would explain the young look of the surface ice—and would open the door for oxygen at the surface to permeate the subsurface ocean.
Oxygen is created when charged particles from Jupiter's magnetic field hit the ice. Given his estimates for the moon's rate of repaving, Greenberg thinks it would have taken one to two billion years for the first surface oxygen to reach the ocean below.
Time to Grow
A few million years after the ice-repaving process had started, oxygen levels in Europan seas reached their current levels—which exceed levels in Earth's oceans—Greenberg speculates.
This timeframe actually improves the chances that life as we know it took root on Europa. For starters, the most primitive life-forms need an absence of oxygen to form, Greenberg said.
"Oxygen tends to cause other molecules to come apart," he said, so genetic material such as DNA can't freely assemble with oxygen present.
"You need the delay so genetic material and structures can take shape," he said. "And then when oxygen arrives, organisms will at least have a fighting chance."
Similarly, a sudden abundance of oxygen can kill simple life-forms that aren't accustomed to the highly reactive element. But if oxygen is introduced slowly, creatures can evolve to tolerate it and even come to depend on it—a process thought to have happened on early Earth.
The Case Against Animals on Europa
Greenberg's generous estimate of oxygen in Europa's ocean—and the resulting speculation that fishlike creatures may exist there—depends on the surface repaving to have happened at a relatively stable rate, in this case, a complete renewal every 50 million years.
But planetary scientist Robert Pappalardo said the process may have been more intermittent, and therefore the oxygen level—and chance for fishlike life—lower.
"Maybe 50 million years ago it was churning away, and now it's slowed down and become much more sluggish," said Pappalardo, a senior research scientist with NASA's Jet Propulsion Laboratory in Pasadena, California.
For example, Pappalardo said, Europa is gravitationally locked with its neighboring moon Io, which has an eccentric orbit around Jupiter. This means Io may be pushing and pulling on Europa in extreme cycles, resulting in periods of high and low tidal friction on Europa.
Even in this scenario, oxygen could reach the seas, though maybe not in quantities that would favor complex life-forms.
Since ice behaves like a fluid over long time frames (think glaciers), he said, surface elements could be reaching Europa's ocean via solid ice.
"Picture a lava lamp: Blobs of warmer material rise, and cooler blobs sink. It's just that in ice it might take a hundred thousand years for a blob to rise."
Meanwhile, if tidal activity on Europa comes in fits and starts, that would change the rates at which heat and nutrients from the rocky mantle become available, he said.
"Say there are microbes down there," Pappalardo added. "What would it mean for their evolution if every hundred thousand years there was much more heat and chemicals? It might lead to much more hardy organisms"—but not necessarily complex life.
Life's chances on Europa would also depend on whether tidal friction heats the moon all the way to its rocky core. If the solid core is in fact warm, he said, "then you may have black smokers pouring out heat and chemicals." If not, the dissolved nutrients needed to sustain life would be much more limited.
In fact, even with vast amounts of oxygen in the water, astrobiologist Cynthia Phillips of the SETI Institute said, it's unlikely for Europa to house anything bigger than microbes, given its probable amounts of life-supporting chemical nutrients.
"While it's really exciting to think of giant squid on Europa, there's not likely to be anything there that size," Phillips said.
Wanted: Ice-Penetrating, Swimming, Sniffing Spacecraft
For some scientists, though, the idea of at least microbial life on Europa is plausible enough that researchers, including Shank of Woods Hole, are already eyeing Earth's superheated hydrothermal vents as possible analogues.
Some microbes can thrive on the gases created from the chemicals spewed out by these vents. On Europa such chemicals could be the basis of a food chain that, with oxygen in the water, might support complex life.
One day spacecraft could be sent to Europa to penetrate the ice and explore the ocean, much as remotely operated vehicles sniff Earth's deep ocean for nutrients released by unseen hydrothermal vents, Shank said.
But first scientists would have to develop sensors that can probe for DNA, RNA, and other chemical signatures of life.
A submersible sent to Europa would also have to be made smaller, lighter, and with better battery life than existing models—while still being able to drill its own way through what may be miles of ice.
Robust communications capabilities would also be essential, Shank said. "It's no good to go down there and find life and not be able to tell anyone about it."
NASA's next step in exploring Europa, however, is more likely to be an orbiter—i.e., no undersea missions—proposed as part of a joint mission with the European Space Agency.
Such a mission, while desirable, would face a number of hurdles, SETI's Phillips noted. At their closest, Jupiter and its moons are roughly 365 million miles (588 million kilometers) from Earth, so getting there can take as long as five to six years with current technology.
At that distance, there's not enough sunlight for a solar-powered probe, so the craft would need to bring its own nuclear power source, Phillips said.
There's also the constant radiation from Jupiter's magnetosphere to contend with.
"If you want to orbit Europa, the radiation is [eventually] going to fry your spacecraft," Phillips said. "Once you finally get to Europa, you can hope to orbit for a couple months if you're lucky."
Pappalardo, the study scientist for the proposed Europa mission, said NASA's planned orbiter should be robust enough to last almost a year before succumbing to radiation or other environmental pressures.
Such a mission, he added, could find concrete evidence for complex life on Europa. But, he said, that's the optimistic view.
"The conservative view would be to ask: Is there enough chemical energy for organisms of any type to thrive?" Pappalardo said.
"It's not out of the question, but first let's go see what's there."
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