Natural Science
This passage is adapted from the article "Could Jupiter Moon Harbor Fish-Sized Life" by Victoria Jaggerd (© 2009 by National Geographic).
Below its icy crust Jupiter's moon Europa is believed to host a global ocean a hundred miles deep, with no land at the surface. And the extraterrestrial ocean is currently being fed more than a hundred times more oxygen than previous models had suggested, according to provocative new research. That amount of oxygen would be enough to support more than just microscopic life-forms: at least three million tons of fishlike creatures could theoretically live and breathe on Europa, said study author Richard Greenberg of the University of Arizona. "There's nothing saying there is life there now," said Greenberg. "But we do know there are the physical conditions to support it1."
Based on what we know about the Jovian moon, parts of Europa's seafloor should greatly resemble the environments around Earth's deep-ocean hydrothermal vents, said deep-sea molecular ecologist Timothy Shank. "I'd be shocked if no life existed on Europa," said Shank, of the Woods Hole Oceanographic Institution.
Despite the promising new estimates, it's too early to do more than speculate about how Europan life might have evolved. A closer look—perhaps by a NASA orbiter now in development—will be needed to tell exactly how chemicals are distributed on Europa and how the moon's geologic history might have contributed to life's chances.
It wasn't until Galileo, the NASA spacecraft, reached the Jupiter system in 1995 that scientists were able to study the moon in detail. 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 Europa2. 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.
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. Europa's smooth surface is marred only by dark, crisscrossing ridges that suggest the icy shell is being stretched and compressed by tidal forces3.
"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. Europa, which is about as big as our moon, also gets tidally stretched, not by the sun but by the gravity of massive Jupiter5. The friction from all this tidal stretching probably heats Europa enough to maintain liquid water, Greenberg said.
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.
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 oceans4—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." 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 regular7 rate, in this case, a complete renewal every 50 million years.
But planetary scientist Robert Pappalardo said the process may have been more intermittent9, 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," said Pappalardo. Europa is gravitationally locked with its neighboring moon Io, which may be pushing and pulling on Europa in extreme cycles, resulting in periods of high and low tidal friction. 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 organisms8—but not necessarily complex life.
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