Beneath the cracked surface, garish bands, jumbled ice and strange impact craters on a distant moon may lurk the greatest scientific prize of all: alien life.
NASA has good indications an ocean may exist beneath the icy crust of Europa, a moon of Jupiter. If so, it may harbor life.The space agency is drafting plans for another probe that would check more closely. The possibility thrills scientists and lay people alike.
Half a billion miles away from Earth, the spacecraft Galileo has been sending back a steady stream of data about Jupiter and its constellation of moons. Surprises have come regularly, ever since the $1 billion unmanned lab arrived in December 1995.
Best of all were the views of the moon that is second-closest to Jupiter, an icy world named Europa. They were so startling that last December the mission was extended two years to focus more attention on Europa.
Jupiter is so far from the sun that, until now, almost nobody believed life could develop on one of its moons. Earthly life needs water and heat. (Another requirement, the right kind of chemistry, was always believed to be present.)
Water: Galileo images prove that Europa is covered with ice from frozen water - not frozen methane or some other material. That is known from studies of the wavelength of light reflected from it.
Heat: They also show a reasonably good likelihood that under the ice, Europa has a liquid ocean. That implies a heat source because otherwise the moon would be frozen solid.
So all three conditions may be met on Europa.
"Because of this reason, NASA has targeted Europa for intensive explorations," Ron Greeley said during a telephone interview. A Galileo Project imaging team member, Greeley is also a geology professor at Arizona State University, Tempe.
The imaging team was responsible for designing and flying Galileo's cameras. Team members are now working to analyze the amazing photos that Galileo is returning, not only of Jupiter and Europa but also other satellites of the giant planet.
"Europa in many respects is unique in the solar system," he said. "It's basically about the size of Earth's moon, and it's composed of rocks. But from its density we know that it has a substantial amount of H2O present."
In fact, enough water is on Europa to form a shell 100 miles thick. The outer part of that shell is frozen. "What we don't know is if there's liquid water beneath that frozen, icy shell," he said.
Galileo is in orbit around Jupiter. But researchers have directed it to swing past Europa whenever it can, sometimes approaching to within a few hundred miles. The closest was on Dec. 16, 1997, when Galileo swung by at a distance of only 124 miles from Europa, taking remarkably detailed pictures.
Icebergs, fractured ice plates that have rotated, chaotic terrain, steep cliffs, thousands of fracture lines and a few craters are indications that water or at least what NASA calls "warm slush" once existed beneath the surface.
"The iceberg-like features that are seen in the pictures are the best indications," said Greeley. "Of course, slabs of the ice crust have broken apart and been pushed into new positions."
For bergs to form or ice plates to rotate, goes the thinking, some material must allow it to rotate. That might be slush or water.
The fact that few craters are seen in the views may mean that the surface has been melting or that liquid has welled up, obliterating the older craters. But that is an iffy point, since nobody knows for sure how common craters would be on Europa without ice. Possibly, most of the meteorites or asteroids that could bombard Europa would be pulled away toward Jupiter's immense gravity sink.
A more certain indication is that some craters look as if they are partly submerged, with only the rims higher than the surrounding plains. Possibly water seeped to the surface when the meteorite hit, flooding the crater floor.
The discoveries were a delightful surprise, Greeley said.
"The features are very fascinating and are very clear evidence of extensive geological activity. As a geologist, that excites me tremendously. One of the reasons a lot of us go into planetary exploration is the opportunity to see new worlds for the first time."
NASA is designing a spacecraft that would orbit only Europa and make observations to determine if that world is frozen solid. It would be launched around 2002. If it follows the same trajectory as Galileo, it would take about six years to arrive.
One of its instruments would measure the precise shape of Europa as it moves elliptically around Jupiter. If it does have a liquid ocean, it probably flexes - bulges out and pulls inward - with tidal action as it approaches and draws away from the king of the planets.
Another instrument would be a radar system like ones used over the Arctic and Antarctic.
"That radar would be able to penetrate down through the ice and tell us if and where the liquid water is," Greeley said.
Should the orbiter prove that Europa has a liquid ocean, the following step would be a lander. The lander might carry a probe that could melt through the ice and examine the ocean directly.
"Just intrigues me no end," said Patrick Wiggins, spokesman for Hansen Planetarium, speaking of the possibility of life in an ocean of Europa.
Even though the surface of Europa is locked in a deep freeze of 260 degrees below zero, the tidal friction may well keep the interior warm enough for an ocean to remain liquid, he said.
To illustrate the point in his astronomy talks, Wiggins likes to bend a wire back and forth many times and allow someone in the audience to touch it. The wire is warm because of flexing.
How could anything live, even in warm water, in the pitch darkness beneath ice?
Many species do live in similar conditions: animals that thrive at the bottom of Earth's oceans, far from the warmth and light of the sun. In utter blackness, whole colonies of tube worms and other creatures have grown up around deep-sea volcanic vents.
Water warmed by the smoking vents, and rich sulfur that spews from the vents, provide the framework for an entire ecology that does not depend on the sun. If it can happen here, maybe it could happen on Europa.
"But I have to also throw in that it could be nothing," said Wiggins. "It may be that, at best, there is a dense slush." A slush layer on Europa probably could not support life.
Still, he added, he has never believed that Earth is the only place with life. To expect that the only creatures in this universe are right here "I think is the height of ego."