SOLAR SYSTEM

Europa: Cracked Up Moon

by Diane Boudreau

The recipe for life requires three critical ingredients: water, organic compounds, and an energy source. Every living thing on Earth, from an amoeba to a geranium to a human being, needs all three to survive. So it makes sense that scientists look for signs of these ingredients on other planets when they're searching for extra-terrestrial life.

"There aren't too many places in the solar system where all these things come together. Europa is one of them," says Ron Greeley, a planetary geologist at Arizona State University.

Europa is just one of 28 moons that orbit the planet Jupiter. Greeley is part of a camera team that studies photos of Europa taken from the Galileo spacecraft. He is also a member of NASA's Virtual Astrobiology Institute. The institute was created to search for life in outer space.

Greeley works with photos from the Solid State Imager on Galileo. Basically, the SSI is a digital camera. It beams data back to Earth where it is picked up by one of the three Deep Space Network satellite dishes. These dishes, located in California, Spain, and Australia, allow constant observation of spacecraft as the Earth rotates.

The photos tell Greeley and other scientists about the geology, history, and composition of Europa. For example, the images show that a thick shell of ice covers this large moon of Jupiter.

Photos of Europa also show objects that look like icebergs, as well as shallow craters in the ice. These craters formed when meteors hit Europa. Greeley says these features mean that there was probably a thin ice layer over liquid or slush at one time. Looking at the photos, scientists can see where the ice was thinner, and possibly still is today.

What the photos don't show is what lies under the ice. Many scientists think a liquid, saltwater ocean lies beneath the surface, but Galileo's instruments cannot give them a definite answer.

"To find out for sure will take another mission," says Greeley.

Photos from Galileo also show brown areas on the moon's surface. This means that there is more than just frozen water on Europa. Infrared measurements, like the kind used with TES, indicate the presence of various salts.

"We know there is interesting chemistry there," says Greeley. "We can't measure the organic compounds right now. But we do know that comets routinely slam into the moons of Jupiter. Comets do have organic chemistry."

What about the third ingredient needed for life-an energy source?

Europa has heat caused by the gravitational pulls of Jupiter and its other moons. These pulls affect Europa just like our moon's gravity affects the ocean tides on Earth.

This tidal energy may have produced black smokers, or undersea volcanic vents, much like the ones found on Earth's ocean floor. Scientists have taken water samples near these hot plumes on Earth and found them teeming with microscopic life. According to one theory, life on Earth began in such environments. If it happened on Earth, why not on Europa?

Galileo's SSI has already beamed back thousands of pictures to Greeley and the science team. The images continue to pour in from space. Recently, Galileo made joint observations with the Cassini spacecraft, which passed by Jupiter between November 2000 and February 2001 on its way to Saturn. This flyby marked the first time that two spacecraft have ever explored a planet at the same time.

Galileo was launched in 1989. The spacecraft did not reach Jupiter's orbit until 1995. Galileo didn't have enough energy to make a straight flight, so it traveled in loops using the gravity of other planets. It got gravity boosts from Venus, Earth, our moon, and even asteroids.

Although Galileo's main mission was to study Jupiter and its moons, the SSI took other important pictures all along the journey. From these pictures, scientists learned that asteroids can have their own moons.

The original Galileo mission was scheduled to last until the end of 1997. However, the spacecraft was still working fine at that time, so scientists asked to extend the mission. Such requests are common on space missions, Greeley says.

"They are usually approved because the biggest investment-getting the spacecraft built and off the ground-has already been made," he explains.

If nothing goes wrong, Galileo could continue its mission for another two years.

"When its mission is complete, Galileo will be programmed to make a death plunge into Jupiter," says Greeley.

Why such a tragic death for this spacecraft? Greeley says NASA wants to be extra careful not to transport Earth germs to other planets.

"It's a very conservative approach. The spacecraft has flown very close to Jupiter. The giant planet has very severe radiation. Anything on the spacecraft has most likely already been radiated beyond concern," he says.

The study of Jupiter will not stop after Galileo's destruction. More missions are already being planned. The Europa Orbiter is scheduled to launch in 2008 and will reach Jupiter in 2010. This spacecraft is being designed to look for signs of a subsurface ocean on Europa using an instrument called a precise altimeter. The device will measure the shape of Europa over time. The measurements will offer clues about what the moon is like beneath the ice.

"Europa is pushed and pulled by the gravity forces of Jupiter's other satellites," explains Greeley. "This affects the shape of the moon, just like if you press on a balloon. The amount of change can tell us something about the makeup of the planet. If the water is solid ice, it won't give much. Thin ice over liquid water will flex more. We can measure how much the planet is distorted and determine if there's liquid water there."

Each bit of information helps Greeley and other scientists understand Europa's geology, chemistry, and history. Bit by bit, scientists are gathering answers to the all-important question: Is Europa cooking up a recipe for life?