Life on Jupiter Moon Likely on Thin (or Thick) Ice

The vast ocean on Jupiter’s moon Europa--considered one of the best hopes for extraterrestrial life in our solar system--may be capped by 12 or more miles of ice, suggesting any life forms that exist underneath could be far stranger and much harder to find than previously believed.

By analyzing the deformation of impact craters and the chaotic patterns on the moon’s icy surface, scientists finally are settling a long-raging debate about just how thick or thin the ice crust is--and what kind of life it could support. Thick is winning.

“This clearly has implications for how organisms might develop and evolve, and how we might land and search for them,” said Paul M. Schenk, a geologist at the Lunar and Planetary Institute in Houston whose findings are published in today’s issue of the journal Nature.

“In a thin shell, you have photosynthetic organisms possibly brewing in the stew,” Schenk said. “In the thick shell, you don’t have that option.”

Europa is a top target of astrobiologists because it is thought to contain three things necessary for life to evolve: a source of heat, organic material and liquid water in an ocean capped by ice.

In recent years, though, a “thick versus thin” debate has raged about whether that ocean is topped by a thin layer of ice that can crack or melt to let sunlight and oxygen reach organisms or is cloaked by a thick, impenetrable sheet of ice that would make the creation and survival of life more challenging.

The newest findings from Schenk show that large meteor craters on Europa do not leave deep pits or raised rims, as they do in harder surfaces. Instead, they leave a flat area and a series of concentric rings on the surface, suggesting that a hole is created in a soft icy area and filled in with ice from below.

Computer analysis of the deformations indicates the hard and soft ice layers must be at least 12 miles thick to produce the distinctive bull’s-eye patterns on Europa.

The new analysis builds on the work of planetary scientists Robert Pappalardo, Louise Prockter and James Head III, who have spent years trying to make sense of Europa’s chaotic surface. The fractured ice is crisscrossed by ridges and bands and pocked by pits, domes and chunky icebergs.

Between these features are gaps that appear to be filled in by newly formed ice and areas that are much redder and darker than the surrounding ice. Adding to the mystery, the surface is relatively free of craters, suggesting that some process--a kind of geological Zamboni machine--is continually resurfacing the ice.

Scientists at first speculated that the turbulent features could be caused by a thin layer of ice atop an ocean. But more recent work suggests that the oddities do not require liquid so near the surface. Instead, Pappalardo said, they are likely to be the product of a thick two-part ice layer.

On top is a five-mile-thick layer of brittle ice several hundred degrees below zero. Underneath is a seven-mile-thick layer of warmer, slushy ice. Scientists think this warmer ice is capable of flowing, as glaciers do. They also think blobs of warmer ice rise upward through the colder surface ice, through convection, causing the strange surface scars.

“The surface seems to cry out that this convection is happening,” said Pappalardo, a planetary scientist at the University of Colorado. “It essentially acts as a lava lamp.”

The thick ice also fits with models created by Dave Stevenson, a planetary scientist at Caltech. A decade ago, Stevenson proposed that Europa is heated by tidal flexing--a squeezing and relaxing of the entire moon caused by the gravitational pull of Jupiter and its other large moons. The internal friction caused by this squeezing creates heat, “just like rubbing your hands together makes them warm,” Stevenson said.

Because many astrobiologists have thought that an ocean open to sunlight and the atmosphere is a more likely place for life, some scientists have leaned toward a controversial proposal by University of Arizona planetary scientist Richard Greenberg, suggesting the icecap is a mere mile or two thick.

A majority of scientists is critical of that idea as more emotional than scientific. “We can’t let our desires for how Europa should act influence our science,” Pappalardo said. A thin cap is unlikely, Stevenson said, because there doesn’t seem to be enough heat on the moon to cause that much melting.

“The only way this question of ice thickness will be resolved is with another mission,” said Ronald Greeley, a planetary scientist at Arizona State University who led a 1999 National Academy of Sciences panel about how best to explore Europa. Measuring its “tidal bulges”--how much it deforms under the pull of Jupiter’s gravity--could yield an answer. A thin ice layer would deform much more.

Studying the frozen world remains a high priority because of NASA’s “follow the water” approach to seeking extraterrestrial life. The combination of ice and ocean that surrounds Europa’s rocky core is estimated to be between 60 miles and 120 miles thick, and contains twice the water of all of Earth’s oceans. “There’s a lot of juice there,” Greeley said.

While it is easy to imagine ways that life could exist if ocean water was exposed to the atmosphere in cracks or melt holes, scientists said it is unlikely that cracks can penetrate from the surface through 12 miles or more of ice cap. Life is not likely to exist on the surface of Europa ice because of the ferocious radiation that pelts the moon, although it could exist in cracks and pores within the ice.

Christopher Chyba, who holds the Carl Sagan chair for the study of life in the universe at the SETI Institute in Mountain View, Calif., is among those who think life may exist in Europa’s ocean.

He believes organisms under the ice cap could exist without sunlight. They could obtain chemical energy, oxidants and other raw ingredients of life from the surface as the ice circulates. “Thin ice is not critical to the possibility of life of Europa,” Chyba said.

This spring, NASA canceled plans to send an orbiter to Europa because of cost overruns. A fat layer of ice also means that a JPL proposal to drill or melt through the icecap with robots and then explore the ocean below with submarines isn’t likely anytime soon.

But planetary scientists still hope to eventually get to the distant moon--thick ice or not.

“It’s a phenomenal place to have in our backyard,” said Rich Terrile, who was the lead JPL scientist on the canceled mission. “It’s got all the elements that just cry out for exploration.”