Solving the Puzzle
The simple assumption was that any pockets of warm, liquid water would drain downward through the ice and refreeze, but Schmidt had read enough studies to know that would not happen on Europa—the ice below was so thick it was virtually impermeable. By contrast, the ice above the water would become relatively unstable, its foundation melted away by the encroaching water. Following that logic, Schmidt concluded that the lid of ice above each water pocket would eventually cave in, crashing onto the liquid below. Through some calculations, she found that large volumes of water could remain trapped for thousands of years or more, enclosed on all sides by thick, insulating slabs of ice.
Schmidt knew her insight had major implications for life. She was suggesting that Europa’s ocean was not its only source of liquid water; the moon also harbored hidden lakes far closer to vital molecules on the surface, perhaps close enough to support miniature habitable ecosystems. But if she was going to persuade her instinctively skeptical colleagues, she would have to prove that this sort of process could actually happen. She needed to follow through with her strategy of
supporting hypotheses about Europa by studying parallel features on Earth.
Schmidt rushed down the hall, past the three-foot-tall multicolored Galileo photomontage of Europa’s fractured surface taped to a column, and burst into Blankenship’s office. She summarized what she had deduced and asked if something like this could be taking place on Earth. He glanced over his glasses and replied, “Grimsvotn.”
In Iceland, the Grimsvotn volcano, buried miles beneath the ice sheet, melts the ice cap above it in the same way that the rising plumes might melt Europa’s shell, causing the surface of the Icelandic ice sheet to cave in. Back at her cluttered desk, Schmidt googled Grimsvotn and uncovered a few papers on the subject. Photos revealed a collapsed, fractured surface eerily similar to some of the chaotic regions of Europa. “I was practically hyperventilating,” she says.
The end of her two-week window came, and although she still had a lot of work to do, Schmidt and Blankenship quickly discussed her model and flew to the agu conference in San Francisco, where she presented it before a crowd of about 100 scientists. Her talk was well received, though some attendees pointed out that her model, like Pappalardo’s, was incomplete. There were still a lot of chaos features to explain.
That was fair criticism, but Schmidt was not done analyzing her Europan lakes, which she felt might explain all the moon’s chaotic topography. A few days later, back at her office whiteboard, she thought about the collapsed sheet of ice above each lake, full of giant fractures like the ones above Grimsvotn. She realized that if any of the fractures stretched deep enough into the ice, the water in the lake would suddenly have somewhere to go. Forced into the fractures of the caved-in ice, it would flow up, toward the surface.
The more Schmidt followed the water, the more compelling her model became. Over the course of weeks, years, or even millennia, she surmised, the tendrils of water leaking upward would refreeze. As they froze, they would also expand, just as ice cubes expand in the freezer. That would wreak havoc on the surrounding ice. Water freezing within the fractures would force the ice sheet apart, causing giant icebergs to break off.
Next, the icebergs would crush the smooth ice around them, creating regions of crunched, mutilated ice. Those regions, full of small cracks and crevices, would draw up even more water from below, which would then freeze and swell into soaring, solid domes of ice—the final piece of the Europa puzzle. By simply tracking water as it melted, migrated, and refroze, Schmidt had, in mere months following a trip to the Antarctic, come up with by far the most complete model of the icy moon’s chaotic surface.
After months of fine-tuning, Schmidt and Blankenship published their model in Nature last November. Pappalardo was skeptical until Schmidt presented it before him and other Europa experts at jpl. “All the pieces just started fitting together,” he now says.
An immediate rush of media attention focused on Europa’s lakes. Schmidt had shown that they could subsist just a mile or two beneath the Europan surface, shallow enough that scientists could plausibly imagine drilling through the ice and accessing them. These lakes could also be hotbeds for life, since molecules embedded in surface ice could easily get dumped into the water when the ice collapses. Schmidt’s paper specifically singled out a chaos region on Europa called Thera Macula, which could conceal a lake containing as much water as all the Great Lakes combined.
To Schmidt, though, the lakes alone are not the big discovery. Her real breakthrough is finding the mechanism by which molecules on Europa’s surface could unite with water and energy in the lakes, and maybe even in the vast, deeper ocean. “You’ve got all this warm water moving up,” she says, “but at the same time all the heavy, cold ice stocked with chemicals is getting pushed down toward the ocean.” Her model describes the ice sheet as a heat-driven conveyor belt enabling the three requirements of life—water, energy, and chemistry—to exist in the same place at the same time. “We’re basically implying the ice is like a washing machine, mixing all those ingredients together,” Schmidt says. “It gets me and a lot of other people really excited.”