Pappalardo’s challenge is hardly over, however. Now he and his fellow Europa boosters have to fight to retain their place in the budget. The space shuttle is slated for retirement next year, and the replacement launcher, called Ares, is taking longer to build and costing more than originally anticipated. Meanwhile, the Obama administration wants NASA to divert more money to Earth observation in order to provide data on global climate change. And the price tags on some of NASA’s more immediate robotic probes—such as the Mars Science Laboratory, now under construction—are soaring. All that puts pressure on the space agency’s limited science budget. Nor is the Ganymede mission a sure bet. Over the next two years, the ESA will pit Ganymede against two other proposed astrophysical observatories.
Scientific competition and jockeying for political support are nothing new in astronomy; they are not even new when it comes to studying Jupiter and its satellites. Four hundred years ago, Galileo Galilei and German astronomer Simon Marius both claimed to have been the first to spot the planet’s four large moons (Europa, Ganymede, Callisto, and Io). Galileo proposed naming the moons after his powerful patrons, the Medicis, in a bid to win favor and funding. Not to be outdone, Marius suggested they be called the Brandenburgian stars, after his patrons. Neither nomenclature caught on.
Years later, at a fair in Regensburg, Germany, Marius ran into the famed astronomer Johannes Kepler, who jokingly suggested the satellites instead be named after Jupiter’s mythological “irregular loves”—three maidens and one youth who were seduced by the king of the gods. Eventually those names stuck (although Galileo, not Marius, does get credit for the discovery in today’s textbooks, because he published first).
For nearly four centuries, astronomers still knew little about the “Galilean” satellites. Never coming closer to Earth than 350 million miles, Europa looked like nothing more than a fuzzy blob through even the most powerful telescopes. The arrival of the Voyager 2 spacecraft in 1979 changed all that. The pictures from Voyager showed an odd and dynamic world. Europa’s smooth, billiard-ball-like surface was covered with cracks, a landscape that eerily resembled Earth’s Arctic ice in winter. The images provided the first observational support for an intriguing idea that, until then, had been solely theoretical: The Jovian moons might have oceans on the inside.
Just one month before Voyager’s arrival, a research team had published the first serious model laying out how a liquid ocean could exist within a planetary body in the chilly outskirts of the solar system. The secret of this warmth is in the stately dance between Jupiter and his four lovers—Io closest to the massive planet, Europa next, followed by Ganymede and finally Callisto. Over billions of years, three of these satellites have settled into a graceful cosmic rhythm. Io completes one circle around Jupiter in half the time it takes Europa, and Europa, in turn, moves through its orbit in half the time it takes Ganymede.
This dance, however, is not perfect. Jupiter’s gravity creates tides—a periodic stretching—on the moons as they circle. Tides, in turn, create friction and therefore heat. There is no question who is leading this dance: The relatively small moons are no match for Jupiter’s enormous girth. On Io, which orbits at about the same distance from the planet as the Earth to the moon, the heat is so intense that it triggers sulfurous plumes from massive volcanic vents, which spew into space. On more distant Ganymede and Callisto, Jupiter’s effects are not so dramatic. There is probably enough sloshing between core and crust to keep those moons’ interiors from freezing completely, but not enough to melt water within a hundred miles of the surface.
Ice that thick effectively seals off the surface—and its potentially life-giving chemicals—from the water below. Europa, however, lies in a zone that planetary scientist Richard Greenberg of the University of Arizona in Tucson calls “the sweet spot,” where Jupiter’s impact makes the moon neither too hot nor too cold.
A year after Galileo arrived in the Jupiter system in December 1995, it swooped within 430 miles of Europa, sending back images resembling a pale blue cracked Easter egg. Among the most intriguing pictures were those of an area called Conamara Chaos. Structures that look like ice rafts are floating in a refrozen matrix; the remarkable absence of craters there points to a geologically active surface and hints that liquid water may have welled up from below in the recent past. That discovery caught the attention of biologists as well as planetary scientists. We know that terrestrial ice sheets can extend half a mile down into the ocean on Earth, yet still support a complex ecology at their base. And sulfur-feeding bacteria flourish at the darkest depths of the oceans, serving as the foundation of a food chain so exotic as to seem virtually extraterrestrial.
Greenberg believes something similar may be happening on Europa. He sees compelling evidence that the moon’s massive tides crack the ice, allowing liquid water to percolate up. And areas like Conamara Chaos offer additional evidence of an ice shell that extends only five or six miles at most, he contends. Such thin ice could screen out Jupiter’s damaging radiation but still admit organic matter—primarily from comet impacts—and some sunlight for photosynthesis. Organisms might find niches in the moon’s many surface cracks amid the regular ebb and flow of Europa’s tides. And below, along possible volcanic vents on the ocean floor, other forms of life could flourish in the sunless depths. “As far as we know, the conditions and ingredients on Europa could sustain substantial complex life,” Greenberg says.
It is a compelling vision, echoing Arthur C. Clarke’s description of Europa’s alien sea creatures bobbing through the dark waters in his novel 2010: Odyssey Two. But most planetary researchers are unconvinced by Greenberg’s theories. They insist that dynamic features like Conamara Chaos could form even in a thick ice crust. Such technical discussions about the depth of the ice shell carry so profound an underlying question—is Europa alive or is it dead?—that they have degenerated into a bitter dispute that at times veers into personal insult and angry countercharges.
At the center of the fracas is Greenberg, who, with his curly hair, thick gray mustache, and gentle manner, hardly fits the image of a scientific bête noire. He earned a degree in physics at MIT, where he went on to receive a doctorate in planetary science in 1972. By the late 1970s he had won a coveted spot on the Galileo imaging team and later landed a rare tenured job in the field. “He’s the oldest of the old boys,” says one younger colleague.
But in his recent book, Unmasking Europa, Greenberg paints himself as a disrespected outsider and accuses colleagues of dishonesty, idea theft, and undermining both his reputation and the careers of his graduate students. “People who don’t adhere to the party line [of thick ice] are shut out,” Greenberg insists. The thick-ice proponents, he charges, have prevented a fair hearing of the thin-ice theory by ignoring and distorting his views.
Many of Greenberg’s colleagues say they are stunned by his unusually personal attack within a small and highly collaborative field. Pappalardo dismisses Greenberg’s charges against him as “absurd and untrue” and adds, “There’s more going on here than scientific debate.” Far from attempting to silence Greenberg, Pappalardo says he has done whatever he could to engage his colleague.
Invited by Pappalardo to write a chapter in a recently published book on Europa, Greenberg declined. “He said he was too busy,” Pappalardo recalls. While bristling over the personal nature of Greenberg’s attack, he insists that he is far from dogmatic on the issue of ice thickness. “Some of his hypotheses make for great challenges—pushing the system is always a good thing.”
A key source of all this frustration is the lack of information about Europa. Detailed maps cover not much more than a tenth of its surface, providing tantalizing but incomplete images of the mysterious moon. That makes for a sort of scientific cabin fever, as researchers reanalyze images, tinker with models, and ponder new ways to explain the complicated world that is, for now, utterly out of grasp. That also can create a herd mentality, say several planetary scientists. “You can get an establishment view if you have limited data,” explains Jonathan Lunine, a planetary scientist at the University of Arizona who worked on the now pushed-back Titan mission.
ASU’s Greeley, who tends toward a thick-ice view and has been another target of Greenberg’s ire, admits that “there is often a bandwagon mentality” that stems from these long waits. He knows, having been involved in NASA lunar and planetary missions for more than four decades. “We’re left with a crystal ball,” says Kevin Hand, a planetary scientist at JPL who is one of the few researchers openly sympathetic to Greenberg’s views. “And people project their own scientific hypothesis onto it.”
The bitter truth is that the new Europa mission, even if it reaches its destination, survives the onslaught of the magnetic field, and successfully returns new data, may still not lay to rest the question of the probability of life. Whether the probe’s radar can penetrate deep into the ice and beyond—much depends on the composition of both ice and ocean—remains to be seen. And since NASA has promised the Titan team that it is next in line for an outer-planet mission, a Europa lander that could plumb the moon’s depths and truly seek out alien life there remains in the realm of science fiction. “All these worlds are yours except Europa,” the HAL 9000 computer informs Earth in Clarke’s 2010. “Attempt no landings there.”
Pappalardo says that the job of the current generation of scientists is to do the hard work to ensure that the next generation will reap the results. Though he may be as gray as Greeley is today when the Europa mission arrives, he intends to fight hard to ensure that a lander will follow, just as Greeley’s generation fought for Galileo. That is something Greenberg, who calls Europa a “fantastic and wonderful place,” can happily agree with. “The stakes are high, and so are the emotions,” adds Lunine, who is still optimistic about the Titan mission despite its defeat. “People go home and dream about this stuff.”
But until this Europa mission is on its way to Jupiter, there is little time for rest.
WHAT LIES BENEATH
Europa’s fractured, frozen surface (left) conceals a global ocean buried below, most scientists now agree. What they do not know is how thick that ice is, a factor that strongly affects the prospects for life on this moon. The artist’s renderings illustrate two possibilities. In both, tides caused by Europa’s eccentric orbit around Jupiter create internal heat that melts the base of the icy crust. If the heat is intense enough (center), it might melt the ice almost all the way to the surface; in that case, organic matter and sunlight from above could reach the ocean, creating promising conditions for biology. If the heat is weaker (right), Europa might have a thick layer of warm ice atop its ocean. That warm ice could slowly rise and flow, much like glaciers on Earth, but it would present a daunting barrier to life.