What lies beneath Europa’s icy crust? Richard Greenberg has been pondering this question for 30-odd years. His new book, Unmasking Europa, describes his view that Europa’s hidden ocean and the life forms it may support are not that far below the surface.
A professor in the Lunar and Planetary Laboratory at the University of Arizona, Greenberg was one of the first to formulate how tidal forces could shape the geology on Jovian moons. He got the opportunity to test his ideas as a member of the imaging team on NASA’s Galileo spacecraft, which orbited Jupiter from 1995 to 2003.
During several flybys, Galileo took hundreds of snapshots of the moon Europa, showing a surface covered with dark spots and crisscrossing lines. In his new book, Greenberg walks readers through the Europa photo gallery like a curator in an art museum. He interprets the meaning of these wonderful images and recounts how he and his colleagues came to see Europa’s strange features as evidence that the outer crust is a thin layer of ice riding over a deep ocean.
This is not the mainstream opinion, however. Most scientists who study Europa believe the ice is much thicker: tens of kilometers as opposed to only a few kilometers. In the course of defending his minority position, Greenberg blames the hierarchical structure of big science projects for creating a politically-motivated "thick ice" cabal that refused to go back on its initial interpretations even when later data seemed to contradict them.
A surface worth visiting
A thin permeable ice crust might be more hospitable to life than a thick crust that seals off and isolates the ocean below. Letting his imagination run free, Greenberg provides a description of a typical day on Europa’s frigid surface.
Life on Europa is still highly speculative, but the notion has gained scientific merit in the last couple of decades. Before Voyager 2 flew by Europa in 1979, most scientists assumed that Europa would be frozen solid. However, the first close-up images showed a relatively small number of impact craters, implying that the moon’s surface has been radically altered within the last 50 million years. Something dynamic must be occurring inside Europa.
As an expert in celestial mechanics, Greenberg was originally interested in the elliptical orbits of the Jovian moons. In the mid-1970s, he highlighted the fact that—due to a three-way resonance between Io, Europa and Ganymede—Europa’s distance to Jupiter fluctuates by about 1 percent during its 85-hour orbit around the giant planet. The resulting change in the gravitational pull from Jupiter causes the moon’s equator to squeeze in and out by as much as 30 meters (in comparison to the Earth’s tides which rise and fall by about a meter). The "breathing" motion generates tidal heating inside Europa, and this presumably keeps the sub-surface ocean from freezing. Greenberg and others proposed early on that this internal heat could explain the geological processes that have recently resurfaced the moon.
Thick or thin blanket?
Europa’s internally-generated heat is continuously escaping out into space, but the question is how. Greenberg and his team believe the heat conducts through the ice (just as heat moves through the metal of a pot) , while most other researchers say warm ice convects heat to the surface (just as water currents carry heat around the ocean). Convection is faster than conduction, but it requires that the ice crust be more than 20 kilometers thick. By contrast, Greenberg thinks the ice extends for no more than a few kilometers.
What is his evidence? Essentially it is the preponderance of ridges and uneven patches, called “chaos,” on the surface of Europa. The ridges, which appear as lines in large scale images of the moon, are cracks in the crust that help relieve some of the stress from tidal stretching, according to Greenberg and his colleagues. Like fault lines on Earth, the ridges are where ice plates meet, and there is some evidence of these plates spreading apart, slamming together and slipping past each other. Greenberg believes such tectonic interactions require a thin crust floating on a liquid ocean.
This thin ice picture could also explain chaotic terrain as those places where the ice melted though and briefly exposed the ocean below. If such were to happen, icebergs would break off and float out into the open sea. These ice blocks would eventually get stuck in random locations when the ocean refroze. Greenberg claims this mechanism explains the appearance of chaos.
The implications of thin ice are perhaps most dramatic when it comes to the potential for life on Europa. Whereas thick ice would relegate any Europan life-forms to deep hydrothermal vents, thin ice would offer open channels for organisms to reach the surface.
These processes would not only provide direct access to nutrients, such as oxygen and sunlight, but they would also offer environmental changes that could drive evolution among Europa’s biota.
How big science gets done
With all the evidence in the book for thin ice, why do most planetary scientists continue to support a thick crust interpretation? Greenberg claims the thin ice model has been suppressed by the scientific community. It might seem hard to believe anyone would bother to stifle a theory about a frozen moon hundreds of millions of miles away, but Greenberg says that Galileo’s team leaders decided prematurely that Europa had thick ice, and afterwards it became politically advantageous to toe that line.
Greenberg dispels these reasons throughout the book. For example, some scientists claimed to have identified "pits, spots and domes" that conform to a convection model of warm ice pushing its way to the surface. Greenberg says that these surface features have not been rigorously defined, and his team’s own inspections have found them to be less uniform in size and spacing than the convection model would predict. Later studies claimed that medium-sized impact craters support thick ice because they show no evidence of having penetrated to the ocean below. But Greenberg disagrees: some of these craters appear to have chaotic terrain at their lowest points, which would imply that the impacts did in fact reach the water level.
He goes on to address other faults he sees in the thick ice paradigm, but a layperson will not have the expertise to judge whether this is a fair assessment or if Greenberg is simply presenting thick ice as a straw man that he can easily tear down. In the end, one might wonder who he is trying to convince. The non-specialist reader may side with Greenberg’s position, but planetary scientists will probably prefer that the thick and thin debate be held in scientific journals rather than the popular press. In any case, we may all agree that this peek under the mask of Europa calls for a closer look.