At a distance, the worlds in our solar system resemble marbles, gleaming spheres suspended in the inkiness of space. Perhaps no world fits this description better than Europa, one of Jupiter’s moons. Europa is covered in a layer of ice—a hardy lid for the salty ocean that scientists suspect churns below, perhaps with microbial life. The icy surface is fractured in places, producing a patchwork of zigzagging cracks, but it lacks craters, cliffs, and mountains, making it one of the smoothest surfaces in the solar system. Such features were likely erased from the moon over time, perhaps as the icy crust settled under its own weight.
Move closer in, however, and the glossy marble of Europa gives way to fields of 50-foot-tall, sharp-edged blades of ice sticking straight up into the sky.
Scientists say parts of Europa could be covered in these icy and snowy formations, known as penitentes. Penitentes can be found on Earth, too, in high-altitude and dry regions like the Chilean desert, pictured at the top of the story, but they usually don’t grow past 20 feet in height. Scientists believe penitentes may emerge on Europa as they do on Earth, through a natural process called sublimation.
During sublimation, when solid ice is exposed to sunlight, it turns into gas and wafts away instead of melting into a liquid. (Think of the spooky effects of frozen carbon dioxide, also known as dry ice.) The transition can dramatically alter a surface. As ice evaporates, pockets of space emerge in the landscape. Eventually, the pockets deepen, producing towering spikes around them.
Daniel Hobley, a scientist at Cardiff University, and his colleagues used data from the Galileo spacecraft, a NASA mission that studied Jupiter and its moons from 1995 to 2003, to examine sublimation rates on Europa. They found that in some parts of the moon, sublimation was overcome by other phenomena, such as a bombardment of tiny meteorites and particles from space, which smooth out the surface. But at the equator, conditions favored sublimation, which does the opposite.
To blossom, penitentes require dry air, cold temperatures, and sustained exposure to sunlight. Europa’s equatorial regions provide all three. Here, the scientists concluded, a jagged terrain of four-story-tall spikes could emerge and survive.
These are, of course, only predictions. But they provide some of the richest descriptions of the texture of Europa, one of the best candidates for extraterrestrial life in the solar system. The most powerful ground and space telescopes, even Hubble, aren’t strong enough to resolve such surface details. The Galileo mission produced some stunning photographs of crisscrossing fissures in Europa’s ice, but only from afar.
“I would hope it would add something to people’s ability to picture what they might see if they were able to actually wander around on the surface—or, I guess, clamber around, given the spikes,” Hobley says. “A bit of a Google image search reveals some cool visualizations of the surface, but these come from people just assuming things would look like Antarctica and Greenland. We’re showing that we can actually think about this scientifically, and we might see something much cooler and more exotic.”
Cynthia Phillips, a planetary geologist at NASA’s Jet Propulsion Laboratory, is hesitant to put too much weight on the researchers’ predictions. She points out that the Galileo mission provided data on sublimation rates on a global scale rather than a regional one. “It’s a bit of a stretch to extrapolate that far,” Phillips says. “That said, it’s the only information we have right now, and so it does make sense to make a prediction and say, ‘Hey, these features are possible.’”
Such predictions are important if, like NASA, you’re planning another trip to the Jovian system. The agency is preparing to send a spacecraft to Europa in 2022 to loop around the moon dozens of times and investigate all kinds of features, from the icy surface to the flowing ocean underneath. The mission, known as Clipper, will be equipped with imaging technology capable of spotting the icy blades at the moon’s equator, if they exist.
Hobley and his fellow researchers suggest that penitentes on Europa could pose a hazard to other types of spacecraft, like landers. NASA has proposed a lander mission, but the concept is still being studied. If penitentes are indeed crowded around the equator, engineers could easily avoid them by landing somewhere else.
It’s also possible they may not be as dangerous as they seem; ice on Europa can be as strong as rock on Earth, but spindly penitentes could be built like sandcastles. Like sunshine-loving plants on Earth, penitentes are always pointed toward the sun. When they’re located farther away from the true equator, penitentes lean over sideways. “It’s hard to be sure exactly what this would do to the strength of them, but doubtless it would make the spikes more likely to break and fall over,” Hobley says. “The bigger they grow, the more likely it is they will break and collapse, too.”
Until a spacecraft arrives at Europa, Hobley’s prediction provides an exciting picture for other scientists who study the moon. “Many of us, including myself, working on other aspects related to Europa haven’t probably really appreciated the kind of complexity and diversity of surface features it might have,” says Xianzhe Jia, a planetary scientist at the University of Michigan who recently used Galileo data for his own research: finding evidence of plumes of vapor shooting out of the Europan surface. “With high-resolution cameras and all the other remote-sensing instruments at different wavelengths, it’s quite possible we’re going to find very new features that we have not thought about possibly existing on Europa.”
Phillips agrees. “I personally think that Europa’s surface is going to be a revolution,” she says. “I think it’s going to look completely unlike anywhere else that we’ve landed.”