Spiky ice spires may stud the equator of Jupiter’s moon Europa

Europa’s frozen landscape could be treacherous territory for future lunar landers.

Jagged spires of ice may stud the Jovian moon’s equator, researchers report October 8 in Nature Geosciences. These structures, called penitentes, could reach heights of 15 meters and occur roughly every 7.5 meters, computer simulations show, potentially rendering parts of the moon unnavigable in future missions.

“All kinds of interesting things might be on the surface” of Europa, says Jeff Moore, a planetary geologist at NASA’s Ames Research Center in Moffett Field, Calif. “You’d want to know about them if you wanted to send a lander.”

Penitentes on Earth are sculpted through a process called sublimation, where sunlight transforms snow or ice in a dry environment into water vapor without first melting it. As parts of the snow or ice sublimate quicker than others, surface depressions form. Those spots concentrate sunlight, speeding up the sublimation process there even more and carving the icy blades.

On Earth, these ice spires grow at high altitudes in the tropics and subtropics where conditions are just right: Penitentes require abundant sunlight, cold temperatures that don’t allow melting and still air, since the ice is easily eroded.

The structures haven’t been directly observed on Europa because the resolution of current imagery is so low. “The best pictures we have of Europa don’t show things any smaller than a house,” Moore says. “You can’t really make out any detail.”

But drawing on data and imagery from previous missions, Moore and his colleagues developed computer simulations to test how easily penitentes could form on the frozen moon. They found that Europan conditions could support the growth of penitente fields along the equator, which receives strong sunlight, and that these structures could stretch as high as a five-story building. It’s cold enough, too: Temperatures, for example, can dip to around –200° Celsius.

Penitentes could explain previous radar observations and odd temperature readings previously collected, the researchers say. Radar signals from the Arecibo Observatory in Puerto Rico that previously mapped Europa’s surface, for example, hinted at uneven features. Radar signals could bounce back and forth between the deep crevices and ridges of the spires, before surging back to their receiver, suggests study coauthor Daniel Hobley, a planetary geologist at Cardiff University in Wales. And abnormally chilly temperature readings gathered during nighttime flybys made by NASA’s Galileo mission may reflect data captured from an oblique angle, measuring only the frozen tips of the penitentes instead of the surface as a whole.

But Kevin Hand, a planetary scientist at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. who was not part of the study, is skeptical of the simulations. For one thing, ice that forms penitentes on Earth may have dust particles, but is largely free of chemicals. “On Europa, we’re talking about ice that contains salts and all sorts of sulfur compounds,” he says. “This changes the sublimation and morphology story significantly.”

Cynthia Phillips, a planetary geologist also at NASA’s Jet Propulsion Laboratory, notes that the data used to create the simulations are “just not precise.” But, she says, “it’s a very testable hypothesis.”

NASA’s Europa Clipper mission, set to launch between 2022 and 2025, will provide more details about the frozen moon. The probe will carry cameras, instruments to measure Europa’s magnetic field and an ice-penetrating radar prepped to chart the frozen moon’s finer details. A lander will eventually follow (SN Online: 2/18/17).

“We’ll be able to go look for [the penitentes], and I think that’s pretty exciting,” Phillips says.

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Editor’s note: This story was updated October 17, 2018, to correct the photo credit.