Ice spikes may cover Jupiter’s moon

Few moons in the solar system are as intriguing as Jupiter’s moon Europa. A global ocean of saltwater almost certainly surrounds the moon– and it holds more water than any ocean on Earth. Above this immense sea, where surface temperatures dip to minus 300 degrees Fahrenheit, a crust of water ice forms a shell. Astronomers predict that Jupiter, which bombards the moon with intense radiation, causes the entire moon to groan with gravity’s tug. Europa’s liquid water is a tempting target for future missions looking for possible alien microbes.

But before a future lander can search for microscopic E.T., the probe might have to contend with a forest of tall, jagged ice spikes. So argues a team of planetary scientists and geomorphologists Monday in the journal Nature Geoscience. Their research suggests Europa is an icy hedgehog world, covered in ice formations rarely found on Earth.

Because Europa’s surface is “incredibly cold,” said Daniel Hobley, a geomorphologist at Cardiff University in the U.K., an author of the study, ice will not melt and refreeze.

Instead, stranger things happen. Ice is not perfectly flat — it’s made of little crystals. “Each of those crystals has potential to lens light,” Hobley said, channeling or refracting light down into ice. Over millions of years, energized by the sun’s radiation, Europa’s ice transforms into gas.

No seasons

Sunlight alone would not be enough to form huge ice blades. Europa is tidally locked with Jupiter, which means its orbit around Jupiter matches its own rotation. Put another way, the same face of Europa always faces Jupiter. That also means that the angle of the sun is constant in Europa’s sky, Hobley said. The moon has no seasons.

“It’s just day-night, day-night, day-night,” Hobley said, year after Jovian year.

Hobley and his colleagues considered where on Earth might approximate Europa. They landed on the Andes, the South American mountain range that stretches from Venezuela to Argentina. At the highest peaks, the Andes has ice — and, in winter, the ice stays too cold to melt. What’s more, near the equator, the sun’s angle is high and constant, just like it is over Europa’s middle.

In the Andes, Hobley and his co-authors knew, an ice formation called penitentes takes shape. Penitentes are named after a religious festival, practiced by some Spanish-speaking Christians, in which monks wear pointed white hats (the “penitent,” hence, penitentes). Charles Darwin, who made some of the first scientific observations of penitentes in the 1830s, imagined wind created the spikes. In 2001, University of Colorado at Boulder physicist Meredith Betterton grew penitentes in a lab via the solid-to-gas process.

At mountain tops, the sun’s light carves pits deep in the ice, leaving behind blades twice as tall as they are wide. Penitentes do not form at high latitudes because the sun is too low in the sky in winter; the ice crystals do not funnel the light so sharply downward.

The bottom of these triangular cavities get hotter, and the sides stay cool. On Europa, they would grow “crazy, crazy slow,” Hobley said, at about a foot every million years. But because Europa’s surface has been largely unchanged for 50 million years, “they’ve got a nice long time to do it.”