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 between 1995 and 2003, to study sublimation rates on Europa. They found that in some parts of the moon, sublimation was overcome by other phenomena, like 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.
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