Jupiter’s icy moon, Europa, is a prime candidate in the search for life elsewhere in our Solar System — but landing a spacecraft on the moon may be even more difficult than we thought. Certain patches of ice on Europa could be rough and jagged, resembling sharp blades, according to a new modeling. And that may make it hard for future probes to touch down gently on the surface.
It’s possible that conditions in areas around Europa’s equator may be just right to form what are known as “penitentes.” These are unique ice formations found here on Earth in places like the Andes Mountains. Penitentes form on Earth when super-cold ice sits in direct sunlight for long periods of time, causing patches in the ice to turn directly from a solid to a gas. In a new study, published today in Nature Geoscience, researchers found that the exact conditions needed to create this phenomena are present on parts of Europa too.
Scientists still hope to confirm the finding with visual evidence of penitentes on Europa. But the new model is a key piece of information that could help inform NASA’s future missions. Right now, the space agency is working on two different missions to the moon. The first, Europa Clipper, is slated to launch sometime around 2022 and will send a spacecraft to fly by Europa and possibly zoom through the world’s plumes — suspected geysers that spew water from a vast ocean below the moon’s icy crust. In the meantime, NASA is in the very early stages of designing a lander that could also travel to Europa someday, touch down on the surface, and then drill into the ice. That way, it could potentially sample the unseen water below.
But if parts of the surface are truly shaped like blades, it would be extremely hazardous for a conventional lander. This new research could help NASA decide which areas to avoid when considering landing spots on Europa. And it’s possible that the upcoming Europa Clipper mission will get even more detailed images of the moon’s surface, to confirm if these formations are actually there. “We’re really hoping that the Clipper mission will tell us one way or the other,” Daniel Hobley, a geologist and planetary scientist at Cardiff University in the UK, as well as lead author on the study, tells The Verge. “We should be able to take pictures of good enough quality to prove it.”
The impetus to look into this came from a simple question: “What would it be like to land on Europa?” Up-close images of the moon, taken in the 1990s by NASA’s old Jupiter probe, Galileo, revealed that the ice world is covered with reddish grooves and ridges. These bands are super wide, but it’s unclear what the terrain is like inside and around these troughs. Is it smooth and slick or jagged and coarse?
To figure this out, Hobley and his team decided to look to Earth as a guide. “On Earth it’s pretty unusual to find completely flat ice,” he says. “Especially if it’s left out in the Sun, that ice will be roughening it up. It won’t just sit there as a beautiful fresh surface of snow. It will have texture on it.” Given this kind of roughness happens on Earth, Hobley wondered if it was something we needed to worry about on Europa, as well.
After researching different types of ice formations, the researchers focused in on penitentes. These jagged formations are rare on Earth, as they require extremely specific conditions to form. Pentientes typically emerge in areas of thin atmosphere, where the air is very still and dry. And these places must be really, really cold, too, so the ice there doesn’t have any risk of melting. That way, when these areas are exposed to direct sunlight, the ice doesn’t turn into a liquid but immediately forms into water vapor. It’s a process known as sublimation, and it occurs when temperatures and pressures are just right so that materials skip the liquid phase of the transition process.
But it’s not enough just to have sunlight in these areas. The sunlight must remain in the same part of the sky for large periods of time, says Hobley. This causes patches of ice to erode away, forming deep pits and high spikes. And when the Sun remains in the same spot overhead, the bottoms of these pits stay illuminated and continue to deepen. This is why penitentes usually only form in South America’s Andes Mountains. At such high altitudes, the air is thin and still; plus these regions are near the equator, so during the winter, the Sun is typically in the same part of the sky for a while. “What you need is light coming straight down in winter, and it needs to not wobble around too much. And that only happens near the equator on Earth,” Hobley says.
Hobley and his team then looked for these conditions on Europa. And sure enough, it has all the right elements for penitentes to grow. “Europa is ideal for that, because it doesn’t have any air essentially,” he says. “There is no atmosphere on Europa.” Plus, temperature is not an issue. It ranges from -210 degrees to -370 degrees Fahrenheit on the moon’s surface. “It’s really cold, so you never have to worry about melting ice,” Hobley says. “You go straight from solid ice into water vapor, because there is no air.”
Based on data gathered by NASA’s old Galileo probe, Hobley and his team estimated how much sublimation is happening on Europa, or how quickly ice is thought to turn into vapor on the moon when exposed to the Sun. Multiple things can cause ice to disappear on Europa, such as charged particles hitting the moon’s surface or small meteorites hitting the ground and ejecting material. And the team wanted to know how sublimation compared to these other processes. They found that ice seems to be turning into gas at a high rate at the equator, making it possible that penitentes are growing there. And they believe these blades could get up to 50 feet tall — about they height of a five-story building.
And this part of Europa has another thing needed for the ice blades: direct sunlight overhead. Because of how Europa orbits Jupiter, the Sun more or less stays in the same spot at the moon’s equator. “The Sun for Europa goes straight overhead at the equator every day, all day long, all years long, for forever,” says Hobley. “It traces the same arc in the sky over and over.” It’s all ideal conditions for ice blades to form at Europa’s tropics.
Of course, we don’t have photographic proof, but Hobley and his team found other signs as well. They say that the penitentes may explain some really weird radar readings and heat signatures that the Galileo probe found at Europa’s equator. That’s because the ice in this region is just structurally different from the rest of the surface.
It all makes for a good theory, one that Hobley has posited before. The researchers have discussed their findings with NASA, and space agency scientists agree it’s a compelling idea. However, not everyone is convinced just yet. Cynthia Phillips, NASA’s Europa project staff scientist, says it’s possible that other explanations may explain the weird readings at the equator. It’s hard to know because Galileo’s instruments weren’t strong enough to tell us for sure. “There are other potential answers for those results,” she says. “It’s just because the data that we have so far is so low-resolution that you can’t really use it to point to one particular explanation or another.”
However, answers will come soon with Europa Clipper, which will fly within 16 miles of Europa’s surface. The spacecraft also has a camera and instruments with higher resolution than Galileo had. “It will be flying over the equatorial region, which is where these features are predicted to exist,” Phillips says. “I think Europa Clipper is well-suited to see any actual evidence for these formations.”
Even if ice blades are found, it’s not a showstopper for a future lander. The new study only found these high sublimation rates occurring in a narrow band around the equator, but areas closer to the poles don’t seem to have the same conditions. “There are still lots of places on the surface of Europa that would be really interesting potential landing sites that are well outside of this band,” says Phillips. “There’s no reason to shoot for the equator over anywhere else.”