There is much debate as to the most promising place to find extraterrestrial life, but even those who don't think Jupiter's moon Europa is top of the list, still place it high. That's why several ideas for missions to study the moon are under consideration, with funding current support for the Europa Clipper flyby. Unfortunately, a new attempt to envisage Europa's surface suggest any attempt to land might risk being spiked by sharp blades of ice as high as four-story buildings.
We're used to ice melting on Earth when exposed to sunlight. The runoff tends to smooth out rough edges, and often pools in places where it refreezes at night. However, Dr Daniel Hobley of Cardiff University points out this isn't always the case.
At high altitudes close to the equator, if conditions are dry enough, ice exposed to the Sun will sometimes sublime, or turn directly to water vapor. This produces blades of ice 1-5 meters (3-16 feet) high known as penitentes. Hobley and co-authors from NASA have noted conditions on Europe could be even more suited to penitente production.
Sublimation is the norm when ice on astronomical objects with minimal atmospheres warms up – such as when comets enter the inner Solar System. Even out as far as Europa, Hobley argues in Nature Geoscience, the Sun's heat is enough to cause ice to sublime, at least in a moon's equatorial belt.
The paper models the rate at which sublimation would occur, and the extremely slow weathering that would smooth ice structures on a world with an atmosphere a trillion times thinner than Earth's. The result, the authors argue, is that Europa's equatorial belt is probably made up of penitentes that dwarf those seen on Earth – 15 meters (50 feet) high and an average of 7.5 meters (25 feet) apart. The authors think something similar occurs on Pluto, where the ice is frozen methane.
Trying to land a spacecraft in this would be like jumping onto a phalanx of medieval soldiers raising pikes. “We suggest that penitentes could pose a hazard to a future lander on Europa,” Hobley and co-authors write dryly.
None of the spacecraft that have visited the Jovinian system have got close enough to Europa to take pictures that would reveal objects of this size, but the authors claim “Radar and thermal data are consistent with our interpretation.” This data shows a previously unexplained difference between the way Europa reflects radar above and below 25° latitude.
On a larger scale, Europa is exceptionally smooth – the smoothest object in the Solar System. The ocean beneath its surface ice has prevented the build-up of mountain ranges and caused the erosion of impact craters, but that may not be enough to make for an easy landing.