The thickness of the ice crust in Europe will be 20 kilometers

With an ocean of liquid water hiding beneath its icy surface, Europa is one of the most fascinating moons in the solar system. Starting in 2027, this satellite of Jupiter will receive a visit from the American probe Europe Clipper. Why this appeal? The subsurface ocean and the presence of organic molecules make Europa an ideal candidate for harboring life forms. To estimate the chances of life arising in this ocean, it is necessary to know its composition and the composition and thickness of the ice. Based on new simulations, Shigeru Wakita of Purdue University in the US and his collaborators conclude that Europe's ice crust is at least 20 kilometers thick… much more than previous estimates.

The expanses of ice covering the surface of Europa are deceptive, because the moon is not just a quiet desert. Signs of plate tectonics have been discovered there, making Europa the only body in the solar system besides Earth where such a process has been detected. Behind this tectonics lie the movements of the subsurface ocean: heat generated by Europa's metallic core and mantle causes ocean water to rise, which then freezes at the surface. Thus, the ice on Europa's surface would be renewed “regularly”: it would be between 20 and 200 million years old, which is very little compared to the 4.5 billion years in the solar system.

If the age of its surface is known to some extent, doubt remains as to its thickness. Until now, studies have mainly relied on two topographic features of Europa's surface. On the one hand, lines, these structures that delineate the ice and which resemble land-based oceanic ridges. On the other hand, chasms are areas where the surface has frozen again after the ice broke up under the influence of ocean movements. Analysis of these areas indicates that the thickness of the ice crust is about 10 to 15 kilometers. But these morphological characteristics arise from modifications of the Earth's crust. Thickness varies locally and may not be representative of the crust as a whole. This was confirmed by Shigeru Wakita, who studied archaeological holes in the ice to determine its thickness.

The idea of ​​using potholes comes precisely from the fact that the surface is “young,” and these structures are as well. Therefore, the craters have changed little since their formation. Since its shape and size are directly related to the mechanical properties of the crust, researchers can analyze it to infer its thickness. With this in mind, Shigeru Wakita set his sights on two large impact craters (called Tire and Calanish), which differ from the two smaller impact craters because they have a concentric ring structure. He and his colleagues conducted numerical simulations in which he changed many factors, most notably the properties of the crust and its thickness. The goal is to find the right combination that makes it possible to reproduce the annular structures of craters as observed in images of Europa collected by the probe in 1998. Galileo. “The best fit is when the crust is at least 20 kilometers thick,” says Shigeru Wakita. If the study makes it possible to deduce the minimum thickness, it says nothing about the maximum thickness.

As for Jupiter's other icy moons, Callisto and Ganymede, their icy crusts are today thought to be about 80 to 100 kilometers across. “But it could also be thicker than we imagine,” Shigeru Wakita predicts.

Knowing the thickness of the surface of these moons is important data, because NASA is studying how to drill them using robots during future missions. For Shigeru Wakita, “Although drilling through 20 kilometers of ice on another celestial body is quite a challenge, it is worth a try if you really want to know what is hidden underneath.”