The European ocean is covered with an icy shell at least 20 kilometers thick

The ice crust that protects the European ocean from the cold of space is at least 20 kilometers thick, according to an analysis of data from the Galileo mission. The conclusion does not shed light on whether that ocean contains life, but indicates how difficult it will be to find a conclusive answer.

Evidence for an ocean in Jupiter’s moon Europa dates back to the Voyager spacecraft, with the Galileo mission strengthening the case. The discovery inspired scientists and science fiction writers alike. We now know that many icy worlds in the outer solar system share this property, but Europa remains of paramount importance, not just because it was the first. The dust thrown away by Io’s nearby volcanoes gives Europa the potential for more complex chemistry than other worlds with internal oceans.

A possible obstacle to Europe’s status as ground zero for astrobiological research would be a crust that is too thick. Previous observations have not been able to determine how close the liquid water gets to the surface; attempts to answer the question have yielded estimates ranging from a few kilometers to ten times that amount. By comparison, the ocean in Enceladus, although much smaller and possibly much younger, escapes into space through geysers at the south pole, providing an opportunity to sample it directly, and suggesting that it may not be that difficult to get a to get a sufficiently flexible robot. down there.

Europa’s surface is the smoothest in the solar system, thanks to the movements of the ice, but nevertheless shows some traces of past impacts from space. A team led by Dr. Shigeru Wakita of MIT realized that two of these, known as Tire and Callanish, could be crucial for determining the thickness of the Earth’s crust.

Tire and Callanish are both multi-ring cymbals. Wakita and co-authors modeled what would happen if asteroids of the right size hit crusts of different thicknesses and concluded that only ice at least 20 kilometers thick would result in something like what we see. The figure is a minimum; Europe’s crust may be a lot thicker. If the crust is 15 kilometers thick, multiring craters of this size would be much deeper.

Europe’s crust may not be the same thickness everywhere. Just as Enceladus’ ocean is thought to be much closer to the surface near the south pole than elsewhere, Europa could also have areas of thinner ice. The authors consider it unlikely that the variation is large, and the similarity of the results they obtain at two locations does not support the idea that we just need to land an oil rig in the right place.

The authors also find that Europa’s ice consists of layers with different thermal properties. The outer part, 6 to 8 kilometers thick, is conductive, while underneath lies relatively warm convective ice.

Previous observations of the central peaks in some of Europa’s craters have been used as evidence that the heat from the impacts did not melt all the way into the ocean below. Using the dimensions of these craters, planetary scientists were able to calculate the heat generated, and therefore the crust must be at least 3 to 4 kilometers thick, but this represented only a minimum. Comparisons of the shapes of small and large craters indicate that the surface’s stiff ice extends only about 4.3 miles (7 kilometers) downward, without revealing whether it rests on softer ice or directly on water.

Multi-ring craters are produced by a complex process in which an impacting object creates a shock wave and produces a series of temporary formations that in turn collapse, leaving a wide basin of surrounding rings. The additional capabilities created by this multi-phase process allow ice models to rule out several pre-existing conditions, including ice less than 20 kilometers deep.

It is also thought that the Jovian moons Ganymede and Callisto may contain internal oceans, but with much thicker shells than Europa. The authors suggest that past estimates of 80–105 kilometers (50–60 mi) may significantly underestimate the crustal thickness of these moons.

The research is open access in Science Advances.

Leave a Reply

Your email address will not be published. Required fields are marked *