Detailed mapping and measurements of impact craters on Jupiter's large icy satellites reveal that Europa's floating ice shell may be at least 19 kilometers thick. These measurements, by Staff Scientist and geologist Dr. Paul Schenk, at Houston's Lunar and Planetary Institute, are reported in the May 23 issue of Nature. The results are based on stereo and topographic analysis of images of impact craters on these satellites acquired from NASA's Galileo spacecraft, currently orbiting Jupiter and heading toward its final plunge into Jupiter in late 2003.

Comparison of impact crater morphology on Ganymede and Callisto (top row) and on Europa (bottom row). These images have been scaled so that each crater appears to be the same size, even though the actual size of the craters increases from left to right. This shows how crater morphology changes as the crater size increases. Note that the scale bars at the bottom of each image represents a distance of 30 kilometers for Ganymede and Callisto and 10 kilometers for the craters on Europa. These images also show clearly how crater morphology on Europa differs from that on its sister satellites.

Geologic and geophysical evidence from Galileo supports the idea that a liquid water ocean exists beneath the icy surface of Europa. Debate now centers on how thick this icy shell is and the implications for life forms that could exist in the ocean. An ocean could melt through a thin ice shell only a few kilometers thick exposing water and anything swimming in it to sunlight (and radiation). A thin ice shell could melt quickly and then refreeze, giving photosynthetic organisms easy access to sunlight. A thick ice shell--tens of kilometers -- would be more difficult to melt through and, since sunlight cannot penetrate more than a few meters into the ice, would preclude photosynthetic organisms. It would also require other processes to expose any ocean material on the surface, where we can search for it.

Dr. Schenk's estimate of the ice thickness is based on a comparison of the topography and morphology of more than 200 impact craters on Europa and on its sister satellites, Ganymede and Callisto. Although both Ganymede and Callisto may have liquid water oceans inside, they also have extremely thick ice shells (roughly 100-200 kilometers). Thus the final surface expression of most craters will be unaffected by the warmer ocean and can be used for comparison with Europa, where the depth to the ocean is uncertain but likely to be much shallower.

Dr.Schenk found that the shapes of Europa's larger craters differ significantly from similar-sized craters on Ganymede and Callisto. His measurements show that this begins with craters larger than 8 kilometers in diameter. The difference is caused by the warming of the lower part of Europa's less-thick ice shell by the ocean. Warm ice is soft and flows relatively quickly (as in glaciers on Earth).

Perspective view of the impact crater Pwyll, showing the rim and central peak complex.  This complex crater is 27 kilometers across.  The topographic data used for this view were based on analysis of stereo and shape-from-shading data. Vertical exaggeration is roughly 50 times normal.

Craters larger than ~30 kilometers show even more dramatic differences. Craters smaller than this are several hundred meters deep and have recognizable rims and central uplifts (standard features of impact craters). Craters on Europa larger than 30 kilometers have no rims or uplifts and have little topographic expression. Instead, they are surrounded by sets of concentric troughs and ridges. This observation implies a fundamental change in the properties of Europa's icy crust at increasing depths. The most logical is a transition from solid to liquid. The concentric rings may be caused by wholesale collapse of the crater floor. As the originally deep crater hole collapses, the material underlying the icy crust rushes in to fill in the void. This inrushing material drags on the overlying crust, fracturing it and forming the observed concentric rings.

Larger impacts penetrate more deeply into the crust and are sensitive to the crustal properties at those depths, providing clues to thickness of the ice shell. Dr. Schenk estimated how big the original crater was and how shallow a liquid layer must be to affect the final shape of the impact crater. Numerical calculations and impact experiments by other researchers were used to produce a "crater collapse model" that is used to convert the observed transition diameter to a thickness for the layer. Hence, a crater 30 kilometers wide is sensing or detecting layers 19-25 kilometers deep. Although there are some uncertainties in this model (10-20% because of the difficulty of duplicating impacts mechanics on Earth), Schenk concludes that the icy shell cannot be only a few kilometers thick, as some have proposed.

Does a thick ice shell mean there is no life on Europa? Dr. Schenk says. "No! Given how little we know about the origins of life and conditions inside Europa, life is still plausible. If organisms inside Europa can survive without sunlight, then the thickness of the shell is of only secondary importance. After all, organisms do quite well on the bottom of Earth's oceans without sunlight, surviving on chemical energy. This could be true on Europa if it is possible for living organisms to originate in this environment in the first place."

He points out that Europa's ice shell could have been much thinner - or even nonexistent -- in the distant past, allowing a variety of organisms to evolve. If the ocean began to freeze over, the organisms could adapt to new environmental niches over time, allowing life of some sort to survive.

A 19-25- kilometer-thick crust will, however, make drilling or melting through the ice with tethered robots impractical! "The challenge will be for us to devise a clever strategy for exploring Europa that won't contaminate what is there yet find it nonetheless. The prospect of a thick ice shell limits the number of likely sites where we might find exposed oceanic material. Most likely, ocean material will be embedded as small bubbles or pockets or as layers within ice that has been brought to the surface by other geologic means," comments Schenk.

Perspective view of Tyre, a multiring impact feature 41 kilometers across, showing concentric ridges and fractures. The original crater rim is located just inside the innermost concentric ring. The original crater collapsed, leaving a smooth plain surrounded by these concentric fractures. Vertical exaggeration is roughly 50 times normal.

He suggests several processes that could allow us to sample ocean material. Impact craters excavate crustal material from depth and eject it out onto the surface, where we might pick it up. Unfortunately, the largest known crater on Europa, Tyre, excavated material from only 3 kilometers deep, not deep enough to get near the ocean. If a pocket or layer of ocean material were frozen into the crust at shallower depth, it might be sampled by an impact. He notes that the floor of Tyre has a color that is slightly more orange than the original crust.

In addition, there is strong evidence that Europa's icy shell is somewhat unstable and has been (or is) convecting (that is, blobs of deep crustal material rise toward the surface where they are sometimes exposed as domes several kilometers wide). Ocean material imbedded within the lower crust could then be exposed to the surface. This process could take thousands of years, and the exposure to Jupiter's lethal radiation would be hostile, but we could investigate and sample what remains behind.

The Galileo imagery recently returned shows clear evidence of resurfacing of wide areas of Europa's surface, where the icy shell has literally torn through and split apart. These areas have been filled with new material from below. Although these areas do not appear to have been flooded by ocean material, but rather by soft warm ice from the lower crust, it is very possible that oceanic material could be found within this new crustal material.

New studies of Galileo imagery and new orbital missions with advanced instruments are needed to investigate these possibilities and to search for potential landing sites on Europa.