Crater measurements show Europa has thick ice shell
LUNAR AND PLANETARY INSTITUTE NEWS RELEASE
Posted: May 27, 2002
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.
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).
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.
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.
DVD is here!|
The first in a series of space DVDs is now available from the Astronomy Now Store. Relive shuttle Columbia's March flight to refurbish the Hubble Space Telescope in spectacular DVD quality.
U.K. & WORLDWIDE STORE
The Apollo 14 Complete Downlink DVD set (5 discs) contains all the available television downlink footage from the Apollo 14 mission. A two-disc edited version is also available.