Why does europa have few impact craters

Voyager mapped only a fraction of the surface at high resolution. The images of Europa's surface strongly resemble images of sea ice on Earth.

Scientists have postulated that a water-ice shell covers Europa and is more than kilometers thick. It is possible that beneath Europa's surface ice there is a layer of liquid water, perhaps as much as 50 km deep, kept liquid by tidally generated heat due to the pull of Jupiter and its other moons.

If so, it would be the only place known in the Solar System besides Earth where liquid water exists in significant quantities. Europa's most striking surface features are the series of dark streaks or cracks that crisscross the entire globe. Cynthia Phillips used this image in a detailed search for current geologic activity on Europa. For a study published in the Journal of Geophysical Research -- Planets last year, she compared the frames that make up this image with similar images taken in by the Voyager spacecraft and found no sign of changes due to geologic activity.

That suggests a minimum surface age for Europa of about 30 million years, though the result does not rule out current geologic activity altogether, as the study was limited by the resolution of 1.

Future images by Europa Orbiter may allow a search for smaller-scale changes on Europa's surface. During its twelfth orbit around Jupiter, on Dec. Highest Resolution Image of Europa. This mosaic of the south polar region of Jupiter's moon Europa shows the northern miles kilometers of a strike-slip fault named Astypalaea Linea. San Andreas-sized Strike-slip Fault on Europa. Europa and Callisto under the Watchful Gaze of Jupiter. This image of Europa, an icy satellite of Jupiter, was obtained from a range of 39, miles 62, kilometers by the Galileo spacecraft during its fourth orbit around Jupiter and its first close Ridges and Plateaus.

The prominent "X" near the center of the mosaic is the junction of two "triplebands," parallel sets of ridges. Prominent "X" on Europa. Craters smaller than 30 kilometers are several hundred meters deep and have recognizable rims and central uplifts these are standard features of impact craters. Pwyll, a crater 27 kilometers across, is one of the largest of these craters. 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 on Europa larger than 30 kilometers, on the other hand, have no rims or uplifts and have negligible topographic expression. Rather they are surrounded by sets of concentric troughs and ridges. These changes in morphology and topography indicate a fundamental change in the properties of the icy crust of Europa.

The most logical change is from solid to liquid. The concentric rings in large Europan craters are probably due to the 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.

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. Where does the 19 to 25 kilometer value come from? Larger impact craters penetrate more deeply into the crust of a planet and are sensitive to the properties at those depths.

Europa is no exception. To use this, we must estimate how big the original crater was and how shallow a liquid layer must be before it can affect the final shape of the impact crater. This is derived from numerical calculations and laboratory experiments into impact mechanics. Hence, craters 30 kilometers wide are sensing or detecting layers kilometers deep. There is some uncertainty in the exact thickness using these techniques.

This is due mostly to uncertainties in the details of impact cratering mechanics, which are very difficult to duplicate in the laboratory.

Could the ice shell have been thinner in the past? There is evidence in the crater topography that the thickness of ice on Ganymede has changed over time, and the same might be true for Europa. Geologic investigations of Europa's surface are underway, and a new spacecraft mission, the Europa Orbiter, is planned.

Impact craters with diameters of 12 miles 20 kilometers and larger are extremely rare on Europa; as of only 7 such features were known. The rarity of larger impact craters on Europa lends greater significance to the discovery of this one. Impact crater counts are often employed to estimate the ages of the exposed surfaces of planets and satellites, and the small number of craters found on Europa implies that the surface may be quite young in geological terms.

Thus the discovery of this feature may provide additional insights into questions about the age and level of geological activity of Europa's surface. Impact craters are expected to form with greater frequency on the "leading" sides of satellites that always turn the same face to their primary planet, in this case, Jupiter. The process is much like the effect of running through a rainstorm.

The "apex" of Europa's leading side is located on the equator at 90 degrees West longitude, only about 10 degrees removed from the feature shown. Europa's leading side does not receive a continuous bombardment by ionized particles carried along by Jupiter's rapidly rotating magnetosphere as is the case for the trailing side , which may allow greater preservation of the chemical signatures of the impacting object.

To the east of the bright ring-like feature are two, or perhaps three, similar but less well-defined quasi-circular features, raising the possibility that this crater is one member of a catena, or chain of craters. This would lend still greater interest to this area as a potential target for focused investigations by later missions such as the Europa Orbiter.

The near-infrared mapping spectrometer on board Galileo obtained this image on May 31, , during that spacecraft's 15th orbital encounter with Europa. The image data was returned to Earth in several segments during both the 15th and the 16th orbital periods. Merging and processing of the full data set was accomplished in