A city-sized impact crater viewed by NASA's Galileo spacecraft may shed new light on the nature of the enigmatic icy surface of Jupiter's moon Europa.
This false-color image reveals the scar of a past major impact of a comet or small asteroid on Europa's surface. The bright, circular feature at center right has a diameter of about 50 miles (80 kilometers), making it comparable in size to the largest cities on Earth. The area within the outer boundary of the continuous bright ring is nearly 2,000 square miles (about 5,000 square kilometers). The diameter of the darker area within the bright ring is about 18 miles (29 kilometers), which is large enough to contain both the city of San Francisco and New York's Manhattan Island, side by side.
The brightest reds in this image correspond to surfaces with high proportions of relatively pure water ice, while the blue colors indicate that non-ice materials are also present. The composition of the darker materials is controversial; they may consist of minerals formed by evaporation of salty brines, or they may be rich in sulfuric acid. The bright ring is a blanket of ejecta that consists of icy subsurface material that was blasted out of the crater by the impact, while the darker area in the center may retain some of the materials from the impacting body. Further study may yield new insights about both the nature of the impactor and the surface chemistry of Europa.
Europa's surface is a question of great interest at present, since an ocean of liquid water may exist beneath the icy crust, possibly providing an environment suitable for life. 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 1999 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, 1998, 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 1999. Analysis and interpretation are ongoing.