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Ida and Dactyl in Enhanced C …
title Ida and Dactyl in Enhanced Color
date 08.28.1993
description This color picture is made from images taken by the imaging system on the Galileo spacecraft about 14 minutes before its closest approach to asteroid 243 Ida on August 28, 1993. The range from the spacecraft was about 10,500 kilometers (6,500 miles). The images used are from the sequence in which Ida's moon was originally discovered, the moon is visible to the right of the asteroid. This picture is made from images through the 4100-angstrom (violet), 7560 A (infrared) and 9680 A (infrared) filters. The color is 'enhanced' in the sense that the CCD camera is sensitive to near infrared wavelengths of light beyond human vision, a 'natural' color picture of this asteroid would appear mostly gray. Shadings in the image indicate changes in illumination angle on the many steep slopes of this irregular body as well as subtle color variations due to differences in the physical state and composition of the soil (regolith). There are brighter areas, appearing bluish in the picture, around craters on the upper left end of Ida, around the small bright crater near the center of the asteroid, and near the upper right-hand edge (the limb). This is a combination of more reflected blue light and greater absorption of near infrared light, suggesting a difference in the abundance or composition of iron-bearing minerals in these areas. Ida's moon also has a deeper near-infrared absorption and a different color in the violet than any area on this side of Ida. The moon is not identical in spectral properties to any area of Ida in view here, though its overall similarity in reflectance and general spectral type suggests that it is made of the same rock types basically. These data, combined with study of further imaging data and more detailed spectra from the Near Infrared Mapping Spectrometer, may allow scientists to determine whether the larger parent body of which Ida, its moon, and some other asteroids are fragments was a heated, differentiated object or made of relatively unaltered primitive chondritic material. The Galileo project, whose primary mission is the exploration of the Jupiter system in 1995-97, is managed for NASA's Office of Space Science by the Jet Propulsion Laboratory. *Image Credit*: JPL
Ida and Dactyl in Enhanced C …
PIA00069
Sol (our sun)
Solid-State Imaging
Title Ida and Dactyl in Enhanced Color
Original Caption Released with Image This color picture is made from images taken by the imaging system on the Galileo spacecraft about 14 minutes before its closest approach to asteroid 243 Ida on August 28, 1993. The range from the spacecraft was about 10,500 kilometers (6,500 miles). The images used are from the sequence in which Ida's moon was originally discovered, the moon is visible to the right of the asteroid. This picture is made from images through the 4100-angstrom (violet), 7560 A (infrared) and 9680 A (infrared) filters. The color is 'enhanced' in the sense that the CCD camera is sensitive to near infrared wavelengths of light beyond human vision, a 'natural' color picture of this asteroid would appear mostly gray. Shadings in the image indicate changes in illumination angle on the many steep slopes of this irregular body as well as subtle color variations due to differences in the physical state and composition of the soil (regolith). There are brighter areas, appearing bluish in the picture, around craters on the upper left end of Ida, around the small bright crater near the center of the asteroid, and near the upper right-hand edge (the limb). This is a combination of more reflected blue light and greater absorption of near infrared light, suggesting a difference in the abundance or composition of iron-bearing minerals in these areas. Ida's moon also has a deeper near-infrared absorption and a different color in the violet than any area on this side of Ida. The moon is not identical in spectral properties to any area of Ida in view here, though its overall similarity in reflectance and general spectral type suggests that it is made of the same rock types basically. These data, combined with study of further imaging data and more detailed spectra from the Near Infrared Mapping Spectrometer, may allow scientists to determine whether the larger parent body of which Ida, its moon, and some other asteroids are fragments was a heated, differentiated object or made of relatively unaltered primitive chondritic material. The Galileo project, whose primary mission is the exploration of the Jupiter system in 1995-97, is managed for NASA's Office of Space Science by the Jet Propulsion Laboratory.
Volcanically Active Regions …
PIA00537
Jupiter
Solid-State Imaging
Title Volcanically Active Regions on Io
Original Caption Released with Image Shown here is a portion of one of the highest-resolution images of Io (Latitude: +10 to +60 degrees, Longitude: 180 to 225 degrees) acquired by the Galileo spacecraft, revealing immense lava flows and other volcanic landforms. Several high-temperature volcanic hot spots have been detected in this region by both the Near Infrared Mapping Spectrometer and the imaging system of Galileo. The temperatures are consistent with active silicate volcanism in lava flows or lava lakes (which reside inside irregular depressions called calderas). The large dark lava flow in the upper left region of the image is more than 400 km long, similar to ancient flood basalts on Earth and mare lavas on the Moon. North is to the top of the picture and the sun illuminates the surface from the left. The image covers an area 1230 kilometers wide and the smallest features that can be discerned are 2.5 kilometers in size. This image was taken on November 6th, 1996, at a range of 245,719 kilometers by the Solid State Imaging (CCD) system on the Galileo Spacecraft. Launched in October 1989, Galileo entered orbit around Jupiter on December 7, 1995. The spacecraft's mission is to conduct detailed studies of the giant planet, its largest moons and the Jovian magnetic environment. The Jet Propulsion Laboratory, Pasadena, CA manages the mission for NASA's Office of Space Science, Washington, DC. This image and other images and data received from Galileo are posted on the Galileo mission home page on the World Wide Web at http://galileo.jpl.nasa.gov. Background information and educational context for the images can be found at URL http://www.jpl.nasa.gov/galileo/sepo
Antum Crater
PIA00854
Jupiter
Near Infrared Mapping Spectr …
Title Antum Crater
Original Caption Released with Image This image shows the location of one of the highest spatial resolution NIMS images acquired. The left image is an airbrush map of the surface of Ganymede from Voyager data. The small square shows the location of Antum crater, target of the high-resolution NIMS image on the right. NIMS spatial resolution is approximately 4 km/pixel and the image is a falsely colored albedo for a single wavelength near 0.8 micrometers. Antum is what is known as a dark ray crater, that is, dark lines emanate from the central bright area. This NIMS image is a close-up of the central area and the dark rays are off the edges of the image. Dark ray craters are fairly unusual and are concentrated in one area of Ganymede's surface. They are thought to be composed of material from the body that impacted Ganymede and created the crater, rather than material brought up from the subsurface. Analysis of the NIMS data will yield compositional and mineralogical information on the dark material. This can help us to understand the nature of bodies that "crash" into the Jupiter system, as did Comet Shoemaker-Levy 9 in 1995, as well as give more information on the history of surface modification on Ganymede. The Jet Propulsion Laboratory, Pasadena, CA manages the mission for NASA's Office of Space Science, Washington, DC. The Jet Propulsion Laboratory, Pasadena, CA manages the mission for NASA's Office of Space Science, Washington, DC. This image and other images and data received from Galileo are posted on the World Wide Web, on the Galileo mission home page at URL http://galileo.jpl.nasa.gov.
Moon As Seen By NIMS
PIA00231
Earth
Near Infrared Mapping Spectr …
Title Moon As Seen By NIMS
Original Caption Released with Image These four images of the Moon are from data acquired by the Galileo spacecraft's Near-Earth Mapping Spectrometer during Galileo's December 1992 Earth/Moon flyby. The part of the Moon visible from Earth is toward the left, and the lunar north pole is near the terminator, upper right. The dark regions to left and below in the black-and-white image at upper left, are lunar Maria, including Mare Imbrium at upper left, Serenitatis and Tranquillitatis, lower left center, and the circular basin to the right is Crisium. The bright areas ringing Crisium and dominating the center of the images are the heavily cratered and mountainous lunar highlands. The black-and-white image used infrared wavelengths just beyond the visible deep red. The false-color map images (upper right and lower right) show the relative strength of silicate-rock absorption of near-infrared sunlight, at about 1-micron wavelength. Blue areas show stronger absorption and generally indicate materials with more pyroxene and olivine (iron-bearing silicate materials), while yellow indicates less absorption, due to original compositional variations. In young fresh craters, absorptions are also stronger due to the absence of meteorite-impact effects. Outlines of previously defined geological units are superimposed in the lower right image. Note correlation with the Maria/highlands features in the black-and-white image. The preliminary mineralogical map at lower left uses infrared band shape and intensity to visualize variations in pyroxene and olivine. Blue is related to low-calcium pyroxene, while green and red indicate high calcium and the iron/magnesium content of pyroxene, as well as olivine. The Galileo project, whose primary mission is the exploration of the Jupiter system in 1995-97, is managed for NASA's Office of Space Science and Applications by the Jet Propulsion Laboratory.
NIMS Observes the Structure …
PIA01224
Sol (our sun)
Near Infrared Mapping Spectr …
Title NIMS Observes the Structure and Composition of Jupiter's Clouds
Original Caption Released with Image Office of Space Science, Washington, DC. This image and other images and data received from Galileo are posted on the World Wide Web, on the Galileo mission home page at URL http://galileo.jpl.nasa.gov., With the NIMS instrument high quality observations are being obtained from all parts of Jupiter. The images in the upper panel are taken at a wavelength of 4.8 microns. At this wavelength thermal radiation from about 100 km deep below the visible cloud deck is escaping, allowing us to study the deep atmospheric region. The overlying cloud deck absorbs a part of the radiation, but there are places where it is thin and more radiation can escape. These are called hot spot regions. Many hotspots regions occur in a zone between the equator and 15 degrees north latitude, the North Equatorial Belt (NEB), but thermal radiation is seen from much of the planet. The uniqueness of NIMS is that it is capable of observing the same spatial region at a maximum of 408 different wavelengths between 0.7 and 5.2 micron simultaneously. Every picture element (pixel) contains a spectrum of up to 408 wavelengths. The gases that compose the atmosphere leave there traces in the spectra. In this particular case, 48 wavelengths were available between 4.6 and 5.2 micron, and we see spectral signatures of water, ammonia, and phosphine. Also, the total amount of radiation is determined by the amount of overlying cloud, characterized by the cloud opacity. By means of model calculations, we can determine the amount of water and the cloud opacity for each individual spectrum. The amount ammonia and phosphine is more difficult to obtain because its influence on the spectra is weaker. The results of these calculations are shown in the form of maps in the next two panels. With NIMS, we can now have a detailed look at the spatial distribution of the water and ammonia amounts and the cloud opacity in the atmosphere. Not all the pixels from the observations have good spectra, so for some data points no reliable determination of the water and cloud opacity could be made. We find that the atmosphere is extremely dry in, and close to, the hot spot, with relative humidities between 0.02 % and 10 %, with the dryest places being inside the hot spot. This corroborates the in-situ Galileo Entry Probe measurements. The Probe entered the atmosphere, on December 5 1995, in a hot spot region. Whereas the Probe obtained only a very localized snapshot, with NIMS we can do observations of larger areas and over longer periods. The spatial distribution of water is more complex than expected. More detailed investigations will be necessary to fully understand these results. Future studies will also allow a better understanding of the dynamics of the Jovian atmosphere, since the spatial distribution of water is thought to be a tracer for atmospheric motions under the cloud deck. NIMS will continue to provide excellent and unique data during the Galileo Europa Mission, planned to last until December 1999. The Jet Propulsion Laboratory, Pasadena, CA manages the mission for NASA's Office of Space Science, Washington, DC. The Jet Propulsion Laboratory, Pasadena, CA manages the mission for NASA's
Sulfuric Acid on Europa
PIA02500
Jupiter
Near Infrared Mapping Spectr …
Title Sulfuric Acid on Europa
Original Caption Released with Image Frozen sulfuric acid on Jupiter's moon Europa is depicted in this image produced from data gathered by NASA's Galileo spacecraft. The brightest areas, where the yellow is most intense, represent regions of high frozen sulfuric acid concentration. Sulfuric acid is found in battery acid and in Earth's acid rain. This image is based on data gathered by Galileo's near infrared mapping spectrometer. Europa's leading hemisphere is toward the bottom right, and there are enhanced concentrations of sulfuric acid in the trailing side of Europa(the upper left side of the image). This is the face of Europa that is struck by sulfur ions coming from Jupiter's innermost moon, Io. The long, narrow features that crisscross Europa also show sulfuric acid that may be from sulfurous material extruded in cracks. Galileo, launched in 1989, has been orbiting Jupiter and its moons since December 1995. JPL manages the Galileo mission for NASA's Office of Space Science, Washington DC. JPL is a division of the California Institute of Technology, Pasadena, CA.
Io's Prometheus Regions as V …
PIA02515
Jupiter
Near Infrared Mapping Spectr …
Title Io's Prometheus Regions as Viewed by Galileo NIMS
Original Caption Released with Image http://galileo.jpl.nasa.gov [ http://galileo.jpl.nasa.gov ]. Background information and educational context for the images can be found at http://galileo.jpl.nasa.gov/gallery/io.cfm [ http://galileo.jpl.nasa.gov/gallery/io.cfm ]., This image shows the region around the Prometheus volcano on Jupiter's moon Io. It was observed by the near-infrared mapping spectrometer on NASA's Galileo spacecraft as it was flying away from a close approach to Io. The area shown is about 1.6 million square kilometers (about 600,000 square miles), roughly the size of the "four corner states" (Arizona, New Mexico, Utah, and Colorado). The spectrometer instrument detects heat emitted by objects that is not visible to the naked eye. The image on the left was taken by Galileo's camera in visible wavelengths, it shows the context for the center and right images. The center and right images show spectrometer data at wavelengths of 1.3 and 4.2 microns respectively. The spectrometer can determine surface composition by measuring the spectrum of reflected sunlight, and can measure thermal emission from Io's hot lava. Prometheus is seen near the center of the three images. The image in the center, taken by the spectrometer at 1.3 microns, shows variations in light and dark surface materials, which result from variations in composition. The spectrometer thermal map (image on right) at 4.2 microns shows where the most heat is being produced from active volcanoes on the surface. The bright colors are used to indicate hot areas. Ten active volcanic regions(hot spots) are seen in this image. Four faint hot spots near the top of the image were not known to be active volcanoes before this image was acquired. All the hot spots correspond to dark areas in the visible wavelengths. This indicates that where the camera shows dark regions on Io, the infrared observations reveal that those regions contain hot lava. A distinct, dark ring can be seen clearly in the spectrometer's 4.2-micron map. The ring indicates a higher local concentration of sulfur dioxide, which appears dark at this wavelength. The dark ring is slightly larger in diameter than the bright ring that can be seen in the visible light camera image and the spectrometer's 1.3-micron image. This contradicts a previous belief that regions rich in sulfur dioxide on Io's surface appear white at visible wavelengths. The Prometheus ring is believed to be composed of fallout from the Prometheus volcanic plume. It is possible that both sulfur and sulfur dioxide are present in the plume, and that the bright white ring represents mostly sulfur deposits. Because sulfur dioxide is more volatile than sulfur, it may not condense and stick to the surface as close to the volcanic vent as sulfur does. Launched in October 1989, Galileo entered orbit around Jupiter on December 7, 1995 on a mission to study the giant planet, its largest moons and its magnetic environment. JPL manages the mission for NASA's Office of Space Science, Washington, DC. JPL is a division of the California Institute of Technology, Pasadena, CA. This image and other images and data received from Galileo are posted on the World Wide Web, on the Galileo mission home page at
Loki as viewed by Galileo NI …
PIA02514
Jupiter
Near Infrared Mapping Spectr …
Title Loki as viewed by Galileo NIMS
Original Caption Released with Image This image shows Loki, the most powerful volcano in the solar system, which has been constantly active on Jupiter's moon Io for at least 20 years. NASA's Galileo spacecraft took these images during its approach to Io on October 10, 1999. One of the spacecraft's instruments, the near infrared mapping spectrometer, was used to capture this observation. The instrument detects heat from objects in the infrared wavelengths not visible to the naked eye. Loki is a volcanic caldera about 200 kilometers (124 miles) across, nearly four times the width of the Yellowstone caldera on Earth. On the left side of the top image is a picture taken in visible light wavelengths by Galileo's camera showing the context of the NIMS image on the right. This thermal map taken by the spectrometer at 4.7 microns shows that heat is being emitted from the areas that are dark in the camera image. The bottom image shows additional spectrometer data obtained as the platform that holds the instrument on the spacecraft was moving toward the next target. This repositioned scan (shown as the zig-zag pattern) allowed the spectrometer to sample the warm, dark floor of the Loki caldera and the cold regions outside the caldera. The thermal map shows that the dark materials on the floor of Loki are cooling lava, near zero degrees Celsius(32 Fahrenheit). This substantially hotter than Io's surface temperature of about -180 degrees Celsius (-300 Fahrenheit). In previous observations, higher lava temperatures have been measured by the spectrometer at Loki, with temperatures similar to those of basaltic lava on Earth. The lighter, colored area in the camera image, which appears to be an island, is cold, which means it has not been active recently. The spectrometer detects both reflected sunlight and thermal emission from hot materials on the surface. This observation was taken on Io's nightside to avoid mixing sunlight with the thermal emission from hot lavas. Launched in October 1989, Galileo entered orbit around Jupiter on December 7, 1995 on a mission to study the giant planet, its largest moons and its magnetic environment. JPL manages the mission for NASA's Office of Space Science, Washington, DC. JPL is a division of the California Institute of Technology, Pasadena, CA. This image and other images and data received from Galileo are posted on the World Wide Web, on the Galileo mission home page at http://galileo.jpl.nasa.gov [ http://galileo.jpl.nasa.gov ]. Background information and educational context for the images can be found at http://galileo.jpl.nasa.gov/gallery/io.cfm [ http://galileo.jpl.nasa.gov/gallery/io.cfm ].
Loki as viewed by Galileo NI …
PIA02514
Jupiter
Near Infrared Mapping Spectr …
Title Loki as viewed by Galileo NIMS
Original Caption Released with Image This image shows Loki, the most powerful volcano in the solar system, which has been constantly active on Jupiter's moon Io for at least 20 years. NASA's Galileo spacecraft took these images during its approach to Io on October 10, 1999. One of the spacecraft's instruments, the near infrared mapping spectrometer, was used to capture this observation. The instrument detects heat from objects in the infrared wavelengths not visible to the naked eye. Loki is a volcanic caldera about 200 kilometers (124 miles) across, nearly four times the width of the Yellowstone caldera on Earth. On the left side of the top image is a picture taken in visible light wavelengths by Galileo's camera showing the context of the NIMS image on the right. This thermal map taken by the spectrometer at 4.7 microns shows that heat is being emitted from the areas that are dark in the camera image. The bottom image shows additional spectrometer data obtained as the platform that holds the instrument on the spacecraft was moving toward the next target. This repositioned scan (shown as the zig-zag pattern) allowed the spectrometer to sample the warm, dark floor of the Loki caldera and the cold regions outside the caldera. The thermal map shows that the dark materials on the floor of Loki are cooling lava, near zero degrees Celsius(32 Fahrenheit). This substantially hotter than Io's surface temperature of about -180 degrees Celsius (-300 Fahrenheit). In previous observations, higher lava temperatures have been measured by the spectrometer at Loki, with temperatures similar to those of basaltic lava on Earth. The lighter, colored area in the camera image, which appears to be an island, is cold, which means it has not been active recently. The spectrometer detects both reflected sunlight and thermal emission from hot materials on the surface. This observation was taken on Io's nightside to avoid mixing sunlight with the thermal emission from hot lavas. Launched in October 1989, Galileo entered orbit around Jupiter on December 7, 1995 on a mission to study the giant planet, its largest moons and its magnetic environment. JPL manages the mission for NASA's Office of Space Science, Washington, DC. JPL is a division of the California Institute of Technology, Pasadena, CA. This image and other images and data received from Galileo are posted on the World Wide Web, on the Galileo mission home page at http://galileo.jpl.nasa.gov [ http://galileo.jpl.nasa.gov ]. Background information and educational context for the images can be found at http://galileo.jpl.nasa.gov/gallery/io.cfm [ http://galileo.jpl.nasa.gov/gallery/io.cfm ].
Loki as viewed by Galileo NI …
PIA02514
Jupiter
Near Infrared Mapping Spectr …
Title Loki as viewed by Galileo NIMS
Original Caption Released with Image This image shows Loki, the most powerful volcano in the solar system, which has been constantly active on Jupiter's moon Io for at least 20 years. NASA's Galileo spacecraft took these images during its approach to Io on October 10, 1999. One of the spacecraft's instruments, the near infrared mapping spectrometer, was used to capture this observation. The instrument detects heat from objects in the infrared wavelengths not visible to the naked eye. Loki is a volcanic caldera about 200 kilometers (124 miles) across, nearly four times the width of the Yellowstone caldera on Earth. On the left side of the top image is a picture taken in visible light wavelengths by Galileo's camera showing the context of the NIMS image on the right. This thermal map taken by the spectrometer at 4.7 microns shows that heat is being emitted from the areas that are dark in the camera image. The bottom image shows additional spectrometer data obtained as the platform that holds the instrument on the spacecraft was moving toward the next target. This repositioned scan (shown as the zig-zag pattern) allowed the spectrometer to sample the warm, dark floor of the Loki caldera and the cold regions outside the caldera. The thermal map shows that the dark materials on the floor of Loki are cooling lava, near zero degrees Celsius(32 Fahrenheit). This substantially hotter than Io's surface temperature of about -180 degrees Celsius (-300 Fahrenheit). In previous observations, higher lava temperatures have been measured by the spectrometer at Loki, with temperatures similar to those of basaltic lava on Earth. The lighter, colored area in the camera image, which appears to be an island, is cold, which means it has not been active recently. The spectrometer detects both reflected sunlight and thermal emission from hot materials on the surface. This observation was taken on Io's nightside to avoid mixing sunlight with the thermal emission from hot lavas. Launched in October 1989, Galileo entered orbit around Jupiter on December 7, 1995 on a mission to study the giant planet, its largest moons and its magnetic environment. JPL manages the mission for NASA's Office of Space Science, Washington, DC. JPL is a division of the California Institute of Technology, Pasadena, CA. This image and other images and data received from Galileo are posted on the World Wide Web, on the Galileo mission home page at http://galileo.jpl.nasa.gov [ http://galileo.jpl.nasa.gov ]. Background information and educational context for the images can be found at http://galileo.jpl.nasa.gov/gallery/io.cfm [ http://galileo.jpl.nasa.gov/gallery/io.cfm ].
Galileo NIMS Observes Amiran …
PIA02516
Jupiter
Near Infrared Mapping Spectr …
Title Galileo NIMS Observes Amirani
Original Caption Released with Image This image is the highest-resolution thermal, or heat image, ever made of Amirani, a large volcano on Jupiter's moon Io. It was taken on October 10, 1999, by the near-infrared mapping spectrometer onboard NASA's Galileo spacecraft. Amirani is on the side of Io that permanently faces away from Jupiter. This image of Amirani was taken at a distance of less than 25,000 kilometers (16,000 miles). The picture scale is approximately 6.5 kilometers (4 miles) per spectrometer pixel. The center and right images show views of Amirani as seen by the spectrometer at two wavelengths, 1.0 and 4.6 microns. These images can be compared with a visible wavelength image (on the left) of the same area obtained by Galileo's camera during a previous orbit. The visible light image shows extensive lava flows and a dark-floored caldera with associated bright red deposits of material fed from the volcano. The spectrometer observation was made in daylight. The center image, taken at a wavelength of 1 micron, shows light and dark areas on the surface that can be used to line up the spectrometer data with the camera image. The image on the right shows the same area at a wavelength of 4.6 microns, which reveals the thermal emission from three separate volcanic areas. The locations of these three "hot spots" correspond to the darkest features in the camera image, reinforcing a previously held belief by Galileo scientists that there is a correlation between the dark areas and the hot spots. The three spectrometer hot spots are located at the eastern edge of the caldera at the bottom of the camera image, and two locations along the massive Amirani flows. These are most likely active lava flows on the surface. Launched in October 1989, Galileo entered orbit around Jupiter on December 7, 1995 on a mission to study the giant planet, its largest moons and its magnetic environment. JPL manages the mission for NASA's Office of Space Science, Washington, DC. JPL is a division of the California Institute of Technology, Pasadena, CA. This image and other images and data received from Galileo are posted on the World Wide Web, on the Galileo mission home page at http://galileo.jpl.nasa.gov [ http://galileo.jpl.nasa.gov ]. Background information and educational context for the images can be found at http://galileo.jpl.nasa.gov/gallery/io.cfm [ http://galileo.jpl.nasa.gov/gallery/io.cfm ].
Europa Impact Crater
PIA02561
Jupiter
Near Infrared Mapping Spectr …
Title Europa Impact Crater
Original Caption Released with Image A newly discovered, 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 80 kilometers (50 miles), making it comparable in size to the largest cities on Earth. The area within the outer boundary of the continuous bright ring is about 5,000 square kilometers (nearly 2,000 square miles). The diameter of the darker area within the bright ring is about 29 kilometers (18 miles), 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 20 kilometers (12 miles) 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. Galileo has been orbiting Jupiter and its moons since December 1995. Its primary mission ended in December 1997, and after that Galileo successfully completed a two-year extended mission. The spacecraft is in the midst of yet another extended journey called the Galileo Millennium Mission. More information about the Galileo mission is available at:http://galileo.jpl.nasa.gov [ http://galileo.jpl.nasa.gov ]JPL manages Galileo for NASA's Office of Space Science, Washington, D.C. JPL is a division of the California Institute of Technology in Pasadena.
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