Crater & spectr The top figure is an image of the crater Melkar on Ganymede, at a wavelength of 0.85 microns, taken by the Near Infrared Mapping Spectrometer (NIMS) on the Galileo spacecraft, The crater is illuminated by the Sun from the left. The finest detail that can be seen is approximately 30 km in size. What is most obvious, and of great interest, are the two concentric ring structures and the central dome. The walls of these rings are in shadow on the left, and are in sunlight on the right. To understand how these rings and central dome are thought to form, consider a pebble dropped into a pond. Ripples spread out from the center, oscillating up and down. The rings and dome forming Melkar are a snapshot of these ripples in the ice of Ganymede, possibly caused by the impact of a comet or asteroid. Similar features on the Moon are only associated with much larger craters as the stronger Moon rock behaves this way only with large impacts. NIMS can obtain images at many different wavelengths from 0.7 to 5.2 microns. The spectrum shows the amount of reflected light as a function of wavelength from the crater floor of Melkar. Several distinct absorption features, caused by water ice, are evident at 1.5 and 2.0 microns. Beyond 3.0 microns the intensity increases again as the longer wavelengths are more sensitive to Ganymede's thermal radiation. The shape of the absorption features suggest that the ice is mixed with hydrated minerals. These relatively dark minerals probably cause the variations in ice brightness seen at visible wavelengths. Impact craters The number of impact craters seen in this image of Ganymede testify to the terrain'sgreat age, dating back several billion years. The image's left edge slices through a 19-kilometer-diameter (12-mile) crater. The dark and bright lines running from lower right to upper left and from top to bottom are deep furrows in the ancient crustof dirty water ice. New over old New terrain overlays older terrain, which overlays still older surface, in this view of part of the surface of Jupiter's moon Ganymede. The key characteristics and relationships of the major terrain types on tectonically activeGanymede are seen at a resolution 16 times better than images taken by the Voyager spacecraft in 1979. Ridges and troughs mosaic of four Galileo high-resolution images of the Uruk Sulcus region of Jupiter's moon Ganymede is shown within the context of an image of the region taken by Voyager 2 in 1979, which in turn is shown within the context of a full-disk image of Ganymede.The image shows details of parallel ridges and troughs that are the principal features in the brighter regions of Ganymede. Science objectives Ganymede Objectives 1. Characterize any volcanism
2. Determine the nature and timing
of any tectonic activity
3. Determine the history of formation and degradation
of impact craters
4. Determine the nature of the surface materials Stereo view New topographic detail is seen in a stereoscopic view of this part of Ganymede. The picture is a computer reconstruction from two Galileo images. The topographic nature of the deep furrows (the trough depth is one kilometer) and impact craters that cover this portion of Ganymede is apparent. The blue-sky above the horizon is artificial. Water and minerals Galileo infrared observations are used to examine Ganymede's surface composition.The central false-colorimage shows the distribution of water ice on Ganymede's surface (brighter areas have more ice). This image shows that water ice is relatively depleted in the regions that appear visually dark in the corresponding Voyager mosaic on the left..The texture differences implied by this false color map in three wavelengths may suggest that water migrates from the equator to the poles.
- Left: Voyager's camera.
- Middle: NIMS, showing water ice on the surface. Dark is less water, bright is more.
- Right: NIMS, showing the locations of minerals in red, and the size of ice grains in shades of blue.
Electric field spectrogram This electric field spectrogram shows the very strong interaction between Ganymede and the Jovian magnetosphere. The wealth and diversity of the wave signatures shown here provide evidence of a small magnetosphere surrounding Ganymede. The band of noise labeled fUH is at the upper hybrid resonance frequency and can be used to determine a plasma density of approximately 100 particles per cubic centimeter. The broadband bursts at the beginning and end of the interaction period are typical of the plasma wave signature for a magnetopause, or boundary of a magnetosphere. The banded emissions after closest approach are electron cyclotron harmonic emissions which are known at Earth to contribute to the generation of the aurora. The bright, broadband emission centered on closest approach and the emissions identified as "chorus" in the spectrogram are called whistler-mode emissions. The maximum frequency of these emissions enable the determination of a maximum in the Ganymede magnetic field traversed by Galileo of about 400 nanoTesla. The narrowband radio emissions extending primary to the right of the Ganymede interaction in the spectrogram are the first known radio emissions from a planetary satellite; these are similar to radio emissions studied at Earth and the outer planets, including Jupiter. Geological mystery This image is centered on an unusual semicircular structure about 33 kilometers (20 miles) across. A 38 kilometer (24 mile) long, remarkably linear feature cuts across its northern extent, and a wide east-west fault system marks its southern boundary. The origin of these features is the subject of much debate among scientists analyzing the data. Was the arcuate structure part of a larger feature? Is the straight lineament the result of internal or external processes? Scientists continue to study this data in order to understand the surface processes occuring on this complex satellite. Interior The cut-out reveals the interior structure of this icy moon. This structure consists of four layers based on measurements of Ganymede's gravity field and theoretical analyses using Ganymede's known mass, size and density. Ganymede's surface is rich in water ice and Voyager and Galileo images show features which are evidence of geological and tectonic disruption of the surface in the past. As with the Earth, these geological features reflect forces and processes deep within Ganymede's interior. Based on geochemical and geophysical models, scientists expected Ganymede's interior to either consist of: a) an undifferentiated mixture of rock and ice or b) a differentiated structure with a large lunar sized 'core' of rock and possibly iron overlain by a deep layer of warm soft ice capped by a thin cold rigid ice crust. Galileo's measurement of Ganymede's gravity field during its first and second encounters with the huge moon have basically confirmed the differentiated model and allowed scientists to estimate the size of these layers more accurately. In addition the data strongly suggest that a dense metallic core exists at the center of the rock core. This metallic core suggests a greater degree of heating at sometime in Ganymede's past than had been proposed before and may be the source of Ganymede's magnetic field discovered by Galileo's space physics experiments. Dark floor craters The dark-floored crater, Khensu, is the target of this image of Ganymede. The solid state imaging camera on NASA's Galileo spacecraft imaged this region as it passed Ganymede during its second orbit through the Jovian system. Khensu is located at 2 degrees latitude and 153 degrees longitude in a region of bright terrain known as Uruk Sulcus, and is about 13 kilometers (8 miles) in diameter. Like some other craters on Ganymede, it possesses an unusually dark floor and a bright ejecta blanket. The dark component may be residual material from the im pactor that formed the crater.Another possibility is that the impactor may have punched through the bright surface to reveal a dark layer beneath. Another large crater named El is partly visible in the top- right corner of the image. This crater is 54 kilometers (34 miles) in diameter and has a small 'pit' in its center. Craters with such a 'central pit' are common across Ganymede and are especially intriguing since they may reveal secrets about the structure of the satellite's shallow subsurface. |