This is the 1st attempt to develop a complete database of craters on the large icy Galilean satellites, Ganymede and Callisto (no craters have been found on Io, but sparsely cratered Europa may be added later). Ganymede and Callisto possess the greatest spectrum of unusual or unique crater morphology types in the solar system. At least 4 crater types are found nowhere else in the solar system. This data base is primarily the result of crater studies completed between 1989 and the present by Paul Schenk, while at the Lunar & Planetary Institute (email@example.com) (and also while at JPL and Washington University). Ejecta measurements were compiled by the author with the assistance of Frankie Ridolfi. Some of the pedestal measurements of Horner and Greeley (1982) are also included. In many cases, however, ejecta deposits have not been measured due to poor lighting or resolution. In some craters, namely palimpsests and some anomalous dome craters, a crater rim could not be identified with confidence. Hence, only a predicted diameter is given, based on studies that relate ejecta deposit dimensions to crater diameter.
A NEW CRATER CLASSIFICATION SCHEME FOR GANYMEDE AND CALLISTO
The database also presents a revised morphological classification system for craters on Ganymede. Lighting conditions and resolution varied considerably across the surface of Ganymede at the time of the Voyager encounters in 1979. This can result in the misidentification of some landforms viewed under high solar illumination. Every effort was made to construct a crater classification scheme that incorporated these variations. As a result, a number of craters (e.g., ZN23:181, Ombus Facula, and Punt Facula) have been reclassified from earlier designations. For example, the large basin at 23 N, 181 W, is frequently designated a palimpsest due to the high sun at the time of exposure (which washed out the complex internal structures). But, this complex feature has all the attributes of a younger penepalimpsest, as defined below, and is not palimpsest.
APPROPRIATE REFERENCES FOR CRATER MORPHOMETRY
-Passey, Q. and G. Shoemaker (1982). Craters and Basins on Ganymede and
Callisto, in Satellites of Jupiter, Univ. Arizona Press, Tucson.
-Schenk, P. (1991). Ganymede and Callisto: Complex craters and planetary crusts, J. Geophys. Res., 96, 15635-15664.
-Horner, V. and R. Greeley (1982). Pedestal craters on Ganymede, Icarus, 51, 549-562.
-Schenk, P. and W. McKinnon (1985). Dark halo craters and the thickness of grooved terrain on Ganymede, J. Geophys. Res. (suppl.), 90, C775-C783.
-Schenk, P. and W. McKinnon (1991). Dark ray and dark floor craters on Ganymede, and the provenance of large impactors in the Jovian system, Icarus, 89, 318-346.
-Moore, J. and M. Malin (1988). Dome craters on Ganymede, Geophys. Res. Letters, 15, 225-228.
-Schenk, P. (1993). Central pit and dome craters: Exposing the interiors of Ganymede and Callisto, J. Geophys. Res., 98, 7475-7498.
-Thomas, P. and S. Squyres (1990). Formation of crater palimpsests on Ganymede, J. Geophys. Res., 95, 19161-19174.
-Schenk, P. (1996). Origin of palimpsests on Ganymede, Submitted to Geophys. Res. Lett.
-McKinnon, W. and H. Melosh (1980). Evolution of planetary lithospheres: Evidence from multi-ringed basins on Ganymede and Callisto, Icarus, 44, 454-471.
-Schenk, P. and W. McKinnon (1987). Ring geometry on Ganymede and Callisto, Icarus, 72, 209-234.
To reference this database, refer to one or more of the above works by Schenk (those efforts has been incorporated in this database, with revisions and new craters). Most of this work and conclusions therein will be used in a planned review article on Ganymede crater morphology (Schenk, P., in preparation (or personal communication), 1996).
The databased should be regarded as ~95% complete, except for simple craters, flat floor craters, and central peak craters, which are not included because of their large numbers. Also, the listings for central pit craters is only ~80% complete. One representative dark ray crater and one dark floor crater on Ganymede (extremely rare on other planetary bodies) are also shown but these craters are not yet represented fully in the data base. Also, approximately 30% of the surfaces of both Ganymede and Callisto were imaged by Voyager with resolution insufficient to map craters. These areas may be sampled (at poor resolution) by Galileo.
In addition to data summaries, each crater subdirectory contains images of most large craters on Ganymede and Callisto, except for some central dome and most central pit craters. Images are GIF rasterfiles named latlon.gif (e.g., 11.0n031.0.gif is the image for the crater Tros, located at 11 N, 31 W.). Allimages are at 1 km/pixel resolution.
The database is downloadable as a tab-delimited text. For the text file, each column is separated by tabs and each row is separated by a new paragraph marker. The accompanying document (gcreadme.html) you are now reading gives details on the entries in the database, and can be downloaded as a seperate text.
DATA ENTRIES IN DATABASE
Below are summarized measurements made for the craters in the database. All distance and location measurements were made from digital Voyager images at various resolution, and are referenced to the USGS Quadrangle Maps of the Jovian Satellites. Errors vary from image to image, but typically are on the order of 1-3 kilometers. All measurements are by the author, except where indicated.
In the Database:
" " - (blank value) measurements not yet obtained or not measureable at Voyager resolution
? - feature in shadow or otherwise obscured from view
N/A - feature not present in this crater type
Given names are official IAU names. For other craters the informal convention is to use the crater's latitude and longitude, i.e., ZS47:132 (located at -47 latitude, 132 longitude).
Note that as a result of study and investigation into the effects of variable solar illumination across Ganymede that a number of craters (e.g., ZN23:181. Ombus Facula, and Punt Facula) have been reclassified here from earlier classifications.
(See the CRATER MORPHOLOGY text for full definitions of each type.)
SC - Simple crater [not included in this database]
Pk - Central peak crater [not included in this database]
FF - Flat floor craters [not included in this database]
CP - Central pit crater
CD - Central dome crater
AD - Anomalous dome crater
PP - Penepalimpsest
PA - Palimpsest
MR - Multiring basin
MV - Multiring basin - "Valhalla-class"
UC - Unclassified
DR - Dark ray crater [not yet fully represented in the database]
DF - Dark floor crater [not yet fully represented in the database]
Latitude of crater center (nearest degree). Locations are referenced to the USGS Misc. Investigations Atlas of Jovian Satellites Quadrangle Maps of Ganymede and Callisto.
Longitude of crater center (nearest degree).
CENTRAL DOME/CENTRAL DEPRESSION:
Measured diameter (km) of the central dome (CD and AD craters), or the smooth central depression (PP, PA, and MR craters).
CENTRAL PIT/INNER RING:
Measured diameter (km) of central pit (CP, CD, and AD craters), or the inner ring structure (PP, PA, and MR craters). Pits can be rimmed or have no raised rim.
Measured structural rim-to-rim diameter (km) of the crater. For palimpsests (PA) and a few anomalous dome craters (AD), crater diameter shown is that estimated from the observed extent of the pedestal facies or continuous ejecta deposits. With the exception of Valhalla and Asgard, crater diameters have not yet been determined for multiring basins (MV-type).
CRATER RIM (PREDICTED):
Crater rim diameter (km) predicted based on dimensions of inner "pedestal" ejecta deposits. This parameter is applicable only to crater types in which the true crater rim is uncertain or not observed (eg., palimpsests, penepalimpsests, some anomalous domes, and Valhalla-class ring structures).
Measured diameter (km) of inner pedestal facies of the continuous ejecta deposit. Pedestals are recognized by an outward-facing scarp or a hummocky texture and form a topographic bench adjacent to the crater rim. Data values marked (*) are from Horner and Greeley (1982) and were made prior to extensive revisions to the coordinate system on Ganymede in the mid-1980's.
Measured diameter (km) of continuous ejecta deposit (also equivalent to inner diameter of secondary crater field). Craters with dark halo ejecta blankets (Schenk and McKinnon, 1985) are indicated in "Note".
Measured (or estimated) diameter (km) of outermost concentric ring arcs in multiring systems. In "Valhalla-class" ring systems, ring arcs consist of concentric graben-like or scarp-like structures termed furrows. In Gilgamesh (Ganymede), rings generally have the form of inward-facing scarps or rings of massifs. A single furrow has been identifed outside the crater rim of Gilgamesh.
Terrace Width (km)
Width of largest terrace observed at Voyager resolution. Usually only one significant terrace can be resolved in larger craters. Unresolved terraces may be present in smaller craters.
Rimwall Width (km)
Width of crater rimwall, measured from rim crest to base of inner rimwall, where the crater floor begins. This width is variable in most craters and the range of measured values is reported here.
Rim Height (km)
Height of rim crest, measured from rim crest to preimpact ground plane or elevation of surrounding terrains.
Measured or estimated depth of crater, as measured from rim crest to crater floor. The floors of many large craters tend to be bowed upward in the center. Depth measurements reported here are maximum depths.
Method for estimating crater depth or rim height.
st - stereoscopic
sh - shadow lengths
pc - photoclinometry
Type of terrain on which crater formed. In some cases, a crater floor or its ejecta overlap two terrain types: both are given.
b - relatively young bright terrain (occurs only on Ganymede)
dc - relatively older dark cratered terrain
mv - multiring basin (Valhalla-class) material (ejecta or floor deposits)
cm - crater or basin material (floor or ejecta blanket of larger crater)
pa - palimpsest material
pp - penepalimpsest material
Best available Voyager image (FDS). (Note: some smaller craters are not yet represented by an image in this version.)
Resolution (km/pixel) of best available Voyager image.
Note indicating presence of unusual features, characteristics, viewing conditions, or future observation plans.