____________________________________________________________________________ CENTRAL PIT CRATERS (CP) Central pit craters are complex craters that have been substantially modified during impact by rim and floor collapse. These craters have raised rims, inward-facing rimwall slopes (ocassionally terraces can be resolved), depressed flat crater floors, and an uplifted central complex. In general, these craters resemble lunar craters, except that the central peak complex found in lunar craters is replaced by a rimmed pit averaging a few to tens of kilometers in diameter. Ejecta and secopndaries are observed. Pedestal deposits, which form an elevated topographic bench adjacent to the rim, are also common. Central pit craters resemble some pit craters observed on Mars. The listing of central pit craters in this database is ~90% complete. Central pit craters range from ~35 km to ~60 km in diameter. STRUCTURE PRESENT ---------------------------- central dome N central pit Y flat floor Y ---------------------------- crater rim Y outer rings N ---------------------------- pedestal facies ejecta Y continuous ejecta Y secondaries Y ---------------------------- REFERENCES Passey and Shoemaker (1982). Craters and Basins on Ganymede and Callisto, in Satellites of Jupiter, Univ. Arizona Press, Tucson. Schenk (1993). Central pit and dome craters: Exposing the interiors of Ganymede and Callisto, J. Geophys. Res., 98, 7475-7498. ____________________________________________________________________________ CENTRAL DOME CRATERS (CD) Central dome craters have been recognized only on Ganymede and Callisto (they may also occur on Titan and Pluto, neither of which have been imaged at suitable resolution). These craters are complex craters that have been substantially modified during impact by rim and floor collapse, resulting in the formation of raised rims, inward-facing rimwall slopes (ocassionally terraces can be resolved), depressed flat crater floors, and an uplifted central complex. The central complex is comprised of a rimmed pit partially filled by a smooth relatively bright dome 10's of kilometers wide. There may be some uncertainty in the proper classification of a few craters in the 50 to 70 km diameter range (i.e., some domes may go undetected in "central pit craters") due to shadows or poor resolution. Central dome craters range from ~60 km to ~180 km in diameter. STRUCTURE PRESENT ---------------------------- central dome Y central pit Y flat floor Y ---------------------------- crater rim Y outer rings N ---------------------------- pedestal facies ejecta Y continuous ejecta Y secondaries Y ---------------------------- REFERENCES Moore and Malin (1988). Dome craters on Ganymede, Geophys. Res. Letters, 15, 225-228. Schenk (1993) Central pit and dome craters: Exposing the interiors of Ganymede and Callisto, J. Geophys. Res., 98, 7475-7498. ____________________________________________________________________________ ANOMALOUS DOME CRATERS (AD) Anomalous dome craters have been recognized only on Ganymede and Callisto (they may also occur on Titan and Pluto, neither of which have been imaged at suitable resolution). These craters consist of relatively bright circular domes several to a few 10's of kilometers across, surrounded by a ring of irregular or chaotic massifs. In high-sun images, this ring shows as a ring of bright spots. Surrounding this central complex is an annulus of smoothed or mottled material, the outer boundary of which is sometimes marked by a discontinuous circular lineament. Ejecta and secondary craters are recognized beyond this boundary, confirming an impact origin. These craters differ from classical central dome craters in that the rim is virtually absent, aside from the discontinuous circular lineament which has negligable topography. (These craters correspond to the Type II penepalimpsests of Passey and Shoemaker (1982) but are reclassified as anomalous dome craters because of their structural link to central dome craters.) Superposed crater counts suggest that they are somewhat older than central dome craters, but not as old as penepalimpsests or palimpsests (Schenk (1996), also abstract in Lunar Planet. Science Conf. 27, p. 1137). The reason for the lack of classical rim morphology is unknown. STRUCTURE PRESENT ---------------------------- central dome Y central pit Y flat floor Y ---------------------------- crater rim N(?) outer rings N ---------------------------- pedestal facies ejecta ? continuous ejecta Y secondaries Y ---------------------------- REFERENCES Passey and Shoemaker (1982). Craters and Basins on Ganymede and Callisto, in Satellites of Jupiter, Univ. Arizona Press, Tucson. ____________________________________________________________________________ PENEPALIMPSESTS (PP) Penepalimpsests have been recognized only on Ganymede and Callisto (they may also occur on Titan and Pluto). These craters consist of circular high albedo spots up to ~300 kilometers wide. Seven have been recognized on Ganymede. Ejecta deposits and secondary craters confirm an impact origin. Several shallow but prominent concentric scarps and ridges occur within penepalimpsests, although which of these structures is the crater rim is not obvious. The outer edge of penepalimpsests is often marked by an outward-facing scarp. This margin has been interpreted as equivalent to the inner pedestal ejecta facies observed in regular impact craters on Ganymede. I have used this relationship to identifiy the probably rim structure in the database. Superposed crater counts and stratigraphic relationships indicate that all penepalimpsests are roughly similar in age to the bright terrain which covers roughly half of Ganymede, and are younger than palimpsests. Penepalimpsests are probably transitional between the highly flattened palimpsests and younger crater landforms. (Under very high sun conditions, a few penepalimpsests may resemble palimpsests. A few palimpsests in this database may prove to be penepalimpsests.) The high albedo and low topography are consistent with impact into a mantle that was warm and rheologically soft due to higher heat flow in Ganymede's early history. The bright material was probably excavated from depth and exposed on the surface during impact. STRUCTURE PRESENT ---------------------------- central dome ? central pit Y(?) flat floor ? ---------------------------- crater rim Y outer rings N ---------------------------- pedestal facies ejecta Y continuous ejecta Y secondaries Y ---------------------------- REFERENCES Passey and Shoemaker (1982). Craters and Basins on Ganymede and Callisto, in Satellites of Jupiter, Univ. Arizona Press, Tucson. Thomas and Squyres (1990). Formation of crater palimpsests on Ganymede, J. Geophys. Res., 95, 19161-19174. Schenk (1996). Origin of palimpsests on Ganymede, Submitted to Geophys. Res. Lett. ____________________________________________________________________________ PALIMPSESTS (PA) Palimpsests have been recognized only on Ganymede and Callisto (they may also occur on Titan and Pluto). These craters consist of circular high albedo spots up to ~500 kilometers wide. Very little topography is preserved, and no ejecta deposits have as yet been recognized. Some faint discontinuous concentric lineaments, including a central smooth spot, are often observed within palimpsests. These may be remnants of central structures, possibly including a remnant crater rim or central dome/pit complex. Superposed crater counts and stratigraphic relationships indicate that all palimpsests are older than the bright terrain, which covers roughly half of Ganymede. Although no obvious ejecta or secondaries can be recognized, they are believed to be ancient impact scars because of their overall similarity to penepalimpsests, which do retain ejecta deposits. The nominal "crater rim" diameter has been estimated here based on the interpretation that the outer margin represents the inner pedestal ejecta facies of the palimpsest. The high albedo and low topography of palimpsests are consistent with impact into a mantle that was warm and rheologically soft due to higher heat flow in Ganymede's early history. The bright material was probably excavated from depth and exposed on the surface during impact. STRUCTURE PRESENT ---------------------------- central dome ? central pit ? flat floor N(?) ---------------------------- crater rim ? outer rings N ---------------------------- pedestal facies ejecta Y continuous ejecta N secondaries N ---------------------------- REFERENCES Passey and Shoemaker (1982). Craters and Basins on Ganymede and Callisto, in Satellites of Jupiter, Univ. Arizona Press, Tucson. Thomas and Squyres (1990). Formation of crater palimpsests on Ganymede, J. Geophys. Res., 95, 19161-19174. Schenk (1996). Origin of palimpsests on Ganymede, Submitted to Geophys. Res. Lett. ____________________________________________________________________________ MULTIRING BASINS (MR) Only 1 definitive classic lunar-like multiring basin has been recognized on Ganymede (and none on Callisto). This basin, Gilgamesh, consists of at least 5 rings, the third and most prominent of which is the crater rim (based on mapping of ejecta deposits and morphology). This ring has a diameter of ~575 kilometers, over twice as large as the next largest crater recognized on Ganymede (among post-bright terrain craters). the inner ring of irregular massifs surrounds a smooth central dome, which may be equivalent to a central dome or may be volcanic. An single outer ring resembles the furrows observed at Valhalla. In planform, Gilgamesh somewhat resembles the well-preserved multiring lunar basin Orientale. Gilgamesh, however, is much flatter, with an estimated depth of only 1 to 3 kilometers. STRUCTURE PRESENT ---------------------------- central dome Y(?) central pit Y(?) flat floor N ---------------------------- crater rim Y outer rings Y ---------------------------- pedestal facies ejecta ? continuous ejecta Y secondaries Y ---------------------------- REFERENCES Passey and Shoemaker (1982). Craters and Basins on Ganymede and Callisto, in Satellites of Jupiter, Univ. Arizona Press, Tucson. ____________________________________________________________________________ MULTIRING BASINS - "VALHALLA-CLASS" (MV) Valhalla-class multiring basins are unique to Ganymede and Callisto. The type example is the ~4000 kilometer wide Valhalla ring system on Callisto, consisting of a set of roughly two dozen concentric ridges and arcuate rimmed fractures or furrows (extensional graben-like structures). In the cases of Valhalla and Asgard, irregular patches of bright material partially cover the centers of the structure. Several similar systems occur on Ganymede, but because of extensive resurfacing, none are completely preserved and all predate the resurfacing associated with bright terrain. These ring systems consist of several to a few dozen arcuate concentric rimmed fractures, or furrows. The largest of these systems is found within Galileo and Marius Regios, two large areas of preserved dark terrain. No classical crater rims have been identified. In the case of Valhalla, the diameter of the "crater rim" has been estimated based on the extent of ejecta deposits and secondary craters. Large furrow systems are most likely the result of large impacts into a rheologically weak target. Flow of asthenospheric material during basin collapse probably fractured a relatively thin lithosphere. STRUCTURE PRESENT ---------------------------- central dome ? central pit ? flat floor ? ---------------------------- crater rim ? outer rings Y ---------------------------- pedestal facies ejecta ? continuous ejecta Y? secondaries Y ---------------------------- REFERENCES Passey and Shoemaker (1982). Craters and Basins on Ganymede and Callisto, in Satellites of Jupiter, Univ. Arizona Press, Tucson. McKinnon and Melosh (1980). Evolution of planetary lithospheres: Evidence from multi-ringed basins on Ganymede and Callisto, Icarus, 44, 454-471. Schenk and McKinnon (1987). Ring geometry on Ganymede and Callisto, Icarus, 72, 209-234. ____________________________________________________________________________ UNCLASSIFIED IMPACT STRUCTURES (UC) Only one major impact structure on Ganymede cannot be tentatively classified, the so-called "Western Equatorial Basin" (WE Basin). Because of a large crater (Ninki) superposed on the interior of this basin, the presence of a central dome or central pit cannot be confirmed. The basin might be a central dome crater or a penepalimpsest. On Callisto, two large craters were poorly observed and are difficult to classify. The large bright ray crater Adlinda has a large bright spot that may be a crater floor deposit. The rim is probably hidden due to poor resolution. A second large bright spot SE of Adlinda may be similar. No concentric structures are obvious in either crater. Also included in this category are representative examples of dark ray craters and dark floor craters (which may be fully represented at a later date). STRUCTURE PRESENT ---------------------------- central dome ? central pit ? flat floor ? ---------------------------- crater rim Y outer rings ? ---------------------------- pedestal facies ejecta Y(?) continuous ejecta Y secondaries Y ---------------------------- REFERENCES Passey and Shoemaker (1982). Craters and Basins on Ganymede and Callisto, in Satellites of Jupiter, Univ. Arizona Press, Tucson. ____________________________________________________________________________ CENTRAL PEAK CRATERS (Pk) Central peak craters are complex craters that have been substantially modified during impact by rim and floor collapse. These craters have raised rims, inward-facing rimwall slopes (ocassionally terraces can be resolved), depressed flat crater floors, and an uplifted central peak complex. At Voyager resolution, these craters resemble lunar central peak craters, except that they are relatively shallow. Central peak craters range from ~5 km to ~35 km in diameter. Because of the large number of this type crater, they are NOT included in this data base. STRUCTURE PRESENT ---------------------------- central peak Y flat floor Y ---------------------------- crater rim Y outer rings N ---------------------------- pedestal facies ejecta Y continuous ejecta Y secondaries Y? ---------------------------- REFERENCES Passey and Shoemaker (1982). Craters and Basins on Ganymede and Callisto, in Satellites of Jupiter, Univ. Arizona Press, Tucson. Schenk (1991). Ganymede and Callisto: Complex craters and planetary crusts, J. Geophys. Res., 96, 15635-15664. ____________________________________________________________________________ SIMPLE CRATERS (SC) Simple craters occur on Ganymede and Callisto, as they do on the other planets. They are bowl-shaped and lack complex structures. On Ganymede (and Callisto) they range up to ~5 km in diameter, and may be relatively shallow compared to lunar simple craters. Because of the large number of this type crater, they are NOT included in this data base. REFERENCES Passey and Shoemaker (1982). Craters and Basins on Ganymede and Callisto, in Satellites of Jupiter, Univ. Arizona Press, Tucson. Schenk (1991). Ganymede and Callisto: Complex craters and planetary crusts, J. Geophys. Res., 96, 15635-15664. ____________________________________________________________________________