Lunar and Planetary Science XXIX 1918.pdf TIME-STRATIGRAPHY AND CRATER RETENTION AGES OF GEOLOGIC UNITS ON CALLISTO. R. Wagner1, U. Wolf1, G. Neukum1, J. E. Klemaszewski2, R. Greeley2, and the Galileo Imaging Team. 1DLR, Institute of Planetary Exploration, D-12489 Berlin, Germany; 2Dept. of Geology, Arizona State University, Tempe AZ 85287, USA. E-mail: Ro- [email protected] Introduction: examined in detail due to limited image resolution [1,8]. During its two years in orbit around Jupiter, the Galileo SSI Schenk [8] mapped areas of bright material, especially in the camera has nearly completed global image coverage of the high latitudes, whose albedo is about a factor of 2 higher than second-largest icy Galilean satellite Callisto at a regional the average albedo of the dark cratered plains. These areas scale of 1 to 2 km/pxl. In addition, selected regions were were covered by either fore-shortened or smeared Voyager imaged at considerably higher resolution down to 30 m/pxl. images, hence are less suitable for crater counts. These re- In this paper, we present a time-stratigraphy of global, re- gions could not be imaged by the Galileo SSI camera. gional and local geologic units on this satellite based on Ages of multi-ring structures. photogeologic mapping and crater size-frequency measure- Large multi-ring basins are the dominant structures on ments in selected regions. Callisto and may be used as time markers in Callisto’s his- Methodology: tory. Many SSI observations ranging in scale from 1 km/pxl Albedo and morphology are used as basic criteria to dis- to 30 m/pxl were targeted to these impact basins. The least criminate geologic units on SSI and Voyager data. Photo- degraded ones are Valhalla (diameter ca. 4000 km), Asgard geologic mapping is carried out on map-projected images. To (diameter ca. 1600 km) and the bright, large crater Lofn easily take into account scale variations, only conformal which also may show ring structures [9]. Furthermore, de- projections are used. Craters are measured on transparencies tailed mapping reveals a number of degraded old multi-ring with a high-resolution Zeiss PS2K stereocomparator which structures, covered by younger dark material [10]. Crater has a maximum error of 5 µm. Since the smallest still meas- size-frequency distributions measured in the central bright urable crater diameters are only 200 µm, the uncertainty in plains and, in some places, in the trough zones of the best diameter measured is less than 2.5%. preserved basins are used to establish a relative time- Earlier measurements of crater size-frequency distribu- stratigraphic sequence of major impact events. Table 1 shows tions on Voyager images showed that the shape of the distri- that Lofn is the youngest of these features, preceded by the butions on Callisto and also on Ganymede are lunar-like, and Valhalla event. Asgard is the oldest among the well- therefore can be fitted by the lunar production function, ac- preserved basins. No measurements were carried out on the counting for different crater scaling on the two satellites degraded basin Adlinda since there is no clear central bright [1,2,3,4]. The derivation of absolute ages on the Galilean area and the spatial resolution in the troughs is not sufficient satellites is highly model-dependent and hence controversial. to determine size-frequency distributions. The cumulative In one of the two models [3,4] (Model I, which is used in this distributions of the three basins are shown in figure 1. If cra- paper), it is assumed that the cratering rate in the Jovian sys- tering chronology model I [3,4] is applied, absolute ages of tem was similar to the inner solar system, i.e. there was a 3.9 b.y. for Lofn, 4.0 b.y. for Valhalla and 4.2 b.y. for Asgard period of heavy bombardment with an exponentional de- are obtained. crease in cratering rate early in the history of the solar sys- Resurfacing. tem. Since then, the cratering rate has become nearly con- No clear evidence was found whether Callisto ever was en- stant [2,3,4]. A second model (Model II), derived by Shoe- dogenically active during its history. Two types of terrain maker and Wolfe [5] and Shoemaker [6], assumes a nearly may be attributed to volcanism: (1) Irregular dark patches constant cratering rate of preferentially comets. several tens of kilometers in length. They were interpreted by Global and regional units: Schenk [8] as extruded material caused by late-stage dark On global and regional scale, Callisto’s surface may be volcanism. Higher resolution SSI images scheduled for orbit mapped into a relatively small number of geologic units, C20 of the Galileo Europa Mission (GEM) should help to primarily dark cratered plains (dcpn) characterized by bright- investigate these dark patches in detail; (2) larger areas, hun- ness differences and differences in crater frequency [1,7,8]. dreds of kilometers in spatial extent and in brightness some- Generally, a darker, less densely cratered variety of plains what between the average dark cratered plains and the high- may be distinguished from a brighter one with higher fre- latitude bright materials, were observed in a small number of quency. The brighter appearance is caused by overlapping, locations and, at Voyager resolution, appear smooth and brighter ejecta but darker intercrater plains are still recogniz- depleted in craters < 30 km in diameter. They were called able [1]. Also, overlapping ejecta tend to subdue or bury smooth plains by different investigators [1,7] and interpreted geologic contacts between these varieties of cratered plains to be volcanically resurfaced [1,7,11]. One of these areas was [1,7]. Crater frequencies of different areas of cratered plains imaged by the SSI camera which revealed a surface that is only differ with factors of about 2 in cumulative frequency. anything but smooth. This target area is currently being in- Table 1 shows relative crater retention ages - cumulative vestigated [10]. densities equal to or larger than diameters of 1 km and 10 km Callisto at high resolution: - of unit dcpn. Embayment of craters by smooth, dark mate- The Galileo SSI camera obtained images of Callisto with rial may be detected on Voyager images but could not be resolutions down to 30 m/pxl, first targeted to the large Lunar and Planetary Science XXIX 1918.pdf TIME-STRATIGRAPHY ON CALLISTO: R. Wagner et al. multi-ring basin Valhalla. Two major surface features were found: (1) Bright material, in crater rims, scarps and ridges linked to the formation and modification of the Valhalla multi-ring basin [12,13], and (2) dark, smooth material [12,13]. As further SSI observations of regions outside Val- halla showed, this dark material may be global in extent [13]. Bright crater walls and scarps show various states of degra- dation caused by e.g. mass wasting or sublimation [13, and references therein]. Measurements of crater size-frequency distributions show that, unexpected from the high density at large crater diameters, the dark smooth material is depleted in craters smaller than about 1 km [12,13] and that the distri- bution is flatter. Although a depletion in smaller projectiles cannot be ruled out [14], we favor erosion and deposition to be the cause for the flat distributions. Two reasons may ac- count for this observation: (1) Small craters whose rims are barely visible in the dark mantling material are found and therefore were part of an original crater distribution; (2) in some areas, cumulative distributions follow a steep produc- tion function - similar to what was measured on Voyager - at different crater frequencies and bend over to flatter slopes at different diameters but fall on the same flat equilibrium dis- tribution at small diameters (figure 2). Sequence of events: According to geologic mapping and crater size-frequency Figure 1: Cumulative size-frequency distributions of measurements, the following sequence of events on Callisto three major impact basins on Callisto: Lofn (diamonds), Val- may be derived: halla (quadrangles), Asgard (hexagons) (curve is lunar pro- (1) Heavy bombardment during which numerous degraded duction function empirically adjusted to Callisto scaling con- ditions [3,4]). basins mostly covered by younger dark material were formed; (2) formation of the oldest dark cratered plains; (3) formation of the Asgard basin; (4) continuing formation of dark cratered plains by heavy bombardment and ero- sion/deposition of the dark material; possible volcanic activ- ity in a small number of locations; (5) formation of the Val- halla basin; (6) heavy bombardment ceases about 3.8 b.y. according to model I. Geologic unit N (D=1 km) N (D=10 km) --------------------------------------------------------------------- dcp3 1.98 . 10-2 2.87 . 10-4 dcp2 2.22 . 10-2 3.22 . 10-4 dcp1 3.75 . 10-2 5.44 . 10-4 --------------------------------------------------------------------- Lofn 2.67 . 10-3 3.88 . 10-5 Valhalla 4.96 . 10-3 7.21 . 10-5 Asgard 2.65 . 10-2 3.85 . 10-4 --------------------------------------------------------------------- Table 1: Crater retention ages of major geologic units and major impact basins on Callisto References: [1] R. Wagner and G. Neukum, Bull. Astron. Soc. Am., 26, 1162, 1994. [2] G. Neukum and B. A. Ivanov, in: Hazards due to comets and aster- oids (ed. T. Gehrels), Univ. of Arizona Press, Tucson, 359-416, 1994. [3] G. Neukum et al., in preparation for Icarus, 1998. [4] G. Neukum, et al., LPSC 29, 1998. [5] E. M. Shoemaker und R. F. Wolfe, in: Satellites of Jupiter (ed. D. Morrison), 277-339, 1982. [6] E. M. Shoemaker, Europa Ocean Confer- ence, San Juan Capistrano , 65-66, 1996. [7] K. Bender et al., U.S. Geol. Surv. Misc.