Lunar and Planetary Science XXX 1818.pdf

AGES OF INDIVIDUAL CRATERS ON THE GALILEAN SATELLITES GANYMEDE AND CALLISTO. R. Wagner1, U. Wolf1, G. Neukum1, and the Galileo SSI Team. 1DLR, Institute of Planetary Exploration, Berlin, Germany. E-mail: [email protected]

Introduction: Craters, palimpsests and multi-ringed used for modeling crater ages on Galilean satellite surfaces. basins are important stratigraphic markers in establishing Model I, as discussed by [3], assumes a lunar-like, prefer- sequences of geologic events by crosscutting relationships. entially asteroidal bombardment of the jovian satellites, Impact structure forms, hence the style of crustal response with a period of heavy bombardment which ended about towards impact deformation, also may have changed 3.8 billion years (b.y.) ago - forming the youngest large through geologic time. Measurements of crater distribu- multi-ring basins, such as Gilgamesh on Ganymede - and a tions mapped in these impact structures and their sur- more or less constant cratering rate since about 3.3 b.y. rounding terrain provide a valuable tool in order to estab- Cratering Model I ages are derived by solving equation (1) lish an age sequence. From the application of cratering for exposition time t (in b.y.). Coefficients A, B and C for chronology models, absolute ages for these impact struc- all three icy Galilean are given in table 1. Age uncertainties tures can be derived also. In this paper we present ages for are on the order of 0.05 b.y. for surfaces older than 3.3 b.y. impact events which created craters and impact structures but may amount to 0.5 b.y. for surfaces younger than 3.3 on Ganymede and Callisto, based on two impact chronol- b.y. ogy models for the jovian system. Ages of impact features . (B.t) . on Europa are treated in [1]. Ncum(D•1km) = A (exp - 1) + C t (1)

Procedure: Geologic mapping and measurements of crater distributions were carried out on Galileo SSI high- Coeff. Europa Ganymede Callisto resolution images with spatial resolutions of less than 100 A 2.173.10-14 1.054.10-14 5.379.10-15 m/pxl, but for some craters and palimpsests Voyager im- B 6.93 6.93 6.93 ages with only 0.9 to 1.5 km/pxl resolution were available. C 3.347.10-4 1.625.10-4 8.28.10-5 Craters were subdivided into geologic units such as (1) continuous ejecta materials, (2) crater floor materials and Table 1: Coefficients used for the lunar-like cratering (3) pit materials. An impact event is reliably given by the chronology (Model I [3]) in equation (1). crater frequencies measured on the ejecta blanket, but cra- ter frequencies on crater floors or pit materials may also be used to date the event unless longer-term resurfacing or Model II, as discussed by [4], assumes impacts prefer- modification subsequent to impact has been going on. For entially by members of the Jupiter-family comet population impact structures such as palimpsests we used the geologic and extrapolates current impact rates of these bodies back- ward in time. For a measured cumulative frequency N for subdivision into (1) interior smooth plains, (2) disorgan- cum ized massif facies, (3) concentric massif facies, and (4) craters •10 km in diameter, and a crater production rate outer deposits, as given by [2]. Uncertainties in measured dC/dt for each satellite, the exposition time t (in b.y.) ac- crater frequencies are on the order of 10-20%. Some craters cording to [4] is given by: do not appear to contain superimposed craters at all. In t’ = [N / (dC/dt)] / 109 (2) these cases, crater frequencies can only be estimated. This cum is done as follows: (a) the area of the crater floor or of the and ejecta deposits is measured; (b) for a given spatial image t = t (1 - exp (-t’/t )) (3) resolution S in km/pxl, the smallest recognizable crater o o should have a diameter of about 3.S km; (c) we assume at with t’, t given in years [yr] and t = 4.56 billion years. least one crater to be present with a diameter just below o D=3.S which is the next lower bin diameter since measured Equation (3) is used to correct secular variations in impact . -14 . - craters are always sorted into bins [3]. Images may be rate with time [4]. The values of dC/dt are 2.5 10 , 5.4 10 14 . -13 -2 -1 zoomed up to a factor of 2 or 3 for this estimation in order and 1.0 10 [km yr ] for Callisto, Ganymede and Eu- to simulate the view in the high-resolution stereo com- ropa respectively. Model II has uncertainties in crater pro- parator which is used for our crater counts [3]. The esti- duction rate of a factor of 5 [4], but a smaller uncertainty of mated age obtained by this procedure gives a maximum age a factor of 3 cannot be ruled out [5]. Two features of Model for the impact event since the craters superimposed on II are noteworthy: (1) the shape of the crater distributions is described by a simple power law N ~ D-2.2 [4] [6], ejecta blanket or floor which could be recognized on cum higher-resolution images may still be smaller than the whereas our measurements show that crater distributions threshold diameter chosen by the image resolution limit. measured on Galilean satellite surfaces follow a complex function, as do lunar ones [3]. Thus, Model II ages derived Cratering models in the jovian system: Currently, from measurements using a simple power law may be up to two subtantially different cratering chronologies may be a factor of 2 higher than the values shown in table 2. (2) Lunar and Planetary Science XXX 1818.pdf

AGES OF INDIVIDUAL CRATERS ON THE GALILEAN SATELLITES R. Wagner et al

Apex-antapex asymmetries for crater frequencies have been distributions could be measured so far are listed in table 3. inferred by [3] and [6]. Such asymmetries, however, could Model I ages for the stratigraphically younger multi-ring not be confirmed in our counts [3] [7]. basins and large impact features such as Valhalla or Lofn of about 4.0 b.y. and 3.9 b.y. resp. are more consistent with a Ganymede: Ages for craters on palimpsests are shown lunar-like bombardment history and a formation of large in table 2. Bright ray craters such as Osiris or Achelous are multi-ring structures that ended about 3.8 b.y. ago. How- relatively young features in Model I, compared to ever it can not be ruled out that at least Lofn was created in Ganymed’s bright and dark terrains (3.6 and 4.2 b.y. resp. more recent times, due to the following reason: The crater according to Model I [3]). The higher age of the presuma- frequency of Lofn’s smooth central plains is very low and bly young bright ray crater Tros results from using Voyager contains only one bright-rimmed crater, maybe 3 or 4 more data rather limited in resolution. In Model II, these bright close to the resolution limit. This larger crater could be a ray craters could be recent features. Palimpsests range from secondary crater from bright young craters superimposed about 4.1 b.y. to 3.8 b.y. in Model I, but may be as young on the (still unnamed) impact structure next to Lofn which as only 0.6 b.y. (Buto Facula) in Model II. Ages of the is seen on low-resolution, highly-foreshortened Voyager younger palimpsests are comparable to those of young images only. Thus, Lofn could postdate the period of Late bright sulci in both chronology models. According to Heavy Bombardment (Model I age: < 3.4 b.y.; Modell II Model I, tectonic activity on Ganymede should have ceased age: < 0.22 b.y. / 0.98 - 0.04). Young bright craters on Cal- about 3.6 b.y. ago (youngest bright sulci [3]), with listo have not been measured yet - mostly due to insuffi- Ganymede’s surface having been modified only by impacts cient medium- and high-resolution image coverage - but ever since, whereas in Model II tectonic deformation as should be present as they are on Ganymede and Europa [1]. well as palimpsest formation could have been going on Galileo’s orbit C20 provides an opportunity to obtain high- until only 500 Million years ago [4]. resolution images of at least one bright ray crater - Bran (ca. 100 km diameter) - during the entire Galileo mission. Impact feature Model I ages Model II ages / (b.y.) range Impact feature Model I ages Modell II ages / range (b.y.) (b.y.) (b.y.) Craters: Craters: OsirisE,V 0.26 0.007 / 0.03 - 0.001 Tindr 3.87 1.35 / 3.77 - 0.31 AchelousE 0.41 0.01 / 0.05 - 0.002 Jalkr 3.96 2.09 / 4.35 - 0.53 TrosE,V 2.89 0.08 / 0.37 - 0.02 G2 palimpsestU 3.98 2.31 / 4.42 - 0.60 Gula 3.70 0.37 / 1.57 - 0.08 Har 4.07 3.32 / 4.55 - 1.04 Neith 3.91 0.99 / 3.23 - 0.22 Basins (MRB): Palimpsests: Lofn 3.88 1.39 / 3.83 - 0.32 Buto Facula 3.82 0.59 / 2.29 - 0.12 Valhalla 3.98 2.31 / 4.45 - 0.60 Zakar 3.83 0.61 / 2.34 - 0.13 Asgard 4.19 4.32 / 4.56 - 2.02 Memphis Facula 4.02 1.80 / 4.19 - 0.44 V Edfu Facula 4.12 2.86 / 4.53 - 0.82 Table 3: Absolute ages for craters, palimpsests and the V Siwah Facula 4.22 3.88 / 4.56 - 1.45 major multi-ring structures (MRB) on Callisto. Ages for Basins (MRB): MRB are from [8]. Measurements on a so far unnamed V Gilgamesh 3.80 0.54 / 2.13 - 0.11 palimpsestU (imaged during G2; lat. = 4° N / long. 282° W) and on dome crater Jalkr (imaged during G8; lat. = 38°S / Table 2: Absolute ages for craters and impact features long. = 83° W) are subject to poor statistics (small areas on Ganymede. For Model II, maximum and minimum ages and/or small numbers of craters superimposed). are given; Ebased on estimated crater frequency, as ex- plained in text; Vbased on Voyager images only. References: [1] G. Neukum et al. (1999a), LPSC 30, this volume; [2] K. Jones et al. (1997), LPSC 28, 679-680; Callisto: Average ages of Callisto’s dark, cratered [3] G. Neukum et al. (1999b), in preparation for Icarus; [4] plains are 4.3 to 4.1 b.y. on the average in Model I [3][8]. K. Zahnle et al. (1998), Icarus 136, 202-222; [5] R. Pap- Crater saturation of Callisto’s surface has been discussed palardo and K. Zahnle, personal communication; [6] E. M. [4][9] but is not confirmed by our measurements, at least Shoemaker et al. (1982), in: Satellites of Jupiter (D. Morri- for craters larger than about 10 km in diameter [3][8]. Cra- son ed.), Univ. of Arizona Press, Tucson, 277-339; [7] R. ter frequency on Callisto, for the diameter range 10 to 100 Wagner and G. Neukum (1994), Icy Galilean Satellites km, is even a factor of 3-4 lower than for the lunar high- Conf., San Juan Capistrano, abstract booklet p. 90-91; [8] lands [10]. Average ages of Callisto’s dark terrains are R. Wagner et al. (1998), LPSC 29, abstract 1918; [9] C. R. rather high in Model II, about 4.56 b.y. to 4.2 b.y. (range Chapman et al. (1998), LPSC 29, abstr. 1927; [10] G. Neu- 4.56 - 1.4 b.y.). Absolute ages for some of Callisto’s im- kum et al. (1998), LPSC 29, abstr. 1742. pact features whose superimposed crater size-frequency