Surface Ages of the Galilean Satellites of Jupiter: the Case of Ganymede

Introduction: The icy Galilean satellites of Jupiter, Europa, Ganymede and Callisto will be the targets of intense study by orbiting spacecraft in the near future: NASA’s Europa Clipper (planned insertion into Jupiter orbit on April 11, 2030 [1]), and ESA’s JUICE (Jupiter orbit and Ganymede orbit insertion 2031 and 2034, respectively [2]). Examining the crater distributions on a planetary body is a general method to infer its geologic evolution. The sparsely cratered, tectonically intensely deformed and comparably young surface of Europa [3] provides a strong contrast to the surfaces of Ganymede [4][5] and Callisto [6] with vast expanses of dark, densely cratered plains most likely dating back to a distant past. Since no surface material from these bodies is available todate, absolute surface ages can only be derived from impact chronology models. In this study we focus on Ganymede featuring dark, densely cratered plains and bright, less cratered tectonized units, using various impact chronology models, including a recently updated model [7][8][9].

Methodology: For geologic mapping and crater counts in mapped geologic units, we follow the guidelines by [10] and use the crater statistics analysis tool craterstats developed and described in [11][12]. Crater size-frequency distributions (abbreviated as CSFDs) are plotted in three types of diagrams: (a) cumulative, (b) differential, or (c) relative distribution [10]. In this study we prefer the cumulative form, representing the logarithm of the cumulative crater frequency larger than, or equal to, a given crater diameter (N_cum(>D)) plotted against the logarithm of crater diameters in (given in kilometers).  

Potential Impactors: The impact craters on the Galilean satellites may have been formed by the following types of bodies: (1) Main Belt Asteroids (MBAs) [7], (2) Comets [8][13], or (3) planetocentric debris (e.g., [13]). Since CSFDs in the ideal case represent the SFD of the impactors which created the craters, the potential impactor family can be inferred from the shape of measured CSFDs (e.g., [14]). Ideal case means: (a) preexisting craters are not superposed by newly formed craters to reach an equilibrium CSFD (or saturation), or (b) an existing CSFD has not been changed by geologic resurfacing (e.g., [14]). Therefore, similarities in the shape of CSFDs measured on different bodies indicate the same impactor family which bombarded these surfaces, while different shapes of CSFDs on different surfaces imply a different projectile family.

Chronology Models and Derived Surface Ages of Ganymede: Impact chronology models are based on the impact rates of members of a potential projectile family. For most planets and satellites in the outer Solar System, the consensus is that cometary bodies from the Kuiper Belt, specifically short-period ecliptic or Jupiter-family comets (ECs or JFCs) are the dominant source of impactors [8][9]. Nearly isotropic or long-period comets (NICs) from the Oort cloud and MBAs are practically negligible, the latter at least at present and in the recent past [8]. Several groups of investigators found CSFDs measured on the Galilean satellites to be distinct from those found on inner Solar System bodies, confirming mainly ECs (JFCs) from the Kuiper Belt (e.g., [8]). An alternative view was presented by G. Neukum and colleagues [7] who found strong similarities between CSFDs on the Galilean satellites and the Earth’s moon instead and concluded MBAs as major impactor source, hence a family of collisionally evolved impactors. This view has been intensely debated and put into doubt (e.g., [13]). However, W. Bottke and colleagues [15] recently discussed a collisional evolution of Kuiper Belt Objects, which supports similarities between the shape of CSFDs from impacts of ECs as well as of MBAs, with a different impactor source, however [15] (see also discussion in [16]). Based on EC (or JFC) impacts, a  Jupiter-family comet chronology model (JCM) was derived by K. Zahnle and colleagues [8] with a more or less constant cratering rate back to ~4 Gyr. In this model chronology, Ganymede’s dark cratered plains (as well as those on Callisto) are older than ~4 Gyr (e.g., [4]). A comprehensive study of the stratigraphy and ages of the tectonically deformed light units on Ganymede by N. R. Baby and colleagues [16] showed that the light terrain unit ages range from ~0.7 Gyr to 4 Gyr and higher, indicating a tectonically active Ganymede for most of its geologic history. The chronology model by G. Neukum and colleagues [7] based on mainly MBA impacts and a lunar-like model chronology (lunar-derived model, LDM) provides a completely different view of Ganymede’s history: the formation of the light units and the tectonically active period was comparably short, on the order of 3.6 – 4 Gyr [16]. Based on the preferential impacts of JFCs, updated impact probabilities for the Galilean satellites (and other outer Solar System bodies) and their time dependence were recently derived by D. Nesvorný and colleagues [9]. In this study, we will discuss how these findings applied to existing CSFD measurements (e.g., [16]) will affect the interpretation of the tectonic evolution of Ganymede.

Summary and Outlook: All existing impact chronology models for bodies in the outer Solar System have high degrees of uncertainty, especially for the first 1-2 Gyr of their geologic histories, and also for the cratering rates (e.g., [8]). Improvements in modeling collisional evolution of impactor families and improving the census of the potential impactor CSFDs by astronomical observations are necessary until Europa Clipper and JUICE reach the Jovian system. The surface age issue, however, can only be solved in the future by a lander capable of carrying out radiometric measurements of surface materials.