OPS3
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Discussion on Slack
Orals: Mon, 19 Sep | Room Andalucia 2
Introduction: Ganymede´s surface can be subdivided into dark and light terrain [1]. The dark terrain covers 35% of the surface, is composed of relatively low albedo material and high crater density. The 65% light terrain is characterized by higher albedo and a lower crater density indicating a relatively younger age [2]. Omnipresent morphotectonic structures such as grooves within the light terrain suggests a more recent activity of this terrain with respect to the dark terrain. The assemblage of lineated grooves has been interpreted as extensional zones similar to the Earth continental rifts, resulting from a global expansion that affected the satellite crust in the past [3]. Figure 1a and b show the spatial distribution of Ganymede’s dark terrain.
Figure 1: Global Mosaic and terrain distribution. (a): Global mosaic of the surface of Ganymede by images of Voyager 1+2 and Galileo Solid State Imagery (SSI) (Becker et al., 2001; Schenk, 2010). (b): Simplified geological map displaying the distribution of the bright and dark terrains as well as impact craters. Data from [1].
Objectives: Neighboring dark terrains (see Fig. 1a and b) often show obvious fitting pattern with respect to their external shape, suggesting that the areas have drifted apart by extension. We intend to reconstruct magnitude and orientation of the finite extension between adjacent dark terrains by restoring their original shape. The kinematic restoration allows to better understand the global tectonic movement pattern and may ultimately help to address the issue whether the individual dark terrains once formed a single terrain covering the entire, smaller moon surface.
Methods: We used the global Ganymede image mosaic that was compiled by using a combination of Voyager 1 and 2 and Galileo images. [4-6]. The mosaic from the USGS can be found at the Annex of the PDS Cartography & Imaging Sciences Node (SIM 3237). Using ESRI's ArcMap 10.7 software polygons of all dark terrains were created in accordance to the geological mapping by [1]. When translating and rotating dark terrain polygons, there is the fundamental problem of distortion according to the projection in use. We therefore defined 32 regions, where dark terrains are particularly abundant and centered the stereographic projection upon every of these regions to minimize the distortion and maintained the angles when shifting the polygons. An example of the translation process is shown in Fig. 2. For every patch a repetitive procedure is chosen to reverse the extensional movements and shift the polygons back together. This repeats until finally all of the dark terrains more or less match up along their borders. The relative movement vectors on the sphere can then be computed by taking the centroid of each polygon both in the original and the final shapefile. The resulting vectors (Fig. 4) represent magnitude and direction of the extension of dark terrains since its break-up.
Results and Discussion: Figure 3 shows the results of the shifting and fitting procedure of the dark terrain polygones. Figure 3a shows the distribution of dark terrains in their present day position. Figure 3b shows the regional closure of smaller rift zones in each of the patches. Figure 3c shows the result of closing large gaps and Figure 3d a step towards a hypothetic “super-dark-terrain”. With increasing distance between the dark terrains, the reconstruction becomes more difficult and is not always unambiguous. Figures 4a and b show the translation vectors centered to the midpoint of each dark terrain and a possible “break-up” center with the deviation of each vector thereof. Vectors are color-coded with respect to the displacement between the present-day position (arrow origin) and the position after restoration (arrowhead). Vectors show a complex displacement pattern that is not solely the result of a ballooning effect but also indicates movements by intrinsic forces.
Figure 2: Example of the translation process of Perrine Regio (Stereographic Projection). (a) Dark terrains with indication of piercing points (arrows). (b) final small-scale restoration. An obvious fitting pattern along the edges of adjacent dark terrains is clearly visible and can be inferred by extensional tectonism.
Figure 3: Palinspastic restoration of Ganymede's dark terrain. (a) Dark terrains at their present location outlined in yellow. (b) Intermediate restoration closing smaller gaps. (c) Final positions after reversing all the extensional features. (d) Hypothetic “super-dark-terrain”. Scale only representative at equator and meridians.
Figure 4: (a): Translation vectors of the centroids between the present-day position (arrow origin) and the restored polygons (arrow head). The color-code represents the absolute distance on a great arc segment and ranges from 0 km up to a maximum of more than 2700 km. (b): Distribution (heatmap) of the intersection points of every two great circles derived from the translation vectors of each polygon and deviation in degrees (color-coded as indicated) of each translation vector from the direct path towards the best-fit center (green dot) inside the area of the most great-circle intersections.
At present, our reconstruction gives no indication of the timing of the rift events because the tectonic signatures of the light terrains have not yet been considered. In addition, the movements are to be understood as relative movements, since an absolute reference system is missing. Due to the uneven spatial resolution of the available data, reconstructions are inaccurate or impossible in some areas. The planned JUICE mission [7] will remedy this situation.
References: [1] Collins G. C. et al. (2013) USGS Sci. Inv. Map #3237, [2] Rossi, C. et al. (2020) J. Maps. 16, 6–16 [3] Pappalardo, R. T., & Greeley, R. (1995). JGR Planets, 100(E9), 18985–19007. [4] Becker, T.L., et al. (2001) 32 LPSC. [5] Schenk, P., 2010. Atlas of the Galilean Satellites. [6] Kersten, E., et al., (2021). Planet. Space Sci. 206, 105310. [7] Stephan et al. (2021) Planet. Space Sci. 208, 105324.
How to cite: Fabi, M., Kenkmann, T., Wulf, G., Baby, N., Stephan, K., and Wagner, R.: Kinematic restoration of Ganymede's dark terrain, Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-134, https://doi.org/10.5194/epsc2022-134, 2022.
Voyager and Galileo observations of Jupiter's largest moon Ganymede revealed numerous fractures and evidence of strike-slip faulting. The formation of these features has been studied by considering gravitational tidal forces and extensional tectonism, but remains enigmatic. In this study, we continue to advance the assessment of Ganymede’s geologic history by examining the high-resolution Galileo Solid State Imaging (SSI) camera images (∼100m/pixel) available for the area of Nippur/Philus Sulci (Figure 1). Here, various geologic units align on a single line of longitude, providing an excellent sampling region of Ganymede’s deformed surface and eliminating the need to correct for possible leading/trailing hemisphere disparities in the cratering record (Figure 2). First, we produced new crater counts and size frequency distributions to define the relative ages of terrains and their individual geologic units. The chronology of tectonic activity implied by mapped crosscutting relationships is consistent with our findings from analyses of crater densities for each unit, revealing three eras of distinct geologic activity: ancient, intermediate and youngest. Moreover, crosscutting bands of light terrain throughout the surface of Ganymede (and at the Nippur/Philus Sulci site) show varying degrees of tectonic distortion related to these eras of activity, ranging from smooth and less distorted bands to highly grooved and deformed terrain with indicators of shear deformation and strike-slip offsets. To further constrain the timing of geologic activity, we conducted a tidal stress analysis to better understand the formation of shear deformation inferred within the region. Previous studies suggest that tidal stresses resulting from a high eccentricity can generate stress fields capable of driving shear failure and promoting tectonism. At present, Ganymede’s diurnal tidal stresses are not large enough to initiate shear failure. However, the Galilean satellites could have been captured for a period of 10 Ma to 1 Ga years in two Laplace-like resonances previous to the current state and eccentricity. In this past phase, the eccentricity could have been as high as e = ∼0.07, driving internal activity as well as subsequent resurfacing. Using examples of shear deformation indicators found in the Nippur/Philus Sulci area, we examined higher values of past eccentricity to evaluate the promotion of shear failure using the SatStress tidal stress model and Coulomb failure theory, where tractions are resolved onto shallow fault planes (100 m depth) with azimuthal orientations consistent with mapped observations (Figure 3). We find that candidate past eccentricities > 0.02 provide optimal conditions for shear failure throughout the Nippur/Philus Sulci region in its intermediate phase of geologic activity, implied by the positioning of shear indicators within intermediate units as identified by the cratering analysis. Moreover, a past higher eccentricity could have produced a stress field conducive to shear failure of the icy upper crust and possibly promoted the formation of the inferred shear features of Nippur/Philus Sulci and beyond. Future high-resolution images, provided by NASA’s Juno mission, as well as future observations by ESA’s Jupiter Icy Moons Explorer (JUICE) mission and NASA’s Europa Clipper mission, may further aid in our understanding of synergies within Ganymede’s cratering record and tectonic history, especially in the far-field regions surrounding this study, for which only lower resolution Voyager data is currently available.
How to cite: Burkhard, L., Costello, E., Smith-Konter, B., and Cameron, M.: Uncovering the tectonic past of Ganymede through crater size frequency distribution and tidal Coulomb failure modeling at Nippur/Philus Sulci, Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-305, https://doi.org/10.5194/epsc2022-305, 2022.
Introduction. Ganymede, Jupiter’s largest moon, is thought to have a salty subsurface ocean underlying an ice shell and may harbor the conditions to support life. The uppermost crust is mainly comprised of two types of terrain: heavily cratered, dark terrains and bright terrains often characterized by grooves [1]. The surface has also been subject to various landform degradation processes, such as mass movement, volatile segregation and migration, impact erosion, and regolith generation [2]. For example, such processes could be responsible for the formation of the dark deposits (layers) observed on Galileo Regio that vary in thickness (and other properties) as a function of their resting slope and position relative to local topographic crests, among other criteria [2]. Landform degradation processes thus can spatially alter the near-surface structure, characterized by different densities, impurities, and thicknesses (from tens to hundreds of meters). We present a radiometric approach for investigating the properties of near-surface layering at Ganymede from the combined observations of radar sounding systems that will reach the Jovian system within the next decade.
Multi-frequency Radar Sounders. The JUpiter ICy moons Explorer (JUICE) from the European Space Agency (ESA) will be orbiting Ganymede by 2034. Its science payload includes the Radar for Icy Moons Exploration (RIME), a 9-MHz central frequency radar sounder with a 1-MHz (low resolution) or 3-MHz (high resolution) bandwidth (noted [9/1 MHz] and [9/3 MHz], respectively) [3]. Nearly concurrently, the Europa Clipper from the National Aeronautics and Space Administration (NASA), will perform flybys of Ganymede once to thrice, providing complementary observations from the Radar for Europa Assessment and Sounding: Ocean to Near-surface (REASON), a dual frequency radar sounder at [60/10 MHz] and [9/1 MHz] [4]. This wealth of multi-frequency and multi-bandwidth observations is uniquely suited to constraining Ganymede’s near-surface properties through differential surface reflectometry.
Reflectometry of Layered Near-surfaces. Reflectometry is related to the measurement of the radar surface echo strength that is sensitive to surface and near-surface properties with a footprint-size spatial resolution of several kilometers (depending on the spacecraft altitude) [5]. The near-surface depth is bounded by the vertical resolution defined from the bandwidth of the transmitted signal. In void, [60/10 MHz], [9/1 MHz] and [9/3 MHz] radar systems have a vertical resolution of 15 m, 50 m and 150 m, respectively [4]. The reflection strength of a homogeneous icy near-surface combined with a smooth surface is nearly independent of the radar system across the RIME-REASON frequency range. However, a sharp dielectric gradient from a horizontal discontinuity in the near-surface affects the effective surface reflection coefficient (Re) because of the phase shift between the surface and the near-surface reflections that are coherently integrated when measured at the receiver. Thin-film theory provides analytical tools to predict Re for layered media as a function of the layers’ thickness and permittivity, as well as the signal wavelength [6].
As an illustration, consider an overburden layer of unknown thickness (zlayer) and permittivity (εlayer), bounded by a void (ε0 = 1), and covering a compact ice substrate (εice = 3.1). Fig. 1 illustrates the field of solutions for Re computed for the three radar systems considered and assuming a synthetic chirp of 100 μs. Each plot on Fig. 1 is characterized by two regimes. When zlayer is larger than the near-surface depth, then the signal strength is independent of zlayer across all frequencies and only dependent on εlayer. Note in particular that as zlayer gets thinner than the near-surface depth, Re exhibits a resonant behavior strongly dependent on zlayer relative to the radio wavelength. Hence, multi-frequency observations can be overlapped, although the solutions are not necessarily unique. However, additional constraints like assumed lateral continuity of the near-surface structure can help resolve most ambiguities.
In addition, surface roughness has a wavelength-dependent effect on surface reflectivity that can be difficult to deconvolve from the effect of near-surface layering. In that regard, because the [9/1 MHz] and [9/3 MHz] radar systems provide the same response to roughness (same radio wavelength), their response should differ only when layer boundaries are between their respective near-surface depths (different bandwidth).
A strategy based on the combination of the reflectivity plots in Fig. 1 for the range of frequencies/bandwidths deployed on Ganymede can then be used to constrain its near-surface properties. An analog application on a terrestrial ice cap is provided in the next section.
Proof-of-concept: Devon Ice Cap, Canadian Arctic. The near-surface of Devon Ice Cap (DIC) is subject to melting and refreezing processes, forming meters-thick ice layers embedded in lower porosity firn. The reflections from these layers from the surface down to the near-surface depth affect the coherent power (Pc) observed by a radar sounder (IPR). Fig. 2 illustrates a transect over DIC, where concurrent ground-penetrating radar (GPR) collected at 500 MHz and airborne IPR observations collected at [60/15 MHz] exist. Superimposed on the GPR radargram is a pink line that represents the frequency/bandwidth-constrained near-surface depth/vertical resolution of the airborne IPR. Results indicate that the behavior of Pc (green line) well correlates with the ice layer thickness within the near-surface depth and decorrelates below the near-surface depth at this frequency.
Figure 1: Coherent surface echo strength (Re) as a function of an overburden thickness (zlayer) and permittivity (εlayer) covering an icy substrate (εice = 3.1) for REASON VHF [60/10 MHz], REASON HF [9/1 MHz], and RIME [9/3 MHz].