Ray and Halo Impact Crater Formation on Ganymede: Probing Crustal Properties via Z-model and Numerical Simulations

Introduction

Ganymede holds a complex geological history with an ocean sandwiched between high-pressure ice layers [1], enhancing its potential for habitability. Consequently, Ganymede has become the primary target of ESA's JUICE mission [2]. The surface of Ganymede is 35% covered by old, low-albedo terrain known as dark terrain, with the remaining characterized by younger, higher-albedo terrain called light terrain [3]. These terrains are dominated by impact craters, each exhibiting distinctive morphology and ejecta patterns (Fig. 1). Studying these impact craters and their ejecta patterns will help to understand the vertical stratigraphy of Ganymede's crust at a local scale and the properties of its dark and light ices.

Data and Methodology

We used the global mosaic from [4] to identify craters with ejecta blankets (Fig. 2). This study focuses on craters with dark ejecta on dark terrain (e.g., Antum), dark ejecta on light terrain (e.g., Kittu), and craters with a dark halo surrounded by bright ejecta (e.g., Nergal). We employed two models to analyze crater formation and excavation depth:

• Maxwell Z-Model: This analytical model examines the excavation flow field during cratering, focusing on near-surface explosion calculations. According to [5], the maximum depth of excavation (D_e) and transient cavity diameter (D_t) relate to the Z value:

D_e = (1/2 D_t) (Z-2 )(Z-1^{) (1-Z) (Z-2)}

Where, D_e is the maximum depth of excavation

D_t is the diameter of the transient crater cavity.

Z=4 is considered in icy targets [6].

• iSALE-2D: iSALE, a multi-rheology, multi-material code based on the SALE hydrocode (e.g., [7]), handles both multi-material and Newtonian fluids and considers porosity, damage, dilatancy. It analyzes material flow and structural collapse under gravity, offering a robust method to recreate and analyze events from projectile impact to crater formation. By incorporating projectile characteristics and ice target properties, iSALE investigates how different sequences of stratigraphic layers interact with impactors (Table 1).

Parameter	Light Ice	Dark Ice
Cohesion (yield strength at zero pressure) (Yi0)	10 MPa	10 MPa
Damaged cohesion (Yd0)	0.2 MPa	0.5 MPa
Limiting strength at high pressure for intact material (Yim)	0.11 GPa	0.11 GPa
Limiting strength at high pressure for damaged material (Ydm)	0.11 GPa	0.11 GPa
Coefficient of internal friction (μi)	2	2
Damaged coefficient of friction (μd)	0.6	0.6
Thermal softening parameter (ξ)	1.2	1.2

Table 1: Summary of the input parameters for the strength model of target (dark ice and light ice).

Results

• Antum: Antum, 15 km in diameter, resides on dark terrain, with  maximum excavation depth of about 2.3 km when Z = 4 (Fig. 3). Its geological features include floor and rim, along with extensive dark rays [6]. Numerical simulations indicate materials are excavated from depths less than 5 km (Fig. 4).

• Kittu: Kittu, 15 km diameter crater located on light terrain (Fig. 4), has also a maximum excavation depth of approximately 2.3 km (Fig. 3). Its main geological features include central peak, bright rim and floor, continuous bright ejecta, and discontinuous dark ejecta [6]. Numerical simulations indicate materials are excavated from depths less than 5 km (Fig. 5).
• Nergal: Nergal is a 9 km diameter crater situated on light terrain. It has  maximum excavation depth of approximately 1.4 km (Fig. 3). The geological features include central peak, rim, and floor, with halo of dark material surrounded by lighter material [6]. Numerical simulations indicate materials are excavated from depths less than 4 km (Fig. 6).

Discussion

Numerical simulations confirm that on Antum, dark ejecta originates from the dark terrain itself (Fig. 4). Simulations on Kittu and Nergal demonstrate the existence of stratigraphic sequence with multiple layers: dark and light terrain layers, excavated from depths less than 5 km (Figs. 5 and 6). The impact crater formation approach from the Z-model [6] aligns well with these simulations. Our study also indicates that dark material exhibits higher cohesion than light ice. However, iSALE has limitations, such as its consideration of only vertical impacts, homogeneous surfaces without topography, and the absence of strain localization in the target material. The numerical simulations show that a nested, two-fold ejecta curtain develops by ice-phase change upon pressure release from the shocked state. This twofold ejecta curtain supports the formation of  double layer ejecta blanket.

Figure 1: (a) Antum and (b) Mir: craters with dark ejecta on dark terrain. (c) Kittu: crater with dark ejecta on light terrain. (d) Nergal and (e) Khensu: craters with a circular halo of dark ejecta surrounded by bright ejecta. (f) Tammuz:  crater with half of the crater floor and ejecta are bright and another half are dark. (g) Melkart: crater with bright ejecta located at the boundary between light and dark terrain. (h) Osiris: bright ray crater.

Figure 2: Global mosaic showing the distribution of different craters. The red-colored ones are those we studied in detail: Antum, Kittu, and Nergal.

 

Figure 3: Schematic illustration of subsurface layer configurations explaining different ejecta patterns [6]. a) Craters in dark terrain with dark ejecta blanket (Antum); b) Craters in light terrain with dark ejecta blanket (Kittu); c) Craters with  dark halo surrounded by bright ejecta (Nergal).

Figure 4  Antum:

Figure 5  Kittu:

Figure 6 Nergal:

(a) The target and projectile before impact. (b) transient cavity formation, and shock waves propagation. (c) shock waves lead to further vaporization of the growing transient cavity. (d) development of voids leads to the formation of nested-crater cavity. (e) nested cavity enlarges and detachment of the layers from the target surface. (f) Final crater look

References:

• [1] Clark, R. N., et al., The science of solar system ices, 3-46, 2013. [2] Grasset, O., et al., Planetary and Space Science, 78, 1-21, 2013. [3] Pappalardo, R. T., et al., Jupiter, 363–396, Cambridge University Press, 2004. [4] Kersten, E., et al., EPSC2022-450, 2022. [5] Maxwell, D. E. Impact and explosion cratering, 1003–1008, Pergamon Press, 1977. [6] Baby, N. R., et al., ESS, 11, e2024EA003541, 2024. [7] Amsden, A., et al., Los Alamos National Laboratories Report, LA-8095, 101, 1980.