Geologic Constraints on the Formation and Evolution of Saturn's Mid-Sized Moons

Abstract

Saturn's mid-sized icy moons have complex relationships with Saturn's interior, the rings, and with each other, which can be expressed in their shapes, interiors, and geology. Observations of their physical states can, thus, provide important constraints on the ages and formation mechanism(s) of the moons, which in turn informs our understanding of the formation and evolution of Saturn and its rings. Here, we describe the cratering records of the mid-sized moons and the value and limitations of their use for constraining the histories of the moons. We also discuss observational constraints on the interior structures of the moons and geologically-derived inferences on their thermal budgets through time. Overall, the geologic records of the moons (with the exception of Mimas) include evidence of epochs of high heat flows, short- and long-lived subsurface oceans, extensional tectonics, and considerable cratering. Curiously, Mimas presents no clear evidence of an ocean within its surface geology, but its rotation and orbit indicate a present-day ocean. While the moons need not be primordial to produce the observed levels of interior evolution and geologic activity, there is likely a minimum age associated with their development that has yet to be determined. Uncertainties in the populations impacting the moons makes it challenging to further constrain their formation timeframes using craters, whereas the characteristics of their cores and other geologic inferences of their thermal evolutions may help narrow down their potential histories. Disruptive collisions may have also played an important role in the formation and evolution of Saturn's mid-sized moons, and even the rings of Saturn, although more sophisticated modeling is needed to determine the collision conditions that produce rings and moons that fit the observational constraints. Overall, the existence and physical characteristics of Saturn's mid-sized moons provide critical benchmarks for the development of formation theories.

Keywords: Cratering; Dione; Enceladus; Geology; Icy moons; Interiors; Mimas; Ocean worlds; Rhea; Saturnian satellites; Tectonics; Tethys.

Fig. 1

(Top left) Key physical properties of Mimas, Enceladus, Tethys, Dione, and Rhea (error bars are 1-sigma); (Top right) comparison of their densities and mean radii to those of other moons in the outer solar system. References for radii and densities are Thomas (2010) and Archinal et al. (2018). (Bottom) Summary of our knowledge of their internal structures. There are still major uncertainties about the state of differentiation of Rhea and Tethys. Modified from Neveu and Rhoden (2019)

Fig. 2

Crater distributions shown on an R-plot, on which a horizontal line would correspond to a −3 slope on a log-log SFD plot (see dotted lines in the left panel). Data for each moon is shown in a different color, with symbols representing the crater counting study from which the data was taken. For Tethys, we show results from Kirchoff and Schenk (2010) only for craters larger than ∼10 km because the size and locations of the areas mapped make the counts less complete than the Ferguson et al. (2020) data set

Fig. 3

Penelope crater on Tethys (left) is elliptical rather than circular, which indicates a relatively slow, oblique impact. Here, we show the major and minor axes of Penelope (center), the relationship between “orientation” and the azimuth of the major axis (right), and the equation for determining the ellipticity of a crater.

Fig. 4

Two styles of histogram show the orientations of the long-axes of elliptical craters on Mimas, Tethys, and Dione, in which the populations are split by latitude. Across all three moons, regions within 30° of the equator contain elliptical craters that are predominantly oriented east-west, although the signal is less apparent on Mimas. On Tethys and Dione, this group makes up the majority of the elliptical craters, with a smaller, more isotropically-oriented group spanning all mapped latitudes. On Mimas, elliptical craters above 30°N are predominantly oriented north-south; this population appears to be unique among the three moons. Image credit: Ferguson et al. (2024)

Fig. 5

a) Cassini ISS image of Enceladus showing numerous viscously relaxed impact craters (yellow arrows, but many more occur throughout the area). Image N1487299402_1 in a local orthographic projection with north up. b) An example of combining observations (black points are measured crater depths on Enceladus relative to their expected depth) with numerical modeling (blue curves) to constrain the heat flux. The simulations shown here assumed a surface temperature of 120 K and a pure ice (non-porous) rheology and thermal conductivity. The modeling suggests a flux in excess of 150 mW m⁻² was necessary to viscously relax Enceladus’ craters. Panel ‘b’ modified from Bland et al. (2012)

Fig. 6

Views of Mimas across its leading and trailing hemispheres. A) Full disk view of Mimas with a focus on the Herschel impact basin (D = 139 km) and its central peak. Image PIA12570. B) Close-up of Herschel’s ejecta blanket. The lower density of craters on the ejecta blanket suggests relatively recent formation of the basin (Ferguson et al. 2024). Image N1644778567_1. C) Trailing hemisphere cratered terrain along with some grooves. Image N1831441742_1. D) Oblique view of the grooves on Mimas’ trailing hemisphere, which are among the only tectonic features so far identified on Mimas. Image N1831443018_1

Fig. 7

A) View of Enceladus’ south polar terrain, with a focus on the Tiger Stripes fractures. Material from within Enceladus can erupt into space via these fractures. Image PIA07800. B) A transitional region between tectonized and cratered terrains. While craters are present on the surface, they are often overprinted by tectonics and appear to have undergone viscous relaxation. Image N1637465264_1. C) “Snowman” craters near the North Pole of Enceladus. While the cratered terrains in the north are likely representative of older surfaces on Enceladus, these craters are often cross-cut by other fractures. Image N1823513163_1. D) Fractured terrain on Enceladus. Due to the lack of impact craters on this surface, it’s inferred that fracture formation occurred relatively recently in geologic history. Image N1604168315_3

Fig. 8

A) Full disk image of the leading hemisphere of Tethys, with a focus on the Odysseus impact basin (D = 445 km). Image PIA08400. B) Closer view of the Odysseus basin and its cratered interior in which the central pit of the basin is clearly visible. Image N1567098978_1. C) Portion of the Ithaca Chasma canyon system, which stretches ∼1800 km across the surface of Tethys. Image N1489061272_1. D) Close up of cratered terrain on Tethys, including some instances of mass wasting. Image N1713137226_1

Fig. 9

A) Full disc view of Dione’s trailing hemisphere showcasing the wispy terrain (PIA08526). B) Cratered terrain on Dione’s leading hemisphere. Pictured off-center are the heavily degraded and modified craters of Murranus (56.8 km) and Metiscus (43.8 km). Image N1820417749_1. C) Oblique view of Erulus crater (120 km), which is thought to be heavily relaxed (White et al. 2017), with a focus on the central peak complex. Image N1665975031_1. D) Amastrus crater (62.4 km) located within the wispy terrain on Dione’s trailing hemisphere, illustrating the combined effects of cratering and tectonic resurfacing on Dione. Image N1507743880_2

Fig. 10

Dione’s craters record variable rates of relaxation, which could mean they are different ages or that the heat flow that led to relaxation was spatially-variable. Here, we overlay the relaxation results from White et al. (2017) on a generic contour map of eccentricity-driven tidal heating (similar to the one shown in Roberts 2015). Warm colors represent higher heat flows relative to the cooler colors. There is good correlation between areas of higher relaxation (and inferred heat flow) and the regions of higher tidal heating, but the fit is not perfect. Generating these high heat flows from tides alone is challenging without both a subsurface ocean and a higher past eccentricity, owing to Dione’s distance from Saturn. It is also possible that the initial depths assumed for the craters on Dione are too large, leading to an overestimate of the past heat flow (e.g., Sect. 3.4)

Fig. 11

A) Full disk image of Rhea with the Tirawa basin located near the center. Image PIA07763. B) Cratered terrain with a linear feature. Image N1567132880_1. C) Obatala crater (62.5 km) overprinted by tectonics. Image N1637518901_1. D) Tectonic features alongside cratered terrain. Image N1637519610_1