Global structural mapping of high-resolution Cassini images of Enceladus reveals a richly varied surface. Most notable are three main regions of deformation each containing multiple structural units. In addition to the well known “South Polar Terrain” (SPT), there are two other large regions of deformation that we term “Leading Hemisphere Terrain” (LHT) and “Trailing Hemisphere Terrain” (THT). Each of these three terrains includes a circumferential belt that encloses one or more other structurally deformed units. Areal extents range from about 80,000 km2 (SPT) to 195,000 km2 (LHT) or 160 to 250 km equivalent circular radius. Based on relative crater densities, the THT is inferred to be older than the LHT; the geologically active SPT is the youngest. The overall similarities in shape and dimension of the three tectonized terrains suggest similar formational processes, plausibly related to broad loading of a thin elastic shell. A viable scenario is that each tectonized terrain formed above a large-scale region of warm upwelling ice, with subsequent downwarping triggered by cooling and/or subsurface melting. However, differences in morphological detail suggest that the specific evolution of each tectonized terrain has been different.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2015JE004818

Structural mapping of Enceladus and implications for formation of tectonized regions

Ocean worlds are prevalent in the solar system. Focusing on Enceladus, Titan, Europa, and Ganymede, I use rotating convection theory and numerical simulations to predict ocean currents and the potential for ice-ocean coupling. When the influence of rotation is relatively strong, the oceans have multiple zonal jets, axial convective motions, and most efficient heat transfer at high latitudes. This regime is most relevant to Enceladus and possibly to Titan, and may help explain their long-wavelength topography. For a more moderate rotational influence, fewer zonal jets form, Hadley-like circulation cells develop, and heat flux peaks near the equator. This regime is predicted for Europa, where it may help drive geologic activity via thermocompositional diapirism in the ice shell, and is possible for Titan. Weak rotational influence allows concentric zonal flows and overturning cells with no preferred orientation. Predictions for Ganymede's ocean span multiple regimes.

https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/2018GL081880

Ocean dynamics of outer solar system satellites

Jupiter’s moon Europa has a subsurface ocean, the chemistry of which is largely unknown. Carbon dioxide (CO2) has previously been detected on the surface of Europa, but it was not possible to determine whether it originated from subsurface ocean chemistry, was delivered by impacts, or was produced on the surface by radiation processing of impact-delivered material. We mapped the distribution of CO2 on Europa using observations obtained with the James Webb Space Telescope (JWST). We found a concentration of CO2 within Tara Regio, a recently resurfaced terrain. This indicates that the CO2 is derived from an internal carbon source. We propose that the CO2 formed in the internal ocean, although we cannot rule out formation on the surface through radiolytic conversion of ocean-derived organics or carbonates. Description Editor’s summary Europa, an icy moon of Jupiter, has a subsurface ocean beneath a crust of water ice. Solid carbon dioxide (CO2) has previously been observed on its surface, but the source was unknown. Two teams analyzed infrared spectroscopy of Europa from the James Webb Space Telescope to investigate the CO2 source. Trumbo and Brown found that the CO2 is concentrated in a region with geology that indicates transport of material to the surface from within the moon, and they discuss the implications for the composition of Europa’s internal ocean. Villanueva et al. also identified an internal origin of the CO2 and measured its 12C/13C isotope ratio. They searched for plumes of volatile material breaching the surface but found a lower activity than earlier observations. Together, these studies demonstrate that there is a source of carbon within Europa, probably in its ocean. —Keith T. Smith The distribution of carbon dioxide ice on Jupiter’s moon Europa indicates that it originates from the subsurface ocean.

pdf/the-distribution-of-co2-on-europa-indicates-an-internal-2rlnyncin3.pdf

The distribution of CO2 on Europa indicates an internal source of carbon

A unified nomenclature for tectonic structures on the surface of Enceladus

Tidal stress modeling of Ganymede: Strike-slip tectonism and Coulomb failure

Ganymede, then and now: How past eccentricity may have altered tidally driven Coulomb failure

2100. [12] Tanaka et al., (2005), USGS Map. [13] Wilhelms (1972), Astrogeology, 55. [14] Tanaka et al. (2011), Planetary Geologic Mapping Handbook. [15] Prockter et al. (1999), JGR, 104, 16531-40. [16] Prockter et al. (2002), LPSC XXXIII, Abstract #1732. [17] Zahnle et al. (1998), Icarus, 136, 202-22. [18] Zahnle et al. (2003), Icarus,

The Europa Global Geologic Map

Diverse Origins of Enceladus's Ridge Terrains Including Evidence for Contraction

Introduction: The existence of global oceans on some icy satellites—Europa and Enceladus, for example—implies the presence of a ductile warm ice layer. However, the role of such a ductile layer in controlling icy-surface deformation has never been systematically investigated nor quantified. We aim to address this issue by combining previous observations from geomorphological mapping of surface features on icy bodies [1] with a unique two-layer analogue model containing an overlying brittle layer and a ductile creeping layer. Although analogue models have been widely used for tectonic studies on Earth [2], they have only rarely been adapted to the studies of the icy-surface deformation [cf., 3] Using the analogue experimental approach and analyzing the effects of a subsurface ductile layer, we will gain understanding of different formation mechanisms for surface features and aid in reconstructing the resurfacing history of icy satellites such as Europa. Procedure: The basic analogue model consists of a ductile, lower viscosity layer underlying a cohesive brittle layer. We initially use therapeutic putty with a measured viscosity of about 10 P•s for our ductile layer and fine-grained sand for our brittle layer. We chose these materials for our initial experiments because they will scale up properly to conditions on Europa. For example, if we scale with the cohesive strength of our sand (~60 Pa) and use well accepted values for Europa [4], we get a scale where about 1 km on Europa corresponds to 1 cm in our model, or 10 scale factor [5]. To set up the experiment, we first layer the putty into a low-walled box. Because the putty is ductile, but still fairly viscous, we let it relax to a flat surface over the course of a few days before adding the desired amount of sand. We also create a 1 cm x 1 cm grid of blue marker sand on top of the experiment to aid us in keeping track of how much extension or contraction is occurring. For experiments where we want to simulate extensional processes, we eliminate one of the box walls causing the ductile material to flow out, creating uniform extension in the surface brittle layer. To simulate compressional processes, we tilt the box at an angle causing the putty to flow towards one end of the box, creating uniform contraction in the brittle surface layer. Initial Results: We began the analogue experiments by varying the thickness of the brittle layer, 0.25 cm, 0.5 cm, and 1.0 cm, to study the effects on the surface features. The experiments are run over the course of about 6 hours, scaling to 10 yrs on Europa [2]. By varying the thickness of the brittle layer, we are effectively studying the effect of varying heat flux on the surface morphology. For example, an icy body with a higher heat flux would have a thinner ice shell and therefore a thinner brittle layer. Extension. In the extension experiments, we see the spacing of resulting normal faults in the brittle layer increase with the increasing thickness of the brittle layer (Fig. 1). We expected this result based on past work on normal fault spacing on Earth [6, 7]; thus this

Analyzing Surface Structures on Icy Satellites: A Physical Analogue Modeling Approach

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