Unified model of tectonics and heat transport in a frigid Enceladus

Abstract

Recent data from the Cassini spacecraft have revealed that Enceladus, the 500-km-diameter moon of Saturn, has a southern hemisphere with a distinct arrangement of tectonic features, intense heat flux, and geyser-like plumes. How did the tectonic features form? How is the heat transported from depth? To address these questions, we formulate a simple model that couples the mechanics and thermodynamics of Enceladus and gives a unified explanation of the salient tectonic features, the plumes, and the transport of heat from a source at a depth of tens of kilometers to the surface. Our findings imply that tiny, icy moons can develop complex surficial geomorphologies, high heat fluxes, and geyserlike activity even if they do not have hot, liquid, and/or convecting interiors. E nceladus, the 500-km-diameter moon of Saturn, is the smallest body in the solar system with erupting plumes (1). On the southern hemisphere, Enceladus displays tectonic features that are closely related to the plumes, a thermal anomaly that straddles the south pole, and high rates of heat flow in coincidence with the thermal anomaly (2, 3). The tectonic features include four 130-km-long fractures that cut across the thermal anomaly and are known as ''tiger stripes'' (2, §). The plumes observed by Cassini gushed from vents located on the tiger stripes (5); these plumes had a total discharge similar to the Old Faithful geyser in Yellowstone National Park and consisted of H 2 O, CH 4 , N 2 , and CO 2 gases laden with ice crystals (1, 2, 6).The cause of plume eruption has been addressed by two antithetic models so far. In one model, called ''Cold Faithful,'' Enceladus has a shell of H 2 O ice with pockets of liquid water at depths as shallow as 7 m (2). Cold Faithful's plumes erupt where small fractures decompress the pockets of liquid water, causing the liquid water to boil at a temperature of at least 273 K. The small fractures may form (and the attendant plumes erupt) at any time, driven by tectonic disturbances. Thus, the plumes of Cold Faithful may remain active over long periods of time.In the other model, called ''Frigid Faithful,'' Enceladus has a shell of H 2 O ice and H 2 O clathrates (CO 2 , CH 4 , N 2 ) topped with a layer of H 2 O-CO 2 ice (Fig. 1) (7). Frigid Faithful is based on the assumption that the composition of the plumes observed by Cassini indicates the composition of the shell where the plumes originate. The plumes of Frigid Faithful erupt where large fractures (the tiger stripes) expose and decompress some clathrates of the clathrate-rich shell, causing these clathrates to absorb heat from a source at depth and then to dissociate at a temperature that might be as low as Ϸ133 K, the average temperature of the hotspots that have been observed on the surface of the south pole (3). The tiger stripes need to form and expose some clathrates of the shell only once, possibly long ago in geologic time. Although clathrate dissociation is, in principle, self-limiting (because the products of dissociation may refreeze), it is also explos...

A unified nomenclature for tectonic structures on the surface of Enceladus

“…Both types of chasmata are observed in the trailing tectonized hemisphere and appear to be associated with the south polar dichotomy based on their relationship with the wedge-shaped protuberances from the SPT boundary. Three chasmata have been identified by Helfenstein et al (2006) and appear to be approximately radial to the south pole (Gioia et al, 2007). Structures that may be interpreted as nascent chasmata are located at 31.5°E, 39.8°S and 40.1°E, 14.3°S.…”

“…Structures classified here as chasmata on Enceladus have been interpreted to be rifts propagating northward from the SPT boundary by Helfenstein et al (2006) and Gioia et al (2007). Narrow chasmata, such as Bishangarh Fossae (Fig.…”

“…Other studies characterized structures as either positive or negative relief features (e.g., Kargel and Pozio, 1996), focused on the SPT and/or the ''tiger stripes'' (e.g., Gioia et al, 2007; Smith-Konter and Martin (2014). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)…”

Researching the Earth—and a Few of Its Neighbors

“…In this model, Enceladus has an icy crust overlying a layer of clathrates that extends to the rocky core. Decomposition of the clathrates drives the plumes through a crust that has been fractured by regional stresses (Gioia et al 2007). Although a number of observations over the past decade have been used to strengthen the arguments for liquid water in Enceladus, there are arguments that the case for a clathrate-rich moon is still open.…”

Geyser

“…2b). (2) (Kieffer et al 2006;Gioia et al 2007). This model is based on the assumption that the plume composition reflects the composition of the subsurface source.…”