Flanking fractures and the formation of double ridges on Europa
Highlights
► Fractures flanking double ridges on Europa suggest lithospheric flexure. ► We evaluate models of double ridge formation by simulating this flexure. ► Only a model with a cryomagmatic sill is consistent with the observation of flexure.
Introduction
Europa, a satellite of Jupiter, is one of the most intriguing worlds in the Solar System. Only slightly smaller than the Moon, Europa possesses a metallic core and a rocky mantle surrounded by a shell of water/water ice ∼100 km thick (Anderson et al., 1998, Schubert et al., 2009). Its dearth of impact craters and plethora of surface morphologies point to a dynamic evolution of its icy shell in geologically recent times (e.g., Greeley et al., 2004, Schenk et al., 2009, Bierhaus et al., 2009). In addition, the preponderance of evidence points to a liquid water ocean beneath the ice shell (e.g., Pappalardo et al., 1999, Kivelson et al., 2000). Because liquid water is a necessary (but insufficient) biological requirement, Europa is one of the prime candidates for extraterrestrial life in the Solar System. This ice shell is of order 10 km thick (see Schenk and Turtle, 2009), but regardless of whether the shell is somewhat thinner or thicker, the heat flow coming out of Europa is likely greater than what can be supplied by radiogenic heating in the silicate portion (see, e.g., Schubert et al., 2004, Ruiz, 2005, Sotin et al., 2009). This fact implicates tidal dissipation as the engine that drives Europa’s diverse (and ongoing?) activity.
Ridges are the most ubiquitous landform on Europa, with multiple generations of ridges cross-cutting each other (for reviews, see Pappalardo et al., 1999, Greeley et al., 2004, Prockter and Patterson, 2009). These features can run remarkably uniformly for more than 1000 km across the surface, a challenge for any model of their formation. A wide spectrum of morphologies has been classified as ridges on Europa, from isolated troughs to ridge complexes that display a series of subparallel features (Head et al., 1999). The most common form is known as the double ridge (Fig. 1); these features are generally ∼0.5–2 km wide, ∼100–300 m tall, and possess a central trough and outer flanks with slopes typically <20° (Head et al., 1999, Coulter et al., 2009, Coulter and Kattenhorn, 2010). These shallow angles on the outer flank are usually interpreted as less than the angle of repose, suggesting non-granular processes are at work. On the other hand, materials can display a wide range of repose angles, including down to ∼10°, depending on the size, shape, and stickiness of the particles (e.g., Zhou et al., 2002), as well as the gravity of the target body (Kleinhans et al., 2011). The outer flanks of double ridges appear to be dominated by mass-wasting processes. Head et al. (1999) reported that the terrain immediately peripheral to a ridge can be traced up the flank of the ridge, suggesting that the ridges represent upwarping of the pre-existing terrain. This interpretation, however, is not unique (Sullivan et al., 1998); if ridges formed by deposition of material on the surface, they would reflect to some degree the topography of the underlying terrain. Furthermore, some of the cracks that persist up the flanks could be due to reactivation along pre-existing structures. A subtle marginal trough a few tens of meters deep is fairly common (see Hurford et al., 2005), and subparallel, presumably tensile fractures can sometimes be found near the outer reaches of these troughs (Fig. 1). Additionally, the marginal troughs can sometimes possess diffuse regions of lower albedo, suggesting burial or processing of the terrain.
Several models have been proposed for the formation of double ridges (to be reviewed in the next section). All of these models appeal to exploitation of a pre-existing crack in the ice shell of Europa. Addressing the source of the initial crack has been beyond the scope of these models (and continues to be so in this current work), but it is thought that the cracks form in response to externally applied stresses (Greeley et al., 2004, Kattenhorn and Hurford, 2009). As pointed out by Greeley et al. (2004), “each model has different implications for the presence and distribution of liquid water at the time of ridge formation.” Given that ridges are the most ubiquitous landform, it is therefore critical to understand ridge formation in order to decipher Europa’s unique history.
A clue to the formation of ridges may be provided by evidence for flexure of the lithosphere in response to a load imposed by the ridge itself. Several groups have interpreted the presence of marginal troughs and subparallel flanking fractures associated with ridges as characteristic of flexure (e.g., Pappalardo and Coon, 1996, Tufts, 1998, Billings and Kattenhorn, 2005, Hurford et al., 2005, Dombard et al., 2007). The addition of a ridge to the surface will cause the lithosphere to warp downward, producing marginal troughs and uplifting a flexural bulge peripheral to these troughs that may be detectible. Between the bulge and the trough, tensile flexural stresses peak and may produce subparallel fractures (arrows in Fig. 1). When this flexure is modeled, a simple elastic lithosphere has usually been assumed (see Turcotte and Schubert, 2002); however as we will discover below, the generally thin lithospheres indicate very high heat flows inconsistent with Europa’s expected thermal budget, which implicates a localized thermal anomaly in the formation of the double ridges (cf. Dombard et al., 2007).
In this paper, we will evaluate models of double ridge formation by determining which ones are consistent with the observation of flexure. As we will discuss, ridges that display evidence of flexure (marginal bulges, troughs, and fractures) are common but not abundant, and those that possess evidence of flexure can provide important constraints. Because the bulge and trough topography is subtle, we will specifically look to see which models, and under what conditions, predict tensile stresses that peak at the right range of distances away from the central ridge axis, and thereby are able to reproduce the flanking fractures. In the next section, we review the various models that have been proposed and discuss the thermal anomalies that may be associated with each one. Then, we discuss a suite of measurements of double ridges, in order to determine the range of distances of the flanking fractures. We subsequently describe our thermal–mechanical finite element simulations, present our results, and discuss the implications.
Section snippets
Models of double ridge formation
Many of the proposed models for double ridge formation are summarized in Fig. 2. In the volcanic model of Kadel et al. (1998), a pre-existing crack provides a pathway for fissure eruptions that build the ridges cryoclastically (Fig. 2a). In the tidal squeezing model of Greenberg et al. (1998), daily tidal forces cause the crack to open and close, squeezing material onto the surface (Fig. 2b). A dike intrusion model (Turtle et al., 1998) posits that injection of melt into the subsurface can
Measurements of double ridges
Following Billings and Kattenhorn (2005), we compare the predicted lateral distance to peak tensile flexural stresses from our simulations to the distances to fractures parallel to double ridges. Thus, important constraints for our method are the distance to flanking fractures and the half-width of the ridge. Billings and Kattenhorn (2005) identified flanking fractures associated with three ridges on Europa: Androgeos Linea, Ridge C2r (Tufts et al., 1999), and Ridge R (Tufts et al., 1999).
Finite element simulations
Our goal is to test which classes of formational models detailed in Section 2 are consistent with observation of lithospheric flexure determined in Section 3. Specifically, we will examine situations that are most favorable to flexure, in order to rule out those classes of models that cannot result in any observable flexure for all reasonable situations. To investigate the deformation of the lithosphere in the presence of these thermal anomalies, we use the commercially available MSC. Marc
Results
Most studies of ridge flexure on Europa do not appeal to local thermal anomalies but implicitly assume a high regional heat flow (e.g., Billings and Kattenhorn, 2005, Hurford et al., 2005). Thus, our first step is to determine the extent of lithospheric flexure under a uniform background heat flow (no anomalies), in order to ascertain the magnitude that would be necessary to produce the observed flanking fractures. A simulation with just a background heat flow of 40 mW m−2 resulted in no
Discussion and conclusion
The central motivation of this work is to evaluate the models that have been proposed for the formation of double ridges on Europa on the basis of the observation of lithospheric flexure. Our results indicate that nearly every model of ridge formation is inconsistent with the observed flexure. If a ridge forms during an episode of very elevated, Io-like heat flow, the lithosphere can flex appropriately, but given how ubiquitous ridges are on Europa, this elevated heat flow state would have to
Acknowledgments
We thank Steven A. Hauck, II for his assistance, as well as 2 anonymous reviewers who helped us refine our arguments and improve our presentation. This research was support by NASA Grant NNX09AP28G to A.J.D.
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