Kattenhorn, S.A., Billings, S.E., Groenleer, J.M., Marshall, S.T., Vetter, J.C. (2006)
Fracture access through the Europan ice shell: Geologic constraints for the selection of an optimal surface entry site
Proceedings of the Europa Focus Group Workshop, NASA Ames Research Center, Moffett Field, California, Feb. 27-28, 2006.
Future exploration of Europa will ultimately attempt to access the likely subsurface ocean beneath the ice shell. Of importance then is the choice of a surface site where access to the ocean is facilitated by optimal ice shell characteristics. A challenging factor in this regard is the ice thickness. Although most estimates have constrained the shell to be less than 40 km thick, and potentially as thin as a few kilometers or less, the difference in these estimates is fairly significant from the standpoint of considering a drilling strategy. Therefore, site selection should focus on locations where the ice shell thickness is likely to be at a minimum. Our estimates of thickness based on flexure analysis along several ridges is typically less than 2.5 km [1] (see a similar analysis in [2]); however, this and other techniques that specifically examine elastic or brittle thicknesses may not be capturing the entire shell thickness which, based on thermal convection and impact cratering models (see references in [1]), is likely to include a deeper, ductile ice layer. Nonetheless, our elastic thickness results indicate that ice thicknesses are likely to be spatially heterogeneous on Europa, which may also explain the range of results obtained by others in different locations containing disparate geologic features. Our minimum thickness estimates were obtained in a type of feature called a smooth band (in contrast to the results of [2]). These dilational features are common on Europa, and appear to represent locations where the ice shell spread apart and was infilled by material from below, forming new crust. The developmental process for bands has been previously compared to that responsible for mid-ocean spreading centers on Earth [3], which are typically locations where the terrestrial lithospheric thickness is at a minimum. Nonetheless, the timing of such features in the geologic history of Europa is crucial as the ice shell likely relaxes and evens out thickness variability over long periods of time. Furthermore, there is evidence to suggest that the ice shell has generally thickened through time [4]. Hence, it would not be feasible to select any smooth band as a surface entry site; rather, only the youngest bands should be considered in a feasibility study, identifiable through rigorous geologic mapping at high resolution [4-8]. Some of the youngest fractures on Europa are cycloids [9-11]. In any area, there may be a succession of cycloid chains that formed over an extended period of time (perhaps several nonsynchronous rotation cycles, amounting to 10s to 100s of thousands of years) [11-12]. Hence, the youngest cycloids may be more likely to have currently open cracks. Some cycloids are crosscut by younger, non-cycloidal fractures [12]. In the E15RegMap01 region, we have identified 22 independent cycloid chains, 14 of which can be placed into a well-constrained fracture sequence based on crosscutting relationships. Although cycloids commonly occur as ridges, some are band-like, implying that the cracks extended sufficiently deep into the ice shell to be intruded by material from below. Cycloids that have widened into bands may perhaps represent locations where the ice shell is relatively thin. Access through the ice shell in such locations may nonetheless need to be partially facilitated by the utilization of open fractures. Prospective sites of open cracks may be associated with regions of shearing-induced, fault-perturbed stresses (which greatly exceed tidal stress magnitudes and may thus create deeply penetrating cracks). Our work on Europan strike-slip faults has revealed a fairly common occurrence of tailcracks which formed at the tips in response to shearing motion [13]. The geometries of tailcracks are a direct indicator of shear fracture displacement characteristics at the time of tailcrack growth and commonly indicate that the shear fracture was partially dilated during the shearing, perhaps not an unexpected result considering the prevalence of tensile tidal stresses. Of particular importance here is the fact that dilated cracks have no frictional resistance to shearing. Therefore, even a small amount of shear stress on an existing crack should be sufficient to induce shearing and the creation of a tailcrack. This phenomenon may be responsible for the development of cusps along cycloid chains [11] via shearing and tailcrack growth at the tip of the most recently-formed cycloid segment. The tailcrack then propagates out into the ambient tidal stress field, forming the next cycloid segment. Continued shearing and tailcrack reworking in the cyclical tidal stress field could result in ice-penetrating cracks at the cycloid cusp. Accordingly, cycloids are often only band-like in close proximity to a cycloid cusp. Some cusps are complex, consisting of horsetail splays of cracks due to repeated cracking of the ice shell in this vicinity. We advocate that such sites of shear-induced tailcracking and band development along cycloids may present promising sites to access the ocean through the ice. It is nonetheless important to consider the possibility of reworking of existing cracks, which could heal cracks or even result in contraction along cracks and resultant ice shell thickening. Lateral offset patterns along typical morphology ridges in Argadnel Regio provide evidence for convergence [14]. Studied examples indicate a ratio of strike-slip shearing to contraction along ridges in the range 3:1 to 5:1. This finding implies that ridges are not necessarily locations of open cracks in the ice shell, providing greater justification for selecting sheared tip regions of young cycloids to locate potentially open and ice-penetrating tailcracks.
References: [1] Billings & Kattenhorn (2005), Icarus 177, 397-412. [2] Hurford et al. (2005), Icarus 177, 380-396. [3] Prockter et al. (2002), JGR 107, 5028, 4-1 - 4-26. [4] Figueredo & Greeley (2004), Icarus 167, 287-312. [5] Spaun et al. (1998), LPSC 29, 1899. [6] Prockter et al. (1999), JGR 104, 16,531-16,540. [7] Figueredo & Greeley (2000), JGR 105, 22,629-22,646. [8] Kattenhorn (2002), Icarus 157, 490-506. [9] Hoppa et al. (1998), Science 285, 1899-1902. [10] Hoppa et al. (2001), Icarus 153, 208-213. [11] Marshall & Kattenhorn (2005), Icarus 177, 341-366. [12] Groenleer & Kattenhorn (2006), 37th LPSC. [13] Kattenhorn (2004), Icarus 172, 582-602. [14] Vetter (2005), MS thesis, Univ. Idaho.
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