Bader, C.E., Kattenhorn, S.A., Schenk, P.M. (2008)
Topographic analysis of Europa's ridges
Eos, Transactions of the American Geophysical Union 89(52), Fall Meeting Supplement, Abstract P23A-1349.
Ridges are the most ubiquitous surface feature on Europa. Here we examine double ridges that have two parallel, raised flanks with a continuous axial trough (referred to as a ridge pair). Characterizing ridge edifices may help us better understand the processes that drive ridge formation and evolution. Because there is no global elevation map for Europa, topography was derived from high resolution (18 to 181 m/pixel) combined stereographic and photoclinometric images to create 265 topographic profiles across 24 features of interest. Ridge topography was examined across 22 ridge pairs (12 with apparent lateral offsets) and 2 ridge complexes, in the Bright Plains, Conamara Chaos, Cilix, Argadnel Regio, Rhadamanthys Linea, and the E17DISSTR01 (northwest of Katreus Linea) areas. Topographic profiles are oriented perpendicular to the strike of each ridge pair to capture height and width variations as well as to highlight asymmetry between adjacent ridges. We characterize ridges using ridge height and width (vertical and horizontal distance from the base of the ridge flank to the ridge peak), average ridge height (average of the individual peaks in a ridge pair), total ridge width (distance between the ridge's outer flanks), and peak-to-peak (PTP) width (distance between peaks in a ridge pair). Height-to-width ratios of 44 individual ridges fall within a wide range that never exceeds 0.53, implying a maximum outer slope of 28 degrees, slightly less than the suggested angle of repose of loose granular ice (~34 degrees). Most slopes are much gentler, between 10 and 20 degrees, which are significantly smaller than those presented in a prior study undertaken early in the Galileo imaging mission. In fact, we have found that ridges can be very wide and low with outer slopes of only a few degrees, implying that very few ridge morphologies are likely to be controlled by granular flow processes down their outer slopes. The ratio of average ridge height to total ridge width has a lower limit of 0.05 that corresponds to the widest ridge examined (4.05 km). Conversely, the upper limit of this ratio is 0.13, corresponding to the highest ridge examined (362 m). This raises the possibility that tall ridges modify their morphology through time through some form of gravitational collapse, thus decreasing the height, increasing the width, and decreasing the ridge slopes. The low slopes and overall low average height to total width ratios suggest predominantly time-dependent viscoplastic gravitational collapse. Variability between ridges may be related to the mechanisms driving ridge development (whether dilational, contractional, or shear heating), which are likely to influence the rate of ridge construction as well as the temperature (and hence rheology) of the icy material involved. We also observe a distinct upper limit of 0.58 for the ratio of average ridge height to PTP width, suggesting that once an active ridge exceeds a certain height, the ridge peaks begin to geomorphically migrate apart in order to maintain a limiting slope of the inner and outer flanks through gravitational collapse (whether it be granular flow or viscoplastic). Lower ratios of average ridge height to PTP width may indicate underdeveloped ridge heights but may also be a sign of dilation across a ridge, causing a tectonic increase in PTP width. Finally, variability in ratios of average height to total width along individual ridges indicates that some ridge pairs may have evolved differently along their lengths, an effect partially related to changes in ridge orientation along the observable length and associated variability in ridge kinematics.
External link: AGU database
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