Global Topography of Enceladus: North-Pole-South-Pole, Deep Basins and TPW Revisited
Authors: Schenk Paul , McKinnon William 
Published: October 2020

Cassini observations have led to several efforts to map the global topography of Enceladus [1-5]. The most prominent large-scale features include 100+ km scale basins (1), and the depressed south polar terrain (SPT) [2]. Here we present a revised and complete stereogrammetric map of Enceladus topography at 0.4-km scales (Fig. 1) including the north polar regions mapped at the end of the mission and that does not require interpolation. We reevaluate the distribution and origin of large-scale topographic deviations and compare north and south polar regions.Stereogrammetric digital elevation models (DEMs) of Enceladus were constructed from Cassini stereo pairs and mosaics using the same software used to map the topography of Pluto, Charon, and other icy bodies [6].&#160; As each stereo pairing has a unique stereo geometry, the vertical precision (or uncertainty) is variable across Enceladus and ranges from 70&#176;S) is depressed ~400 m relative to the best-fit ellipsoid (confirming [2]), but the NPT (>70&#176;N) are elevated ~0.7 km above. The north polar imaging confirms that tectonic features with fresh blue-ice signatures do cross the NPT. The apparent broad doming of the NPT might suggest that either the ice shell is relatively thick there (and perhaps more resistant to tidal heating relative to the original southern terrain) or that incipient resurfacing is preceded by doming before the crustal replacement cycle begins.3. Global Aspects and ConclusionsA global hypsogram of Enceladus (Fig. 2) using our new complete DEM (Fig. 1) shows that global topographic deviations from the first-order triaxial ellipsoid [2] are at the 95th percentile within &#177;1 km of the triaxial ellipsoid, confirming earlier incomplete maps [1,4]. Two &#8216;humps&#8217; can be identified in the hypsogram at -0.4 km (representing the SPT) and at -1 km (representing the basins). This is possibly the narrowest global topographic signature in the Solar System for an ellipsoidal body [11], although it might be matched by Europa, Ganymede and Triton (work in progress), and is consistent with expected subdued topography of an active icy ocean world. The preservation of the deep 1-1.5 km deep basins is an unsolved problem [1,3], but might reflect patterns of heat flow from the core [12]. &#160;Enceladus&#8217; topographic signature is complicated, with indications that older cratered terrains and the three younger resurfaced terrains responded differently to stresses.&#160; The distribution of large deep basins is enigmatic, and although they do not form a great circle, lower-lying topography is more prevalent at mid-northern latitudes.&#160; More complete analyses of higher harmonics are underway and may yield additional insights.References[1] Schenk, P., and McKinnon, W., GRL 36, L16202, 2009.&#160;[2] Thomas, P.C, et al., Icarus 190, 573&#8211;584, 2007.[3] Nimmo, F., et al., JGR 116, E11001, 2011.[4] Tajeddine, R., et al., Icarus 295, 46-60, 2017.[5] Bland, M., et al., 4th Planetary Data Workshop, abs. #7048, 2019.[6]. Schenk, P.M., et al., Icarus, 314, 400-433, 2018a.[7] Thomas, P.C, et al., Icarus 264, 37-47, 2016.[8] Barr A.C., and McKinnon, W.B., GRL 34, L09202, 2007.[9]. Crow-Willard, E., R. Pappalardo, JGR., 120, 928-950.[10] Nimmo, F., et al., in Enceladus and the Icy Moons of Saturn, UA Press, pp. 79-94, 2018.[11] Schenk, P., et al., in Enceladus and the Icy Moons of Saturn, UA Press, pp. 237-265, 2018.[12] &#268;adek, O., et al., Icarus 319, 476-484, 2019.