EPSC Abstracts Vol. 13, EPSC-DPS2019-813-1, 2019 EPSC-DPS Joint Meeting 2019 c Author(s) 2019. CC Attribution 4.0 license.

Fractures and Furrows on Occator’s Lobate Flows: Morphologic Evidence of Ice Content

Debra L. Buczkowski (1), Britney Schmidt (2), Margaret Landis (3), Hanna Sizemore (3), Jennifer E.C. Scully (4), Paul Schenk (5), Thomas Prettyman (3), Julie Castillo-Rogez (4), Carol A. Raymond (4), and Chris T. Russell (6) (1) Johns Hopkins Applied Physics Laboratory, Maryland, USA, (2) Georgia Institute of Technology, Georgia, USA (3) Planetary Science Institute, Arizona, USA; (4) NASA Jet Propulsion Laboratory, California, USA; (5) Lunar and Planetary Institute, Texas, USA; (6) University of California, Los Angeles, California, USA. Abstract 2. Rock glaciers

We present geomorphic evidence from the highest On Earth a mixture of rock debris interspersed with resolution XM2 data of Ceres that suggests that the ice in cavities is referred to as a “rock glacier”. They movement and deformation of the Occator lobate form as either permafrost flow phenomena, due to the features is consistent with the flow of interstitial ice. continuous freezing of talus, or where an ice glacier covered by extensive debris has retreated. These types of glaciers move downslope by the deformation 1. Introduction of the ice within them, but are typically much slower NASA’s Dawn spacecraft [1] was captured into orbit than ice glaciers. Flow features form by: 1) by the dwarf planet (1) Ceres on March 6, 2015. Deformation of the ice core; 2) Movement of the During the Approach phase capture was preceded debris cover along its interface with the ice; or 3) and followed by a series of optical navigation and Deformation during a period of advance. Multiple rotation characterization observations by Dawn’s overlapping flow lobes are frequent, and terminal Framing Camera (FC) [2], which provided the first embankments can be up to 60 m high. images of Ceres’ surface. Crevasses, deep V-shaped clefts in the upper brittle Multiple geologic analyses of Ceres have been part of a glacier, form as a result of ice undergoing performed using Dawn spacecraft [1] Framing extension. This brittle deformation occurs when the Camera (FC) [2] mosaics from late Approach (1.3 ice cannot creep fast enough to accommodate the km/px), Survey (415 m/px), the High Altitude driving stresses. Longitudinal crevasses are oriented Mapping Orbit (HAMO - 140 m/px) and the Low more or less parallel to the long axis of a glacier and Altitude Mapping Orbit (LAMO – 35 m/px) orbits, typically open when the glacier becomes wider. including clear filter and color images and digital Transverse crevasses are oriented more or less terrain models derived from stereo images. However, perpendicular to the long axis of a glacier and Dawn’s second extended mission (XM2) utilized low typically open when the terrain becomes steeper. elliptical orbits with <50 km perihelion altitude to Splaying crevasses typically form in the lower parts gain a ground sampling distance as low as ~5 m/pixel of the glacier in the zone of compression, over some regions of Ceres, including Occator crater. approximately parallel to ice flow. A bergschrund is an irregular crevasse where active glacier ice pulls Occator is the 92 km diameter crater that hosts the away from ice adhering to the steep mountainside “Bright Spot 5” that was identified in Hubble Space and usually run across an ice slope in the Telescope data [3], which is actually comprised of accumulation area. En echelon crevasses are a series multiple bright spots on the crater floor. The floor of of crevasses oriented at an angle to the glacier margin; Occator is cut by linear fractures, while they form as a result of rotational strain within the ice circumferential fractures are found in the ejecta and along the glacier's edge. Chevron crevasses form on the crater walls. The bright spots are noticeably where there is strong lateral drag against the valley associated with the floor fractures, although the walls; they often form near the edge of a glacier brightest spot is associated with a central pit [4]. where interactions with underlying or marginal rock Multiple lobate flows are observed on the crater floor; impede flow and are angled obliquely upvalley. hypotheses for the formation of these flows have ranged from impact melt [5-7] to volcanism [8-10]. 3. Occator’s Lobate Flows and/or debris-covered glaciers (ice-cored). This morphology does not prove or disprove the formation Lobate material covers almost the entire interior of of these flows by either impact melt or Occator crater [5, 6]. The large, thick sheet of lobate cryovolcanism, but rather is indicative of their material in the northeastern quadrant of the crater enrichment in ice. The detection of water ice within was visible even in HAMO data, but in XM2 it Occator by GRaND at 1 m depth but not by VIR became evident there is a veneer of lobate material (~cms depth) is consistent with these flows having coating the majority of the terraces [5, 6]. Also either interstitial ice or ice under a deep layer of visible in XM2 data are the multiple instances of debris/talus. Meanwhile, lobate flows in the western lobate flows superposing each other with overlapping wall of Occator do not share the ice-rich lobes. These multi-lobed and overlapping flow fronts morphologies, but rather are more consistent with are highly reminiscent of the morphology of (relatively) dry debris falls. terrestrial rock glaciers. Acknowledgements Extensive fracturing of the large NE lobate material was mappable in both HAMO and LAMO. However, Support of the Dawn Instrument, Operations, and new smaller-scale fractures and furrows became Science Teams is gratefully acknowledged. This evident with the higher-resolution XM2 data. work is supported by grants from NASA through the Dawn project, and from the German and Italian One example is a 16 km long lobate flow in north- Space Agencies. central Occator that appears to be flowing to the south around an older, higher-standing flow. References Longitundal fractures only observable in XM2 data are found in the center of flow while en echelon [1] Russell, C.T. and Raymond, C.A.: Space Sci. Rev., 163, fractures are identified at the edges of flow. The flow 3-23. 2012. ends in a series of overlapping flows marked by a steep terminal embankment and splaying fractures [2] Sierks H. et al.: Space Sci. Rev., 163, 263-328, 2012. just behind the flow front. Flow fronts and deep furrows also mark the termination of the older flow, [3] Li J.Y. et al. (2006) Icarus, 182, 143-160. and suggest that it flowed from east to west; pitted cones (possible pingos?) are found on its surface. [4] Schenk, P. et al. (2015) EPSC2015-527.

A second example is found where lobate material is [5] Scully J. et al. [2019] LPSC #1619. putatively perched on top of terrace material. Chevron fractures are identified in a region where [6] Scully J. et al. (in review) Nature. the ice-rich material would be flowing downslope to [7] Schenk P. et al. (in review) Nature. join the main flow to the west. [8] Krohn K. et al. (2016) GRL, doi: These morphologic analyses are supported by new 10.1002/2016GL070370. XM2 GRaND data, which determined that there was hydrogen enrichment within Occator crater and its [9] Nathues A. et al. [2019] LPSC #1814. eastern ejecta [11, 12]. The enrichment is consistent with the presence of subsurface ice within the lobate [10] Nathues A. et al. (in review) Nature. deposits at

4. Summary and Conclusions Fracture and furrow morphology of the lobate flows within Occator are consistent with that of ice-rich flows on Earth, either as rock glaciers (interstitial ice)