FRACTURES AND FURROWS ON OCCATOR’S LOBATE FLOWS: MORPHOLOGIC EVIDENCE OF ICE CONTENT

Dawn’s second extended mission (XM2) utilized low elliptical orbits to gain a resolution of ~5 m/pixel over some regions of Ceres, including Occator crater. Multiple fractures and lobate flows are observed on the floor of Occator; hypotheses for the formation of the flows have ranged from impact melt to volcanism. We present geomorphic evidence from the highest resolution XM2 data that shows that the fracture and furrow morphology of the lobate flows are consistent with that of ice-rich flows on Earth, either as rock glaciers (interstitial ice) and/or debris-covered glaciers (ice-cored). This morphology does not prove or disprove the formation of these flows by either impact melt or cryovolcanism, but rather is indicative of their enrichment in ice.

The large, thick sheet of lobate material in the NE quadrant of the crater was visible even in HAMO data, but in XM2 it became evident there is a veneer of lobate material coating the majority of the terraces. Also visible in XM2 data are the multiple instances of lobate flows superposing each other with overlapping lobes, highly reminiscent of the morphology of terrestrial rock glaciers.

Extensive fracturing of the large NE lobate material was mappable in HAMO and LAMO, but new smaller-scale fractures and furrows became evident with the higher-resolution XM2 data. These fractures are consistent with those that occur in flowing ice-rich material on Earth. One example is a 16.4 km long lobate flow in north-central Occator, which appears to be flowing south from the crater rim. Longitundal fractures only observable in XM2 data are found in the center of flow while en echelon fractures are identified at the edges. The flow ends in a series of overlapping flow fronts marked by a steep terminal embankment and splaying fractures.

More examples of small-scale fractures are found in a 14.9 km lobate flow originating from crater terrace material, and flowing south around other terrace outcrops. En echelon fractures are found where the lobate material flows around outcrops of highstanding terrace material. Chevron fractures are identified in a region where ice-rich material perched on top of terrace material would be flowing downslope to join the main flow. The flow terminates on top of the first flow; splaying fractures are observed near where the material is terminating in the direction of flow.