HIGH-RESOLUTION GEOLOGIC MAPPING OF URVARA CRATER, CERES: UNIT DEFINITIONS & PRELIMINARY CHRONOLOGY

We are developing a geologic map and accompanying chronostratigraphy of Urvara crater, Ceres, based on high-resolution images (3.5-20 m/px) acquired at the end of the Dawn mission. Crater diameter: 170 km. Map region: -128° to -93° E longitude, -59° to -35° N latitude. Publication scale: 1:250,000. Digitization scale: 1:50,000.

A major landscape development question that arose from the Dawn mission at Ceres was: What was the role of impacts in facilitating the development of putative cryovolcanic landforms? Reconstructing the chronological evolution of individual crater interiors is key to addressing this question. Analysis of Low Altitude Mapping Orbit (LAMO) image data (35 m/px) identified Ahuna Mons as a location of cryvolcanic extrusion on the dwarf planet. Geologic mapping of Occator crater (diameter = 92 km), based on high-resolution images (>3 m/px), produced a detailed chronological sequence of crater floor evolution, building the case for prolonged hydrothermal activity and/or cryovolcanism at a second location. We are conducting a comparable analysis at Urvara crater to evaluate whether similar processes occurred inside this larger, older crater, which may constitute a third region of extrusive activity.

We previously produced a ~40000 x 57000 pixel basemap mosaic at 5 m/px resolution, using ~1600 Level 2b images of the Urvara region from the XM2 Dawn mission phase. We then developed a complete crater database for diameters larger than ~100 m. We also produced crater density estimates (KDE) for crater diameters >400 m and >300 m.

We have identified 10 geologic units in the map region: talus material (tlm), crater material (cm), hummocky floor material, incised (hfmi), hummocky material (hm), terrace material (trm), terrace material, smooth (tms), Urvara central peak material (Ucpm), Urvara floor material, smooth (Ufms), Urvara smooth material (Usm), and Urvara ejecta material (Uem). Age constraints based on the Lunar Derived Model (LDM) range from range from 180 + 7 Ma for the terrace material (the presumed crater formation age) to 100 + 4 Ma for Ufsm, a smooth, flat-lying unit with lobate margins that covers most of the southern basin floor. This chronostratigraphy suggests a long history of post-impact modification at Urvara, and bolsters the hypothesis that the crater hosts cryovolcanic deposits.