11th Planetary Crater Consortium 2020 (LPI Contrib. No. 2251) 2079.pdf

HYDROTHERMAL DEPOSITION AT OCCATOR CRATER, CERES, IN A PLANETARY CONTEXT. P. Schenk1, D. Buczkowski2, J. Scully3, A. Neesemann4, C. Pieters5, J. Castillo-Rogez3, C. Russell6, C. Raymond3, 1Lunar & Planetary Institute, Houston, TX ([email protected]). 2JHU-APL, Laurel, MD, 3JPL/Caltech, Pasadena, CA, 4Freie Univ. Berlin, Ger., 5Brown Univ. Providence, RI, 6Univ. Calif. LA, CA.

INTRODUCTION: Hydrothermal carbonates [1] major spots have surprisingly different morphologies. at the 92-km-diameter pristine Occator crater, the ‘Ty- Many of these surfaces are heavily pitted. The three cho’ of the Asteroid Belt, emphasize the importance of largest bright spots to the west (Figs. 1, 2) are closely impact-induced hydrothermal processes on planetary correlated with topography and effectively “line” but do bodies. Dawn XM2 extended mission focused exten- not “fill” the depressions in which they form. The east- sively on Occator with the objective of obtaining high ern spots tend to cover larger areas with little or con- resolution imaging, topography, compositional and flicting preferences for elevation and slope. The only gravity measurements over the crater and its unusual evidence of discrete bright constructional features is a bright carbonate deposits [1], and testing hypotheses for depression ~1 km wide with a 20-m-high lobate dome.

their origins [2]. XM2 topography and stratigraphy (as revealed through stereo images and derived DEMs) [3] has proven critical for testing hypotheses how these and other floor deposits formed. STEREO & TOPOGRAPHY: Global mapping of all of Ceres was acquired at ~35 m pixel scales and in stereo, with global DEM mapping at similar scales. In XM2, hundreds of images were acquired over the east- ern 2/3rds of Occator at nominal pixel scales of ~3 to ~8 m, a factor of ~10 improvement. Overlapping orbital coverage acquired at variable look angles, and occa- sional parallel use of both cameras, provides excellent opportunities for stereo analysis and DEM construction, with variable stereo parameters and DEM quality. XM2 stereo pairs of Occator were processing by the author at LPI and combined to produce regional topography (Fig. 1). Two primary target areas for production (at present) are the central pit and dome and bright deposits of Ce- realia Facula, and the bright deposits on the crater floor at Vinalia Faculae. Individual DEMs have also been produced at isolated sites to focus on features of interest, such as impact melt flows. Figure 1. Mosaic of Vinalia Faculae bright spots and LOBATE FLOOR DEPOSITS: Mantling by central region of lobate floor deposits covering large cliff-forming units is extensive, indicating that impact parts of Occator, color-coded for topography (blue low, melt deposits are widespread across Occator. Within the red high). Relief shown is ~500 m. lobate floor deposits (LFD), relief is low, with shallow pits, pit clusters, low bright mounds <100 m high, and CEREALIA FACULA: The central pit and dome scarps up to 200 m high (Fig. 2). Lobate flow features of Occator are topographically complex. The western with 10s to >100 m of relief are found across the floor pit exhibit several peculiar very narrow arcuate benches of Occator, but many have higher relief than lunar im- or ‘terraces’ at various levels up the outer pit walls, pos- pact melt deposits, likely due to the different (i.e., ice, sibly artifacts of the pit formation process. Eastern mas- carbonate and salt rich) composition [1,4]. sifs flanking the pit structure (Fig. 3) include two frac- VINALIA FACULAE: The Vinalia bright depos- tured flat-topped mesas 350 and 600 m high (the inner its (Fig. 1) form on a low ridged plain that constitutes mesa slopes inward, capped by bright material). most of the northern extension of Occator’s lobate floor Bright materials of Cerealia Facula are found on the deposits [2]. The ridges have amplitudes of 50 to 100 tops of fault blocks across the outer central pit, some of m depending on location. The prominent V-shaped frac- which are tilted. The most prominent of these is the 0.7 tures that cross this region are 50 to 150 m deep. While by 1.3 km wide ledge or plateau complex SW from the proximal to several prominent floor fractures, these ap- central dome (Fig. 3). A contiguous bright deposit co- pear to post-date the deposits. The 10 or so individual vers ~75% of this structure, and correlates with the outer 11th Planetary Crater Consortium 2020 (LPI Contrib. No. 2251) 2079.pdf

edge but not the inner edge (which forms a series of lo- Endogenic pits on Occator floor are different in mor- bate features), indicating a thickness of only a few me- phology and significantly less frequent and less tightly ters at most. spaced than on Mars (3, 5) (Fig. 2), suggesting that ei- ther Ceres melts were less gas-rich or volatiles were more inhibited from reaching the surface relative to Mars conditions. The thicker flow morphologies in some areas of Occator are suggestive of higher yield strengths. Thinner flow units (Fig. 2) are more likely to be mud flows produced during impact or post-impact differentiation of impact deposits that on Ceres behave like lava flows (6).

Figure 3. Perspective view of western margins of Cere- alia Facula and central pit of Occator, showing bright covered ledge described in text (left) and how bright material partially flanks the lower pit walls.

Bright and dark material stratigraphy in Cerealia Facula is complex, with indications that both materials may be constructional in many areas. Spatial relation- ships are consistent with localized deposition from min- erals dissolved in ground waters extruded onto the sur- face, and that flow gradients might have been locally asymmetric, leading to asymmetric deposition with re- Figure 2 Examples of endogenic features within Occa- spect to local topography. Asymmetric formation due tor crater (left) with comparison to Mars (right). Fea- to ballistic emplacement is not vigorously supported by tures include irregular non-impact pits (p), irregular de- the data. Most of the Vinalia Faculae deposits are on bris mounds (large arrow), thin mud flow units (arrows), the order of a few to 10 or so meters thick, and form sinuous troughs (3b, 3f) and smooth units (s and else- isolated spots and sinuous topographically controlled where). deposits. These were likely emplaced via brine seepage at innumerable isolated vents, forming small deposits Dark materials within Cerealia Facula form both that coalesced where vent sources were more concen- negative and positive relief features. The negative relief trated, at the 10 or so major centers (Fig. 1) (3). features are generally associated with downslope mass References: [1] De Sanctis, M. et al., Nature, 528, 241, wasting, positive features may be either uncovered rem- 2015. [2] Scully, J., et al., Icarus, 320, 7, 2019. [3] nant pit structures or constructional edifices. Schenk, P., et al., Nature Comm, in press, 2020. [4] Er- OBSERVATIONS: Dawn XM2 observations of makov, A., et aL., J. Geophys. Res., 123, 2267, 2017. Occator reveal a wealth of features related to large im- [5] Tornabene, L., et al., Icarus, 220, 348, 2012; pact formation in an ice-rich, partially differentiated Mouginis-Mark, P. and J. Boyce, Chemie der Erde, 72, body. While these features share morphologies and pat- 1, 2012; Boyce, J. et al., Icarus, 221, 262, 2012. [6] terns similar to those of fresh lunar craters such as Jack- Broz, P., et al., Nature Geo., 13, 2020. son and Tycho, major differences are also evident.