Adrian Neesemann

Cryovolcanism on Ceres

INTRODUCTION Classic volcanism prevalent on terrestrial planets and volatile-poor protoplanets, such as asteroid Vesta, is based on silicate chemistry and is often expressed by volcanic edifices (unless erased by impact bombardment). In ice-rich bodies with sufficiently warm interiors, cryovolcanism involving liquid brines can occur. Smooth plains on some icy satellites of the outer solar system have been suggested as possibly cryovolcanic in origin. However, evidence for cryovolcanic edifices has proven elusive. Ceres is a volatile-rich dwarf planet with an average equatorial surface temperature of ~160 K. Whether this small (~940 km diameter) body without tidal dissipation could sustain cryovolcanism has been an open question because the surface landforms and relation to internal activity were unknown. RATIONALE The Framing Camera onboard the Dawn spacecraft has observed >99% of Ceres’ surface at a resolution of 35 m/pixel at visible wavelengths. This wide coverage and resolution were exploited for geologic mapping and age determination. Observations with a resolution of 135 m/pixel were obtained under several different viewing geometries. The stereo-photogrammetric method applied to this data set allowed the calculation of a digital terrain model, from which morphometry was investigated. The observations revealed a 4-km-high topographic relief, named Ahuna Mons, that is consistent with a cryovolcanic dome emplacement. RESULTS The ~17-km-wide and 4-km-high Ahuna Mons has a distinct size, shape, and morphology. Its summit topography is concave downward, and its flanks are at the angle of repose. The morphology is characterized by (i) troughs, ridges, and hummocky areas at the summit, indicating multiple phases of activity, such as extensional fracturing, and (ii) downslope lineations on the flanks, indicating rockfalls and accumulation of slope debris. These morphometric and morphologic observations are explained by the formation of a cryovolcanic dome, which is analogous to a high-viscosity silicic dome on terrestrial planets. Models indicate that extrusions of a highly viscous melt-bearing material can lead to the buildup of a brittle carapace at the summit, enclosing a ductile core. Partial fracturing and disintegration of the carapace generates slope debris, and relaxation of the dome’s ductile core due to gravity shapes the topographic profile of the summit. Modeling of this final phase of dome relaxation and reproduction of the topographic profile requires an extruded material of high viscosity, which is consistent with the mountain’s morphology. We constrained the age of the most recent activity on Ahuna Mons to be within the past 210 ± 30 million years. CONCLUSION Cryovolcanic activity during the geologically recent past of Ceres constrains its thermal and chemical history. We propose that hydrated salts with low eutectic temperatures and low thermal conductivities enabled the presence of cryomagmatic liquids within Ceres. These salts are the product of global aqueous alteration, a key process for Ceres’ evolution as recorded by the aqueously altered, secondary minerals observed on the surface. Perspective view of Ahuna Mons on Ceres from Dawn Framing Camera data (no vertical exaggeration). The mountain is 4 km high and 17 km wide in this south-looking view. Fracturing is observed on the mountain’s top, whereas streaks from rockfalls dominate the flanks. Volcanic edifices are abundant on rocky bodies of the inner solar system. In the cold outer solar system, volcanism can occur on solid bodies with a water-ice shell, but derived cryovolcanic constructs have proved elusive. We report the discovery, using Dawn Framing Camera images, of a landform on dwarf planet Ceres that we argue represents a viscous cryovolcanic dome. Parent material of the cryomagma is a mixture of secondary minerals, including salts and water ice. Absolute model ages from impact craters reveal that extrusion of the dome has occurred recently. Ceres’ evolution must have been able to sustain recent interior activity and associated surface expressions. We propose salts with low eutectic temperatures and thermal conductivities as key drivers for Ceres’ long-term internal evolution.

Cratering on Ceres: Implications for its crust and evolution

INTRODUCTION Thermochemical models have predicted that the dwarf planet Ceres has, to some extent, formed a mantle. Moreover, due to viscous relaxation, these models indicate that Ceres should have an icy crust with few or no impact craters. However, the Dawn spacecraft has shown that Ceres has elevation excursions of ~15 km, cliffs, graben, steep-sided mountains, and a heavily cratered surface. RATIONALE We used Dawn’s Framing Camera to study the morphology, size frequency, and spatial distribution of the craters on Ceres. These data allow us to infer the structure and evolution of Ceres’ outer shell. RESULTS A large variety of crater morphologies are present on Ceres, including bowl-shaped craters, polygonal craters, floor-fractured craters, terraces, central peaks, smooth floors, flowlike features, bright spots, secondary craters, and crater chains. The morphology of some impact craters is consistent with water ice in the subsurface. Although this might have favored relaxation, there are also large unrelaxed craters. The transition from bowl-shaped simple craters to modified complex craters occurs at diameters of about 7.5 to 12 km. Craters larger than 300 km are absent, but low-pass filtering of the digital elevation model suggests the existence of two quasi-circular depressions with diameters of ~570 km (125.56°E and 19.60°N) and ~830 km (24.76°W and 0.5°N). Craters are heterogeneously distributed across Ceres’ surface, with more craters in the northern versus the southern hemisphere. The lowest crater densities are associated with large, well-preserved southern hemisphere impact craters such as Urvara and Yalode. Because the low crater density (LCD) terrain extends across a large latitude range in some cases (e.g., Urvara and Yalode: ~18°N and 75°S; Kerwan: ~30°N and 46°S), its spatial distribution is inconsistent with simple relaxation driven by warmer equatorial temperatures. We instead propose that impact-driven resurfacing is the more likely LCD formation process, although we cannot completely rule out an internal (endogenic) origin. We applied two different methodologies to derive absolute model ages from observed crater size-frequency distributions. The lunar-derived model adapts the lunar production and chronology functions to impact conditions on Ceres, taking into account impact velocities, projectile densities, current collision probabilities, and surface gravity. The asteroid-derived model derives a production function by scaling the directly observed object size-frequency distribution from the main asteroid belt (extended to sizes <5 km by a collisional model) to the resulting size-frequency distribution of cerean craters, using similar cerean target parameters as the lunar-derived model. By dating a smooth region associated with the Kerwan crater, we determined absolute model ages of 550 million and 720 million years, depending on which chronology model is applied. CONCLUSION Crater morphology and the simple-to-complex crater transition indicate that Ceres’ outer shell is likely neither pure ice nor pure rock but an ice-rock mixture that allows for limited relaxation. The heterogeneous crater distribution across the surface indicates crustal heterogeneities and a complex geologic evolution of Ceres. There is evidence for at least some geologic activity occurring in Ceres’ recent history. Spatial density of craters larger than 20 km on Ceres. Crater rims are shown as black solid circles. Blue indicates areas with LCDs; yellow and red represent more highly cratered areas. The smallest dashed ellipse denotes the idealized former rim of an extremely degraded impact crater at 48.9°E and 44.9°S, which is barely recognizable in imagery but apparent from the global digital elevation model. Also shown as dashed circles are the outlines of two large putative basins. Unambiguously recognized basins >300 km in diameter are missing, and there are several areas with LCDs associated with large impact craters (e.g., Yalode, Urvara, Kerwan, Ezinu, Vinotonus, Dantu, and two unnamed craters northeast and southeast of Oxo). Areas A and B are topographic rises with central depressions that also show LCDs. Thermochemical models have predicted that Ceres, is to some extent, differentiated and should have an icy crust with few or no impact craters. We present observations by the Dawn spacecraft that reveal a heavily cratered surface, a heterogeneous crater distribution, and an apparent absence of large craters. The morphology of some impact craters is consistent with ice in the subsurface, which might have favored relaxation, yet large unrelaxed craters are also present. Numerous craters exhibit polygonal shapes, terraces, flowlike features, slumping, smooth deposits, and bright spots. Crater morphology and simple-to-complex crater transition diameters indicate that the crust of Ceres is neither purely icy nor rocky. By dating a smooth region associated with the Kerwan crater, we determined absolute model ages (AMAs) of 550 million and 720 million years, depending on the applied chronology model.

Timing of optical maturation of recently exposed material on Ceres

On Ceres, multispectral imaging data from the Dawn spacecraft show a distinct bluish characteristic for recently exposed material from the subsurface in, for example, crater ejecta. Ejecta blankets of presumably old craters show a more reddish spectrum. We selected areas in which fresh material from the Cerean subsurface was exposed at a specific time in the past, and no later geologic process is expected to have changed its surface composition or its cratering record. For each area, we determined two color ratios and the crater retention age. The measured color ratios show an exponential diminishment of the bluish characteristic over time. Although the cause of the color change remains uncertain, the time-dependent change in spectral properties is evident, which could help identify the process.

Pitted terrains on (1) Ceres and implications for shallow subsurface volatile distribution

Abstract Prior to the arrival of the Dawn spacecraft at Ceres, the dwarf planet was anticipated to be ice‐rich. Searches for morphological features related to ice have been ongoing during Dawns mission at Ceres. Here we report the identification of pitted terrains associated with fresh Cerean impact craters. The Cerean pitted terrains exhibit strong morphological similarities to pitted materials previously identified on Mars (where ice is implicated in pit development) and Vesta (where the presence of ice is debated). We employ numerical models to investigate the formation of pitted materials on Ceres and discuss the relative importance of water ice and other volatiles in pit development there. We conclude that water ice likely plays an important role in pit development on Ceres. Similar pitted terrains may be common in the asteroid belt and may be of interest to future missions motivated by both astrobiology and in situ resource utilization.

Hydrological and sedimentary analyses of well‐preserved paleofluvial‐paleolacustrine systems at Moa Valles, Mars

Moa Valles is a well-preserved, likely Amazonian (younger than 2 Ga old), paleodrainage system that is nearly 300 km long and carved into ancient highland terrains west of Idaeus Fossae. The fluvial system apparently originated from fluidized ejecta blankets, and it consists of a series of dam breach paleolakes with associated fan-shaped sedimentary deposits. The paleolakes are interconnected and drain eastward into Liberta crater, forming a complex and multilobate deltaic deposit exhibiting a well-developed channelized distributary pattern with evidence of switching on the delta plain. A breach area, consisting of three spillover channels, is present in the eastern part of the crater rim. These channels connect the Liberta crater to the eastward portion of the valley system, continuing toward Moa Valles with a complex pattern of anabranching channels that is more than 180 km long. Based on hydrological calculations of infilling and spillover discharges of the Liberta crater lake, the formation of the whole fluvial system is compatible with short to medium (<1000 year) timescales, although the length and morphology of the observed fluvial-lacustrine features suggest long-term periods of activity based on terrestrial analogs. Water for the 300 km long fluvial system may have been primarily sourced by the melting of shallow ice due to the thermal anomaly produced by impact craters. The occurrence of relatively recent (likely Amazonian) hydrological activity, which could have been primarily supported by groundwater replenishment, supports the hypothesis that hydrological activity could have been possible after the Noachian-Hesperian boundary, which is commonly considered as the onset epoch of the present cold-dry climate.

Ring‐Mold Craters on Ceres: Evidence for Shallow Subsurface Water Ice Sources

One of the main tasks of the Dawn mission is to characterize the potentially ice-rich crust of the dwarf planet Ceres. Ongoing studies reveal morphological features related to ice-rich material such as pits or particular landslides. Here we report the identification of ring-mold craters within the huge impact crater Occator. The Cerean ring-mold craters exhibit strong morphological similarities to the ring-mold craters on Mars, where ice-rich material is thought to be involved in such crater development. We discuss the occurrence of water ice reservoirs in the subsurface and assume that ice-rich material likely plays an important role in the development of ring-mold craters on Ceres. The occurrence of ring-mold craters on the surface of Ceres is not only a sign of water ice reservoirs in the subsurface but can also be used for the study of habitable zones on planetary bodies. Plain Language Summary One of the main tasks of the Dawn mission is to characterize the potentially ice-rich crust of the dwarf planet Ceres. Ongoing studies reveal morphological features related to ice-rich material such as pits or particular landslides. Here we report the identification of a special type of craters, so-called ring-mold craters. The craters are found within the huge impact crater Occator. The Cerean ring-mold craters exhibit strong morphological similarities to the ring-mold craters on Mars, where ice-rich material is thought to be involved in such crater development. We discuss the occurrence of water ice reservoirs in the subsurface and assume that ice-rich material likely plays an important role in the development of ring-mold craters on Ceres. The occurrence of ring-mold craters on the surface of Ceres is not only a sign of water ice reservoirs in the subsurface but can also be used for the study of habitable zones on planetary bodies.

THE HAMO-BASED GLOBAL GEOLOGIC MAP OF CERES FROM NASA’S DAWN MISSION

This abstract discusses current results from the 1:2.5M-scale High Altitude Mapping Orbit (HAMO)-based global geologic mapping effort of Ceres using image, spectral and topographic data from the Dawn mission. Mapping base materials include the Dawn Framing Camera (FC) HAMO mosaic and individual images (∼140 m/pixel), the global HAMO DTM (137 m/pixel) derived from FC stereo images, and FC color mosaics (0.44-0.96 μm). These data are used to identify contacts and features, and for unit characterization. Geologic units are discriminated primarily by differences in albedo and surface texture; FC color images are used to spectrally constrain and characterize units. The map displays contacts and linear features (e.g., structures) represented by polylines, and singular features (e.g., albedo spots) represented by points. Because of map scale, only geologic units greater than 100 km2 in area, impact craters greater than 20 km in diameter, and linear features greater than 20 km in length are shown. Through geologic mapping we have defined several widespread units: cratered terrain, smooth material, and units of the Urvara/Yalode system. Cratered terrain forms the largest unit exposed on Ceres and contains rugged surfaces derived largely from the structures and deposits of impact features. This unit includes the oldest terrains exposed on Ceres, but the geologic materials likely consist of crustal materials mixed with impact materials. Smooth material forms a large deposit of nearly flat-lying to hummocky plains that fill and surround Kerwan basin, and embay the cratered terrain. Geologic materials related to the Urvara and Yalode basins consist of floor, rim, and ejecta deposits. Urvara ejecta consists of a rugged and a smooth facies; Yalode ejecta is distinguished by its smooth and rolling to stucco-like texture. Stratigraphic relations show that ejecta deposits and structures from Urvara superpose Yalode, indicating it is younger. Impact craters are the most prevalent features on the surface of Ceres, and appear to have caused most of the visible modification of the surface [1]. Impact craters exhibit sizes ranging from the limits of resolution to larger impact basins such as Urvara (170 km), Yalode (260 km), and Kerwan (284 km). Impact craters also exhibit a range of preservation states. Many craters of all sizes appear morphologically “fresh” to moderately degraded, with nearly circular rims that are raised above the surrounding terrain. Small fresh craters (<15 km) display simple bowl shapes, whereas larger fresh craters display steep walls and flat (sometimes fractured) floors [2], and most contain hummocky or irregular-shaped deposits on their floors. Many craters exhibit irregularly shaped, sometimes scalloped, rim structures, and contain debris lobes on their floors, suggesting instability in surface materials [1]. We are currently engaged in crater-based age dating, determining superposition relations, and using these to interpret Ceres chronostratigraphy, which will be presented at EGU. Support of the Dawn Instrument, Operations, & Science Teams is acknowledged. This work is supported by grants from NASA, DLR and MPG.