Katharina A. Otto

The geomorphology of Ceres

INTRODUCTION Observations of Ceres, the largest object in the asteroid belt, have suggested that the dwarf planet is a geologically differentiated body with a silicate core and an ice-rich mantle. Data acquired by the Dawn spacecraft were used to perform a three-dimensional characterization of the surface to determine if the geomorphology of Ceres is consistent with the models of an icy interior. RATIONALE Instruments on Dawn have collected data at a variety of resolutions, including both clear-filter and color images. Digital terrain models have been derived from stereo images. A preliminary 1:10 M scale geologic map of Ceres was constructed using images obtained during the Approach and Survey orbital phases of the mission. We used the map, along with higher-resolution imagery, to assess the geology of Ceres at the global scale, to identify geomorphic and structural features, and to determine the geologic processes that have affected Ceres globally. RESULTS Impact craters are the most prevalent geomorphic feature on Ceres, and several of the craters have fractured floors. Geomorphic analysis of the fracture patterns shows that they are similar to lunar Floor-Fractured Craters (FFCs), and an analysis of the depth-to-diameter ratios shows that they are anomalously shallow compared with average Ceres craters. Both of these factors are consistent with FFC floors being uplifted due to an intrusion of cryomagma. Kilometer-scale linear structures cross much of Ceres. Some of these structures are oriented radially to large craters and most likely formed due to impact processes. However, a set of linear structures present only on a topographically high region do not have any obvious relationship to impact craters. Geomorphic analysis suggests that they represent subsurface faults and might have formed due to crustal uplift by cryomagmatic intrusion. Domes identified across the Ceres surface present a wide range of sizes (<10 km to >100 km), basal shapes, and profiles. Whether a single formation mechanism is responsible for their formation is still an open question. Cryovolcanic extrusion is one plausible process for the larger domes, although most small mounds (<10-km diameter) are more likely to be impact debris. Differences in lobate flow morphology suggest that multiple emplacement processes have operated on Ceres, where three types of flows have been identified. Type 1 flows are morphologically similar to ice-cored flows on Earth and Mars. Type 2 flows are comparable to long-runout landslides. Type 3 flows morphologically resemble the fluidized ejecta blankets of rampart craters, which are hypothesized to form by impact into ice-rich ground. CONCLUSION The global trend of lobate flows suggests that differences in their geomorphology could be explained by variations in ice content and temperature at the near surface. Geomorphic and topographic analyses of the FFCs suggest that cryomagmatism is active on Ceres, whereas the large domes are possibly formed by extrusions of cryolava. Although spectroscopic analysis to date has identified water ice in only one location on Ceres, the identification of these potentially ice-related features suggests that there may be more ice within localized regions of Ceres’ crust. Dawn high-altitude mapping orbit imagery (140 meters per pixel) of example morphologic features. (A) Occator crater; arrows point to floor fractures. (B) Linear structures, denoted by arrows

The structure of the regolith on 67P/Churyumov-Gerasimenko from ROLIS descent imaging

The structure of the upper layer of a comet is a product of its surface activity. The Rosetta Lander Imaging System (ROLIS) on board Philae acquired close-range images of the Agilkia site during its descent onto comet 67P/Churyumov-Gerasimenko. These images reveal a photometrically uniform surface covered by regolith composed of debris and blocks ranging in size from centimeters to 5 meters. At the highest resolution of 1 centimeter per pixel, the surface appears granular, with no apparent deposits of unresolved sand-sized particles. The thickness of the regolith varies across the imaged field from 0 to 1 to 2 meters. The presence of aeolian-like features resembling wind tails hints at regolith mobilization and erosion processes. Modeling suggests that abrasion driven by airfall-induced particle “splashing” is responsible for the observed formations.

Cryogenic flow features on Ceres: Implications for crater‐related cryovolcanism

Craters on Ceres, such as Haulani, Kupalo, Ikapati, and Occator show postimpact modification by the deposition of extended plains material with pits, multiple lobate flows, and widely dispersed deposits that form a diffuse veneer on the preexisting surface. Bright material units in these features have a negative spectral slope in the visible range, making it appear bluish with respect to the grey-toned overall surface of Ceres. We calculate the drop height-to-runout length ratio of several flow features and obtain a coefficient of friction of <0.1: The results imply higher flow efficiency for flow features on Ceres than for similar features on other planetary bodies with similar gravity, suggesting low-viscosity material. The special association of flow features with impact craters could either point to an impact melt origin or to an exogenic triggering of cryovolcanic processes.

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.

An investigation of the bluish material on Ceres

The dwarf planet Ceres shows spatially well-defined regions, which exhibit a negative (blue) spectral slope between 0.5 and 2.5 µm. Comparisons with planetary bodies known to exhibit a blue slope and spectral properties of materials identified on Ceres’ surface based on infrared wavelength signatures indicate the spectral changes could be related to physical properties of the surface material rather than variations in its composition. The close association of bluish surface regions to fresh impact craters implies a possible relationship to an impact-triggered alteration and/or space weathering processes. The bluish regions could be linked with blankets of ultra-fine grains and partly amorphous phyllosilicates, which form larger agglomerates due to the sticky behavior of impact induced phyllosilicate dust and/or the amorphization of the ejecta material during the impact process. Space weathering processes (micro-meteoritic impacts, temperature changes) cause a reversal of the agglutination process and a re-crystallization of the surface material with time resulting in a reddening of the spectral slope.

The vanishing cryovolcanoes of Ceres

Ahuna Mons is a 4-km-tall mountain on Ceres interpreted as a geologically young cryovolcanic dome. Other possible cryovolcanic features are more ambiguous, implying that cryovolcanism is only a recent phenomenon or that other cryovolcanic structures have been modified beyond easy identification. We test the hypothesis that Cerean cryovolcanic domes viscously relax, precluding ancient domes from recognition. We use numerical models to predict flow velocities of Ahuna Mons to be 10–500 m/Myr, depending upon assumptions about ice content, rheology, grain size, and thermal parameters. Slower flow rates in this range are sufficiently fast to induce extensive relaxation of cryovolcanic structures over 108–109 years, but gradual enough for Ahuna Mons to remain identifiable today. Positive topographic features, including a tholus underlying Ahuna Mons, may represent relaxed cryovolcanic structures. A composition for Ahuna Mons of >40% ice explains the observed distribution of cryovolcanic structures because viscous relaxation renders old cryovolcanoes unrecognizable.

Seasonal Mass Transfer on the Nucleus of Comet 67P/Chuyumov-Gerasimenko

We collect observational evidence that supports the scheme of mass transfer on the nucleus of comet 67P/Churyumov-Gerasimenko. The obliquity of the rotation axis of 67P causes strong seasonal variations. During perihelion the southern hemisphere is four times more active than the north. Northern territories are widely covered by granular material that indicates back fall originating from the active south. Decimetre sized chunks contain water ice and their trajectories are influenced by an anti-solar force instigated by sublimation. OSIRIS observations suggest that up to 20 per cent of the particles directly return to the nucleus surface taking several hours of travel time. The back fall covered northern areas are active if illuminated but produce mainly water vapour. The decimetre chunks from the nucleus surface are too small to contain more volatile compounds such as CO 2 or CO. This causes a north-south dichotomy of the composition measurements in the coma. Active particles are trapped in the gravitational minimum of Hapi during northern winter. They are ‘shock frozen’ and only re-activated when the comet approaches the sun after its aphelion passage. The insolation of the big cavity is enhanced by self-heating, i. e. reflection and IR radiation from the walls. This, together with the pristinity of the active back fall, explains the early observed activity of the Hapi region. Sobek may be a role model for the consolidated bottom of Hapi. Mass transfer in the case of 67P strongly influences the evolution of the nucleus and the interpretation of coma measurements.

Close-up images of the final Philae landing site on comet 67P/Churyumov-Gerasimenko acquired by the ROLIS camera

After coming to rest on the night side of comet 67P/Churyumov-Gerasimenko, the ROLIS camera on-board Rosetta’s Philae lander acquired five images of the surface below the lander, four of which were with the aid of LED illumination of different colors. The images confirm that Philae was perched on a sloped surface. A local horizon is visible in one corner of the image, beyond which we can see the coma. Having spent a full day on the surface Philae was commanded to lift and rotate, after which a final, sixth, LED image was acquired. The change in perspective allowed us to construct a shape model of the surface. The distance to the foreground was about 80 cm, much larger than the nominal 30 cm. This caused stray light, rather than directly reflected LED light, to dominate the image signal, complicating the analysis. The images show a lumpy surface with a roughness of apparently fractal nature. Its appearance is completely different from that of the first landing site, which was characterized by centimeter to meter-sized debris (Mottola et al., 2015). We recognize neither particles nor pores at the image resolution of 0.8 mm per pixel and large color variations are absent. The surface has a bi-modal brightness distribution that can be interpreted in terms of the degree of consolidation, a hypothesis that we support with experimental evidence. We propose the surface below the lander to consist of smooth, cracked plates with unconsolidated edges, similar to terrain seen in CIVA images.

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 Coriolis effect on mass wasting during the Rheasilvia impact on asteroid Vesta

We investigate the influence of the Coriolis force on mass motion related to the Rheasilvia impact on asteroid Vesta. Observations by the NASA Dawn mission revealed a pattern of curved radial ridges, which are related to Coriolis-deflected mass-wasting during the initial modification stage of the crater. Utilizing the projected curvature of the mass-wasting trajectories, we developed a method that enabled investigation of the initial mass wasting of the Rheasilvia impact by observational means. We demonstrate that the Coriolis force can strongly affect the crater formation processes on rapidly rotating objects, and we derive the materials velocities (28.9 +/- 22.5 m/s), viscosities (1.5-9.0 × 106 Pa s) and coefficients of friction (0.02-0.81) during the impact modification stage. The duration of the impact modification stage could be estimated to (1.1 +/- 0.5) h. By analyzing the velocity distribution with respect to the topography, we deduce that the Rheasilvia impactor hit a heterogeneous target and that the initial crater walls were significantly steeper during the modification stage.