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Spectral investigation of quadrangle AC-H 3 of the dwarf planet Ceres – The region of impact crater Dantu
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Spectral investigation of quadrangle AC -H 3 of the dwarf planet Ceres – The region of impact crater Dantu K. Stephan a,∗, R. Jaumann a,b, F. Zambon c, F.G. Carrozzo c, M.C. De Sanctis c, F. Tosi c, A. Longobardo c, E. Palomba c, E. Ammannito d, L.A. McFadden e, K. Krohn a, D.A. Williams f, A. Raponi c, M. Ciarnello c, J.-P. Combe g, A. Frigeri c, T. Roatsch a, K.-D. Matz a, F. Preusker a, C.A. Raymond h, C.T. Russell d a
DLR, Institute of Planetary Research, Berlin, Germany Free University of Berlin, Germany c INAF-IAPS, Rome, Italy d UCLA, Institute of Geophysics and Planetary Physics, Los Angeles, CA, USA e NASA Goddard Space Flight Center, Greenbelt, MD, USA f Arizona State University, Tempe, AZ, USA g Bear Fight Institute, Winthrop, WA, USA h NASA-JPL Pasadena, CA, USA b
a r t i c l e
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Article history: Received 26 April 2017 Revised 3 July 2017 Accepted 25 July 2017 Available online xxx
a b s t r a c t Mapping Ceres’ surface composition in the Dantu region, located between 21°–66°N and 90°–180°E, offers the unique possibility to investigate changes in the surface composition related to different stratigraphic levels of Ceres’ crust. Dantu is located in a huge depression named Vendimia Planitia, which possibly represents a completely degraded impact basin formed in the beginning of Ceres’ geological history. Most parts of this depression are characterized by strong phyllosilicate absorptions, which are stronger than elsewhere on Ceres’ surface. This spectral signature possibly is related to the material emplaced at the time of the Vendimia impact event excavating material from deeper regions of Ceres’ crust. Subsequent impacts in this basin reach far deeper into Ceres’ crust than any impact events outside of Vendemia Planitia, which could explain the spectral signature of Dantu, possibly pointing to a higher concentration of ammonium-bearing phyllosilicates in Ceres’ deeper crust. Spectral differences with respect to the small fresh craters on Dantu’s floor are probably related to grain size effects causing a bluish visible slope as observed by fresh impact craters on other places on Ceres. The local enrichment of carbonates in the Dantu area could also be associated with the impact event and may have been formed by additional impact-triggered and/or post-impact alteration processes. © 2017 Elsevier Inc. All rights reserved.
1. Introduction Since NASA’s Dawn spacecraft arrived at Ceres in March 2015, it has shown that the surface composition of the dwarf planet Ceres is dominated by a mixture of ammoniated phyllosilicates, Mg-phyllosilicates, carbonates and some dark materials (De Sanctis et al., 2015). The composition has been found to be rather homogeneous on a global scale, implying a globally widespread endogenous formation of the surface material, i.e. an aqueous alteration of silicates (Ammannito et al., 2016b). The abundance and/or physical properties of the individual surface compounds, however, appears
∗
Corresponding author. E-mail address: [email protected] (K. Stephan).
to be quite variable on a regional and local scale, which could be evidence of a vertically stratified upper crust (Ammannito et al., 2016b) or could result from specific surface processes (Stephan et al., 2017a). In order to improve our understanding of the compositional variations in Ceres’ crustal material, the Dawn Science Team conducted a campaign for mapping the spectral properties of Ceres’ surface based on data acquired by the instruments aboard the Dawn spacecraft, i.e. Dawn’s Framing Camera (FC) (Sierks et al., 2011) as well as its Visible and Infrared Spectrometer (VIR) (De Sanctis et al., 2011). The resulting spectral maps provide the unique opportunity to investigate Ceres’ surface composition in comparison to its geology and topography, which is essential to resolve the origin of the surface compounds and the processes responsible for their formation.
http://dx.doi.org/10.1016/j.icarus.2017.07.019 0019-1035/© 2017 Elsevier Inc. All rights reserved.
Please cite this article as: K. Stephan et al., Spectral investigation of quadrangle AC -H 3 of the dwarf planet Ceres – The region of impact crater Dantu, Icarus (2017), http://dx.doi.org/10.1016/j.icarus.2017.07.019
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Fig. 1. Global HAMO mosaic with nomenclature in Mollweide projected (adapted from Roatsch et al., (2016a)). The Dantu region of quadrangle AC -H 3 is highlighted by the red box.
In this study we explore the AC -H 3 quadrangle, including Ceres’ surface located between 21° and 66°N as well as 90° and 180°E (Fig. 1), named after its dominating surface feature Dantu, a large complex impact crater with an extended ejecta blanket situated in the southern portion of this quadrangle. It is not only an impressive geological feature but is also spectrally unique. Furthermore, Dantu is located at a geographic longitude, where Kuppers et al. (2014) suggested water vapor was being released from Ceres, based on ESA Herschel space telescope observations. Its investigation might offer a key to further our understanding about the compositional and geological evolution of Ceres’ crust (Kneissl et al., 2016). 2. Data basis and methodology The Dawn mission at Ceres consists of four principle phases performed at different altitudes of the spacecraft from the surface in order to maximize coverage and to meet the viewing conditions of the requirements of mapping Ceres’ surface: the Survey mission (altitude of 4900 km), the High Altitude Mapping Orbit (HAMO, altitude of 1950 km), and Low Altitude Mapping Orbit (LAMO) 1and 2 (altitude of ∼850 km) (Russell et al., 2016). In order to investigate the composition of Ceres’ surface in detail, we mainly used FC images and hyperspectral VIR cubes acquired during the Survey, HAMO and LAMO 1 mission phase offering the highest pixel ground resolution possible. 2.1. VIR data The VIR instrument detects Ceres’ surface between 0.25 and 1.05 μm (VIS) and between 1.0 and 5.1 μm (IR), respectively. The spectral resolution of each VIR channel is 1.8 nm and 9.8 nm per spectral channel, respectively, which allows the identification of major surface compounds and changes in their spectral parameters like the strength and wavelength position of individual and/or overlapping absorptions (De Sanctis et al., 2011). The spatial resolution of the VIR cubes reach 360 m per pixel during HAMO and 90 m per pixel during LAMO, which enables mapping local changes in Ceres’ surface composition. VIR data have been calibrated by the VIR team following the procedure of Filacchione et al. (2011) and calibration artifacts have been removed by Carrozzo et al. (2016). In
order to study the spectral characteristics of Ceres spectra at wavelengths longer than 3 μm, which are strongly affected by Ceres’ thermal signal, a correction of the thermal signal has been performed as described in Raponi (2015). Further information of the mapping process can be found in Frigeri et al. (this issue). During the Dawn mission, the VIR instruments measured distinct absorptions and spectral parameters including depth and position, which are indicative of specific surface compounds. The spectral signature of Ceres is generally dominated by a mixture of NH4 -montmorillonite or NH4 -annite, antigorite, Mg-carbonate, and a featureless, dark, possibly carbon-rich component (De Sanctis et al., 2015; Hendrix et al., 2016). Ammoniated phyllosilicates exhibit prominent absorptions at ∼2.7 μm indicative of OH− , and at 3.1 μm, indicative of NH4 − . Carbonates like magnesite (MgCO3 ) show relatively broad absorptions at 3.4 and 3.9 μm (De Sanctis et al., 2016), while sodium carbonates, such that identified in Cerealia facula (De Sanctis et al., 2016), have absorptions at longer wavelength (at about 4.01 μm). The absorption at 3.4 μm, however, was also found to be associated with areas enriched in organics (De Sanctis et al., 2017b). Finally, H2 O-ice, with its distinct absorptions at 1.04, 1.25, 1.5 and 2 μm, has been locally identified on Ceres’ surface in the vicinity of small (less than 20 km in diameter) morphologically fresh impact craters, such as Oxo (Combe et al., 2016; Combe et al., 2017) and Juling (De Sanctis et al., 2017a; Raponi et al., 2017a). The variations in the spectral parameters of these spectral signatures as well as the slope of the overall spectral continuum vary depending on the abundance as well as the physical properties of the specific surface compound. Detailed mapping of these variations enables the investigation of Ceres’ surface composition depending on its location on the body as well as the surface geology and topography, which is essential to reveal their origin and surface processes responsible for their existence. Measurements of the band depths (BDs) of individual absorptions have been performed following the procedure of Clark (2003), and described in detail by Ammannito et al. (2016a). 2.2. FC data Images from the Framing Camera, which offer the geological context of the Dantu region, reach spatial resolutions up to
Please cite this article as: K. Stephan et al., Spectral investigation of quadrangle AC -H 3 of the dwarf planet Ceres – The region of impact crater Dantu, Icarus (2017), http://dx.doi.org/10.1016/j.icarus.2017.07.019
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∼140 m/pixel (HAMO) and ∼35 m/pixel (LAMO). In order to study the relationship between surface composition and geology in general, a HAMO mosaic with a mean resolution of 140 m per pixel (Roatsch et al., 2016b) has been used (Fig. 1). The comparison of local spectral changes and geology was done using individual FC images acquired during the LAMO 1 phase, preferentially acquired at a similar time of day as the corresponding VIR observation and thus with similar illumination conditions. In each mission phase, Ceres has been mapped completely under similar illumination conditions but different viewing condition, which enables the construction of digital terrain models (DTM). Thus, in addition to the FC images, a stereogrammetry-based DTM derived from HAMO data (135 m/pixel) (see Preusker et al., (2016)) is available (Fig. 1). FC data, however, do not only offer the geologic context for our spectral investigation but also provide spectral information from its 7 color filters that are sensitive in a wavelength range between 0.4 and 1.1 μm (Sierks et al., 2011). For each filter a global photometrically corrected HAMO mosaic is available (Roatsch et al., 2016b; Schröder et al., 2015). Although these images do not offer a continuous spectrum like the VIR observations, they are useful to see changes in the visible albedo and spectral slope at a much higher spatial resolution – revealing correlations between composition and geology at a local scale. Fig. 1 shows the enhanced true color mosaic combining the mosaics derived from images of filters F5, F2 and F8, sensitive at 965, 555 and 438 nm, respectively, and a classification map of the spectral slope, i.e. the ratio of the FC filters F8 and F3, sensitive at 438 and 749 nm as demonstrated in Stephan et al. (2017a). 3. Geological overview The geology of the Dantu area (Fig. 2) has been studied in detail by Kneissl et al. (2016) and Williams et al. (2016), including the investigation of the surface ages of the specific geologic units as derived by measurements of crater size frequency distributions using the lunar-derived (LDM) and the asteroid-derived (ADM) cratering model (Hiesinger et al., 2016; Schmedemann et al., 2016). We summarize here the major geological facts relevant for comparison with the spectral properties. Most parts of the area lie in a large-scale (diameter of 750 km) depression named Vendimia Planitia (Marchi et al., 2016), which extends between 20°S/85°E and 65°N/185°E (Fig. 1). Numerous impact craters of various sizes and fresh to highly degraded morphology characterize the area (Fig. 2). The northern and southeastern parts of the quadrangle are dominated by geologically old cratered terrain, including the largest crater in this area, Vinotonus (d=∼140 km; 43.0°N/95.1°E). Vinotonus, located at the western quadrangle boundary, is highly degraded without any sign of a former ejecta blanket. The southern and southwestern parts of the area are dominated by the partially smooth ejecta blankets of the impact craters Dantu (d=∼126 km; 24.3°N/138.2°E) and Gaue (d=∼80 km; 31.8°N/86.2°E). Gaue is located at the border to quadrangle AC -H 2 (Raponi et al., 2017b) (Fig. 1). Only the eastern portion of its ejecta reaches into the Dantu area. High-resolution measurements of crater size-frequency distributions (Hiesinger et al., 2016) imply a similar surface age for both impact features. Schmedemann et al. (2016) and Pasckert et al. (2017) derived a LDM surface age of ∼162 Ma and 220 – 260 Ma for Gaue, respectively. Kneissl et al. (2016) measured surface ages ranging between ∼72–150 Ma for different parts of Dantu’s ejecta blanket and crater interior. The Dantu crater is the most prominent surface feature and extends over the quadrangle boundary into quadrangle AC -H 4 (Kerwan) (Williams et al., 2016). The crater has a quite complex morphology (Fig. 2) with a central peak similar to Ceres’ famous impact feature Occator (19.8°N/239.3°E), but which apparently has
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collapsed during the crater modification phase. Further, pitted terrain can be recognized on its floor (Kneissl et al., 2016). The crater itself exhibits a distinct topography with up to ∼4 km high crater walls that are characterized by extended slump terraces. Linear features interpreted as fractures of possible extensional origin are confined to the crater floor of Dantu. It is not clear, however, if these fractures result from the Dantu impact event or have been formed by post-impact processes (Kneissl et al., 2016). FC images reveal that smooth flow-like material interpreted as impact melt (Krohn et al., 2016a) superpose these fractures. Other lobate features are also associated with small impact craters north of Dantu such as Jaja (diameter = 22.0 km, 52.1°N/125.3°E) and Xochipili (diameter: 22.7 km, 56.7°N/93.2°E). Superimposed onto Dantu’s crater floor are two small fresh impact craters – a 5-km large unnamed impact crater located near 23.1°N/131.7°E and the 6 km large impact crater Centeotl (18.9°N/141.2°E) with the latter already situated in quadrangle AC -H 7 (Williams et al., 2016). Several bright spots appear on the crater floor but also at the crater wall and outside the crater as part of the ejecta blanket (Kneissl et al., 2016). Although their origin is not fully understood yet, they possibly represent the youngest surface deposits in the Dantu area. 4. Overview of the spectral characteristics within the Dantu-quadrangle The VIR-derived spectral characteristics of the Dantu region in general mirror the average surface composition of Ceres (De Sanctis et al., 2015). The abundance and/or the physical properties of the individual surface compounds such as phyllosilicates and carbonates, however, changes throughout the region causing variations in the band depths (BDs) and/or wavelength position of the individual characteristic absorptions. Fig. 3 shows the endmember spectra, which reflect the spectral variations identified in the Dantu region. Non- and ammoniated phyllosilicates with the diagnostic absorptions at 2.7 and 3.1 μm, respectively, and carbonates, which are responsible for the absorptions at 3.4 and 3.9 μm, appear in every VIR spectrum. Changes in the depth of the two absorptions at 2.7 and 3.1 μm in the endmember spectra of the Dantu region appear to be well correlated and support their origin in the same material, i.e. phyllosilicates. In comparison to Ceres’ average spectrum (spectrum #3), relatively weak absorptions occur in the endmember spectra #5 and #6 and deep absorptions in the endmember spectrum #4 (Fig. 3). On the contrary, relatively deep carbonate absorptions at 3.4 and 4 μm can be observed in the endmember spectra #1 and #2 in Fig. 3 and appear uncorrelated from the variations in the other spectral parameters. In connection to the enrichment in carbonates, a slight shift of the band center to longer wavelengths (from 2.73 to 2.77 μm) and broadening of the OH absorption at 2.7 μm are apparent. A similar shift has been also reported in Occator’s bright deposits (De Sanctis et al., 2016; Longobardo et al., 2017) and in few bright pixels in the area of the impact crater Kupalo (39.4°S/173.2°E) (De Sanctis et al., 2017a), suggesting a similar change in the mineralogy of the phyllosilicate species. H2 O-ice as observed in limited locations on Ceres’ surface (Combe et al., 2016; Combe et al., 2017) with its distinct absorptions at 1.04, 1.25, 1.5 and 2 μm has also been identified in one small high-albedo area of the AC -H 3 quadrangle (155°E/30°N) (Combe et al., 2017) (see below). The variations in the spectral properties often correspond to changes in the visible albedo as well as the overall spectral slope as derived from the FC images. The ratio of the filters F8 and F3 varies between ∼0.8 and ∼1.2, indicating changes in the visible spectral slope from reddish (positive) in the carbonate-rich spectrum (#1) to bluish (negative) in the spectrum with the weakest phyllosilicate absorptions at 2.7 and 3.1 μm (#4) (Fig. 3). The visible albedo, measured at 550 nm in the Dantu area, changes between
Please cite this article as: K. Stephan et al., Spectral investigation of quadrangle AC -H 3 of the dwarf planet Ceres – The region of impact crater Dantu, Icarus (2017), http://dx.doi.org/10.1016/j.icarus.2017.07.019
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Fig. 2. Geological context of the Dantu region: (top) FC Clear filter mosaic, (center) the DTM, and (bottom) the geological map adapted from Kneissl et al. (2016) including the names of the major geological features.
∼2.9% and 9.1%. The highest albedo corresponds to the carbonaterich spectra. The lowest albedo is associated with spectrum #4 with the bluish slope and the weak absorptions at 2.7 and 3.1 μm (Fig. 3). These relationships become obvious in the maps of the individual spectral parameters of the Dantu region, which will be described in the next section.
5. Distribution of spectral units In the spectral parameter maps, i.e. the band depth maps of the absorptions at 2.7 and 3.1 μm as well as the FC color composite and the spectral slope classification map (Figs. 4 and 5), two major areas can be distinguished. Similar to what can be seen in the
Please cite this article as: K. Stephan et al., Spectral investigation of quadrangle AC -H 3 of the dwarf planet Ceres – The region of impact crater Dantu, Icarus (2017), http://dx.doi.org/10.1016/j.icarus.2017.07.019
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Fig. 3. Spectral endmembers of the Dantu-region on Ceres over the wavelength range detected by the VIR-IR instrument identified in specific locations: spectra #1 and #2 bright spots (Figs. 7 and 9), average spectrum #3 of the Dantu region, spectrum #4 located Dantu’s central pit (Figs. 7 and 8), spectrum #5 of an unnamed small fresh crater located within Dantu (Fig. 7), spectrum #6 of impact crater Ialonos (Fig. 11). The spectrum of the only H2 O-ice rich spot in the Dantu region is shown in Fig. 6.
Fig. 4. Maps of the Dantu-quadrangle illustrating: (top) FC color composite, (bottom) FC Slope Classification.
geological map of Fig. 1, the southern portion of our region of interest is dominated by the spectral signature of the impact crater Dantu and its ejecta blanket which is characterized by a generally higher visible albedo (∼3.6 – 4.5%) with a red-to-neutral visible spectral slope similar to the neighboring geologically old terrain, which exhibits a reddish tint in the FC color composite (Fig. 4).
Intriguingly, the bright region associated with Dantu and its ejecta blanket also exhibits deep absorptions at 2.7 and 3.1 μm (band depth of ∼23% and 14%, respectively). Indeed, when looking at the entire surface of Ceres, both absorptions are deepest in the Dantu area [Stephan et al. this issue] implying the highest amount of phyllosilicates (Ammannito et al., 2016b). Thus, a red visible
Please cite this article as: K. Stephan et al., Spectral investigation of quadrangle AC -H 3 of the dwarf planet Ceres – The region of impact crater Dantu, Icarus (2017), http://dx.doi.org/10.1016/j.icarus.2017.07.019
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Fig. 5. VIR-derived maps of the Dantu-quadrangle illustrating the band depth (BD) of the phyllosilicate absorptions at 2.7 μm (top) and 3.1 μm (bottom).
spectral slope does not always indicate a similar visible albedo and similar spectral properties in the near-infrared wavelength range. Nevertheless, small fresh impact craters superimposed on Dantu as well as outside Dantu always appear bluish in the FC color composite. The northern highly cratered terrain shows a higher variegation in the spectral properties, but generally, a slightly lower visible albedo and weaker absorptions at 2.7 and 3.1 μm imply a lower abundance of phyllosilicates in this area (Ammannito et al., 2016b). As mentioned above, regions associated with fresh impact craters such as Ialonos show a blue slope as well as a bluish color in the FC color composite and the weakest absorptions at 2.7 μm and 3.1 μm, similar to other blue sloped young craters observed on Ceres (Stephan et al., 2017a). The remaining cratered terrain exhibits a reddish visible slope similar to Dantu’s central pit. This terrain also exhibits a reddish color in the color composite of the Dantu region and a relatively low visible albedo. This is contrary to what can be seen in the vicinity of Dantu, possibly pointing to compositional differences but also perhaps similarities in the physical surface properties between these two terrain types. Changes in the appearance and/or abundance of carbonates do not occur on a global/regional scale. Only locally a few small bright spots could be detected in the Dantu area that show a slightly stronger carbonate signature (discussed below), and only one local bright spot associated to the wall of a ∼275 m large impact crater shows evidence of H2 O ice (Combe et al., 2017) (Fig. 6). In particular, the ratio of the VIR spectra selected for the bright spot and the average VIR spectrum of the surrounding region show the diagnostic H2 O-ice absorptions at 1.04, 1.25, 1.5 and 2 μm (Fig. 6e).
Every FC-image of this region taken during the LAMO and HAMO mission phase shows this bright spot, which is bright in the images of all 7 color filters. The bright spot is visible in images acquired from September 2015 (HAMO) to October 2016 (LAMO2) with a diameter of about ∼120 m. No significant time-dependent changes could be recognized with respect to its size and albedo. FC images acquired during the Survey mission phase do not show the bright spot. However, it cannot be excluded that at that time the bright spot already existed, and that the lower pixel ground resolution (∼400 m) of the images prevented its detection. Hayne and Aharonson (2015) estimated that ice at these latitudes (0 to 30°) is stable with solar illumination for at least about 10 to 100 years. Since Prettyman et al. (2017) predicted extended ice deposits in Ceres’ surface, the observed icy spot could represent excavated subsurface ice during a recent impact event or slumping. Possibly, the icy material is part of Dantu’s ejecta material since the bright spot lies completely within the region of Dantu’s ejecta blanket (Fig. 1). However, this does not explain, why this icy spot is only one detected so far in the Dantu area (Combe et al., 2017). Either additional spots are below the detection limit or the H2 O-ice is of exogenic origin with the H2 O ice being a remnant of an ice-rich impactor.
5.1. Impact crater Dantu Impact crater Dantu displays a wide variety of colors in the FC color filters and in the VIR derived spectral properties (Fig. 7). All spectral endmembers derived for the Dantu-region (Fig. 3) can be
Please cite this article as: K. Stephan et al., Spectral investigation of quadrangle AC -H 3 of the dwarf planet Ceres – The region of impact crater Dantu, Icarus (2017), http://dx.doi.org/10.1016/j.icarus.2017.07.019
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Fig. 6. Close-up view of the small H2 O-ice rich location identified by (Combe et al., 2017): (a) full and (b) subset of VIR observation 494,522,179 (HAMO) with (c) the full and (d) subset of the corresponding FC composite including LAMO images of filter 5 (R), 2 (G), and 8 (B) with the H2 O-ice rich spot (indicated by the green point or arrow) evident (e) by the H2 O-ice absorptions at 1.04, 1.25, 1.5 and 2 μm.
observed in the vicinity of this complex impact feature. Local spectral variations are associated to specific inter-crater features. Significant parts of the crater floor are covered by material emplaced by small impacts (one of these small craters named Centeotl is situated in quadrangle AC -H 7 (Kerwan) investigated by Palomba et al. (2017) (Fig. 7). These impact features are characterized by a slightly blue slope and weaker absorptions at 2.7 μm and 3.1 μm than the average depths measured on Ceres (spectral endmember #1 in Fig. 3). This corresponds to the spectral properties of other young impact craters on Ceres’ surface, which exhibit a blue spectral slope as discussed by Stephan et al., (2017a). Intriguingly, Dantu’s central pit and portions of the eastern crater wall and floor show a distinct visible red-sloped visible spectrum. These red-sloped areas also nicely correspond to the regions, where the deepest OH and NH4 absorptions at 2.7 and 3.1 μm, respectively, have been measured (Figs. 7 and 8). LAMO FC images illustrate that these deep absorptions occur in the interior of the pit and con-
tinue in topographically low-lying areas surrounding the pit. The red-slope regions continue especially in the eastern direction up to the crater wall. Less deep but still distinct deeper absorptions can also be found in the western portions of the impact ejecta. The remaining areas of the crater floor show the typical average surface composition and a neutral (neither red nor blue spectral slope) of Ceres. Usually, red-sloped spectra characterize geologically old weathered and/or slumping material on Ceres’ surface (Stephan et al., 2017a). Also, the old cratered terrain surrounding Dantu shows a reddish visible slope associated with a reddish color of the surface in the FC color composite and a VIR spectrum similar to Ceres’ average spectral signature (spectrum #4 in Fig. 3). The red-sloped regions associated with Dantu itself appear bright in the FC color composite, and the corresponding VIR spectrum is characterized by deep absorptions at 2.7 and 3.1 μm, which does not correspond fully to a typically red spectrum. Thus, the comparison of the FC Color Composite and FC slope map with the
Please cite this article as: K. Stephan et al., Spectral investigation of quadrangle AC -H 3 of the dwarf planet Ceres – The region of impact crater Dantu, Icarus (2017), http://dx.doi.org/10.1016/j.icarus.2017.07.019
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Fig. 7. Spectral properties of impact crater Dantu: a) FC image, b) FC DTM, c) FC slope classes, d) BD at 2.7 and e) BD at 3.1 μm. The location of close-up views presented in Figs. 7 and 8 as well as the border between the Dantu region of quadrangle AC -H 3 discussed in this work and the southern AC -H 7 quadrangle are highlighted. Black arrows point out small fresh impact craters superimposed onto Dantu’s floor (Fig. 3, spectrum #5) with Centeotl located in quadrangle AC -H 7 (Palomba et al., 2017). Red arrows point to local bright spots dominated by carbonates (Fig. 3, spectra #1 and #2).
VIR-derived spectral parameter maps imply that a red-sloped area is not always correlated to the spectral signature of Ceres’ surface material in the infrared wavelength region. However, slumping material related to young impact craters can also show a red-slope pointing to a relationship to physical properties of the surface material (Stephan et al., 2017a). Possibly, red-sloped material within Dantu, including the material of the collapsed central pit, represents slumped material as can partly be seen along Dantu’s crater walls. Few locally, very limited bright spots are visible at the crater walls (indicated by the red arrows in Fig. 7a). These bright deposits show weak absorptions at 2.7 and 3.1 μm but not a blue slope like the small fresh impact craters (spectral endmember #1 in Fig. 3). Instead they show a slightly broader absorption at 2.7 μm centered at slightly longer wavelengths and distinct absorptions at 3.4 and 3.9 μm, indicative of enrichment in carbonates (De Sanctis et al., 2016). Apparently this material became excavated very recently due to mass wasting processes at the very steep crater walls (Fig. 9). Additional, bright carbonate-rich spots are also apparent on Dantu’s crater floor and ejecta blanket, which are mostly associated to a visible red slope (Fig. 7). The highest concentration appears in Dantu’s ejecta blanket located in the neighboring quadrangle AC -H 7 (Kerwan) discussed in detail by Williams et al. (2016) and Palomba et al. (2017). Appearance and spectral properties of these bright spots are similar to the unique bright spots seen at Occator consisting of large amounts of sodium carbonate and interpreted to be of endogenous origin (De Sanctis et al., 2016;
Longobardo et al., 2017). The heat source may be triggered by impact heating, or Ceres’ internal temperatures may be above the eutectic temperature of subsurface brines, in which case fluids may exist at certain depths on Ceres today (De Sanctis et al., 2016). Geologic investigations of the Dantu area by Williams et al. (2016) and Kneissl et al. (2016) suggest that the bright carbonate-rich material is part of Dantu’s impact ejecta but cannot exclude its emplacement by cryovolcanic venting. Cryovolcanic activity or the outgassing of volatiles such as CO2 in the Dantu area is also supported by the numerous pits on Dantu’s crater floor. Furthermore, the concentration and orientation of fractures in the southern portion of Dantu (Fig. 2) (Buczkowski et al., 2016) could imply extensional forces formed by a bulge over a concentration of volatiles. Stratigraphic relationships of the fractures to other morphological features in the Dantu region imply that these fractures have been formed subsequently to the formation of the Dantu crater. These fractures are confined to the crater but are covered by, i.e. older than, smooth terrain around the central pit and along the crater walls (bright crater floor material in the geological map of Fig. 2) and the fresh small impact craters such as Centeotl (Fig. 7). Although the dimensions of these fractures are too small to be completely resolved by VIR, the spectral signatures indicate dominant bluish signatures in this region (Fig. 7c) similar to that observed at the impact crater Centeotl and might point to a possible reactivation due to this impact event. The only surface feature in the Dantu region of quadrangle AC H 3, that shows deep absorptions at 2.7 and 3.1 as Dantu, is the
Please cite this article as: K. Stephan et al., Spectral investigation of quadrangle AC -H 3 of the dwarf planet Ceres – The region of impact crater Dantu, Icarus (2017), http://dx.doi.org/10.1016/j.icarus.2017.07.019
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Fig. 8. Close-up view of the spectral and geological properties of Dantu’s central pit: a) FC image, b) FC slope classes, c) BD at 2.7 and d) BD at 3.1 μm. The central portion of the pit corresponds to spectrum #4 in Fig. 3.
Fig. 9. Close-up view of the spectral and geological properties of located spots enriched in carbonates at Dantu’s western crater wall (Fig. 3, spectrum #1) and close to a small fresh impact crater (Fig. 3, spectrum #5): a) FC image, b) FC slope classes, c) BD at 2.7 and d) BD at 3.1 μm.
Please cite this article as: K. Stephan et al., Spectral investigation of quadrangle AC -H 3 of the dwarf planet Ceres – The region of impact crater Dantu, Icarus (2017), http://dx.doi.org/10.1016/j.icarus.2017.07.019
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Fig. 10. Impact crater Aristaneous as seen by (a) FC clear filter, (b) in the DTM and (c) the visible spectral slope, as well as in the VIR-derived maps of the (d) BD 2.7 μm and (e) BD 3.1 μm.
35.8 km large impact crater Aristaneous (Fig. 10) located at 23.4°N/ 97.7°E with the highest concentration not in the crater but only in its ejecta blanket. In contrast, the spectral slope of Aristaneous and its ejecta is not red but rather neutral-blue like the adjacent impact crater Gaue and its ejecta (Raponi et al., 2017b). The latter, however, exhibits rather weak absorptions at 2.7 and 3.1 μm as seen in the case of other fresh impact features (Stephan et al., 2017a). This indicates that the cause of the spectral slope is not always directly correlated to the variations in the other spectral signatures. It cannot be excluded that the spectral signature of Aristaneous’ ejecta possibly represents re-excavated material emplaced by the Dantu impact event. Other fresh impact craters in the northern regions of the quadrangle like Ialonos (Fig. 11) show a typical blue slope and the weakest OH- and NH4 -absorption, such as the small impact craters within Dantu. Also impact crater Gaue, which is mainly located in the neighboring quadrangle AC -H 2 (Coniraya) (Raponi et al., 2017b) but with its ejecta reaching far into the Dantu region, shows the typically blue signature and weak absorptions at 2.7 and 3.1 μm. This corresponds nicely to the blue sloped impact craters described by Stephan et al. (2017a) with the difference in their spectral properties to their surroundings explained by grain size differences of the surface material, i.e. the phyllosilicates. FC images reveal peculiar mass wasting deposits, i.e. accumulation of material in lobes trending downslope or mounds of material on crater floors or other topographic lows associated with Dantu and other impact features like Jaja and Xochipili. They have been interpreted as surface material that has been transported downhill by either mass wasting, emplacement of impact melt, or ground ice flow (Schmidt et al., 2016). Although these features can be well defined in FC images, no apparent variations in albedo and no unique spectral signature of these lobes compared to surrounding units and/or associated impact crater material could be identi-
fied. Usually, these lobate features show spectral signatures similar to the ones of the impact craters they are associated with, i.e. bluecyan colors in the color composites and slope classification maps implying similar chemical as well as physical properties.
6. Discussion: Dantu as part of Vendimia Planitia The observed spectral variations in the Dantu region have been found to appear directly correlated to the regional geology and/or topography. Morphologically fresh impact craters exhibit a visible bluish slope and mostly relatively weak phyllosilicate absorptions at 2.7 and 3.1 μm, which could be explained by changes in the physical properties of the phyllosilicates (Stephan et al., 2017a). Usually a reddening of the visible slope and an equalization of the absorptions strengths to the average values of the Ceres’ surface material can be observed with increasing geological age of the surface area but also appears in association with recent mass wasting deposits such as slumping material. The steepest red visible slope occurs in the vicinity of the small carbonate-rich spots within the Dantu area, which show similarly weak absorptions at 2.7 and 3.1 μm, such as the small fresh impact craters but with a slight shift of the 2.7 μm-absorption toward longer wavelengths. Intriguingly, Dantu’s spectral properties do not fit this trend. Dantu’s visible slope is generally rather red-to-neutral, which could be explained due to its higher age. However, the measured phyllosilicate absorptions are stronger than observed in corresponding areas and actually are the deepest phyllosilicate absorptions measured on Ceres so far, pointing to an unique surface composition of this area. The area of the enhanced signature of phyllosilicates also includes Kerwan (Palomba et al., 2017), the oldest impact basin on Ceres (Williams et al., 2016) and some fresh impact craters such as Rao (diameter: 12 km, located at 8.1°N/119.0°E) (Stephan et al., 2017b). Both impact features lie in the large-scale
Please cite this article as: K. Stephan et al., Spectral investigation of quadrangle AC -H 3 of the dwarf planet Ceres – The region of impact crater Dantu, Icarus (2017), http://dx.doi.org/10.1016/j.icarus.2017.07.019
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Fig. 11. Impact crater Ialonos as seen by (a) FC clear filter, and VIR: (b) BD 2.7 μm, (c) BD 3.1 μm, FC slope. The blue-sloped region of the impact crater (FC ratio classes 1–3) corresponds to the spectrum #6 in Fig. 3.
depression of Vendimia Planitia, which has been interpreted as a possible huge impact basin formed early in Ceres’ history excavating material from much deeper regions of Ceres’ crust (Hiesinger et al., 2016; Marchi et al., 2016), which is only visible in the surface topography (Fig. 2). If the composition of Ceres’ crust changes with increasing depth, Ceres’ impact craters could reveal differences in the surface composition depending on their excavation depth influencing the measured spectral parameters. Subsequent impacts in this basin reach far deeper into Ceres’ crust than the events resulting in any other blue impact crater. Although distinctly older than the blue-sloped impact craters on Ceres such as Oxo (0.5 ± 0.2 Ma; Schmedemann et al. (2016)), Haulani (1.67/1.96 Ma; Krohn et al. (2016b)) and Occator (∼14.3/∼31.8 Ma; Hiesinger et al. (2016)) based on the LDM and ADM, respectively, impact crater Dantu is with an absolute model age of 72–150 Ma (LDM) and ∼25 Ma using the ADM (Kneissl et al., 2016) the youngest of the large impact features located in this basin, which could explain why the phyllosilicate absorptions are strongest here. The two other ancient basins postulated by Marchi et al. (2016) are mainly located in the quadrangles AC -H 10 (Platz et al., 2017; Zambon et al., 2017) and AC -H 11 (De Sanctis et al., 2017a; Schulzeck et al., 2016). Although global VIR maps show slightly deeper absorptions at 2.7 and 3.1 μm in the vicinity of these basins (Stephan et al., this issue), they are much less pronounced than in the Dantu region. These basin, though, are mainly covered by geologically relatively old impact features (Platz et al., 2017; Schulzeck et al., 2016) and do not exhibit relatively fresh and large, i.e. deeply excavating, impact features such as Dantu. This could possibly explain the unique spectral signature of Dantu and its surroundings. The small fresh impact craters superimposed on Dantu’s floor, however, again show the typical blue-sloped spectral signature. Usually no significant spectral differences could be observed between “blue” craters of different sizes (Stephan et al., 2017a). Possible changes in the surface composition within Dantu are very small (except for the carbonate-rich spots) and grain size differences become dominant in the spectral signature. Thus small fresh
craters on Dantu’s floor can appear bluish again if grain size differences occur. In summary, differences in the spectral signature of Dantu could not be explained by different physical surface properties but by variations in the composition, i.e. the abundance of phyllosilicates. The excavation of material from stratigraphically deeper lying regions of Vendimia could offer an explanation of the observed spectral differences as an indicator of a higher concentration of phyllosilicates or less altered material (without a significant contribution of carbonates) in Ceres’ deeper crust. Alteration processes of surface material rich in phyllosilicates forming carbonates have been already observed on Mars (Brown et al., 2010). Local enrichment of carbonates in the Dantu area could be associated with Dantu’s impact event such as the possible fracturing of its crater floor and could possibly be formed by additional impact-triggered and/or post-impact alteration processes, changing portions of the ubiquitous phyllosilicates into carbonates. References Ammannito, E., De Sanctis, M.C., Carrozzo, F.G., Zambon, F., Ciarniello, M., Combe, J.P., Frigeri, A., Longobardo, A., Raponi, A., Tosi, F., Fonte, S., Giardino, M., McFadden, L.A., Palomba, E., Stephan, K., Raymond, C.A., Russell, C.T., 2016a. Composition of the Urvara–Yalode region on Ceres. In: Proceedings GSA Annual Meeting, Denver, Colorado, USA, Volume 48. Ammannito, E., DeSanctis, M.C., Ciarniello, M., Frigeri, A., Carrozzo, F.G., Combe, J.-P., Ehlmann, B.L., Marchi, S., McSween, H.Y., Raponi, A., Toplis, M.J., Tosi, F., Castillo-Rogez, J.C., Capaccioni, F., Capria, M.T., Fonte, S., Giardino, M., Jaumann, R., Longobardo, A., Joy, S.P., Magni, G., McCord, T.B., McFadden, L.A., Palomba, E., Pieters, C.M., Polanskey, C.A., Rayman, M.D., Raymond, C.A., Schenk, P.M., Zambon, F., Russell, C.T., 2016b. Distribution of phyllosilicates on the surface of Ceres. Science 353 (6303). Brown, A.J., Hook, S.J., Baldridge, A.M., Crowley, J.K., Bridges, N.T., Thomson, B.J., Marion, G.M., de Souza Filho, C.R., Bishop, J.L., 2010. Hydrothermal formation of Clay-Carbonate alteration assemblages in the Nili Fossae region of Mars. Earth Planet. Sci. Lett. 297, 174–182. Buczkowski, D.L., Schmidt, B.E., Williams, D.A., Mest, S.C., Scully, J.E.C., Ermakov, A.I., Preusker, F., Schenk, P., Otto, K.A., Hiesinger, H., O’Brien, D., Marchi, S., Sizemore, H., Hughson, K., Chilton, H., Bland, M., Byrne, S., Schorghofer, N., Platz, T., Jaumann, R., Roatsch, T., Sykes, M.V., Nathues, A., De Sanctis, M.C., Raymond, C.A., Russell, C.T., 2016. The geomorphology of Ceres. Science 353.
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Stephan, K., Jaumann, R., Krohn, K., Schmedemann, N., Zambon, F., Tosi, F., Carrozzo, F.G., McFadden, L.A., Otto, K., De Sanctis, M.C., Ammannito, E., Matz, K.D., Roatsch, T., Preusker, F., Raymond, C.A., Russell, C.T., 2017a. An investigation of the bluish material on Ceres. Geophys. Res. Lett. 44, (4), 1660–1668. Stephan, K., Wagner, R., Jaumann, R., Carrozzo, F.G., Palomba, E., Mc Fadden, L.A., Zambon, F., Matz, K.-D., Williams, D., De Sanctis, C., Combe, J.-P., Ammannito, E., Roatsch, T., Raymond, C., Russell, C.T., 2017b. Close up view onto Dantu’s spectral properties. Meteor. Planet. Sci. (submitted). Williams, D.A., Mest, S.C., Kneissl, T., Pasckert, J.H., Hiesinger, H., Schmedemann, N.,
Please cite this article as: K. Stephan et al., Spectral investigation of quadrangle AC -H 3 of the dwarf planet Ceres – The region of impact crater Dantu, Icarus (2017), http://dx.doi.org/10.1016/j.icarus.2017.07.019
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Zambon, F., Carrozzo, F.G., Tosi, F., Ciarniello, M., Combe, J.-P., Frigeri, A., Raponi, A., Ammannito, E., De Sanctis, M.C., Longobardo, A., McFadden, L.A., Palomba, E., Stephan, K., Raymond, C.A., Russell, C.T., 2017. Spectral analysis of the quadrangle AC -H-10 Rongo on Ceres. Icarus, this issue.
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Spectral investigation of quadrangle AC -H 3 of the dwarf planet Ceres – The region of impact crater Dantu K. Stephan a,∗, R. Jaumann a,b, F. Zambon c, F.G. Carrozzo c, M.C. De Sanctis c, F. Tosi c, A. Longobardo c, E. Palomba c, E. Ammannito d, L.A. McFadden e, K. Krohn a, D.A. Williams f, A. Raponi c, M. Ciarnello c, J.-P. Combe g, A. Frigeri c, T. Roatsch a, K.-D. Matz a, F. Preusker a, C.A. Raymond h, C.T. Russell d a
DLR, Institute of Planetary Research, Berlin, Germany Free University of Berlin, Germany c INAF-IAPS, Rome, Italy d UCLA, Institute of Geophysics and Planetary Physics, Los Angeles, CA, USA e NASA Goddard Space Flight Center, Greenbelt, MD, USA f Arizona State University, Tempe, AZ, USA g Bear Fight Institute, Winthrop, WA, USA h NASA-JPL Pasadena, CA, USA b
a r t i c l e
i n f o
Article history: Received 26 April 2017 Revised 3 July 2017 Accepted 25 July 2017 Available online xxx
a b s t r a c t Mapping Ceres’ surface composition in the Dantu region, located between 21°–66°N and 90°–180°E, offers the unique possibility to investigate changes in the surface composition related to different stratigraphic levels of Ceres’ crust. Dantu is located in a huge depression named Vendimia Planitia, which possibly represents a completely degraded impact basin formed in the beginning of Ceres’ geological history. Most parts of this depression are characterized by strong phyllosilicate absorptions, which are stronger than elsewhere on Ceres’ surface. This spectral signature possibly is related to the material emplaced at the time of the Vendimia impact event excavating material from deeper regions of Ceres’ crust. Subsequent impacts in this basin reach far deeper into Ceres’ crust than any impact events outside of Vendemia Planitia, which could explain the spectral signature of Dantu, possibly pointing to a higher concentration of ammonium-bearing phyllosilicates in Ceres’ deeper crust. Spectral differences with respect to the small fresh craters on Dantu’s floor are probably related to grain size effects causing a bluish visible slope as observed by fresh impact craters on other places on Ceres. The local enrichment of carbonates in the Dantu area could also be associated with the impact event and may have been formed by additional impact-triggered and/or post-impact alteration processes. © 2017 Elsevier Inc. All rights reserved.
1. Introduction Since NASA’s Dawn spacecraft arrived at Ceres in March 2015, it has shown that the surface composition of the dwarf planet Ceres is dominated by a mixture of ammoniated phyllosilicates, Mg-phyllosilicates, carbonates and some dark materials (De Sanctis et al., 2015). The composition has been found to be rather homogeneous on a global scale, implying a globally widespread endogenous formation of the surface material, i.e. an aqueous alteration of silicates (Ammannito et al., 2016b). The abundance and/or physical properties of the individual surface compounds, however, appears
∗
Corresponding author. E-mail address: [email protected] (K. Stephan).
to be quite variable on a regional and local scale, which could be evidence of a vertically stratified upper crust (Ammannito et al., 2016b) or could result from specific surface processes (Stephan et al., 2017a). In order to improve our understanding of the compositional variations in Ceres’ crustal material, the Dawn Science Team conducted a campaign for mapping the spectral properties of Ceres’ surface based on data acquired by the instruments aboard the Dawn spacecraft, i.e. Dawn’s Framing Camera (FC) (Sierks et al., 2011) as well as its Visible and Infrared Spectrometer (VIR) (De Sanctis et al., 2011). The resulting spectral maps provide the unique opportunity to investigate Ceres’ surface composition in comparison to its geology and topography, which is essential to resolve the origin of the surface compounds and the processes responsible for their formation.
http://dx.doi.org/10.1016/j.icarus.2017.07.019 0019-1035/© 2017 Elsevier Inc. All rights reserved.
Please cite this article as: K. Stephan et al., Spectral investigation of quadrangle AC -H 3 of the dwarf planet Ceres – The region of impact crater Dantu, Icarus (2017), http://dx.doi.org/10.1016/j.icarus.2017.07.019
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Fig. 1. Global HAMO mosaic with nomenclature in Mollweide projected (adapted from Roatsch et al., (2016a)). The Dantu region of quadrangle AC -H 3 is highlighted by the red box.
In this study we explore the AC -H 3 quadrangle, including Ceres’ surface located between 21° and 66°N as well as 90° and 180°E (Fig. 1), named after its dominating surface feature Dantu, a large complex impact crater with an extended ejecta blanket situated in the southern portion of this quadrangle. It is not only an impressive geological feature but is also spectrally unique. Furthermore, Dantu is located at a geographic longitude, where Kuppers et al. (2014) suggested water vapor was being released from Ceres, based on ESA Herschel space telescope observations. Its investigation might offer a key to further our understanding about the compositional and geological evolution of Ceres’ crust (Kneissl et al., 2016). 2. Data basis and methodology The Dawn mission at Ceres consists of four principle phases performed at different altitudes of the spacecraft from the surface in order to maximize coverage and to meet the viewing conditions of the requirements of mapping Ceres’ surface: the Survey mission (altitude of 4900 km), the High Altitude Mapping Orbit (HAMO, altitude of 1950 km), and Low Altitude Mapping Orbit (LAMO) 1and 2 (altitude of ∼850 km) (Russell et al., 2016). In order to investigate the composition of Ceres’ surface in detail, we mainly used FC images and hyperspectral VIR cubes acquired during the Survey, HAMO and LAMO 1 mission phase offering the highest pixel ground resolution possible. 2.1. VIR data The VIR instrument detects Ceres’ surface between 0.25 and 1.05 μm (VIS) and between 1.0 and 5.1 μm (IR), respectively. The spectral resolution of each VIR channel is 1.8 nm and 9.8 nm per spectral channel, respectively, which allows the identification of major surface compounds and changes in their spectral parameters like the strength and wavelength position of individual and/or overlapping absorptions (De Sanctis et al., 2011). The spatial resolution of the VIR cubes reach 360 m per pixel during HAMO and 90 m per pixel during LAMO, which enables mapping local changes in Ceres’ surface composition. VIR data have been calibrated by the VIR team following the procedure of Filacchione et al. (2011) and calibration artifacts have been removed by Carrozzo et al. (2016). In
order to study the spectral characteristics of Ceres spectra at wavelengths longer than 3 μm, which are strongly affected by Ceres’ thermal signal, a correction of the thermal signal has been performed as described in Raponi (2015). Further information of the mapping process can be found in Frigeri et al. (this issue). During the Dawn mission, the VIR instruments measured distinct absorptions and spectral parameters including depth and position, which are indicative of specific surface compounds. The spectral signature of Ceres is generally dominated by a mixture of NH4 -montmorillonite or NH4 -annite, antigorite, Mg-carbonate, and a featureless, dark, possibly carbon-rich component (De Sanctis et al., 2015; Hendrix et al., 2016). Ammoniated phyllosilicates exhibit prominent absorptions at ∼2.7 μm indicative of OH− , and at 3.1 μm, indicative of NH4 − . Carbonates like magnesite (MgCO3 ) show relatively broad absorptions at 3.4 and 3.9 μm (De Sanctis et al., 2016), while sodium carbonates, such that identified in Cerealia facula (De Sanctis et al., 2016), have absorptions at longer wavelength (at about 4.01 μm). The absorption at 3.4 μm, however, was also found to be associated with areas enriched in organics (De Sanctis et al., 2017b). Finally, H2 O-ice, with its distinct absorptions at 1.04, 1.25, 1.5 and 2 μm, has been locally identified on Ceres’ surface in the vicinity of small (less than 20 km in diameter) morphologically fresh impact craters, such as Oxo (Combe et al., 2016; Combe et al., 2017) and Juling (De Sanctis et al., 2017a; Raponi et al., 2017a). The variations in the spectral parameters of these spectral signatures as well as the slope of the overall spectral continuum vary depending on the abundance as well as the physical properties of the specific surface compound. Detailed mapping of these variations enables the investigation of Ceres’ surface composition depending on its location on the body as well as the surface geology and topography, which is essential to reveal their origin and surface processes responsible for their existence. Measurements of the band depths (BDs) of individual absorptions have been performed following the procedure of Clark (2003), and described in detail by Ammannito et al. (2016a). 2.2. FC data Images from the Framing Camera, which offer the geological context of the Dantu region, reach spatial resolutions up to
Please cite this article as: K. Stephan et al., Spectral investigation of quadrangle AC -H 3 of the dwarf planet Ceres – The region of impact crater Dantu, Icarus (2017), http://dx.doi.org/10.1016/j.icarus.2017.07.019
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∼140 m/pixel (HAMO) and ∼35 m/pixel (LAMO). In order to study the relationship between surface composition and geology in general, a HAMO mosaic with a mean resolution of 140 m per pixel (Roatsch et al., 2016b) has been used (Fig. 1). The comparison of local spectral changes and geology was done using individual FC images acquired during the LAMO 1 phase, preferentially acquired at a similar time of day as the corresponding VIR observation and thus with similar illumination conditions. In each mission phase, Ceres has been mapped completely under similar illumination conditions but different viewing condition, which enables the construction of digital terrain models (DTM). Thus, in addition to the FC images, a stereogrammetry-based DTM derived from HAMO data (135 m/pixel) (see Preusker et al., (2016)) is available (Fig. 1). FC data, however, do not only offer the geologic context for our spectral investigation but also provide spectral information from its 7 color filters that are sensitive in a wavelength range between 0.4 and 1.1 μm (Sierks et al., 2011). For each filter a global photometrically corrected HAMO mosaic is available (Roatsch et al., 2016b; Schröder et al., 2015). Although these images do not offer a continuous spectrum like the VIR observations, they are useful to see changes in the visible albedo and spectral slope at a much higher spatial resolution – revealing correlations between composition and geology at a local scale. Fig. 1 shows the enhanced true color mosaic combining the mosaics derived from images of filters F5, F2 and F8, sensitive at 965, 555 and 438 nm, respectively, and a classification map of the spectral slope, i.e. the ratio of the FC filters F8 and F3, sensitive at 438 and 749 nm as demonstrated in Stephan et al. (2017a). 3. Geological overview The geology of the Dantu area (Fig. 2) has been studied in detail by Kneissl et al. (2016) and Williams et al. (2016), including the investigation of the surface ages of the specific geologic units as derived by measurements of crater size frequency distributions using the lunar-derived (LDM) and the asteroid-derived (ADM) cratering model (Hiesinger et al., 2016; Schmedemann et al., 2016). We summarize here the major geological facts relevant for comparison with the spectral properties. Most parts of the area lie in a large-scale (diameter of 750 km) depression named Vendimia Planitia (Marchi et al., 2016), which extends between 20°S/85°E and 65°N/185°E (Fig. 1). Numerous impact craters of various sizes and fresh to highly degraded morphology characterize the area (Fig. 2). The northern and southeastern parts of the quadrangle are dominated by geologically old cratered terrain, including the largest crater in this area, Vinotonus (d=∼140 km; 43.0°N/95.1°E). Vinotonus, located at the western quadrangle boundary, is highly degraded without any sign of a former ejecta blanket. The southern and southwestern parts of the area are dominated by the partially smooth ejecta blankets of the impact craters Dantu (d=∼126 km; 24.3°N/138.2°E) and Gaue (d=∼80 km; 31.8°N/86.2°E). Gaue is located at the border to quadrangle AC -H 2 (Raponi et al., 2017b) (Fig. 1). Only the eastern portion of its ejecta reaches into the Dantu area. High-resolution measurements of crater size-frequency distributions (Hiesinger et al., 2016) imply a similar surface age for both impact features. Schmedemann et al. (2016) and Pasckert et al. (2017) derived a LDM surface age of ∼162 Ma and 220 – 260 Ma for Gaue, respectively. Kneissl et al. (2016) measured surface ages ranging between ∼72–150 Ma for different parts of Dantu’s ejecta blanket and crater interior. The Dantu crater is the most prominent surface feature and extends over the quadrangle boundary into quadrangle AC -H 4 (Kerwan) (Williams et al., 2016). The crater has a quite complex morphology (Fig. 2) with a central peak similar to Ceres’ famous impact feature Occator (19.8°N/239.3°E), but which apparently has
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collapsed during the crater modification phase. Further, pitted terrain can be recognized on its floor (Kneissl et al., 2016). The crater itself exhibits a distinct topography with up to ∼4 km high crater walls that are characterized by extended slump terraces. Linear features interpreted as fractures of possible extensional origin are confined to the crater floor of Dantu. It is not clear, however, if these fractures result from the Dantu impact event or have been formed by post-impact processes (Kneissl et al., 2016). FC images reveal that smooth flow-like material interpreted as impact melt (Krohn et al., 2016a) superpose these fractures. Other lobate features are also associated with small impact craters north of Dantu such as Jaja (diameter = 22.0 km, 52.1°N/125.3°E) and Xochipili (diameter: 22.7 km, 56.7°N/93.2°E). Superimposed onto Dantu’s crater floor are two small fresh impact craters – a 5-km large unnamed impact crater located near 23.1°N/131.7°E and the 6 km large impact crater Centeotl (18.9°N/141.2°E) with the latter already situated in quadrangle AC -H 7 (Williams et al., 2016). Several bright spots appear on the crater floor but also at the crater wall and outside the crater as part of the ejecta blanket (Kneissl et al., 2016). Although their origin is not fully understood yet, they possibly represent the youngest surface deposits in the Dantu area. 4. Overview of the spectral characteristics within the Dantu-quadrangle The VIR-derived spectral characteristics of the Dantu region in general mirror the average surface composition of Ceres (De Sanctis et al., 2015). The abundance and/or the physical properties of the individual surface compounds such as phyllosilicates and carbonates, however, changes throughout the region causing variations in the band depths (BDs) and/or wavelength position of the individual characteristic absorptions. Fig. 3 shows the endmember spectra, which reflect the spectral variations identified in the Dantu region. Non- and ammoniated phyllosilicates with the diagnostic absorptions at 2.7 and 3.1 μm, respectively, and carbonates, which are responsible for the absorptions at 3.4 and 3.9 μm, appear in every VIR spectrum. Changes in the depth of the two absorptions at 2.7 and 3.1 μm in the endmember spectra of the Dantu region appear to be well correlated and support their origin in the same material, i.e. phyllosilicates. In comparison to Ceres’ average spectrum (spectrum #3), relatively weak absorptions occur in the endmember spectra #5 and #6 and deep absorptions in the endmember spectrum #4 (Fig. 3). On the contrary, relatively deep carbonate absorptions at 3.4 and 4 μm can be observed in the endmember spectra #1 and #2 in Fig. 3 and appear uncorrelated from the variations in the other spectral parameters. In connection to the enrichment in carbonates, a slight shift of the band center to longer wavelengths (from 2.73 to 2.77 μm) and broadening of the OH absorption at 2.7 μm are apparent. A similar shift has been also reported in Occator’s bright deposits (De Sanctis et al., 2016; Longobardo et al., 2017) and in few bright pixels in the area of the impact crater Kupalo (39.4°S/173.2°E) (De Sanctis et al., 2017a), suggesting a similar change in the mineralogy of the phyllosilicate species. H2 O-ice as observed in limited locations on Ceres’ surface (Combe et al., 2016; Combe et al., 2017) with its distinct absorptions at 1.04, 1.25, 1.5 and 2 μm has also been identified in one small high-albedo area of the AC -H 3 quadrangle (155°E/30°N) (Combe et al., 2017) (see below). The variations in the spectral properties often correspond to changes in the visible albedo as well as the overall spectral slope as derived from the FC images. The ratio of the filters F8 and F3 varies between ∼0.8 and ∼1.2, indicating changes in the visible spectral slope from reddish (positive) in the carbonate-rich spectrum (#1) to bluish (negative) in the spectrum with the weakest phyllosilicate absorptions at 2.7 and 3.1 μm (#4) (Fig. 3). The visible albedo, measured at 550 nm in the Dantu area, changes between
Please cite this article as: K. Stephan et al., Spectral investigation of quadrangle AC -H 3 of the dwarf planet Ceres – The region of impact crater Dantu, Icarus (2017), http://dx.doi.org/10.1016/j.icarus.2017.07.019
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Fig. 2. Geological context of the Dantu region: (top) FC Clear filter mosaic, (center) the DTM, and (bottom) the geological map adapted from Kneissl et al. (2016) including the names of the major geological features.
∼2.9% and 9.1%. The highest albedo corresponds to the carbonaterich spectra. The lowest albedo is associated with spectrum #4 with the bluish slope and the weak absorptions at 2.7 and 3.1 μm (Fig. 3). These relationships become obvious in the maps of the individual spectral parameters of the Dantu region, which will be described in the next section.
5. Distribution of spectral units In the spectral parameter maps, i.e. the band depth maps of the absorptions at 2.7 and 3.1 μm as well as the FC color composite and the spectral slope classification map (Figs. 4 and 5), two major areas can be distinguished. Similar to what can be seen in the
Please cite this article as: K. Stephan et al., Spectral investigation of quadrangle AC -H 3 of the dwarf planet Ceres – The region of impact crater Dantu, Icarus (2017), http://dx.doi.org/10.1016/j.icarus.2017.07.019
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Fig. 3. Spectral endmembers of the Dantu-region on Ceres over the wavelength range detected by the VIR-IR instrument identified in specific locations: spectra #1 and #2 bright spots (Figs. 7 and 9), average spectrum #3 of the Dantu region, spectrum #4 located Dantu’s central pit (Figs. 7 and 8), spectrum #5 of an unnamed small fresh crater located within Dantu (Fig. 7), spectrum #6 of impact crater Ialonos (Fig. 11). The spectrum of the only H2 O-ice rich spot in the Dantu region is shown in Fig. 6.
Fig. 4. Maps of the Dantu-quadrangle illustrating: (top) FC color composite, (bottom) FC Slope Classification.
geological map of Fig. 1, the southern portion of our region of interest is dominated by the spectral signature of the impact crater Dantu and its ejecta blanket which is characterized by a generally higher visible albedo (∼3.6 – 4.5%) with a red-to-neutral visible spectral slope similar to the neighboring geologically old terrain, which exhibits a reddish tint in the FC color composite (Fig. 4).
Intriguingly, the bright region associated with Dantu and its ejecta blanket also exhibits deep absorptions at 2.7 and 3.1 μm (band depth of ∼23% and 14%, respectively). Indeed, when looking at the entire surface of Ceres, both absorptions are deepest in the Dantu area [Stephan et al. this issue] implying the highest amount of phyllosilicates (Ammannito et al., 2016b). Thus, a red visible
Please cite this article as: K. Stephan et al., Spectral investigation of quadrangle AC -H 3 of the dwarf planet Ceres – The region of impact crater Dantu, Icarus (2017), http://dx.doi.org/10.1016/j.icarus.2017.07.019
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Fig. 5. VIR-derived maps of the Dantu-quadrangle illustrating the band depth (BD) of the phyllosilicate absorptions at 2.7 μm (top) and 3.1 μm (bottom).
spectral slope does not always indicate a similar visible albedo and similar spectral properties in the near-infrared wavelength range. Nevertheless, small fresh impact craters superimposed on Dantu as well as outside Dantu always appear bluish in the FC color composite. The northern highly cratered terrain shows a higher variegation in the spectral properties, but generally, a slightly lower visible albedo and weaker absorptions at 2.7 and 3.1 μm imply a lower abundance of phyllosilicates in this area (Ammannito et al., 2016b). As mentioned above, regions associated with fresh impact craters such as Ialonos show a blue slope as well as a bluish color in the FC color composite and the weakest absorptions at 2.7 μm and 3.1 μm, similar to other blue sloped young craters observed on Ceres (Stephan et al., 2017a). The remaining cratered terrain exhibits a reddish visible slope similar to Dantu’s central pit. This terrain also exhibits a reddish color in the color composite of the Dantu region and a relatively low visible albedo. This is contrary to what can be seen in the vicinity of Dantu, possibly pointing to compositional differences but also perhaps similarities in the physical surface properties between these two terrain types. Changes in the appearance and/or abundance of carbonates do not occur on a global/regional scale. Only locally a few small bright spots could be detected in the Dantu area that show a slightly stronger carbonate signature (discussed below), and only one local bright spot associated to the wall of a ∼275 m large impact crater shows evidence of H2 O ice (Combe et al., 2017) (Fig. 6). In particular, the ratio of the VIR spectra selected for the bright spot and the average VIR spectrum of the surrounding region show the diagnostic H2 O-ice absorptions at 1.04, 1.25, 1.5 and 2 μm (Fig. 6e).
Every FC-image of this region taken during the LAMO and HAMO mission phase shows this bright spot, which is bright in the images of all 7 color filters. The bright spot is visible in images acquired from September 2015 (HAMO) to October 2016 (LAMO2) with a diameter of about ∼120 m. No significant time-dependent changes could be recognized with respect to its size and albedo. FC images acquired during the Survey mission phase do not show the bright spot. However, it cannot be excluded that at that time the bright spot already existed, and that the lower pixel ground resolution (∼400 m) of the images prevented its detection. Hayne and Aharonson (2015) estimated that ice at these latitudes (0 to 30°) is stable with solar illumination for at least about 10 to 100 years. Since Prettyman et al. (2017) predicted extended ice deposits in Ceres’ surface, the observed icy spot could represent excavated subsurface ice during a recent impact event or slumping. Possibly, the icy material is part of Dantu’s ejecta material since the bright spot lies completely within the region of Dantu’s ejecta blanket (Fig. 1). However, this does not explain, why this icy spot is only one detected so far in the Dantu area (Combe et al., 2017). Either additional spots are below the detection limit or the H2 O-ice is of exogenic origin with the H2 O ice being a remnant of an ice-rich impactor.
5.1. Impact crater Dantu Impact crater Dantu displays a wide variety of colors in the FC color filters and in the VIR derived spectral properties (Fig. 7). All spectral endmembers derived for the Dantu-region (Fig. 3) can be
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Fig. 6. Close-up view of the small H2 O-ice rich location identified by (Combe et al., 2017): (a) full and (b) subset of VIR observation 494,522,179 (HAMO) with (c) the full and (d) subset of the corresponding FC composite including LAMO images of filter 5 (R), 2 (G), and 8 (B) with the H2 O-ice rich spot (indicated by the green point or arrow) evident (e) by the H2 O-ice absorptions at 1.04, 1.25, 1.5 and 2 μm.
observed in the vicinity of this complex impact feature. Local spectral variations are associated to specific inter-crater features. Significant parts of the crater floor are covered by material emplaced by small impacts (one of these small craters named Centeotl is situated in quadrangle AC -H 7 (Kerwan) investigated by Palomba et al. (2017) (Fig. 7). These impact features are characterized by a slightly blue slope and weaker absorptions at 2.7 μm and 3.1 μm than the average depths measured on Ceres (spectral endmember #1 in Fig. 3). This corresponds to the spectral properties of other young impact craters on Ceres’ surface, which exhibit a blue spectral slope as discussed by Stephan et al., (2017a). Intriguingly, Dantu’s central pit and portions of the eastern crater wall and floor show a distinct visible red-sloped visible spectrum. These red-sloped areas also nicely correspond to the regions, where the deepest OH and NH4 absorptions at 2.7 and 3.1 μm, respectively, have been measured (Figs. 7 and 8). LAMO FC images illustrate that these deep absorptions occur in the interior of the pit and con-
tinue in topographically low-lying areas surrounding the pit. The red-slope regions continue especially in the eastern direction up to the crater wall. Less deep but still distinct deeper absorptions can also be found in the western portions of the impact ejecta. The remaining areas of the crater floor show the typical average surface composition and a neutral (neither red nor blue spectral slope) of Ceres. Usually, red-sloped spectra characterize geologically old weathered and/or slumping material on Ceres’ surface (Stephan et al., 2017a). Also, the old cratered terrain surrounding Dantu shows a reddish visible slope associated with a reddish color of the surface in the FC color composite and a VIR spectrum similar to Ceres’ average spectral signature (spectrum #4 in Fig. 3). The red-sloped regions associated with Dantu itself appear bright in the FC color composite, and the corresponding VIR spectrum is characterized by deep absorptions at 2.7 and 3.1 μm, which does not correspond fully to a typically red spectrum. Thus, the comparison of the FC Color Composite and FC slope map with the
Please cite this article as: K. Stephan et al., Spectral investigation of quadrangle AC -H 3 of the dwarf planet Ceres – The region of impact crater Dantu, Icarus (2017), http://dx.doi.org/10.1016/j.icarus.2017.07.019
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Fig. 7. Spectral properties of impact crater Dantu: a) FC image, b) FC DTM, c) FC slope classes, d) BD at 2.7 and e) BD at 3.1 μm. The location of close-up views presented in Figs. 7 and 8 as well as the border between the Dantu region of quadrangle AC -H 3 discussed in this work and the southern AC -H 7 quadrangle are highlighted. Black arrows point out small fresh impact craters superimposed onto Dantu’s floor (Fig. 3, spectrum #5) with Centeotl located in quadrangle AC -H 7 (Palomba et al., 2017). Red arrows point to local bright spots dominated by carbonates (Fig. 3, spectra #1 and #2).
VIR-derived spectral parameter maps imply that a red-sloped area is not always correlated to the spectral signature of Ceres’ surface material in the infrared wavelength region. However, slumping material related to young impact craters can also show a red-slope pointing to a relationship to physical properties of the surface material (Stephan et al., 2017a). Possibly, red-sloped material within Dantu, including the material of the collapsed central pit, represents slumped material as can partly be seen along Dantu’s crater walls. Few locally, very limited bright spots are visible at the crater walls (indicated by the red arrows in Fig. 7a). These bright deposits show weak absorptions at 2.7 and 3.1 μm but not a blue slope like the small fresh impact craters (spectral endmember #1 in Fig. 3). Instead they show a slightly broader absorption at 2.7 μm centered at slightly longer wavelengths and distinct absorptions at 3.4 and 3.9 μm, indicative of enrichment in carbonates (De Sanctis et al., 2016). Apparently this material became excavated very recently due to mass wasting processes at the very steep crater walls (Fig. 9). Additional, bright carbonate-rich spots are also apparent on Dantu’s crater floor and ejecta blanket, which are mostly associated to a visible red slope (Fig. 7). The highest concentration appears in Dantu’s ejecta blanket located in the neighboring quadrangle AC -H 7 (Kerwan) discussed in detail by Williams et al. (2016) and Palomba et al. (2017). Appearance and spectral properties of these bright spots are similar to the unique bright spots seen at Occator consisting of large amounts of sodium carbonate and interpreted to be of endogenous origin (De Sanctis et al., 2016;
Longobardo et al., 2017). The heat source may be triggered by impact heating, or Ceres’ internal temperatures may be above the eutectic temperature of subsurface brines, in which case fluids may exist at certain depths on Ceres today (De Sanctis et al., 2016). Geologic investigations of the Dantu area by Williams et al. (2016) and Kneissl et al. (2016) suggest that the bright carbonate-rich material is part of Dantu’s impact ejecta but cannot exclude its emplacement by cryovolcanic venting. Cryovolcanic activity or the outgassing of volatiles such as CO2 in the Dantu area is also supported by the numerous pits on Dantu’s crater floor. Furthermore, the concentration and orientation of fractures in the southern portion of Dantu (Fig. 2) (Buczkowski et al., 2016) could imply extensional forces formed by a bulge over a concentration of volatiles. Stratigraphic relationships of the fractures to other morphological features in the Dantu region imply that these fractures have been formed subsequently to the formation of the Dantu crater. These fractures are confined to the crater but are covered by, i.e. older than, smooth terrain around the central pit and along the crater walls (bright crater floor material in the geological map of Fig. 2) and the fresh small impact craters such as Centeotl (Fig. 7). Although the dimensions of these fractures are too small to be completely resolved by VIR, the spectral signatures indicate dominant bluish signatures in this region (Fig. 7c) similar to that observed at the impact crater Centeotl and might point to a possible reactivation due to this impact event. The only surface feature in the Dantu region of quadrangle AC H 3, that shows deep absorptions at 2.7 and 3.1 as Dantu, is the
Please cite this article as: K. Stephan et al., Spectral investigation of quadrangle AC -H 3 of the dwarf planet Ceres – The region of impact crater Dantu, Icarus (2017), http://dx.doi.org/10.1016/j.icarus.2017.07.019
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Fig. 8. Close-up view of the spectral and geological properties of Dantu’s central pit: a) FC image, b) FC slope classes, c) BD at 2.7 and d) BD at 3.1 μm. The central portion of the pit corresponds to spectrum #4 in Fig. 3.
Fig. 9. Close-up view of the spectral and geological properties of located spots enriched in carbonates at Dantu’s western crater wall (Fig. 3, spectrum #1) and close to a small fresh impact crater (Fig. 3, spectrum #5): a) FC image, b) FC slope classes, c) BD at 2.7 and d) BD at 3.1 μm.
Please cite this article as: K. Stephan et al., Spectral investigation of quadrangle AC -H 3 of the dwarf planet Ceres – The region of impact crater Dantu, Icarus (2017), http://dx.doi.org/10.1016/j.icarus.2017.07.019
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Fig. 10. Impact crater Aristaneous as seen by (a) FC clear filter, (b) in the DTM and (c) the visible spectral slope, as well as in the VIR-derived maps of the (d) BD 2.7 μm and (e) BD 3.1 μm.
35.8 km large impact crater Aristaneous (Fig. 10) located at 23.4°N/ 97.7°E with the highest concentration not in the crater but only in its ejecta blanket. In contrast, the spectral slope of Aristaneous and its ejecta is not red but rather neutral-blue like the adjacent impact crater Gaue and its ejecta (Raponi et al., 2017b). The latter, however, exhibits rather weak absorptions at 2.7 and 3.1 μm as seen in the case of other fresh impact features (Stephan et al., 2017a). This indicates that the cause of the spectral slope is not always directly correlated to the variations in the other spectral signatures. It cannot be excluded that the spectral signature of Aristaneous’ ejecta possibly represents re-excavated material emplaced by the Dantu impact event. Other fresh impact craters in the northern regions of the quadrangle like Ialonos (Fig. 11) show a typical blue slope and the weakest OH- and NH4 -absorption, such as the small impact craters within Dantu. Also impact crater Gaue, which is mainly located in the neighboring quadrangle AC -H 2 (Coniraya) (Raponi et al., 2017b) but with its ejecta reaching far into the Dantu region, shows the typically blue signature and weak absorptions at 2.7 and 3.1 μm. This corresponds nicely to the blue sloped impact craters described by Stephan et al. (2017a) with the difference in their spectral properties to their surroundings explained by grain size differences of the surface material, i.e. the phyllosilicates. FC images reveal peculiar mass wasting deposits, i.e. accumulation of material in lobes trending downslope or mounds of material on crater floors or other topographic lows associated with Dantu and other impact features like Jaja and Xochipili. They have been interpreted as surface material that has been transported downhill by either mass wasting, emplacement of impact melt, or ground ice flow (Schmidt et al., 2016). Although these features can be well defined in FC images, no apparent variations in albedo and no unique spectral signature of these lobes compared to surrounding units and/or associated impact crater material could be identi-
fied. Usually, these lobate features show spectral signatures similar to the ones of the impact craters they are associated with, i.e. bluecyan colors in the color composites and slope classification maps implying similar chemical as well as physical properties.
6. Discussion: Dantu as part of Vendimia Planitia The observed spectral variations in the Dantu region have been found to appear directly correlated to the regional geology and/or topography. Morphologically fresh impact craters exhibit a visible bluish slope and mostly relatively weak phyllosilicate absorptions at 2.7 and 3.1 μm, which could be explained by changes in the physical properties of the phyllosilicates (Stephan et al., 2017a). Usually a reddening of the visible slope and an equalization of the absorptions strengths to the average values of the Ceres’ surface material can be observed with increasing geological age of the surface area but also appears in association with recent mass wasting deposits such as slumping material. The steepest red visible slope occurs in the vicinity of the small carbonate-rich spots within the Dantu area, which show similarly weak absorptions at 2.7 and 3.1 μm, such as the small fresh impact craters but with a slight shift of the 2.7 μm-absorption toward longer wavelengths. Intriguingly, Dantu’s spectral properties do not fit this trend. Dantu’s visible slope is generally rather red-to-neutral, which could be explained due to its higher age. However, the measured phyllosilicate absorptions are stronger than observed in corresponding areas and actually are the deepest phyllosilicate absorptions measured on Ceres so far, pointing to an unique surface composition of this area. The area of the enhanced signature of phyllosilicates also includes Kerwan (Palomba et al., 2017), the oldest impact basin on Ceres (Williams et al., 2016) and some fresh impact craters such as Rao (diameter: 12 km, located at 8.1°N/119.0°E) (Stephan et al., 2017b). Both impact features lie in the large-scale
Please cite this article as: K. Stephan et al., Spectral investigation of quadrangle AC -H 3 of the dwarf planet Ceres – The region of impact crater Dantu, Icarus (2017), http://dx.doi.org/10.1016/j.icarus.2017.07.019
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Fig. 11. Impact crater Ialonos as seen by (a) FC clear filter, and VIR: (b) BD 2.7 μm, (c) BD 3.1 μm, FC slope. The blue-sloped region of the impact crater (FC ratio classes 1–3) corresponds to the spectrum #6 in Fig. 3.
depression of Vendimia Planitia, which has been interpreted as a possible huge impact basin formed early in Ceres’ history excavating material from much deeper regions of Ceres’ crust (Hiesinger et al., 2016; Marchi et al., 2016), which is only visible in the surface topography (Fig. 2). If the composition of Ceres’ crust changes with increasing depth, Ceres’ impact craters could reveal differences in the surface composition depending on their excavation depth influencing the measured spectral parameters. Subsequent impacts in this basin reach far deeper into Ceres’ crust than the events resulting in any other blue impact crater. Although distinctly older than the blue-sloped impact craters on Ceres such as Oxo (0.5 ± 0.2 Ma; Schmedemann et al. (2016)), Haulani (1.67/1.96 Ma; Krohn et al. (2016b)) and Occator (∼14.3/∼31.8 Ma; Hiesinger et al. (2016)) based on the LDM and ADM, respectively, impact crater Dantu is with an absolute model age of 72–150 Ma (LDM) and ∼25 Ma using the ADM (Kneissl et al., 2016) the youngest of the large impact features located in this basin, which could explain why the phyllosilicate absorptions are strongest here. The two other ancient basins postulated by Marchi et al. (2016) are mainly located in the quadrangles AC -H 10 (Platz et al., 2017; Zambon et al., 2017) and AC -H 11 (De Sanctis et al., 2017a; Schulzeck et al., 2016). Although global VIR maps show slightly deeper absorptions at 2.7 and 3.1 μm in the vicinity of these basins (Stephan et al., this issue), they are much less pronounced than in the Dantu region. These basin, though, are mainly covered by geologically relatively old impact features (Platz et al., 2017; Schulzeck et al., 2016) and do not exhibit relatively fresh and large, i.e. deeply excavating, impact features such as Dantu. This could possibly explain the unique spectral signature of Dantu and its surroundings. The small fresh impact craters superimposed on Dantu’s floor, however, again show the typical blue-sloped spectral signature. Usually no significant spectral differences could be observed between “blue” craters of different sizes (Stephan et al., 2017a). Possible changes in the surface composition within Dantu are very small (except for the carbonate-rich spots) and grain size differences become dominant in the spectral signature. Thus small fresh
craters on Dantu’s floor can appear bluish again if grain size differences occur. In summary, differences in the spectral signature of Dantu could not be explained by different physical surface properties but by variations in the composition, i.e. the abundance of phyllosilicates. The excavation of material from stratigraphically deeper lying regions of Vendimia could offer an explanation of the observed spectral differences as an indicator of a higher concentration of phyllosilicates or less altered material (without a significant contribution of carbonates) in Ceres’ deeper crust. Alteration processes of surface material rich in phyllosilicates forming carbonates have been already observed on Mars (Brown et al., 2010). Local enrichment of carbonates in the Dantu area could be associated with Dantu’s impact event such as the possible fracturing of its crater floor and could possibly be formed by additional impact-triggered and/or post-impact alteration processes, changing portions of the ubiquitous phyllosilicates into carbonates. References Ammannito, E., De Sanctis, M.C., Carrozzo, F.G., Zambon, F., Ciarniello, M., Combe, J.P., Frigeri, A., Longobardo, A., Raponi, A., Tosi, F., Fonte, S., Giardino, M., McFadden, L.A., Palomba, E., Stephan, K., Raymond, C.A., Russell, C.T., 2016a. Composition of the Urvara–Yalode region on Ceres. In: Proceedings GSA Annual Meeting, Denver, Colorado, USA, Volume 48. Ammannito, E., DeSanctis, M.C., Ciarniello, M., Frigeri, A., Carrozzo, F.G., Combe, J.-P., Ehlmann, B.L., Marchi, S., McSween, H.Y., Raponi, A., Toplis, M.J., Tosi, F., Castillo-Rogez, J.C., Capaccioni, F., Capria, M.T., Fonte, S., Giardino, M., Jaumann, R., Longobardo, A., Joy, S.P., Magni, G., McCord, T.B., McFadden, L.A., Palomba, E., Pieters, C.M., Polanskey, C.A., Rayman, M.D., Raymond, C.A., Schenk, P.M., Zambon, F., Russell, C.T., 2016b. Distribution of phyllosilicates on the surface of Ceres. Science 353 (6303). Brown, A.J., Hook, S.J., Baldridge, A.M., Crowley, J.K., Bridges, N.T., Thomson, B.J., Marion, G.M., de Souza Filho, C.R., Bishop, J.L., 2010. Hydrothermal formation of Clay-Carbonate alteration assemblages in the Nili Fossae region of Mars. Earth Planet. Sci. Lett. 297, 174–182. Buczkowski, D.L., Schmidt, B.E., Williams, D.A., Mest, S.C., Scully, J.E.C., Ermakov, A.I., Preusker, F., Schenk, P., Otto, K.A., Hiesinger, H., O’Brien, D., Marchi, S., Sizemore, H., Hughson, K., Chilton, H., Bland, M., Byrne, S., Schorghofer, N., Platz, T., Jaumann, R., Roatsch, T., Sykes, M.V., Nathues, A., De Sanctis, M.C., Raymond, C.A., Russell, C.T., 2016. The geomorphology of Ceres. Science 353.
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Please cite this article as: K. Stephan et al., Spectral investigation of quadrangle AC -H 3 of the dwarf planet Ceres – The region of impact crater Dantu, Icarus (2017), http://dx.doi.org/10.1016/j.icarus.2017.07.019
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Neesemann, A., Buczkowski, D.L., Scully, J.E.C., Marchi, S., Schenk, P., Jaumann, R., Roatsch, T., Preusker, F., Nathues, A., Schaefer, M., Hoffmann, M., Raymond, C.A., Russell, C.T., 2016. Geologic mapping of the AC -H-7 Kerwan quadrangle of Ceres from NASA dawn mission. In: Lunar and Planetary Science Conference, Volume 47, p. 1522.
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Zambon, F., Carrozzo, F.G., Tosi, F., Ciarniello, M., Combe, J.-P., Frigeri, A., Raponi, A., Ammannito, E., De Sanctis, M.C., Longobardo, A., McFadden, L.A., Palomba, E., Stephan, K., Raymond, C.A., Russell, C.T., 2017. Spectral analysis of the quadrangle AC -H-10 Rongo on Ceres. Icarus, this issue.
Please cite this article as: K. Stephan et al., Spectral investigation of quadrangle AC -H 3 of the dwarf planet Ceres – The region of impact crater Dantu, Icarus (2017), http://dx.doi.org/10.1016/j.icarus.2017.07.019