Icarus 318 (2019) 111–123 Contents lists available at ScienceDirect Icarus journal homepage: www.elsevier.com/locate/icarus Spectral investigation of quadrangle A C -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 b 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 a r t i c l e i n f o a b s t r a c t Article history: Mapping Ceres’ surface composition in the Dantu region, located between 21 °–66 °N and 90 °–180 °E, offers Received 26 April 2017 the unique possibility to investigate changes in the surface composition related to different stratigraphic Revised 3 July 2017 levels of Ceres’ crust. Dantu is located in a huge depression named Vendimia Planitia, which possibly Accepted 25 July 2017 represents a completely degraded impact basin formed in the beginning of Ceres’ geological history. Most Available online 25 July 2017 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 to be quite variable on a regional and local scale, which could be evidence of a vertically stratified upper crust ( Ammannito et al., Since NASA’s Dawn spacecraft arrived at Ceres in March 2015, 2016b ) or could result from specific surface processes ( Stephan it has shown that the surface composition of the dwarf planet et al., 2017a ). In order to improve our understanding of the com- Ceres is dominated by a mixture of ammoniated phyllosilicates, positional variations in Ceres’ crustal material, the Dawn Science Mg-phyllosilicates, carbonates and some dark materials ( De Sanctis Team conducted a campaign for mapping the spectral properties et al., 2015 ). The composition has been found to be rather homo- of Ceres’ surface based on data acquired by the instruments aboard geneous on a global scale, implying a globally widespread endoge- the Dawn spacecraft, i.e. Dawn’s Framing Camera (FC) ( Sierks et al., nous formation of the surface material, i.e. an aqueous alteration of 2011 ) as well as its Visible and Infrared Spectrometer (VIR) ( De silicates ( Ammannito et al., 2016b ). The abundance and/or physical Sanctis et al., 2011 ). The resulting spectral maps provide the unique properties of the individual surface compounds, however, appears opportunity to investigate Ceres’ surface composition in compar- ison to its geology and topography, which is essential to resolve the origin of the surface compounds and the processes responsible ∗ Corresponding author. for their formation. E-mail address: [email protected] (K. Stephan). http://dx.doi.org/10.1016/j.icarus.2017.07.019 0019-1035/© 2017 Elsevier Inc. All rights reserved. 112 K. Stephan et al. / Icarus 318 (2019) 111–123 Fig. 1. Global HAMO mosaic with nomenclature in Mollweide projected (adapted from Roatsch et al., (2016a) ). The Dantu region of quadrangle A C -H 3 is highlighted by the red box. In this study we explore the AC -H 3 quadrangle, including order to study the spectral characteristics of Ceres spectra at wave- Ceres’ surface located between 21 ° and 66 °N as well as 90 ° and lengths longer than 3 μm, which are strongly affected by Ceres’ 180 °E ( Fig. 1 ), named after its dominating surface feature Dantu, a thermal signal, a correction of the thermal signal has been per- large complex impact crater with an extended ejecta blanket situ- formed as described in Raponi (2015) . Further information of the ated in the southern portion of this quadrangle. It is not only an mapping process can be found in Frigeri et al. (this issue). impressive geological feature but is also spectrally unique. Further- During the Dawn mission, the VIR instruments measured dis- more, Dantu is located at a geographic longitude, where Kuppers tinct absorptions and spectral parameters including depth and po- et al. (2014) suggested water vapor was being released from Ceres, sition, which are indicative of specific surface compounds. The based on ESA Herschel space telescope observations. Its investiga- spectral signature of Ceres is generally dominated by a mixture tion might offer a key to further our understanding about the com- of NH4 -montmorillonite or NH4 -annite, antigorite, Mg-carbonate, positional and geological evolution of Ceres’ crust ( Kneissl et al., and a featureless, dark, possibly carbon-rich component ( De Sanc- 2016 ). tis et al., 2015; Hendrix et al., 2016 ). Ammoniated phyllosilicates exhibit prominent absorptions at ∼2.7 μm indicative of OH −, and − 2. Data basis and methodology 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 The Dawn mission at Ceres consists of four principle phases al., 2016 ), while sodium carbonates, such that identified in Cerealia performed at different altitudes of the spacecraft from the surface facula ( De Sanctis et al., 2016 ), have absorptions at longer wave- in order to maximize coverage and to meet the viewing conditions length (at about 4.01 μm). The absorption at 3.4 μm, however, was of the requirements of mapping Ceres’ surface: the Survey mission also found to be associated with areas enriched in organics ( De (altitude of 4900 km), the High Altitude Mapping Orbit (HAMO, al- Sanctis et al., 2017b). Finally, H2 O-ice, with its distinct absorptions titude of 1950 km), and Low Altitude Mapping Orbit (LAMO) 1and at 1.04, 1.25, 1.5 and 2 μm, has been locally identified on Ceres’ 2 (altitude of ∼850 km) ( Russell et al., 2016 ). In order to investi- surface in the vicinity of small (less than 20 km in diameter) mor- gate the composition of Ceres’ surface in detail, we mainly used phologically fresh impact craters, such as Oxo ( Combe et al., 2016; FC images and hyperspectral VIR cubes acquired during the Sur- Combe et al., 2017 ) and Juling ( De Sanctis et al., 2017a; Raponi et vey, HAMO and LAMO 1 mission phase offering the highest pixel al., 2017a ). ground resolution possible. The variations in the spectral parameters of these spectral sig- natures as well as the slope of the overall spectral continuum vary 2.1. VIR data depending on the abundance as well as the physical properties of the specific surface compound. Detailed mapping of these varia- The VIR instrument detects Ceres’ surface between 0.25 and tions enables the investigation of Ceres’ surface composition de- 1.05 μm (VIS) and between 1.0 and 5.1 μm (IR), respectively. The pending on its location on the body as well as the surface geology spectral resolution of each VIR channel is 1.8 nm and 9.8 nm per and topography, which is essential to reveal their origin and sur- spectral channel, respectively, which allows the identification of face processes responsible for their existence. Measurements of the major surface compounds and changes in their spectral parame- band depths ( BD s) of individual absorptions have been performed ters like the strength and wavelength position of individual and/or following the procedure of Clark (2003) , and described in detail by overlapping absorptions ( De Sanctis et al., 2011 ). The spatial res- Ammannito et al. (2016a) . olution of the VIR cubes reach 360 m per pixel during HAMO and 90 m per pixel during LAMO, which enables mapping local changes 2.2. FC data in Ceres’ surface composition. VIR data have been calibrated by the VIR team following the procedure of Filacchione et al. (2011) and Images from the Framing Camera, which offer the geologi- calibration artifacts have been removed by Carrozzo et al. (2016) . In cal context of the Dantu region, reach spatial resolutions up to K. Stephan et al. / Icarus 318 (2019) 111–123 113 ∼140 m/pixel (HAMO) and ∼35 m/pixel (LAMO). In order to study collapsed during the crater modification phase. Further, pitted ter- the relationship between surface composition and geology in gen- rain can be recognized on its floor ( Kneissl et al., 2016 ). The crater eral, a HAMO mosaic with a mean resolution of 140 m per pixel itself exhibits a distinct topography with up to ∼4 km high crater ( Roatsch et al., 2016b ) has been used ( Fig.