Geomorphology and structural geology of Saturnalia Fossae and adjacent structures in the northern hemisphere of Vesta
Introduction
Vesta is the second most massive asteroid in the Solar System, located in the asteroid belt, with axes measuring 286.3 ± 0.1 km, 278.6 ± 0.1 km and 223.2 ± 0.1 km when approximated by an ellipsoid (Russell et al., 2012). Vesta is the only asteroid known to have differentiated to form a core, a mantle and a crust, and is consequently referred to as a protoplanet (e.g. Russell et al., 2012). Vesta is thought to have remained intact since its formation at ∼4.56 Ga (McSween et al., 2011), dated by crystallization ages of meteorites interpreted to derive from Vesta: the Howardite, Eucrite and Diogenite (HED) meteorites (McCord et al., 1970).
Vesta has been studied remotely since its discovery by H.M. Wilhelm Olbers in 1807. In 1970, the HED meteorites were proposed to originate from Vesta (McCord et al., 1970). Later, Hubble Space Telescope observations of Vesta identified a large southern impact basin, which could be the source region of the HEDs (Thomas et al., 1997). Further observations showed spectral heterogeneity across the vestan surface (Binzel et al., 1997, Gaffey, 1997, Li et al., 2010). The proposed connection between the HEDs and Vesta was recently confirmed by the observations of the Dawn spacecraft (Russell et al., 2012, De Sanctis et al., 2012, Prettyman et al., 2012, Reddy et al., 2012a). Further introductory information about Vesta is included in the Introductory paper to this Special Issue (Williams et al., 2014).
This paper focuses on the first-order Saturnalia Fossae and second-order adjacent structures, which occur in Vesta’s northern hemisphere. Section 2, Background, discusses Vesta’s place in structural and geological studies of planetary bodies of different scales. Section 3, Methods, describes the techniques used in this study. Section 4, Results, presents the geological maps produced in this work, along with observations and interpretations of the structural features and geological units identified. Section 5, Discussion, considers interpretations of the main findings in this study, with emphasis on the temporal relationships between the mapped structures and the related impact events. Section 6, Conclusions, summarizes the main findings of this work and their implications for Vesta and broader studies in planetary sciences.
Section snippets
Background
Vesta’s size and surface gravity, 0.25 m/s2, place it in an intermediate Solar System category between that of the terrestrial planets and small asteroids (e.g. Jaumann et al., 2012). Unlike Earth or Mars, Vesta lacks a protective atmosphere and consequently the dominant geologic process is impact cratering (e.g. Russell et al., 2012, Jaumann et al., 2012, Schenk et al., 2012). On account of this, Vesta’s surface is highly cratered (Marchi et al., 2012). The two most prominent impact features
Data sources: basemaps and ancillary data
The Vesta science phase of the Dawn mission lasted from May 3rd 2011 to August 26th 2012. During this time data were acquired by the Dawn spacecraft in six stages: (1) Approach, (2) Survey, (3) High Altitude Mapping Orbit 1 (HAMO 1), (4) Low Altitude Mapping Orbit (LAMO), (5) High Altitude Mapping Orbit 2 (HAMO 2) and (6) Departure. Data were collected by Dawn’s three instruments: (1) the Framing Camera (FC) (Sierks et al., 2011, Schröder et al., 2013), (2) the Visible and Infrared Spectrometer
Maps
In accordance with the other papers in this Special Issue, the geological maps of Caparronia quadrangle (Fig. 5a) and Domitia quadrangle (Fig. 5b) contain all of the mapped features apart from craters <6 km in diameter. In this paper, versions of the geological maps with craters <6 km in diameter are also included (Fig. 6a and b). In these figures the quadrangle maps are overlain on the LAMO–HAMO 2 composite quadrangle mosaics. Mapping of the structural features, which consist of the fossae (Fig.
Discussion
Our study, based mainly on photogeologic structural mapping, leads to the following main findings:
- 1.
Eight geomorphic units are classified in Caparronia and Domitia quadrangles, based on morphology, cross-cutting relationships, spectral properties and topographic properties. Each geomorphic unit is heavily influenced by impact cratering and/or impact-related processes.
- 2.
The Saturnalia Fossae are the first-order dominant structure in Vesta’s northern hemisphere, and are mainly located in Domitia
Conclusions
Vesta’s low gravity and internal differentiation have resulted in the surface being dominated by impact-cratering and impact-related processes. Each of the eight geomorphic units identified in Caparronia and Domitia quadrangles, (1) Vestalia Terra unit, (2) cratered highlands unit, (3) Saturnalia Fossae trough unit, (4) Saturnalia Fossae cratered unit, (5) undifferentiated ejecta unit, (6) dark lobate unit, (7) dark crater ray unit and (8) lobate crater unit, are either modified by impact
Acknowledgments
We thank NASA and the Dawn Flight Teams at JPL for the development, cruise, orbital insertion and operations of the Dawn spacecraft at Vesta. We also thank the instrument teams at the Max Planck Institute, German Aerospace Center (DLR), Italian National Institute for Astrophysics (INAF) and Planetary Science Institute for the acquisition and processing of Dawn data used in this work. The data used in this paper are available from the website http://dawndata.igpp.ucla.edu .
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