Elsevier

Icarus

Volume 244, December 2014, Pages 23-40
Icarus

Geomorphology and structural geology of Saturnalia Fossae and adjacent structures in the northern hemisphere of Vesta

https://doi.org/10.1016/j.icarus.2014.01.013 Get rights and content

Highlights

  • A mapping study of a portion of the northern hemisphere of Vesta is presented.

  • Eight geomorphic units are classified, along with their relative age relationships.

  • The fossae are interpreted to be impact-induced graben and half-graben.

  • Structures adjacent to the fossae are minor ridges, grooves and crater chains.

  • Many impact-related formation mechanisms are interpreted for adjacent structures.

Abstract

Vesta is a unique, intermediate class of rocky body in the Solar System, between terrestrial planets and small asteroids, because of its size (average radius of ∼263 km) and differentiation, with a crust, mantle and core. Vesta’s low surface gravity (0.25 m/s2) has led to the continual absence of a protective atmosphere and consequently impact cratering and impact-related processes are prevalent. Previous work has shown that the formation of the Rheasilvia impact basin induced the equatorial Divalia Fossae, whereas the formation of the Veneneia impact basin induced the northern Saturnalia Fossae. Expanding upon this earlier work, we conducted photogeologic mapping of the Saturnalia Fossae, adjacent structures and geomorphic units in two of Vesta’s northern quadrangles: Caparronia and Domitia. Our work indicates that impact processes created and/or modified all mapped structures and geomorphic units. The mapped units, ordered from oldest to youngest age based mainly on cross-cutting relationships, are: (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. The Saturnalia Fossae consist of five separate structures: Saturnalia Fossa A is the largest (maximum width of ∼43 km) and is interpreted as a graben, whereas Saturnalia Fossa B-E are smaller (maximum width of ∼15 km) and are interpreted as half grabens formed by synthetic faults. Smaller, second-order structures (maximum width of <1 km) are distinguished from the Saturnalia Fossae, a first-order structure, by the use of the general descriptive term ‘adjacent structures’, which encompasses minor ridges, grooves and crater chains. For classification purposes, the general descriptive term ‘minor ridges’ characterizes ridges that are not part of the Saturnalia Fossae and are an order of magnitude smaller (maximum width of <1 km vs. maximum width of ∼43 km). Shear deformation resulting from the large-scale (diameter of <100 km) Rheasilvia impact is proposed to form minor ridges (∼2 km to ∼25 km in length), which are interpreted as the surface expression of thrust faults, as well as grooves (∼3 km to ∼25 km in length) and pit crater chains (∼1 km to ∼25 km in length), which are interpreted as the surface expression of extension fractures and/or dilational normal faults. Secondary crater material, ejected from small-scale and medium-scale impacts (diameters of <100 km), are interpreted to form ejecta ray systems of grooves and crater chains by bouncing and scouring across the surface. Furthermore, seismic shaking, also resulting from small-scale and medium-scale impacts, is interpreted to form minor ridges because seismic shaking induces flow of regolith, which subsequently accumulates as minor ridges that are roughly parallel to the regional slope. In this work we expand upon the link between impact processes and structural features on Vesta by presenting findings of a photogeologic, structural mapping study which highlights how impact cratering and impact-related processes are expressed on this unique, intermediate Solar System body.

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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