Elsevier

Icarus

Volume 244, December 2014, Pages 74-88
Icarus

The geology of the Marcia quadrangle of asteroid Vesta: Assessing the effects of large, young craters

https://doi.org/10.1016/j.icarus.2014.01.033 Get rights and content

Highlights

  • We produced a geologic map Vesta’s Marcia quadrangle to determine regional geologic history.

  • The ∼58 km diameter Marcia crater is youngest large crater on Vesta.

  • It marks a major vestan time-stratigraphic event and is one of the geologically youngest formations.

  • Marcia reveals pristine bright/dark materials, and terrains suggestive of impact melts and release of volatiles.

  • Aricia Tholus is a fragment of an ancient impact basin rim topped by a dark-rayed impact crater.

Abstract

We used Dawn spacecraft data to identify and delineate geological units and landforms in the Marcia quadrangle of Vesta as a means to assess the role of the large, relatively young impact craters Marcia (∼63 km diam.) and Calpurnia (∼53 km diam.) and their surrounding ejecta field on the local geology. We also investigated a local topographic high with a dark-rayed crater named Aricia Tholus, and the impact crater Octavia that is surrounded by a distinctive diffuse mantle. Crater counts and stratigraphic relations suggest that Marcia is the youngest large crater on Vesta, in which a putative impact melt on the crater floor ranges in age between ∼40 and 60 Ma (depending upon choice of chronology system), and Marcia’s ejecta blanket ranges in age between ∼120 and 390 Ma (depending upon choice of chronology system). We interpret the geologic units in and around Marcia crater to mark a major vestan time-stratigraphic event, and that the Marcia Formation is one of the geologically youngest formations on Vesta. Marcia crater reveals pristine bright and dark material in its walls and smooth and pitted terrains on its floor. The smooth unit we interpret as evidence of flow of impact melts and (for the pitted terrain) release of volatiles during or after the impact process. The distinctive dark ejecta surrounding craters Marcia and Calpurnia is enriched in OH- or H-bearing phases and has a variable morphology, suggestive of a complex mixture of impact ejecta and impact melts including dark materials possibly derived from carbonaceous chondrite-rich material. Aricia Tholus, which was originally interpreted as a putative vestan volcanic edifice based on lower resolution observations, appears to be a fragment of an ancient impact basin rim topped by a dark-rayed impact crater. Octavia crater has a cratering model formation age of ∼280–990 Ma based on counts of its ejecta field (depending upon choice of chronology system), and its ejecta field is the second oldest unit in this quadrangle. The relatively young craters and their related ejecta materials in this quadrangle are in stark contrast to the surrounding heavily cratered units that are related to the billion years old or older Rheasilvia and Veneneia impact basins and Vesta’s ancient crust preserved on Vestalia Terra.

Introduction

The asteroid Vesta displays a variety of impact crater-related features as revealed by NASA’s Dawn spacecraft (Russell et al., 2012, Jaumann et al., 2012), including a region of overlapping large, relatively fresh craters with low albedo ejecta fields adjacent to the Vestalia Terra highlands, as revealed by 1:500,000 scale global geologic mapping (Yingst et al., in press). As part of a quadrangle mapping program devised to support the Dawn Science Team with geologic and stratigraphic analyses of Vesta’s surface, we produced a 1:250,000 scale geologic map of the Av-8 Marcia quadrangle. In this paper we discuss insights into the geologic features and stratigraphic relations of the Marcia quadrangle using our new map, which was derived from the higher spatial resolution Low Altitude Mapping Orbit (LAMO) basemap than was used for the global map.

Section snippets

Regional setting

The Av-8 Marcia quadrangle straddles the 180° longitude in the equatorial region of Vesta, covering 144–216°E longitude and ±21° latitude. Global geologic mapping of Vesta (Yingst et al., in press) recognized that the Marcia quadrangle occurs at the confluence of several units: (1) the south polar and southern latitude, relatively lightly-cratered floors of the Rheasilvia and Veneneia impact basins (called the Rheasilvia Formation); (2) the equatorial, highly-cratered ridge and trough terrain

Methods

Detailed discussion of the basemap and supplemental materials used to make the quadrangle maps of Vesta, as well as the techniques and tools used, is given in the introductory paper (Williams et al., 2014). Briefly, we conducted iterative mapping of the Av-8 Marcia quadrangle using successively higher spatial resolution Framing Camera image mosaics obtained during the Survey (∼260 m/pixel), High Altitude Mapping Orbit (HAMO, ∼70 m/pixel), and LAMO (∼25 m/pixel, Fig. 1) phases of the Dawn mission.

Results

Geologic mapping of the Av-8 Marcia quadrangle has identified 15 units in this region (Fig. 4a, Fig. 4b and 5). In the following sections we discuss each of these map units, from oldest to youngest, based on correlation of stratigraphic relations (Fig. 6). Where possible, we have applied assessments of crater densities to obtain cratering model ages of crater units (Table 1) using both asteroid flux-based (Marchi et al., 2012a, Marchi et al., 2012b, Marchi et al., 2013, O’Brien et al.,

Geologic history of Marcia quadrangle

The geologic history of Marcia quadrangle is outlined graphically in the Correlation of Map Units (Fig. 6). After Vesta’s formation the crust crystallized from a magma ocean and was bombarded by impacts, forming various cratered highlands units, possibly including old craters such as Minucia. The Veneneia and Rheasilvia impacts at the south pole disrupted almost half of Vesta, deposited ejecta into the northern latitudes, and led to the formation of the Divalia Fossae Formation in the

Conclusions

Geologic mapping has illuminated the geologic history of the Marcia quadrangle of Vesta. In juxtaposition to the ancient cratered highlands and the large areas affected by the Veneneia and Rheasilvia impact events billions of years ago, the Marcia quadrangle is dominated by the formation of younger, larger (>25 km) impacts. In particular, the event that formed Marcia crater disrupted the relatively large region of dark, hydrated material found in this hemisphere of Vesta, exposing both buried

Acknowledgments

The authors thank James Skinner, Wesley Patterson, and editor David Blewett for productive and helpful reviews. The authors also thank the NASA Dawn Science and Flight Teams at the Jet Propulsion Laboratory for their tireless work that enabled the successful Vesta encounter, and the instrument teams at the Max Planck Institute, the German Aerospace Center (DLR), the Italian National Institute of Astrophysics (INAF), and the Planetary Science Institute for collecting and processing the data that

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