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

Abstract

a b s t r a c tWe 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 http://dx

Geologic map of the northern hemisphere of Vesta based on Dawn Framing Camera (FC) images

“…Our absolute model age of Marcia can be compared only with those obtained in the same geologic unit and with the same functions. We found that our AMA of the Marcia ejecta blanket calculated with the functions of Schmedemann et al (in preparation) is just within the error estimates of the youngest AMA presented in the work of Williams et al (2014b) (101 ± 13 Ma versus 123 ± 9 Ma). We consider the counting area on the Marcia ejecta blanket to be more representative of the formation age than a counting area within the crater itself.…”

“…We consider the counting area on the Marcia ejecta blanket to be more representative of the formation age than a counting area within the crater itself. The latter counting area, which is presented in Marchi et al (submitted for publication) and Williams et al (2014b), corresponds to a possible impact melt pond. Thus, the derived age of the pond could be underestimated due to target property effects as observed on the Moon and Mars (e.g., Dundas et al, 2010;Hiesinger et al, 2012;van der Bogert et al, 2010van der Bogert et al, , 2013.…”

“…In the study of Williams et al (2014b), different ages of Marcia are presented with different counting areas and different production and chronology functions. Our absolute model age of Marcia can be compared only with those obtained in the same geologic unit and with the same functions.…”

“…This impact has been dated at 60 Ma in the work of Williams et al (2014b) and Marchi et al (2013). Differences between crater size-frequency measurements and isochron precluded derivation of reliable AMA for units older than unit uc.…”

Introduction: The geologic mapping of Vesta

“…Williams et al (2014) focus on the stratigraphic relations of the large, young fresh craters and associated dark ejecta field found in the Av-8 (Marcia) quadrangle, and Buczkowski et al (2014) focus on the diversity of units on the highland of Vestalia Terra in Av-9 (Numisia) quadrangle. Garry et al (2014) have used the geologic map of the Av-10 (Oppia) quadrangle to investigate the stratigraphic relations and the context of high-albedo crater ejecta surrounding Oppia crater, building on the study of Le Corre et al (2013).…”