Imprint of the Rheasilvia impact on Vesta – Geologic mapping of quadrangles Gegania and Lucaria

Abstract

a b s t r a c tWe produced two 1:250,000 scale geologic maps of the adjacent quadrangles Av-6 Gegania and Av-7 Lucaria, located in the equatorial region of (4) Vesta (0-144°E, 22°S to 22°N). The mapping is based on clear and color filter images of the Framing Camera (FC) onboard the Dawn spacecraft, which has captured the entire illuminated surface of Vesta with high spatial resolution (up to $20 m/pixel), and on a digital terrain model derived from FC imagery. Besides the geologic mapping itself, a secondary purpose of this work is to investigate one of the most prominent morphological features on Vesta, namely the aggregation of several giant equatorial troughs termed the Divalia Fossae, most probably formed during the Rheasilvia impact near Vesta's south pole. The up to 465 km long and 22 km wide troughs show height differences of up to 5 km between adjacent troughs and ridges. Another imprint of the Rheasilvia impact is the >350 km long and $250 km wide swath of ejecta crossing quadrangle Av-6 Gegania. This lobe shows a distinct appearance in FC color ratios and a high albedo in FC images, indicating a mineralogical similarity to material typically found within the Rheasilvia basin, in particular composed of diogenite-rich howardites. Almost the entire northern half of the mapping area shows the oldest surface, being dominated by upper crustal basaltic material. To the south, increasingly younger formations related to the Rheasilvia impact occur, either indicated by the troughs formed by Rheasilvia or by the Rheasilvia ejecta itself. Only medium sized impact craters with diameters less than 22 km occur within the two mapped quadrangles. Some of the craters exhibit ejecta blankets and/or distinctly dark or bright ejecta material in ejecta rays outside and exposures within the crater, and mass-wasting deposits down crater slopes, forming the youngest surfaces.

The chronostratigraphy of protoplanet Vesta

“…However, the distribution of the Rheasilvia-Veneneia ejecta is not homogeneous in either extent or depth, and these deposits did not erase the largest (few tens of km diameter) craters (Schenk et al, 2012;Marchi et al, 2012aMarchi et al, , 2012b. Additionally, FC color data (Reddy et al, 2012a(Reddy et al, , 2012b) and the GRaND global iron map show a large lobe of apparent Rheasilvia ejecta extending across the Gegania and Lucaria quadrangles (Schaefer et al, 2014). There is also evidence that small (<5-km-diameter) lobes of either Rheasilvia or Veneneia ejecta extend well into the northern hemisphere (Williams et al, in press-a).…”

“…Global geologic mapping (Jaumann et al, 2012;Yingst et al, in press) identified two separate sets of prominent ridges and troughs, one set encircling much of Vesta's equatorial region (Divalia Fossae: see also Schaefer et al, 2014) and the other preserved in the heavily cratered northern terrain (Saturnalia Fossae). Structural analysis of these ridge-and-trough systems indicated that they are likely a tectonic response to the formation of the south polar basins: the Veneneia impact led to the formation of the Saturnalia Fossae, with the Rheasilvia impact leading to the Divalia Fossae (Jaumann et al, 2012;Buczkowski et al, 2012;Bowling et al, 2014).…”

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

“…16). The distribution of this diffuse mantling unit is not symmetrical around the crater, but rather extends $190 km to the west-northwest (Le Corre et al, 2013) and into the Av-7 Lucaria quadrangle (Schäfer et al, 2014). To date, FC color and VIR data analysis has not resulted in a specific mineralogical composition of this diffuse mantling material.…”

Introduction: The geologic mapping of Vesta

“…In the equatorial latitudes, Schaefer et al (2014) use a combined map of quadrangles Av-6 (Gegania) and Av-7 (Lucaria) to study the dimensions and nature of the equatorial troughs of the Divalia Fossae Formation. 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.…”