Lobate Ejecta Deposits in Mercury's South Polar Quadrangle (H15)

International audienceIntroduction: During geological mapping of Mercury's south polar quadrangle (H15), we have discovered unrecorded lobate ejecta deposits, one in particular at Nairne crater (70.36° S, 1.65° E) exhibiting a unique and distinctive morphology. Lobate ejecta deposits are relatively rare on Mercury; Xiao and Komatsu compiled a global database of only 7 craters with ejecta flows [1], and Blance et al. [2] have since found more. However, the timing of emplacement has remains ambiguous. Understanding the formation of the lobate ejecta deposits on Mercury would reveal insights into the factors that control crater morphology, and the characteristics of crater deposits can better our understanding of impact crater formation mechanisms operating on Mercury. Methods: We mapped using a monochrome basemap (166 m/pixel) in conjunction with individual narrow-angle camera (NAC) images at higher resolutions. Additionally, we used 665 m/pixel enhanced color mosaic and 665 m/pixel digital elevation model (DEM) mosaics to distinguish units by colour and to assist in the mapping of topographic features. All these data are products of the Mercury Surface, Space Environment, Geochemistry and Ranging (MESSENGER) Mercury Dual Imaging System (MDIS) instrument. Observations: Morphology. Nairne crater, from which two ejecta lobes extend, is relatively fresh (c4 [3]) and was formed by an impact onto the rim of a much larger and now significantly more degraded unnamed crater (c1 [3]). It exhibits asymmetrical terracing, a bright blue central peak and bright red impact ejecta. The two distinct lobes that propagate SSW into the floor of the preexisting crater exhibit steep fronts and appear to have greater thickness and surface roughness compared to normal continuous ejecta deposits on Mercury. We also identified very smooth localized deposits perched on topographic lows on top of the larger of the two lobes. Discussion: Formation of fluidized ejecta: Key factors affecting the formation of ejecta flows on Mercury include th