The Timing and Origin of Lobate Ejecta Forms at Craters in Mercury's South Polar Region

We present six newly identified examples of lobate ejecta in Mercury's south polar quadrangle (H15), providing the first evidence of syn-impact formation through our observation of perched impact melt at Nairne and Magritte craters. This finding challenges the idea that lobate forms could have developed post-impact via mass wasting or landslide processes, suggesting instead that many, if not all, lobate ejecta deposits formed contemporaneously with the impact event. We present detailed morphostratigraphic maps of two exemplary case studies: Nairne and Magritte, for which we used shadow measurements to better constrain the morphology of their lobes. Many examples globally, including Nairne, have been formed by deposition of the lobe material into a topographically lower antecedent crater. While we confirm that topography plays a significant role in the formation of lobate ejecta, it cannot be the sole factor, as similar impacts onto pre-existing craters do not always produce these features and not all lobate ejecta exhibit evidence for a topographic control. Our study also highlights the frequent association between lobate ejecta and characteristics of oblique impacts. Considering analogous features at Mars and the Moon in particular, lobate ejecta on Mercury is likely the result of either fluidized or dry granular flows, with minimal delay between impact and emplacement. Our findings suggest that lobate ejecta are more widespread on Mercury than previously recognized, and future studies and missions, such as a comprehensive global survey with higher resolution data from BepiColombo, will continue to constrain their formation mechanisms and prevalence. Plain Language Summary We discovered six new examples of unusual landforms surrounding impact craters at Mercury, referred to as lobate forms/ejecta. We found these six examples in Mercury's south polar region, an area referred to as the Bach quadrangle or H15. We present evidence that these landforms likely developed during the impact events that formed the host crater, rather than developing later by processes similar to those producing landslides. This study almost doubles the known examples of lobate forms/ejecta on Mercury, presents maps to better constrain the shape and structure of the crater and ejecta, and calculates the heights of the lobes. We confirm that the shape of the land onto which the impactor hits strongly affects their formation, though it is not the only factor. We also suggest that these landforms are linked to oblique impacts, where the angle between the trajectory of the impactor is less than the typical 45 degrees, causing radial symmetry to be lost, and that the lobate forms were produced at the same time as the impact occurred. The study concludes that craters with lobate ejecta are more common on Mercury than previously thought. Future missions will provide more detailed data to better understand them.