Lithologies of Dimorphos revealed by boulder morphological classes

The stony/Sq-type binary system (65803) Didymos consists of two rubble pile bodies, Didymos, the primary, and its moonlet Dimorphos. In 2022, the first planetary defense mission Double Asteroid Redirection Test (DART) reached Dimorphos and collected unprecedented high-resolution images of its surface. They revealed a variegated surface completely covered by stacked boulders and cobbles with different shapes and textures. Their morphological heterogeneity likely reflects the lithologies originally present within the Didymos and Dimorphos' parent body, before its fragmentation. We present a lithologic study of Dimorphos' surface, based on a morphological analysis of its boulders. Our approach considers each boulder's 2D outline, perceived 3D shape, and surface texture characteristics. Based on these features, we identified two main morphological classes. A total of 178 boulders were classified as Angular morphology covering 38.5 % of the mapped area. This type is characterized by cohesive boulders with straight and angular outlines as well as sharp three-dimensional edges, which form well-defined sub-planar facets. Additionally, the texture of this kind of rocks show low-to-mid Roughness and the presence of lineations on some boulders. On the other hand, 210 boulders were categorized as Hummocky morphology, which covers 61.5 % of the area. Such morphology appears more friable and features boulders with rounded shaped and ragged and irregular 2D perimeter. Their 3D perceived shape appears as flat-to-curve, with rough and hummocky surface textures due to embedded clasts. We interpret the Hummocky morphology as breccia lithology. On the other hand, although Angular morphology can be associated with multiple potential lithologies, we interpret it as either an achondritic/igneous lithology or a highly metamorphosed chondritic lithology. The breccias are likely characteristic of the parent body's outer layers, actively involved in impact and sedimentary processes. Conversely, the lithologies associated with the An morphology should represent deeper regions of the plantesimal, possibly exhumed by impacts. All these results will be further complemented by data from the Hera mission, which will arrive at (65803) Didymos system at the end of 2026. • Lithological analysis based on boulder's morphological and geological observations. • Two morphological classes have been found: Hummocky and Angular morphologies. • Sphericity and circularity parameters are consistent with two morphologies. • We found breccia and achondritic or ultra-metamorphic chondritic lithologies. • Results suggest an evolved parent body near the differentiation stage.