Lithologically diverse yardangs in the circum-isidis region : implications for yardang evolution controls and in situ study at the mars 2020 landing site

Introduction: Parallel, wind-carved ridges called yardangs are pervasive features on Mars [1], with rare and hypothesized occurrences, respectively, on Earth and Venus. Despite recent advances in quantitative study of terrestrial yardangs [2], much remains unknown about the controls on yardang formation and evolution on planetary surfaces, including the combined influences of substrate friability, material heterogeneity, and structural bedding orientation on the morphologies of yardang fields from initiation to demise [2–4]. The rarity of yardangs on Earth and data limitations on Mars have restricted insight into the interplay of the diverse factors in yardang development. The circum-Isidis region of Mars, including Nili Fossae and Libya Montes, hosts a lithologically and structurally diverse stratigraphy of Hesperianto Noachian-aged rocks [5,6] that has been previously suggested to have eroded into yardangs. For instance, qualitatively parallel ridges that are nearly ubiquitously associated with a circum-Isidis olivine-rich unit [7] have been locally interpreted as yardangs [8,9]. Parallel ridges are common in the landing site for the Mars 2020 rover at Jezero crater and in potential extended mission targets in NE Syrtis [10]. The potential existence of yardangs eroded from the region’s lithologically and structurally variable stratigraphy makes the region’s geomorphology valuable for constraining substrate influences on yardang evolution. However, it remains unclear whether these features are yardangs or are other erosional, depositional, or structural features. Methods: We constrained the origin of qualitatively parallel ridges in Nili Fossae and Libya Montes through measurements and observations using CTX imagery mosaics [11] and HiRISE imagery and digital elevation models [12]. We mapped the extent of potential yardang fields (Fig. 1), tested the potential yardangs’ degree of consolidation with observations of rock textures, and tested whether they are bedforms by comparing their stratigraphic position and orientations with previously identified bedding. We also measured orientations of thousands of ridges, which we compared with those of adjacent crater wind streaks. Results: Ridge Classification. We observe several types of parallel ridges in the circum-Isidis region. Libya Montes hosts the following ridge types associated with the olivine-rich unit: 1) large (10s to 100s of meters) inverted ship-hull ridges (Fig. 2), 2) small (meters to 10s of meters) elongate mounds, and 3) large (10s of meters) low-aspect-ratio mounds. These features variably qualitatively resemble “whale-back,” “saw-tooth,” or “hogback” yardangs [4]. In Nili Fossae, ridges are commonly 100s of meters in length and have high aspect ratio, irrespective of their association with the olivine-rich unit (Fig. 3), mafic capping unit, or Jezero delta (Fig. 4), resembling “long-ridge” yardangs [4]. Ridges in the floor material of Jezero, which includes a rock unit correlative with the olivine-rich unit [14], resemble other Nili Fossae ridges but have lower relief and shorter lengths (Fig. 4B,D). Ridges composed of basement rock in NE Syrtis, referred to as “Long Linear Features” [15] exhibit a paired-ridge, branching morphology strongly dissimilar from yardangs observed on Earth or Mars and are not considered further.