The mysterious location of Maryland on 2014 MU69 and the reconfiguration of its bilobate shape
Authors: Hirabayashi Masatoshi , Trowbridge Alexander , Bodewits Dennis 
Published: October 2020

Introduction: The flyby observations of 2014 MU69 (also known as Arrokoth) by the New Horizons spacecraft in 2019 showed that Arrakoth possesses a bilobate shape with unique surface morphologies [e.g., 1-3]. Arrokoth is likely to consist of an icy and porous structure [1]. High-resolution images captured a circular depression, Maryland. This ~7-km-diameter circular feature may be developed by a meteoroid impact process [1,3]. Assuming that Maryland is geologically younger than Arrokoth&#8217;s bilobate shape, here, we investigate the structural condition of Arrokoth, focusing on its neck.We combine two models to analyze the neck&#8217;s structural sensitivity during the formation of Maryland. The first model computes the crater formation condition by using impact scaling relationships (Model I), while the second model characterizes the structural and dynamics conditions (Model II). To model Arrokoth&#8217;s shape [4], we assume that this body consists of a triaxial ellipsoid with a size of 22 km x 20 km x 7 km and a sphere with an equivalent radius of 6.3 km [4]. Below is the summary of these models.Model I applies the pi-scaling relationships [5] to compute the Maryland formation condition on the small lobe. We define two end-member target materials. End-member A consists of a porous sand material [6] in the gravity regime and an icy, porous material [7] in the strength regime. End-member B is characterized by a water-ice material [8] in the gravity regime and an icy, porous material [7] in the strength regime.Model II consists of two parts. The first part calculates the minimum level of the cohesive strength that the neck can avoid structure failure by the Maryland impact. The structural failure condition is calculated based on the Mohr-Coulomb yield criterion with an angle of friction of &#160;[9]. Impact driven loadings are computed with the assumptions of a zero-obliquity impact and zero momentum transfer, although these conditions may likely be different [e.g., 10, 11].We integrate these models to compute the Maryland formation condition and determine the minimum cohesive strength of the neck, given the size of Maryland on Arrokoth.Results: Figures 1 illustrates the bulk cohesive strength for the neck to resist structural failure. The cohesive strength needs to be higher than a few kPa, depending on material and structural conditions. It increases with the bulk density because the internal stress directly depends on the bulk density. The results imply that the predicted bulk cohesive strength for Arrokoth is higher than the typical values of the bulk cohesive strengths of small bodies. Small bodies observed at high resolutions were predicted to have bulk cohesive strengths of ~300 Pa [13, 14], except that there are a few rubble pile bodies having bulk cohesive strengths up to 1 kPa [15]. The Deep Impact ejecta also implied that the bulk cohesive strength of the nucleus of comet 9P/Tempel 1 might be less than~340 Pa [16]*. Also, ice rubbles may have a cohesive strength of ~1 kPa [17].Discussion: Our analysis showed that Arrokoth needs much higher bulk cohesive strength than observed rubble pile small bodies. This discrepancy leads to two explanations. The first explanation is that Arrokoth has a high cohesive strength to resist structural failure during the Maryland formation. While this explanation may be plausible (depending on its structural condition), it is not supported by recent reports. If this is the case, it is necessary to review the mechanisms of cohesion. The second explanation is that the Maryland formation broke the neck&#8217;s structure. In this case, the neck lost mechanical strength. The two lobes could move freely, reaching a new stable shape configuration [14] while avoiding a complete separation (i.e., they fly away). Arrokoth needs a specific energy level to reach the current shape configuration at which the small lobe rests along the longest axis of the large lobe [18]. This condition constraints the relationships between the shape and the bulk density (Figure 2). Considering the current spin, we obtain that Arrokoth&#8217;s bulk density is between 300 kg/m3 and 500 kg/m3, which is consistent with [19] and the bulk density of the nucleus of 67P [20, 21].*We note that [16] obtained an upper limit of the effective strength as 10 kPa, leading to a bulk cohesive strength of this nucleus less than ~340 Pa.Acknowledgments: M.H. thanks support from NASA/SSW (NNH17ZDA001N/80NSSC19K0548). A.J.T. is supported by NESSF (80NSSC18K1265). M.H. and D.B. are also supported by Auburn University/Intramural Grant Program. 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