Examining Arrokoth The New Horizons spacecraft flew past the Kuiper Belt object (486958) Arrokoth (also known as 2014 MU69) in January 2019. Because of the great distance to the outer Solar System and limited bandwidth, it will take until late 2020 to downlink all the spacecraft's observations back to Earth. Three papers in this issue analyze recently downlinked data, including the highest-resolution images taken during the encounter (see the Perspective by Jewitt). Spencer et al. examined Arrokoth's geology and geophysics using stereo imaging, dated the surface using impact craters, and produced a geomorphological map. Grundy et al. investigated the composition of the surface using color imaging and spectroscopic data and assessed Arrokoth's thermal emission using microwave radiometry. McKinnon et al. used simulations to determine how Arrokoth formed: Two gravitationally bound objects gently spiraled together during the formation of the Solar System. Together, these papers determine the age, composition, and formation process of the most pristine object yet visited by a spacecraft. Science, this issue p. eaay3999, p. eaay3705, p. eaay6620; see also p. 980 Simulations show that Arrokoth (2014 MU69) formed by the gentle inspiral of a binary system in the early Solar System. INTRODUCTION The close flyby of the Kuiper Belt object (486958) Arrokoth (formerly 2014 MU69) by NASA’s New Horizons spacecraft revealed details of the body’s structure, geology, and composition. Arrokoth is a member of the cold classical component of the Kuiper Belt, a population of dwarf planets and smaller bodies thought to be only modestly dynamically or collisionally disturbed, unlike the asteroids of the inner Solar System, comets, or other groups of Kuiper Belt objects. Data from this flyby provides the opportunity to observe the results of primordial planetesimal accretion, largely unobscured by later geological or dynamical processes. RATIONALE Planetesimal formation is an unsolved problem in planetary science. Many mechanisms have been proposed in which small solid particles (dust and pebbles) agglomerate into planetesimals and ultimately into planets. The flyby of Arrokoth provides data that constrain planetesimal formation theories and allow us to construct models of Arrokoth’s specific physical characteristics. The accretion processes that operated in the cold classical region of the Kuiper Belt during the formation of the Solar System are expected to have also occurred elsewhere in the protosolar nebula. Arrokoth is a contact binary about 35 km long composed of two unequally sized lobes. Each lobe is flattened or lenticular in shape, and the planes of flattening of both (determined from their principal axes) are closely aligned, to within 5°. The smaller lobe is slightly oblong, with its long axis pointing down the long axis of the binary as a whole (to within 5°). The surface and overall structure of Arrokoth do not display any obvious signs of catastrophic or subcatastrophic collision, and the join or neck between the two lobes is narrow. Each lobe is compositionally similar to within the precision of spectral measurements. RESULTS We show that stresses in the neck region today are compatible with the structural integrity of Arrokoth for densities (several 100 kg m−3) and material strengths (a few kilopascals) similar to those observed in comets, but at mass scales ~1000 times the mass of typical cometary nuclei. We performed numerical simulations of collisions between two bodies on the scale of the two lobes of Arrokoth, assuming those density and strength parameters. We found that impacts at or greater than their mutual escape speed (a few meters per second) would have been highly damaging. The close geometric alignment of the lobes is highly unlikely to the be the result of a chance collision alone but can be readily understood as the result of tidal evolution of a tight, co-orbiting binary. This requires a mechanism to extract angular momentum from the binary orbit, causing the orbit to shrink, and the two components to gently merge. Numerical models show that overdense concentrations of particles in the protosolar gas nebula can become gravitationally unstable and collapse to form planetesimals. The angular momentum in the simulated pebble clouds is high enough that formation of co-orbiting binaries is efficient and with binary characteristics that are a good match to binaries observed in the Kuiper Belt today. We examined a range of mechanisms to extract or transfer angular momentum from a co-orbiting binary and drive an ultimate merger, including mutual tides, tidal effects of the Sun (Kozai-Lidov oscillations), collisions with smaller Kuiper Belt objects, the ejection of third bodies, asymmetric radiation forces, and gas drag. We found that for bodies the size of Arrokoth, gas drag may be most effective in this merger process over the lifetime of the protosolar nebula. CONCLUSION We show that models of Arrokoth’s formation and evolution support accretion of the binary through the gravitational collapse of an overdense pebble cloud in the presence of protosolar nebular gas, either as a contact binary initially or as a co-orbiting binary that later inspiraled and gently merged. Similar accretional processes and binary planetesimal formation likely occurred throughout the early Solar System. Simulated maximum accelerations experienced by particles during low-velocity impact of two spheres, approximating the scale of the two lobes of Arrokoth. Spheres are modeled as granular aggregates with bulk densities of 500 kg m−3 and an impact speed of 2.9 m s−1 at a tangent angle of 80°; such gentle collisional conditions are necessary to preserve Arrokoth’s overall undamaged shape. Extreme reds and blues correspond to the greatest and least maximum accelerations experienced, respectively. The maximum disturbance is concentrated in the narrow contact area, or neck, between the two bodies. The New Horizons spacecraft’s encounter with the cold classical Kuiper Belt object (486958) Arrokoth (provisional designation 2014 MU69) revealed a contact-binary planetesimal. We investigated how Arrokoth formed and found that it is the product of a gentle, low-speed merger in the early Solar System. Its two lenticular lobes suggest low-velocity accumulation of numerous smaller planetesimals within a gravitationally collapsing cloud of solid particles. The geometric alignment of the lobes indicates that they were a co-orbiting binary that experienced angular momentum loss and subsequent merger, possibly because of dynamical friction and collisions within the cloud or later gas drag. Arrokoth’s contact-binary shape was preserved by the benign dynamical and collisional environment of the cold classical Kuiper Belt and therefore informs the accretion processes that operated in the early Solar System.

中文翻译：

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柯伊伯带原始接触双星 (486958) Arrokoth 的太阳星云起源

检查 Arrokoth 新视野号航天器于 2019 年 1 月飞过柯伊伯带天体 (486958) Arrokoth（也称为 2014 MU69）。由于与外太阳系的距离很远且带宽有限，因此要到 2020 年末才能下行所有航天器的观测返回地球。本期中的三篇论文分析了最近的下行数据，包括相遇期间拍摄的最高分辨率图像（参见 Jewitt 的观点）。斯宾塞等人。使用立体成像检查了 Arrokoth 的地质和地球物理学，使用撞击坑确定了地表的年代，并制作了一张地貌图。格兰迪等人。使用彩色成像和光谱数据研究了表面的组成，并使用微波辐射测定法评估了 Arrokoth 的热发射。麦金农等。使用模拟来确定 Arrokoth 是如何形成的：在太阳系形成过程中，两个受引力束缚的物体轻轻地盘旋在一起。这些论文共同确定了航天器迄今为止访问过的最原始物体的年龄、组成和形成过程。科学，这个问题 p。eaay3999，第。eaay3705，第。eaay6620; 另见第 980 模拟表明，Arrokoth (2014 MU69) 是由早期太阳系双星系统的温和启发形成的。介绍 NASA 的新视野号航天器近距离飞越柯伊伯带天体 (486958) Arrokoth（原 2014 MU69）揭示了天体结构、地质和成分的详细信息。Arrokoth 是柯伊伯带冷经典成分的成员，与内太阳系的小行星、彗星或其他柯伊伯带天体群不同，一群矮行星和较小的天体被认为只会受到适度的动态或碰撞干扰。来自这次飞越的数据提供了观察原始微星吸积结果的机会，这些吸积在很大程度上没有被后来的地质或动力过程所掩盖。基本原理 小行星的形成是行星科学中一个未解决的问题。已经提出了许多机制，其中小的固体颗粒（灰尘和鹅卵石）聚集成星子并最终聚集成行星。Arrokoth 的飞越提供了限制星子形成理论的数据，并使我们能够构建 Arrokoth 特定物理特征的模型。在太阳系形成期间在柯伊伯带寒冷的经典区域进行的吸积过程预计也发生在原太阳星云的其他地方。Arrokoth 是一个大约 35 公里长的接触双星，由两个大小不等的瓣组成。每个叶呈扁平状或透镜状，两者的扁平平面（根据它们的主轴确定）紧密对齐，误差在 5° 以内。较小的瓣略呈椭圆形，其长轴整体指向双星长轴下方（5°以内）。Arrokoth 的表面和整体结构没有表现出任何明显的灾难性或亚灾难性碰撞的迹象，并且两个裂片之间的连接或颈部很窄。在光谱测量的精度范围内，每个波瓣的成分相似。结果我们表明，今天颈部区域的应力与 Arrokoth 的结构完整性相容，其密度（100 kg m-3）和材料强度（几千帕）与在彗星中观察到的相似，但质量尺度约为 1000 倍典型彗核的质量。我们在 Arrokoth 的两个叶瓣的尺度上对两个物体之间的碰撞进行了数值模拟，假设这些密度和强度参数。我们发现，以或大于它们的相互逃逸速度（每秒几米）的撞击将具有很高的破坏性。叶瓣的紧密几何排列极不可能仅是偶然碰撞的结果，但可以很容易地理解为紧密的共轨双星潮汐演化的结果。这需要一种机制从双星轨道中提取角动量，导致轨道收缩，两个分量轻轻合并。数值模型表明，原太阳气体星云中粒子的密度过高会变得重力不稳定并坍塌形成小行星。模拟卵石云中的角动量足够高，以至于形成共轨双星是有效的，并且双星特征与今天在柯伊伯带观察到的双星非常匹配。我们研究了一系列从共轨双星中提取或转移角动量并驱动最终合并的机制，包括相互潮汐、太阳潮汐效应（Kozai-Lidov 振荡）、与较小柯伊伯带天体的碰撞、第三体、不对称辐射力和气体阻力。我们发现，对于 Arrokoth 大小的天体，在原太阳星云的整个生命周期中，气体阻力在这种合并过程中可能最有效。结论 我们表明，Arrokoth 的形成和演化模型支持在原太阳星云气体存在的情况下，通过密度过大的卵石云的引力坍缩，双星的吸积，无论是最初作为接触双星还是作为后来受到轻微启发的共轨双星合并。类似的吸积过程和双星小行星的形成很可能发生在整个早期太阳系。模拟粒子在两个球体的低速撞击过程中经历的最大加速度，近似于 Arrokoth 的两个叶瓣的尺度。球体被建模为颗粒聚集体，体积密度为 500 kg m-3，冲击速度为 2。9 ms−1，切角为 80°；这种温和的碰撞条件对于保持 Arrokoth 整体未损坏的形状是必要的。极端的红色和蓝色分别对应于最大和最小的最大加速度。最大的干扰集中在两个身体之间狭窄的接触区域或颈部。新视野号航天器与寒冷的经典柯伊伯带天体 (486958) Arrokoth（临时编号 2014 MU69）的相遇揭示了一个接触双星小行星。我们调查了 Arrokoth 是如何形成的，并发现它是早期太阳系温和、低速合并的产物。它的两个透镜状裂片表明，在引力坍缩的固体粒子云中，许多较小的星子以低速聚集。叶瓣的几何排列表明它们是一个共轨双星，经历了角动量损失和随后的合并，可能是因为云内的动力摩擦和碰撞或后来的气体阻力。Arrokoth 的接触双星形状被冷的经典柯伊伯带的良性动力和碰撞环境保留下来，因此为早期太阳系中的吸积过程提供了信息。