The geology and geophysics of Kuiper Belt object (486958) Arrokoth,Science

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The geology and geophysics of Kuiper Belt object (486958) Arrokoth
Science ( IF 44.7 ) Pub Date : 2020-02-13 , DOI: 10.1126/science.aay3999
J R Spencer ₁ , S A Stern ₁ , J M Moore ₂ , H A Weaver ₃ , K N Singer ₁ , C B Olkin ₁ , A J Verbiscer ₄ , W B McKinnon ₅ , J Wm Parker ₁ , R A Beyer _{2,

6} , J T Keane ₇ , T R Lauer ₈ , S B Porter ₁ , O L White _{2,

6} , B J Buratti ₉ , M R El-Maarry _{10,

11} , C M Lisse ₃ , A H Parker ₁ , H B Throop ₁₂ , S J Robbins ₁ , O M Umurhan ₂ , R P Binzel ₁₃ , D T Britt ₁₄ , M W Buie ₁ , A F Cheng ₃ , D P Cruikshank ₂ , H A Elliott ₁₅ , G R Gladstone ₁₅ , W M Grundy _{16,

17} , M E Hill ₃ , M Horanyi ₁₈ , D E Jennings ₁₉ , J J Kavelaars ₂₀ , I R Linscott ₂₁ , D J McComas ₂₂ , R L McNutt ₃ , S Protopapa ₁ , D C Reuter ₁₉ , P M Schenk ₂₃ , M R Showalter ₆ , L A Young ₁ , A M Zangari ₁ , A Y Abedin ₂₀ , C B Beddingfield ₆ , S D Benecchi ₂₄ , E Bernardoni ₁₈ , C J Bierson ₂₅ , D Borncamp ₂₆ , V J Bray ₂₇ , A L Chaikin ₂₈ , R D Dhingra ₂₉ , C Fuentes ₃₀ , T Fuse ₃₁ , P L Gay ₂₄ , S D J Gwyn ₂₀ , D P Hamilton ₃₂ , J D Hofgartner ₉ , M J Holman ₃₃ , A D Howard ₃₄ , C J A Howett ₁ , H Karoji ₃₅ , D E Kaufmann ₁ , M Kinczyk ₃₆ , B H May ₃₇ , M Mountain ₃₈ , M Pätzold ₃₉ , J M Petit ₄₀ , M R Piquette ₁₈ , I N Reid ₄₁ , H J Reitsema ₄₂ , K D Runyon ₃ , S S Sheppard ₄₃ , J A Stansberry ₄₁ , T Stryk ₄₄ , P Tanga ₄₅ , D J Tholen ₄₆ , D E Trilling ₁₇ , L H Wasserman ₁₆

Affiliation

Southwest Research Institute, Boulder, CO 80302, USA.
NASA Ames Research Center, Moffett Field, CA 94035-1000, USA.
Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA.
Department of Astronomy, University of Virginia, Charlottesville, VA 22904, USA.
Department of Earth and Planetary Sciences and McDonnell Center for the Space Sciences, Washington University, St. Louis, MO 63130, USA.
SETI Institute, Mountain View, CA 94043, USA.
Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA.
National Science Foundation's National Optical Infrared Astronomy Research Laboratory, Tucson, AZ 26732, USA.
Jet Propulsion Laboratory, California Institute of Technology Pasadena, CA 91109, USA.
Department of Earth and Planetary Sciences, Birkbeck, University of London, London WC1E 7HX, UK.
University College London, Gower St, Bloomsbury, London WC1E 6BT, UK.
Independent Consultant, Washington, D.C., USA.
Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
Department of Physics, University of Central Florida, Orlando, FL 32816, USA.
Southwest Research Institute, San Antonio, TX 78238, USA.
Lowell Observatory, Flagstaff, AZ 86001, USA.
Department of Astronomy and Planetary Science, Northern Arizona University, Flagstaff, AZ, 86011, USA.
Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO 80303, USA.
NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA.
National Research Council of Canada, Victoria, BC V9E 2E7, Canada.
Independent Consultant, Mountain View, CA 94043, USA.
Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA.
Lunar and Planetary Institute, Houston, TX 77058, USA.
Planetary Science Institute, Tucson, AZ 85719, USA.
Earth and Planetary Science Department, University of California, Santa Cruz, CA 95064, USA.
Decipher Technology Studios, Alexandria, VA 22314, USA.
Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA.
Independent Science Writer, Arlington, VT 05250, USA.
University of Idaho, Moscow, ID 83844, USA.
Universidad de Chile, Centro de Astrofísica y Tecnologías Afines, Santiago, Chile.
Kashima Space Technology Center, National Institute of Information and Communications Technology, Kashima, Ibaraki 314-8501, Japan.
Department of Astronomy, University of Maryland, College Park, MD 20742, USA.
Center for Astrophysics, Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA.
Department of Environmental Sciences, University of Virginia, Charlottesville, VA 22904, USA.
National Institutes of Natural Sciences, Tokyo, Japan.
Marine, Earth, and Atmospheric Sciences, North Carolina State University, Raleigh, NC 27695, USA.
Independent Collaborator, Windlesham GU20 6YW, UK.
Association of Universities for Research in Astronomy, Washington, DC 20004, USA.
Rheinisches Institut für Umweltforschung an der Universität zu Köln, Cologne 50931, Germany.
Institut Univers, Temps-fréquence, Interfaces, Nanostructures, Atmosphère et environnement, Molécules, Unité Mixte de Recherche, Centre National de la Recherche Scientifique, Universite Bourgogne Franche Comte, F-25000 Besancon, France.
Space Telescope Science Institute, Baltimore, MD 21218, USA.
Independent Consultant, Holland, MI 49424, USA.
Department of Terrestrial Magnetism, Carnegie Institution for Science, Washington, DC 20015, USA.
Roane State Community College, Oak Ridge, TN 37830, USA.
Université Côte d'Azur, Observatoire de la Côte d'Azur, Laboratoire Lagrange/ Centre National de la Recherche Scientifique, Unité Mixte de Recherche 7293, 06304 Nice Cedex 4, France.
Institute for Astronomy, University of Hawaii, Honolulu, HI 96822, USA.

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 Stereo images of Arrokoth (2014 MU69) are used to map its geological units and date its surface using impact craters. INTRODUCTION On 1 January 2019, the New Horizons spacecraft passed 3538 km from Kuiper Belt object (KBO) (486958) Arrokoth. Arrokoth is a contact binary consisting of two distinct lobes, connected by a narrow neck. Its orbital parameters, albedo, and color make Arrokoth a typical cold classical KBO (CCKBO). CCKBOs are the most dynamically and physically primitive population of small Solar System bodies known. RATIONALE Since the publication of initial results from the flyby, additional data have been downlinked and analyzed. This paper describes the resulting analysis of Arrokoth’s shape, geological evolution, and satellite and ring constraints. RESULTS Improved stereo imaging constrains the object’s shape and topography and allows us to generate a stereographic terrain model. Typical relief on both lobes (away from the neck region) is ~0.5 km or smaller. Arrokoth’s rotational period is 15.92 ± 0.02 hours, with its rotational pole pointing to right ascension = 317.5 ± 1°, declination = −24.9 ± 1°, J2000 equinox. The object consists of two roughly ellipsoidal lobes with overall dimensions of 36 km by 20 km by 10 km. The maximum dimensions of the two lobes are 20.6 km by 19.9 km by 9.4 km and 15.4 km by 13.8 km by 9.8 km, with uncertainties of 0.5 km by 0.5 km by 2.0 km. The total volume is equal to a sphere of diameter 18.3 ± 1.2 km, and the volume ratio of the two lobes is 1.9 ± 0.5. Global bulk density must be >290 kg m−3 if the neck is not in tension. Assuming a bulk density of 500 kg m−3, as measured for comets, the mean surface gravity is ~1 mm s−2, and the compressive strength of the neck must be >2.3 kPa. The two lobes are closely aligned. The maximum axis of inertia of the large lobe is aligned within <5° of that of the small lobe. The equatorial planes of the two lobes are also almost coincident in space. The small lobe’s surface is marked by complex albedo patterns, often with sinuous margins and no detectable topographic signature, whereas the large lobe’s surface is dominated by clusters of low dark hills superposed on brighter, smoother terrain. The large lobe’s surface is divided into distinct subunits, which may represent smaller bodies that accreted to form it, though the overall smoothness of the surface, and the youthful appearance of many boundaries, which are sometimes undetectable or cross-cut by clusters of hills, suggest a more complex postformation history. If the subunits did accrete first, the smoothness of their mutual boundaries suggests subsequent accretion of additional material and later reactivation of the boundaries. We identify ~40 possible impact craters on Arrokoth, though only about 10 with high confidence. The largest crater, nicknamed Maryland, is about 7 km in diameter, and the rest are smaller than 1 km. Their size-frequency distribution is consistent with a single power law. Crater densities are lower than on many other small bodies but are consistent with a surface age of >4 billion years. No satellites or rings are detected: Satellite diameter upper limit is 180 m out to 8000-km radius from Arrokoth. CONCLUSION Arrokoth’s smooth, lightly cratered surface is unlike that of other Solar System bodies and appears to date from the period of planetary accretion. The alignment of its two lobes constrains the processes that formed this contact binary. Because its orbit, albedo, color, and rotation are typical of other CCKBOs, Arrokoth can likely be used to understand the cold classical belt as a whole. Stereo image pair of Arrokoth. The left and center images can be viewed cross-eyed, or the right and center by direct viewing. The Cold Classical Kuiper Belt, a class of small bodies in undisturbed orbits beyond Neptune, is composed of primitive objects preserving information about Solar System formation. In January 2019, the New Horizons spacecraft flew past one of these objects, the 36-kilometer-long contact binary (486958) Arrokoth (provisional designation 2014 MU69). Images from the flyby show that Arrokoth has no detectable rings, and no satellites (larger than 180 meters in diameter) within a radius of 8000 kilometers. Arrokoth has a lightly cratered, smooth surface with complex geological features, unlike those on previously visited Solar System bodies. The density of impact craters indicates the surface dates from the formation of the Solar System. The two lobes of the contact binary have closely aligned poles and equators, constraining their accretion mechanism.

中文翻译：

柯伊伯带天体的地质和地球物理学 (486958) Arrokoth

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 Arrokoth (2014 MU69) 的 980 立体图像用于绘制其地质单元并使用撞击坑确定其表面日期。简介 2019 年 1 月 1 日，新视野号航天器从柯伊伯带天体 (KBO) (486958) Arrokoth 飞过 3538 公里。Arrokoth 是一个接触双星，由两个不同的叶组成，由一个狭窄的颈部连接。它的轨道参数、反照率和颜色使 Arrokoth 成为典型的冷经典 KBO (CCKBO)。CCKBO 是已知的小型太阳系天体中最动态和物理上最原始的群体。基本原理自从飞越的初步结果公布以来，其他数据已被下载和分析。本文描述了对 Arrokoth 的形状、地质演化以及卫星和环约束的结果分析。结果改进的立体成像限制了物体的形状和地形，并允许我们生成立体地形模型。两个叶（远离颈部区域）的典型浮雕约为 0.5 公里或更小。Arrokoth 的自转周期为 15.92 ± 0.02 小时，其自转极指向赤经 = 317.5 ± 1°，赤纬 = -24.9 ± 1°，J2000 春分点。该物体由两个大致椭圆形的瓣组成，总尺寸为 36 公里乘 20 公里乘 10 公里。两个瓣的最大尺寸为 20.6 公里乘 19.9 公里乘 9.4 公里和 15.4 公里乘 13.8 公里乘 9.8 公里，不确定性为 0.5 公里乘 0.5 公里乘 2.0 公里。总体积等于一个直径为18.3±1.2km的球体，两瓣的体积比为1.9±0.5。如果颈部未处于紧张状态，整体体积密度必须 >290 kg m-3。假设彗星的体积密度为 500 kg m-3，则平均表面重力为 ~1 mm s-2，并且颈部的抗压强度必须 >2.3 kPa。两个瓣紧密对齐。大叶的最大惯性轴与小叶的最大惯性轴在 <5° 内对齐。两个瓣的赤道平面在空间上也几乎重合。小叶的表面有复杂的反照率图案，通常有弯曲的边缘，没有可检测的地形特征，而大叶瓣的表面则是由叠加在更明亮、更平滑的地形上的一簇簇低矮的暗山丘所主导。大叶的表面被分成不同的亚基，它们可能代表形成它的较小天体，尽管表面的整体光滑，以及许多边界的年轻外观，有时无法检测到或被成群的山丘横切，暗示更复杂的后形成历史。如果亚基确实首先吸积，则它们相互边界的平滑表明随后会吸积额外的材料，然后再重新激活边界。我们在 Arrokoth 上确定了大约 40 个可能的撞击坑，尽管只有大约 10 个具有高可信度。最大的陨石坑，绰号马里兰，直径约 7 公里，其余均小于 1 公里。它们的大小频率分布符合单一幂律。陨石坑的密度低于许多其他小天体，但与大于 40 亿年的地表年龄一致。未检测到卫星或环：卫星直径上限为 180 m，从 Arrokoth 到 8000 公里半径。结论 Arrokoth 的光滑、带有轻微陨石坑的表面不同于其他太阳系天体的表面，它似乎可以追溯到行星吸积时期。它的两个瓣的对齐限制了形成这种接触二进制的过程。因为它的轨道、反照率、颜色和旋转是其他 CCKBO 的典型特征，所以 Arrokoth 很可能被用来理解整个冷经典带。Arrokoth 的立体图像对。可以用斗鸡眼查看左侧和中间的图像，或者直接查看右侧和中间的图像。冷古典柯伊伯带是一类在海王星以外不受干扰的轨道上运行的小天体，由保存有关太阳系形成信息的原始天体组成。2019 年 1 月，新视野号航天器飞越了其中一个物体，即 36 公里长的接触双星 (486958) Arrokoth（临时编号 2014 MU69）。飞越的图像显示，Arrokoth 在 8000 公里的半径内没有可探测到的环，也没有卫星（直径大于 180 米）。Arrokoth 有一个浅坑、光滑的表面，具有复杂的地质特征，这与之前访问过的太阳系天体不同。撞击坑的密度表明其表面的年代可追溯到太阳系的形成。接触双星的两个瓣具有紧密对齐的两极和赤道，限制了它们的吸积机制。

更新日期：2020-02-13

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