Research Article

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

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Science  28 Feb 2020:
Vol. 367, Issue 6481, eaay3999
DOI: 10.1126/science.aay3999

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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

Structured Abstract

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.

Abstract

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.

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