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Register a new userThe Geology and Geophysics of Kuiper Belt Object (486958) Arrokoth
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J.R. Spencer
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The Cold Classical Kuiper Belt, a class of small bodies in undisturbed orbits beyond Neptune, are primitive objects preserving information about Solar System formation. The New Horizons spacecraft flew past one of these objects, the 36 km long contact binary (486958) Arrokoth (2014 MU69), in January 2019. Images from the flyby show that Arrokoth has no detectable rings, and no satellites (larger than 180 meters diameter) within a radius of 8000 km, and 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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The outer Solar System object (486958) Arrokoth (provisional designation 2014 MU$_{69}$) has been largely undisturbed since its formation. We study its surface composition using data collected by the New Horizons spacecraft. Methanol ice is present along with organic material, which may have formed through radiation of simple molecules. Water ice was not detected. This composition indicates hydrogenation of carbon monoxide-rich ice and/ or energetic processing of methane condensed on water ice grains in the cold, outer edge of the early Solar System. There are only small regional variations in color and spectra across the surface, suggesting Arrokoth formed from a homogeneous or well-mixed reservoir of solids. Microwave thermal emission from the winter night side is consistent with a mean brightness temperature of 29$pm$5 K.
On January 1st 2019, the New Horizons spacecraft flew by the classical Kuiper belt object (486958) Arrokoth (provisionally designated 2014 MU69), possibly the most primitive object ever explored by a spacecraft. The I/F of Arrokoth is analyzed and fit with a photometric function that is a linear combination of the Lommel-Seeliger (lunar) and Lambert photometric functions. Arrokoth has a geometric albedo of p_V = 0.21_(-0.04)^(+0.05) at a wavelength of 550 nm and ~0.24 at 610 nm. Arrokoths geometric albedo is greater than the median but consistent with a distribution of cold classical Kuiper belt objects whose geometric albedos were determined by fitting a thermal model to radiometric observations. Thus, Arrokoths geometric albedo adds to the orbital and spectral evidence that it is a cold classical Kuiper belt object. Maps of the normal reflectance and hemispherical albedo of Arrokoth are presented. The normal reflectance of Arrokoths surface varies with location, ranging from ~0.10-0.40 at 610 nm with an approximately Gaussian distribution. Both Arrokoths extrema dark and extrema bright surfaces are correlated to topographic depressions. Arrokoth has a bilobate shape and the two lobes have similar normal reflectance distributions: both are approximately Gaussian, peak at ~0.25 at 610 nm, and range from ~0.10-0.40, which is consistent with co-formation and co-evolution of the two lobes. The hemispherical albedo of Arrokoth varies substantially with both incidence angle and location, the average hemispherical albedo at 610 nm is 0.063 +/- 0.015. The Bond albedo of Arrokoth at 610 nm is 0.062 +/- 0.015.
The New Horizons space probe led the first close flyby of one of the most primordial and distant objects left over from the formation of the solar system, the contact binary Kuiper Belt object (486958) Arrokoth, which is composed of two progenitors, the lobes nicknamed Ultima and Thule. In the current work, we investigated Arrokoths surface in detail to identify the location of equilibrium points and also explore each lobes individual dynamic features. We assume Arrokoths irregular shape as a homogeneous polyhedra contact binary. We numerically explore its dynamic characteristics by computing its irregular binary geopotential to study its quantities, such as geometric height, oblateness, ellipticity, and zero-power curves. The stability of Arrokoth Hill was also explored through zero-velocity curves. Arrokoths external equilibrium points have no radial symmetry due to its highly irregular shape. We identified even equilibrium points concerning its shape and spin rate: i.e., four unstable external equilibrium points and three inner equilibrium points, where two points are linearly stable, with an unstable central point that has a slight offset from its centroid. Moreover, the large and small lobes each have five equilibrium points with different topological structures from those found in Arrokoth. Our results also indicate that the equatorial region of Arrokoths lobes is an unstable area due to the high rotation period, while its polar locations are stable resting sites for surface particles. Finally, the zero-power curves indicate the locations around Arrokoth where massless particles experience enhancing and receding orbital energy.
The New Horizons spacecrafts encounter with the cold classical Kuiper belt object (486958) Arrokoth (formerly 2014 MU69) revealed a contact-binary planetesimal. We investigate how it formed, finding 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, solid particle cloud. The geometric alignment of the lobes indicates the lobes were a co-orbiting binary that experienced angular momentum loss and subsequent merger, possibly due to dynamical friction and collisions within the cloud or later gas drag. Arrokoths contact-binary shape was preserved by the benign dynamical and collisional environment of the cold classical Kuiper belt, and so informs the accretion processes that operated in the early Solar System.
We consider the history of New Horizons target (486958) Arrokoth in the context of its sublimative evolution. Shortly after the Suns protoplanetary disk (PPD) cleared, the newly intense sunlight sparked a sublimative period in Arrokoths early history that lasted for ~10-100 Myr. Although this sublimation was too weak to significantly alter Arrokoths spin state, it could drive mass transport around the surface significant enough to erase topographic features on length scales of ~10-100 m. This includes craters up to ~50-500 m in diameter, which suggests that the majority of Arrokoths craters may not be primordial (dating from the merger of Arrokoths lobes), but rather could date from after the end of this sublimative period. Thereafter, Arrokoth entered a Quiescent Period (which lasts to the present day), in which volatile production rates are at least 13 orders of magnitude less than the ~10^24 molecules/s detection limit of the New Horizons spacecraft (Lisse et al. 2020). This is insufficient to drive either mass transport or sublimative torques. These results suggest that the observed surface of Arrokoth is not primordial, but rather dates from the Quiescent Period. By contrast, the inability of sublimative torques to meaningfully alter Arrokoths rotation state suggests that its shape is indeed primordial, and its observed rotation is representative of its spin state after formation.
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