Color, composition, and thermal environment of Kuiper Belt object (486958) Arrokoth

By W. M. Grundy, M. K. Bird, D. T. Britt, J. C. Cook, D. P. Cruikshank, C. J. A. Howett, S. Krijt, I. R. Linscott, C. B. Olkin, A. H. Parker, S. Protopapa, M. Ruaud, O. M. Umurhan, L. A. Young, C. M. Dalle Ore, J. J. Kavelaars, J. T. Keane, Y. J. Pendleton, S. B. Porter, F. Scipioni, J. R. Spencer, S. A. Stern, A. J. Verbiscer, H. A. Weaver, R. P. Binzel, M. W. Buie, B. J. Buratti, A. Cheng, A. M. Earle, H. A. Elliott, L. Gabasova, G. R. Gladstone, M. E. Hill, M. Horanyi, D. E. Jennings, A. W. Lunsford, D. J. McComas, W. B. McKinnon, R. L. McNutt Jr., J. M. Moore, J. W. Parker, E. Quirico, D. C. Reuter, P. M. Schenk, B. Schmitt, M. R. Showalter, K. N. Singer, G. E. Weigle II, A. M. Zangari

Science 28 Feb 2020:
Vol. 367, Issue 6481, eaay3705
DOI: 10.1126/science.aay3705



The New Horizons spacecraft flew past (486958) Arrokoth (provisional designation 2014 MU69) on 1 January 2019. Arrokoth is a member of the subclass of trans-neptunian or Kuiper Belt objects (KBOs), known as the cold classical KBOs (CCKBOs). Most KBOs formed in a disk of planetesimals that extended to about 30 AU from the Sun. Neptune eventually disrupted that disk by migrating outward through it, with the migration halted by the sparseness of the disk beyond 30 AU. That event eliminated most members of the planetesimal disk, but a minority were emplaced into dynamically excited orbits in the present-day Kuiper belt. CCKBOs differ from those objects in having formed well beyond the 30-AU edge of the main planetesimal disk. They remain approximately where they formed, on low-inclination, near-circular orbits between 42 and 47 AU from the Sun, relicts of the early Solar System. Their distributions of colors, albedos, sizes, and binarity differ from those of the more excited KBOs.

Initial results from the exploration of Arrokoth were published previously. More data have since been received from the spacecraft, allowing a more detailed analysis. We analyze a high–spatial resolution color imaging observation, near-infrared spectral imaging, and microwave radiometry of Arrokoth. The infrared spectral data have been processed to compensate for the changing range and scale during the observation. Our multiple scattering radiative transfer models provide compositional constraints from the infrared spectral imagery. Microwave thermal radiometry at 4.2-cm wavelength is combined with heat transport models that account for the bilobate shape of Arrokoth and for self-radiation.

At visual wavelengths, Arrokoth’s reflectance rises toward longer wavelengths. This red coloration is typical of the broader CCKBO population that has been studied using telescopic observations. Color differences across the surface of Arrokoth correspond to geological features. These color differences are subtle, with deviations of just a few percent around the prevailing red coloration. Some of the color variations are associated with albedo markings, such as the bright neck between the two lobes, bright splotches associated with a large pit or crater on the smaller lobe, and poorly resolved small bright spots. Methanol ice (CH3OH) and complex organic tholins dominate the near-infrared reflectance spectrum, with H2O ice contributing little or no detectable absorption. At the 4.2-cm microwave wavelength of New Horizons’ radio system, Arrokoth’s winter night side glows with an average brightness temperature of 29 ± 5 K. This emission probably emerges from below the cold winter surface, at depths where warmth from the previous summer lingers. Our models show that self-radiation more than compensates for self-shadowing in the neck region between the two lobes, resulting in warmer temperatures in that region, by up to a few kelvin.

The nearly uniform coloration across Arrokoth is consistent with expectations for an object that accreted too quickly for the composition of the available nebular solids to have changed during the course of its accretion. Radiolysis and photolysis from long exposure to space radiation would be expected to result in a dark, space-weathered surface veneer that is distinct from the more pristine interior, but there is little evidence for such a coating, perhaps because radiolytically processed material is eroded away faster than it accumulates. The abundance of CH3OH ice and apparent scarcity of H2O ice appear to be signatures of a distinct environment in the cold, dust-shaded midplane of the outer nebula during formation of the Solar System. In this region, temperatures would have been low enough that volatile CO and CH4 could freeze onto dust grains, enabling production of CH3OH and perhaps also destruction of H2O. When the nebular dust dissipated some time after Arrokoth’s formation, exposure to sunlight would have raised its temperature, rapidly driving off condensed CO and CH4. The temperature has remained too cold to crystallize amorphous H2O. Volatile species may remain trapped in amorphous H2O ice within Arrokoth’s interior, but the infrared spectrum shows little evidence for such ice at the surface. Although the neck region gets slightly warmer than the rest of Arrokoth’s surface, this effect is small relative to the winter-summer temperature contrast and is thus unlikely to account for the distinctly higher albedo and slightly less red material that is seen there. A more plausible explanation for the neck’s albedo and color contrasts involves texture changes induced by the merger of the two lobes or subsequent downslope movement of material there.”