Research Article

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

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Science  13 Feb 2020:
eaay3705
DOI: 10.1126/science.aay3705

Abstract

The outer Solar System object (486958) Arrokoth (provisional designation 2014 MU69) 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 irradiation of simple molecules. H2O ice is not detected. This composition indicates hydrogenation of CO-rich ice and/or energetic processing of CH4+H2O ices 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 ± 5 K.

The New Horizons spacecraft flew past (486958) Arrokoth at the beginning of 2019 (1). Arrokoth rotates with a 15.9 hour period, about a spin axis inclined 99.3° to the pole of its 298 year orbit, at a mean distance from the Sun of 44.2 au (2, 3). Its near-circular orbit, with a mean eccentricity of 0.03 and inclination of 2.4° to the plane of the Solar System, makes it a Kuiper belt object (KBO) and specifically a member of the “kernel” sub-population of the cold classical KBOs (CCKBOs) (4). CCKBOs have distinct origins and distinct properties from KBOs on more excited orbits, which are thought to have formed closer to the Sun before being perturbed outward by migrating giant planets early in Solar System history (5). CCKBOs still orbit where they formed in the protoplanetary nebula (the accretion disk of gas and dust around the young Sun). CCKBOs have a high fraction of binary objects (6), a uniformly red color distribution (7, 8), a size-frequency distribution deficient of large objects (9, 10), and higher albedos (11, 12) than other KBOs. These properties arise from the environment at the outermost edge of the protoplanetary nebula, from a distinct history of subsequent evolution of CCKBOs compared to other KBOs, or of some combination of these two. Arrokoth provides a record of the process of forming planetesimals, the first generation of gravitationally bound bodies, that has been minimally altered by subsequent processes such as heating and impactor bombardment (3). Its distinctive bi-lobed, 35 km-long shape with few impact craters favors formation via rapid gravitational collapse, rather than scenarios involving more gradual accretion via piece-wise agglomeration of dust particles to assemble incrementally larger aggregates (13). We study Arrokoth’s color, composition, and thermal environment using data from the New Horizons flyby and discuss the resulting implications for its formation and subsequent evolution.

Instruments and data

New Horizons encountered Arrokoth when it was 43.28 au from the Sun, collecting data with a suite of scientific instruments. Color and compositional remote sensing data were provided by the Ralph color camera and imaging spectrometer, sensitive to wavelengths between 0.4 and 2.5 μm (14). Over this wavelength range, all light observed from Arrokoth is reflected sunlight, with the wavelength-dependence of the reflectance indicative of surface composition and texture. Ralph’s two focal planes share a single 75-mm aperture telescope using a dichroic beam-splitter. The Multi-spectral Visible Imaging Camera (MVIC) provides panchromatic and color imaging in four color filters, “BLUE” (400-550 nm), “RED” (540-700 nm), “NIR” (780-975 nm), and “CH4” (860-910 nm) (15). The highest spatial resolution MVIC color observation of Arrokoth, identified as “CA05”, was obtained on 2019 January 1 at 05:14 coordinated universal time (UTC), from a range of 17,200 km, at an image scale of 340 m per pixel, and phase angle 15.5°. This provides more spatial detail than the 860 m per pixel “CA02” MVIC color scan (1).

Ralph’s Linear Etalon Infrared Spectral Array (LEISA) images its target scene through a linear variable filter covering wavelengths from 1.2 to 2.5 μm at a spectral resolving power of about 240. To capture each location at each wavelength of the filter, frames are recorded while the spacecraft scans LEISA’s field of view across the scene. The highest spatial resolution LEISA observation, identified as “CA04”, was executed around 04:58 UTC, shortly before the CA05 MVIC observation, from a phase angle 12.6°, and mean range of 31,000 km, resulting in a mean image scale of 1.9 km per pixel (15).

New Horizons’s panchromatic LOng-Range Reconnaissance Imager ((16) LORRI) is co-aligned with Ralph and can record images while the spacecraft is scanning for a Ralph observation. Such LORRI observations, referred to as “riders”, are limited to short integration times to minimize image smear from scan motion, but multiple images can be recorded and combined in post-processing, providing for longer effective integration times (3). LORRI rider observations were obtained during both the CA04 and CA05 observations, providing higher spatial resolution context images for the Ralph observations.

New Horizons’ Radio Science Experiment ((17) REX), was used to observe thermal emission in the X-band (4.2 cm wavelength, 7.2 GHz) from Arrokoth’s Sun-oriented face on approach and then from its anti-Sun oriented face on departure. The two REX observations, identified as “CA03” and “CA08”, respectively, were obtained January 1 at mean times of 04:34 and 05:52 UTC, phase angles of 11.9° and 162.0°, and ranges of 52,000 and 16,700 km. At those distances, Arrokoth was unresolved, appearing much smaller than the 1.2° width (at 3 dB) of the high-gain antenna beam. Two independent receivers recorded the radio flux density in different polarizations at a sampling rate of 10 Hz. The REX A receiver recorded right circularly polarized flux while REX B recorded left circularly polarized flux.

Visual wavelength color

The CA02 MVIC color scan (1) had shown Arrokoth to be red, but revealed little spatial variation in color. The higher resolution CA05 observation allows us to better quantify Arrokoth’s regional color variations. Figure 1 shows the CA05 color image, compared with the contemporaneous LORRI panchromatic rider image. Color slopes, computed by fitting a linear model to the MVIC BLUE, RED, and NIR filter reflectance data, are shown in Fig. 1C. All of Arrokoth’s surface is red in color, with a mean color slope of 27% rise per 100 nm relative (at 550 nm). This quantification of color slopes is commonly used for KBOs, being convenient for comparison of colors obtained using different filter sets (18). Even in the higher spatial resolution of the CA05 observation, the color distribution is largely uniform across the observed face of Arrokoth, with a standard deviation in slope of only ±2.7% per 100 nm.

Fig. 1 The CA05 color observation of (486958) Arrokoth.

(A) LORRI panchromatic context image obtained as a rider during the CA05 observation, so the geometry is nearly identical to the MVIC observation, but with a finer spatial scale of 83 m pixel–1. Abbreviations and informal feature nicknames mentioned in the text are (clockwise from the top): SL smaller lobe, MD Maryland, LA Louisiana, dr depressed region, LL larger lobe, bm bright material, ND North Dakota, and sp spots. These features can be seen more clearly in higher resolution LORRI images [(3), their figure 1]. (B) Color observation CA05, at a spatial scale of 340 m pixel–1. The BLUE filter (400-550 nm) is displayed in blue, RED filter (540-700 nm) in green, and NIR filter (780-975 nm) in red. (C) Color slope map obtained by fitting a linear model to the BLUE, RED, and NIR reflectance values.

Subtle regional color differences correspond to specific geological and albedo features discussed in a companion paper (3). The smaller lobe (SL) appears slightly redder on average than the larger lobe (LL) (28 ± 2% average slope versus 27 ± 2% for LL, where the ±2 values represent the variance across each lobe, rather than the uncertainty in the measurement of the mean slopes, which is much smaller for averages over many pixels (15)). That difference appears to be mostly due to the redder rim (color slope 30 ± 2%) of a 6-km diameter depression on SL, a possible impact crater informally identified as Maryland (MD, all place names are informal nicknames). Statistically significant color differences tend to be on similarly small (or smaller) spatial scales. Several slightly less red regions appear as blue in the color bar used in Fig. 1C. These include the brighter neck region where the two lobes intersect (25 ± 1% slope), and several regions on LL. Two regions that were not resolved in the earlier color data are a depression near the neck nicknamed Louisiana (LA, 23 ± 2% slope) and a linear depression or groove North Dakota (ND, 24 ± 1% slope), labeled in Fig. 1A. Bright material (bm) in the geological map (3) is in some places more and in others less red than average, suggesting that that unit is composed of two or more distinct materials. Another depressed region (dr) on LL is slightly redder than the average (29 ± 2% slope). Some bright spots (sp) appear to have distinct colors as well, though they are not all the same; some are a little redder than average while others are a little less red. The lack of a consistent color pattern for these spots suggests they may have resulted from delivery of diverse material in impacts rather than by impact excavation of a uniform subsurface material. However, the nature of these spots remains ambiguous (3).

We performed a principal components analysis (PCA) of the color data (Fig. 2). This analysis projects the data into an orthogonal basis set, with the first axis corresponding to the axis of maximum variance within the data. The second axis corresponds to the maximum remaining variance after collapsing the data along the first axis, and so forth. Principal component 1 (PC1, Fig. 2A) corresponds with variations in brightness due to lighting and albedo, accounting for 97% of the total variance in the data. The corresponding eigenvector is flat across all filters. PC2 corresponds to redness (Fig. 2B), closely resembling the color slope map in Fig. 1C, and accounts for 1.8% of the total variance. PC3 and PC4 correspond to contrasts between the NIR and CH4 filters and to spectral curvature between BLUE, RED, and NIR filters, respectively. They account for only 1% of the variance between them, much of it due to image noise rather than real color variations across the surface of Arrokoth.

Fig. 2 Principal components analysis of the CA05 color data.

(A to D) Principal components images (PCs) 1 through 4 above, along with the corresponding eigenvector below. The images show the regional distribution of the spectral characteristic indicated by the eigenvector. For example, the PC2 eigenvector shows a red slope (B). The corresponding image shows high values where Arrokoth is reddest and low values where it is least red. The y axes of the eigenvector plots give the contribution of each filter to that PC. (E) False color image highlighting color contrasts across Arrokoth with PC2, PC3, and PC4 displayed in the red, green, and blue channels, respectively, with shading from the PC1 image.

Red coloration on planetary bodies is often attributed to the presence of tholins (19). These are a broad class of refractory macromolecular polymer-like organic solids, commonly produced in laboratory simulations of energetic radiation acting on various combinations of simpler molecules (2022). The precursors can be in gaseous form (23) or frozen solid (24, 25). Figure 3 compares the color of Arrokoth with other KBOs and related populations. Arrokoth’s color slope is consistent with other CCKBOs (8). It is less red than the reddest KBOs and the red group of Centaurs (26), whose red coloration is generally interpreted as due to tholins (19). Arrokoth is much more red than the gray Centaurs and various classes of asteroids (27), including the red D-type asteroids that dominate the Jupiter Trojan population (28). Using the Sloan g, r, and z filters, KBO colors can be placed on a color-color plot of gr and rz color differences, which has revealed a distinction between CCKBOs and more dynamically excited KBO populations (8). The latter appear to follow two color tracks (29) (Fig. 3B) while the CCKBOs cluster below and to the right, owing to their red slopes becoming less steep at z-band wavelengths (0.82 to 0.96 μm). Arrokoth’s red slope continues into MVIC’s NIR band (0.78 to 1 μm). After converting to Sloan colors, Arrokoth lies above the main clump of CCKBO colors in Fig. 3B, although this is consistent with the overall CCKBO color distribution.

Fig. 3 Comparison of Arrokoth’s color with other populations.

(A) Histogram of Arrokoth color slopes shown in Fig. 1C. Typical color ranges for other Solar System small body populations are indicated via horizontal bars, e.g., (8, 18, 2629). (B) Color-color plot using Sloan g, r, and z filters, with the additional wavelength providing sensitivity to spectral curvature in addition to slope. In this color-color space, KBOs form three distinct zones (8). The gray tracks are populated by KBOs on dynamically excited (higher inclination and/or eccentricity) orbits while CCKBOs occupy the red zone. Arrokoth lies above the center of the CCKBO clump, because its red slope declines less into the z-band than the average for CCKBOs. The yellow Sun symbol indicates solar color, corresponding to neutral spectral reflectance at visible wavelengths.

Near-infrared spectral reflectance

The LEISA data were processed into a spectral cube, with spatial dimensions along two axes and wavelength along the third axis (15). The cube-building algorithm we adopted accounts for changes in spacecraft range over the course of the CA04 LEISA scan [unlike (1)]. The spatial resolution of the LEISA data are considerably coarser than the corresponding LORRI rider (Fig. 4), and the signal/noise ratio is considerably lower. Noise in the LEISA data are dominated by instrumental effects, which are as large as the signal from sunlit areas on Arrokoth, so spectral features can only be revealed by averaging over multiple pixels and/or wavelengths. The spatially averaged spectrum (Fig. 4C) lacks the strong absorption features that were seen in the Pluto system, e.g., (30). The red color slope seen in the visible flattens with increasing wavelength to become neutral by around 1.5 μm. We constructed Hapke reflectance models (15, 31) for various combinations of potential surface components. The data support inclusion of amorphous carbon and tholins in the models. Combinations of these materials match the overall albedo and spectral shape. Although we used tholins made in conditions simulating the atmosphere of the moon Titan (20), the data do not support singling out any of the various tholins with published optical constants. None have been made under simulated outer nebular conditions, so they may not be particularly good analogs for the tholins on Arrokoth. Amorphous carbon has no diagnostic spectral features in this wavelength range, so it cannot be specifically identified. Any dark, spectrally neutral material would be equally consistent with the data.

Fig. 4 The CA04 infrared spectral observation of Arrokoth.

(A) The LORRI CA04 rider image for context, with a spatial scale of 138 m pixel–1. (B) LEISA image with regions of interest (ROIs) indicated, with a much coarser mean spatial scale of 1.9 km pixel–1. (C) Average and ROI spectra (points) compared with the Hapke model fitted to the average spectrum (black curves). Vertical gray lines indicate the wavelengths of the two strongest CH3OH ice absorptions. Purple arrows mark other possible features discussed in the text. I/F is defined as the ratio of the bidirectional reflectance to that of a perfectly diffusely scattering surface illuminated normally (31).

Shallow absorption bands can be discerned near 1.5–1.6, 1.8, 2.0–2.1, 2.27 and 2.34 μm (Fig. 4C). Including ices of methanol (CH3OH), water (H2O), and ammonia (NH3) in the models can match many of these features, but not the one at 1.8 μm. With the low signal to noise ratio of the LEISA spectrum, even in the global average, we must consider the information content and limit the model free parameters to those that can be statistically justified. Of the molecular ices we tried, only the addition of CH3OH produces a sufficiently large improvement in χ2 to constitute a confident detection (15). This does not preclude the presence of H2O or NH3 ices, but the available data do not provide statistically significant evidence for their presence. Adding more than a trace of them to the models makes χ2 worse, but the increase can be minimized with large grain sizes that limit the projected area of H2O or NH3 grains to a small fraction of the total. Small amounts of other ices, such as H2CO, CO2 or C2H6, could also be compatible with the data, as could the silicate and metallic phases that have been seen in comets and interplanetary dust particles. The 1.8 μm feature in Arrokoth’s spectrum is not matched by available ices or tholins and remains unidentified. It could be an artifact.

To search for spectral contrasts across the surface of Arrokoth, we selected several regions of interest (ROIs) as shown in Fig. 4B. These include LL, the brighter neck region (n), and a pair of ROIs on the left and right sides of SL (sl and sr), with sr incorporating the redder material on the rim of MD and sl representing portions of SL unrelated to MD. Spectra of these regions are shown in Fig. 4C. In averaging over fewer pixels, the signal/noise ratios in the ROI spectra are correspondingly poorer than the global average. However, the ROI spectra all look very similar to the average, and fitting Hapke models shows that tholin, carbon, and CH3OH are favored, without statistically significant evidence for H2O or NH3 ices, just as with the average.

Our confidence in the detection of CH3OH is increased by the appearance of two distinct absorption bands of methanol ice, at 2.271 μm, attributed to (ν1 + ν11) or (ν1 + ν7) vibrational combination modes, and 2.338 μm, attributed to (ν1 + ν8) (32). These spectral characteristics have been seen in Earth-based spectra of the Centaur 5145 Pholus (33) and the resonant KBO (55638) 2002 VE95 (34). Additional, weaker CH3OH absorption bands at 1.6 and 2.1 μm are not visible in Arrokoth’s spectrum. In the case of Pholus, the spectrum from 0.45 to 2.45 μm was fitted with a radiative transfer model incorporating solid CH3OH and H2O, in addition to an iron-bearing olivine (forsterite Fo 82) and tholin (33), similar to our models for Arrokoth, except that H2O ice is not required in the Arrokoth models.

To assess spectral contrasts in a way that does not depend on multiple scattering models, a PCA was performed on the LEISA cube, with results shown in Fig. 5. Due to the low signal-to-noise ratio, we first binned the wavelengths down to 28 channels, producing an effective spectral resolving power of 39. We also discarded pixels along the edge where jitter during the scan is prone to producing artifacts. As with the MVIC colors, PC1 is sensitive to the overall light variation from shading and albedo, with the eigenvector flat across all wavelengths (Fig. 5A). PC1 accounts for 72% of the total variance in the LEISA data. PC2 captures only 2.3% of the variance, but the eigenvector shows pronounced dips around 1.5 and 2 μm, where H2O ice has its strongest absorption bands within LEISA’s spectral range, suggesting that regional variations in H2O ice absorption could be the next most prominent source of spectral variance across the surface of Arrokoth, perhaps being most abundant around Maryland crater. However, the absence of strong H2O absorptions in any of our extracted spectra (including the sr ROI that covers this region) reduces confidence in that conclusion. Subsequent PCs account for even lower fractions of the total variance. The lack of spatial coherence in the images coupled with eigenvectors that are not suggestive of absorption by likely surface constituents, suggests that the higher PCs are responding mostly to instrumental noise rather than to signal in the LEISA data.

Fig. 5 Principal components analysis of the CA04 infrared spectral imaging data.

Same as Fig. 2, but for the LEISA observation in Fig. 4. Edge pixels have been removed; the white outlines show the full extent of Arrokoth.

Thermal environment

The CA03 REX observation was performed on approach, observing Arrokoth’s day side while the CA08 observation was done after closest approach, looking back at Arrokoth’s night side. The microwave sky background is shown in Fig. 6A (35, 36). The CA03 observation was performed with a fixed staring geometry. A later observation of the same field was obtained for background subtraction, but the system antenna temperature drifts over time, making it difficult to separate out the flux from Arrokoth. The CA08 night side observation was obtained under more favorable geometry, from a closer range, and with the antenna scanned along the uncertainty ellipse for Arrokoth’s location. Scanning instead of staring facilitated calibration against the later background observation despite the drift in system response. The flux measurements are shown Fig. 6B. The radiometric signal was converted to radio brightness temperature using procedures developed earlier in the mission, accounting for the solid angle subtended by Arrokoth within the antenna beam (15, 17, 36, 37). Accounting for the 414.4 km2 cross section of Arrokoth and noise from instrument and background, we obtain a mean brightness temperature TB, averaged across the night-side visible face of Arrokoth, of TB = 29 ± 5 K, which is within the range of brightness temperatures estimated from an earlier analysis (1). To translate that brightness temperature to a kinetic temperature requires knowledge of the X-band emissivity of Arrokoth’s surface, which is not known, but most likely lies in the range from 0.7 to 0.9 (36).

Fig. 6 Microwave radiometry of Arrokoth.

(A) 7.1 GHz microwave sky background based on an all sky radio map (35) sampled at REX’s 4.2 cm wavelength and smoothed to REX’s 1.2° beam width (36), indicating the locations of the CA03 and CA08 observations. Reproduced from (36) with permission. (B) Observed flux during the CA08 scan in green, with the later background scan in blue. Shaded areas were used to calibrate the two observations for background subtraction. The black curve is a model response for a TB = 30 K source with the 414 km2 projected area of Arrokoth. (C) CA07 LORRI image (3) obtained 10 minutes before the mid-time of the REX scan, at nearly identical lighting geometry but about a 10° shift in viewing geometry, showing more of the lit crescent than was visible at the time of the CA08 REX observation.

Thermophysical models were used to assess the implications of this TB measurement (15). For each surface element of the three dimensional shape model (3), we balanced radiative losses and thermal conduction against insolation (received sunlight) and also re-radiation from other parts of Arrokoth’s surface visible from that location. Accounting for self-shadowing and surface re-radiation makes this modeling inherently global in scope (38). Subsurface thermal evolution was simulated with a 1D thermal diffusion prescription (15). For simplicity, we assume Arrokoth’s obliquity does not precess and its orbit is circular, with a semimajor axis of 44.2 au and period of 298 years. At this distance the incident solar radiation flux F is 0.7 W m–2. Given Arrokoth’s 99.3° obliquity (2), seasonal effects are strong. We determined the subsolar latitude along approximately 300 equally spaced temporal nodes over one orbital period (15). During the New Horizons flyby the subsolar latitude was approximately –62°. At each orbital node, the daily averaged (15.9 hours period) solar insolation was calculated, accounting for self-shadowing. With these diurnally averaged insolation profiles, we determined the surface temperature on every element over the course of an orbit. We assumed that the subsurface thermal response is in the time-asymptotic limit, meaning there is no net gain or loss of thermal energy into or out of the interior over the course of one orbit (39). This assumption requires heat from radioactive decay inside Arrokoth to be negligible and for the interior to have reached thermal equilibrium with the Sun over the course of the lifetime of the Solar System (see further below).

We assume that the low bond albedo (AB = 0.06 (1, 3)) surface of Arrokoth is characterized by a very low thermal inertia (Γ = 2.5 J m–2 s–1/2 K–1) typical of loosely consolidated granular material, as inferred from infrared observations of KBOs (40). The thermal inertia is given by ΓkρCp, where k is the thermal conductivity, ρ is the density, and Cp is the specific heat at constant pressure. Arrokoth’s bulk density must be at least 290 kg m–3 (3) and densities of small KBOs and comet nuclei tend to be higher than that, but generally less than 1000 kg m–3 (41, 42). The density near the surface that matters for Arrokoth’s thermal response is even more uncertain, and could differ substantially from the bulk density. We assume a generic density of ρ = 500 kg m–3 (3), and that Cp = 350 J kg–1 K–1. Under these assumptions the corresponding thermal conductivity is 3.6×10−5 W m–1 K–1, very low compared to values determined for surfaces in the inner Solar System. These values correspond to a seasonal thermal skin depth λ = 0.55 m, where λkτs(ρCp2π)1, and τs is the 298 year seasonal period (43). This value is similar to the electrical skin depth (36). The sub-solar equilibrium temperature T* = 58 K was obtained from εσT*4 = (1 – AB)F, where ε is the emissivity, commonly assumed to be 0.9 [e.g., (40, 43)], σ is the Stefan-Boltzmann constant (5.67×10−8 W m–2 K–4), and F is the solar flux at 44.2 au. The thermal parameter Θ ≡ Γω01/2σ–1T*–3 characterizes the efficiency of the energy transport rate (per K) of thermal conduction across one seasonal skin depth relative to radiative losses (43), which is about Θ ≡ 0.02 for Arrokoth. Such a small value of Θ indicates that the surface layers are highly insulating, leading to extreme variations in surface temperature over the course of a year. Winter surface temperatures are about 1/6th of peak summer temperatures. The low conductivity might only pertain to a surficial layer. If deeper below the surface the texture is more compacted with greater granular contact, the conductivity would likely be higher. We can estimate the body’s thermalization timescale by calculating the thermal wave propagation time (tthermalization ≡ 2πρCpR2k–1) across a length scale R corresponding to the characteristic radius of the short axes of both lobes (~3.5 km). For values of k greater than 10−4 W m–1 K–1, this timescale is less than the age of the Solar System, supporting our assumption of a time-asymptotic state.

Figure 7A shows the insolation averaged over an orbit from two viewing positions. The flattened shape and high obliquity lead to the equator receiving less energy on average (~0.1 W m–2) than the poles (~0.2 W m–2). Owing to self-shadowing, the neck region generally receives less energy than the equatorial zone (ranging from 0.06-0.08 W m–2). Figure 7B shows the additional radiation received from thermal emission from other parts of Arrokoth itself, again averaged over an orbit. Our model indicates that the neck region is warmed by this trapping process, receiving about 0.025-0.04 W m–2 from self-re-radiation, which partially offsets the effect of shadowing. Figure 7C shows the orbital average of the warming due to self-reradiation. The neck region experiences the greatest amount of relative warming, in the range of ~1-3 K. MD crater also receives enhanced thermal re-radiation but the relative warming in that region is slight, about 0.5 K. Figure 7D shows the predicted observable surface temperature at the time of the New Horizons encounter: typical temperatures are in the range of 50-57 K near the poles of each lobe, falling to ~40 K near the equator. Parts of the neck region that are not shaded at times of high subsolar latitude (–62° at the time of the encounter) stand out as having among the warmest surface temperatures during the encounter, as high as 60 K.

Fig. 7 Models of insolation and temperature across the surface of Arrokoth.

(A) The Insolation averaged over Arrokoth’s orbit viewed from two different orientations. (B) Re-radiation received from other portions of the shape on the same scale and from the same orientations as (A), also averaged over the orbit. The top color bar applies to (A) and (B). (C) Seasonally averaged additional warming resulting from re-radiation is shown as a temperature increase above that which would have been computed in the absence of self-radiation. (D) Summer day side temperature distribution as seen from New Horizons during LORRI observation CA06 (3). (E) Winter night side temperature distribution as seen from New Horizons during the REX CA08 scan. In all panels, the x, y, and z axes correspond to the principal axes of inertia with the origin at the center of mass (13).

Figure 7E shows the predicted surface temperature at the time and viewing geometry of the CA08 REX observation, during which the sub spacecraft point was latitude +44°, longitude E 78°. From this orientation the model global averaged surface temperature is 16.1 K. While the surface temperature within the bulk of the body’s winter night side is predicted to be in the range of 12-14 K, the contribution to the average from the viewable part of the lit equatorial region (top of Fig. 7E; T ~ 40-55 K) raises the average temperature only slightly. The observed thermal emission seen by REX yields a much warmer mean brightness temperature of 29 K. If the model is correct, this discrepancy implies that the 4.2 cm radiation emerges primarily from the warmer subsurface, which is consistent with the expectation that the 4.2 cm thermal emission samples many wavelengths into the surface (36). Higher thermal inertias than we have assumed would also permit warmer winter surface temperatures (15).

Implications for formation

The distribution of orbits in the present-day Kuiper belt was strongly influenced by an outward migration of Neptune early in Solar System history resulting from dynamical interaction between Neptune and the disk of planetesimals, e.g., (44). Neptune’s migration stopped at 30 au from the Sun, indicating a break in the distribution of planetesimals in the disk, beyond which there was insufficient mass to drive Neptune’s migration further outward (45). Most KBOs that have been studied spectroscopically [e.g., (46, 47)], are not CCKBOs; they originated in the denser planetesimal disk from inside 30 au. Likewise, most comets that can be studied in detail, due to their proximity to the Sun and Earth, likely do not sample the outer planetesimal disk from beyond 30 au. Arrokoth may contain a record of conditions in the outer part of the nebula where it formed. Constraints include the evidence for methanol ice and the lack of evidence for water ice, which is unlike the high abundance of H2O in many outer Solar System bodies and interstellar grains.

CCKBOs appear to have formed in the outer solar nebula via the gravitational collapse of pebble-size particles, concentrated aerodynamically (13, 48). In this scenario, microscopic dust grains coagulate into larger particles (49, 50). As particles approach pebble sizes they decouple from the gas, causing them to spend most of their time near the cold disk midplane and allowing them to become concentrated in dense clumps that can gravitationally collapse into planetesimals (51, 52). The (original) bulk composition of CCKBOs should reflect the make-up of the solids present in the midplane of the solar nebula at the time and location of their formation. It remains unclear, however, how long the dust coagulation phase lasted and/or how far pebbles were able to move radially inward, e.g., (53), before they formed planetesimals.

When the solar nebula formed, the chemical composition of the ices present in the outer regions was set by a combination of inheritance from the parent molecular cloud and chemistry taking place during formation of the disk. In the midplane of the outer disk, the resulting CH3OH/H2O ratio on cold grain surfaces likely did not exceed a few percent (54). During the subsequent disk evolution, spatial variations in physical conditions such as temperature, density, and radiation environment, coupled with ongoing chemistry (55) and transport/mixing processes (56), result in gas-phase and ice compositions changing over time.

In regions where it was cold enough for highly-volatile CO to freeze as ice onto grains (57), methanol could be formed through successive addition of hydrogen atoms to CO ice. Both interstellar and outer nebular environments are potential settings for this chemistry (5860). The midplane of the disk was shaded from direct sunlight and extremely cold prior to loss of nebular gas and dust, favoring condensation of CO in its outermost portions. Simulations of protoplanetary disks indicate that methanol can be produced in this way (consuming CO) on ~Myr timescales at Arrokoth’s distance from the Sun (61). Intermediate steps include formation of formaldehyde (H2CO) and radicals (e.g., CH3O). Radiolytic destruction of CH3OH can produce H2CO, but the band at 2.27 μm remains prominent even after irradiation (62).

Another potential CH3OH formation mechanism involves radiolysis of mixed H2O and CH4 ices (6366). Again, low temperatures consistent with the shaded midplane are required for CH4 to be frozen onto grains, although CH4 is not quite as volatile as CO. If such radiolytic production occurs with an excess of CH4, the H2O could be consumed, providing a possible explanation for the lack of evidence for H2O at Arrokoth. Such a radiolytic process would also efficiently form simple hydrocarbons such as C2H4 and C2H6, which are known to be precursors of complex organic tholins, e.g., (20, 67).

Gas-phase methanol has been detected at low abundance in protoplanetary disks around nearby stars (68). This is consistent with methanol ice formation on grain surfaces, with a small fraction subsequently released to the gas through non-thermal desorption mechanisms, e.g., (60, 69). This would also be consistent with the observation that many of these disks are also depleted in gaseous CO (70, 71), requiring a combination of sequestration on pebbles and chemical processing (61, 72, 73).

Although H2O was not detected on Arrokoth, it could be present, but somehow masked or hidden from view, such as by materials produced through radiolysis or photolysis of CH3OH ice and perhaps other, undetected precursor materials. Preferential removal of H2O ice from the uppermost surface by a process such as sputtering is another possibility, though it is unclear that H2O should be more susceptible to such removal than CH3OH is. Spectra of some larger KBOs also lack H2O absorption features (46, 47). H2O ice absorption is considerably weaker in the spectrum of Arrokoth than seen on Pholus and (55638) 2002 VE95, the two other objects with strong CH3OH signatures, but those objects are much larger than Arrokoth (74, 75). They likely formed in the closer, more densely populated planetesimal disk originally inside 30 au, as did other, large KBOs where strong H2O ice signatures have been detected spectroscopically. It is hard to envision a mechanism that preferentially masks or removes H2O from the surface of Arrokoth but not the H2O on these other objects. Their contrasting compositions suggests that the observed surface composition of these bodies is reflective of their bulk compositions, and Arrokoth’s composition is distinct from those of planetesimals that formed closer to the Sun. A contrast in planetesimal composition driven by nebular chemistry enabled by CO and/or CH4 frozen on grains may be connected to the transition at 30 au that halted Naptune’s outward migration at that distance.

Although regional variations in tholin and ice abundance could cause albedo, color, and spectral variations, the subtle variations that are seen at Arrokoth do not require compositional differences. Reflectance also depends on mechanical properties such as the particle size distribution and degree of compaction (31). The merger of the two lobes (13) could have mechanically modified the material in the neck region. After the formation of Arrokoth from the nebula, low speed impacts of residual debris could locally modify surface textures, which might account for some of the spots with slightly contrasting colors and albedos.

Surface and interior evolution

The surface features of comets are dominated by geologically rapid volatile loss and sublimation erosion, while the surfaces of larger asteroids are dominated by high-energy impacts. In contrast, Arrokoth and the CCKBOs are distinct in inhabiting an environment with very little energy input from interstellar, solar, and micrometeorite sources that require long timescales to modify the surface. Depending on the thermal parameters, surface temperatures range from as low as 10 to 20 K in winter to 50 to 60 K in summer, with the neck region getting at most a few degrees warmer due to self-radiation. Summer surface temperatures are warm enough to drive off volatiles such as CO, CH4, and N2, but are not warm enough to crystallize amorphous H2O ice, or to sublimate it. We therefore expect little thermally-driven evolution of the surface, except early in Arrokoth’s history when the volatile ices would have been lost soon after nebular dust cleared, allowing sunlight to illuminate the surface. Galactic and solar energetic photons and charged particles can break bonds and drive chemical reactions that produce refractory macromolecular tholins (19, 24, 25). Photolysis and radiolysis of solid methanol also produces formaldehyde, which may subsequently polymerize (76, 77). While formaldehyde polymer (paraformaldehyde) shows some spectral structure in the 2-2.5 μm region, it does not match the bands seen in Pholus (33) or Arrokoth. Macromolecular tholins seen on Arrokoth’s surface could have originated in the pre-solar gas and dust cloud. They could also arise from photolysis and radiolysis of hydrocarbons and associated nitrogen- and oxygen-bearing components in the nebula, especially where material is transported to regions near the surface of the nebula exposed to radiation from the forming Sun or other astrophysical sources (56). Radiolysis and photolysis of Arrokoth’s surface components could also produce such tholins, as discussed above. All these sources likely contributed to Arrokoth’s inventory of complex organics, with the products of the first two mechanisms being distributed throughout the body, while the products of the third should only occur at the surface. The flux of Solar System and interstellar micrometeorites at Arrokoth’s location is highly uncertain (78, 79), but such bombardment could produce up to several meters of mechanical erosion over the age of the Solar System (80, 81). If this erosion operates faster than space weathering by energetic radiation, the visible surface could be representative of the deep interior. If it is slower, the surface should accumulate a lag deposit enriched in more refractory materials through loss or destruction of more volatile and fragile molecules.

It is not obvious from the encounter data whether a distinct surface veneer exists on Arrokoth. Albedo and color contrasts corresponding to ancient features such as the neck suggest that such contrasts are not quickly masked by a space weathering processes. If compositionally distinct interior material was exposed at the neck, it might weather differently and thus maintain a contrasting appearance, but the LEISA data show no evidence for a distinct composition in the neck region. The warmer thermal environment of the neck would be another potential reason for distinct evolution there, but the temperature difference is too small to produce outcomes that differ substantially from the rest of the surface. No obviously fresh craters expose distinct-looking interior material (with the possible exception of Maryland), and color trends do not appear to correspond to down-slope transport, which is generally from equators to the poles of the lobes, and ultimately to the neck (3). Less-altered interior material should be preferentially exposed at high elevations, but we do not see obvious color differences in high-standing regions along the equators. Brighter material in the neck and in Maryland may accumulate in topographic lows, suggestive of textural rather than compositional contrasts. Among the CCKBO population, the diversity of colors coupled with similar colors of different sized components of binaries have been used to argue against the importance of size-dependent factors such as the balance between erosion and space weathering in altering their surface colors (82).

The evolution of Arrokoth’s interior is shaped by even lower energy inputs than its surface. Subsequent to the loss of shading from nebular dust, insolation would have raised Arrokoth’s equilibrium temperature. As that thermal wave slowly propagated inward, highly volatile species such as N2, CO, and CH4 would have become unstable, at least as condensed ices. Early outgassing of these species may have produced what appear to be collapse or outgassing pits at the boundaries of terrain units (1, 3). Such features may be analogous to pits or sinkholes on comet 67P/Churyumov-Gerasimenko, e.g., (83, 84). Localization of the pits to certain regions may arise from variable permeability of surface deposits that would favor volatiles escaping through weaker zones at unit boundaries. However the equilibrium temperature in Arrokoth’s interior would never have been high enough for amorphous ice to crystallize and expel its payload of trapped volatiles, e.g., (85, 86). Apart from loss of these volatile species, Arrokoth’s interior may have undergone little alteration or processing since accretion, and could thus preserve many characteristics of the original accretion such as layering (as observed on comets (87, 88)), very high porosity, and an intimate mixture of nebular ices, organics, and silicate dust grains.

Supplementary Materials

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Materials and Methods

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Data S1

References (8998)

References and Notes

  1. See supplementary materials.
Acknowledgments: We gratefully thank the many hundreds of people who worked together on the New Horizons team. Their hard work enabled the encounter. Funding: Supported by NASA’s New Horizons project via contracts NASW-02008 and NAS5-97271/TaskOrder30. JJK was supported by the National Research Council of Canada. SK acknowledges the support of a Hubble Fellowship, program number HST-HF2-51394.002 provided by NASA through a grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA contract NAS5-26555. MR acknowledges support by the NASA Astrobiology Institute under Cooperative Agreement Notice NNH13ZDA017C issued through the Science Mission Directorate. BS, LG, and EQ acknowledge support by the French Centre National d’Etudes Spatiales (CNES). OMU acknowledges NASA Astrophysics Theory Program Grant Number NNX17AK59G for partial support of this work. Author contributions: WMG led authorship of the manuscript. WMG, CJAH, CBO, AHP, and SP analyzed the MVIC color data and wrote that section. WMG, AHP, SP, JCC, and DPC analyzed the LEISA spectral data and wrote that section. MKB and IRL analyzed the REX data and wrote that section. OMU did the thermal modeling and wrote that section, with input from WMG, CJAH, and LAY. WMG, DPC, DTB, SK, and MR wrote the implications section. CMDO, JJK, JTK, YJP, SBP, FS, JRS, SAS, AJV, and HAW contributed to data analysis and development of scientific ideas. All authors played roles in experimental design and/or provided feedback and insights on drafts of the manuscript. Competing interests: We declare no competing interests. Data and materials availability: All images, spacecraft data, and the shape model used in this paper are available at https://doi.org/10.6084/m9.figshare.c.4819110 and at https://doi.org/10.6084/m9.figshare.11485443. The three Charon observations used in flat-fielding the LEISA data are available at https://pds-smallbodies.astro.umd.edu/holdings/nh-p-leisa-3-pluto-v3.0/data/20150714_029914/lsb_0299146219_0x54b_sci.fit, https://pds-smallbodies.astro.umd.edu/holdings/nh-p-leisa-3-pluto-v3.0/data/20150714_029917/lsb_0299171308_0x53c_sci.fit, and https://pds-smallbodies.astro.umd.edu/holdings/nh-p-leisa-3-pluto-v3.0/data/20150714_029917/lsb_0299175509_0x53c_sci.fit. Additional fully calibrated New Horizons data and higher-order data products will be released by the NASA Planetary Data System at https://pds-smallbodies.astro.umd.edu/data_sb/missions/newhorizons/index.shtml in a series of stages in 2020 and 2021, due to the time required to fully downlink and calibrate the data.
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