ABSTRACT We here study the level of albedo variegation on the nucleus of Comet 67P/Churyumov–Gerasimenko. This is done by fitting the parameters of a standard photometric phase function model to disc–average radiance factor data in images acquired by the Rosetta/OSIRIS Narrow Angle Camera in the orange filter. Local discrepancies between the observed radiance factor and the disc–average solution are interpreted as a proxy $\mathcal {W}$ of the local single–scattering albedo. We find a wide range $0.02 \lesssim \mathcal {W}\lesssim 0.09$ around an average of $\mathcal {W}=0.055$. The observed albedo variegation is strongly correlated with nucleus morphology – smooth terrain is brighter, and consolidated terrain is darker, than average. Furthermore, we find that smooth terrain darken prior to morphological changes, and that stratigraphically low terrain (with respect to the centre of each nucleus lobe) is brighter than stratigraphically high terrain. We propose that the observed albedo variegation is due to differences in porosity and the coherent effect: compaction causes small brighter particles to act collectively as larger optically effective particles that are darker. Accordingly, we consider the dark consolidated terrain materials more compacted than smooth terrain materials, and darkening of the latter is due to subsidence.

Observations of Triton’s rotational light curve from Table Mountain Observatory in 2017 show that volatile transport is continuing on its surface. Changes in the light curve are consistent with sustained increases in albedo in two patches near the equator at longitudes of ∼120° and ∼280°, as first reported by Bauer et al. based on observations obtained with the Hubble Space Telescope. An early observation of Triton’s visual albedo shows a substantially lower albedo in the past, when the northern polar region was visible. This result confirms the hypothesis of Moore & Spencer and Spencer & Moore of a net flux of volatile material into the southern hemisphere of Triton over much of the satellite’s history and up to the present. The averaged V-band geometric albedo of Triton has been stable over the past three decades. The color of Triton has also been stable during the past decade.

Observations of the opposition surges on the main moons of Saturn (Mimas, Enceladus, Tethys, Dione, Rhea, and Iapetus) during Cassini's prime and extended missions are reduced and analyzed. The main data set comes from the Visual Infrared Mapping Spectrometer (VIMS) with wavelength coverage in the visible and near-infrared out to 3.60 μm, covering 99% of the solar spectrum. Imaging Science Subsystem images and Ultraviolet Imaging Spectrometer data augment the VIMS data set. Hapke models are fit to Dione and Rhea, and Irvine's simpler shadowing model is fit to the sparser data sets of Enceladus, Dione, and Rhea. The high porosities (∼95% void space in the optically active portion of the regolith) and forward-scattering properties of the surfaces are similar to other icy moons and to Pluto. A change in the character of their opposition surge at 3.60 μm can be attributed largely to the noninteraction of long-wavelength photons with small particles on these moons’ surfaces. The opposition surge on the low- and high-albedo regions of Dione are similar. However, the low-albedo terrain of Iapetus exhibits a less robust surge than the high-albedo regions, which we attribute to differences in surface texture. The low-albedo hemisphere of Iapetus seems to be akin to the small number of bodies in the solar system that lack an appreciable opposition surge, possibly due to the accumulation of low-albedo dust. With observations over the range of the solar spectrum, we compute new values for the bolometric Bond albedos of these moons.

Near‐infrared observations of the Pluto Charon system were captured with the Palomar High Angular Resolution Observer (PHARO) adaptive optics (AO) system on the 200‐inch Hale telescope during the historically small solar phase angles in 2018–2019. Both objects exhibit large opposition surges of ∼30%–35% in the last half degree of solar phase angle, which is among the largest observed for icy moons and other Kuiper Belt Objects. In addition, Pluto's surge is exceptionally steep. Pluto's unusual phase curve may be due to an unusual surface texture caused by seasonal volatile transport and active geologic processes. These observations enable accurate determination of Pluto's geometric albedo in the JHK filter system, which we find to be 0.86 ± 0.04, 0.59 ± 0.05, and 0.39 ± 0.04 for Pluto, respectively, and 0.68 ± 0.06 for Charon in the J filter.

Observations of Pluto from New Horizons have been combined with previous ground-based observations and fit to a radiative transfer model based on Chandrasekhar’s planetary problem and Hapke theory to simultaneously derive the physical properties of the dwarf planet’s surface and atmosphere. We derive the macroscopic roughness, single-scattering albedo, and directional scattering properties of the surface, and the single-scattering albedo, optical depth, and single-scattering phase function of Pluto’s haze. The haze particles are small, with best-fit sizes in the range of ∼0.41–1.14 μ m. We find that Pluto’s haze is more similar to that of Titan, rich in organic compounds and highly forward scattering, than that of Triton. With organic compounds and a likely subsurface water ocean, Pluto may harbor sustainable habitable environments. Our model, which includes the coherent backscatter effect, fits the anomalously large opposition surge recently discovered on Pluto.

The New Horizons spacecraft observed Pluto and Charon at solar-phase angles between 16° and 169°. In this work, we use the Multispectral Visible Imaging Camera (MVIC) observations to construct multiwavelength phase curves of Pluto’s atmosphere, using the limb scatter technique. Observational artifacts and biases were removed using Charon as a representative airless body. The size and distribution of the haze particles were constrained using a Titan fractal aggregate phase function. We find that monodispersed and log-normal populations cannot simultaneously describe the observed steep forward scattering, indicative of wavelength-scale particles, and the non-negligible backscattering indicative of particles much smaller than the wavelength. Instead, we find it necessary to use bimodal or power-law distributions, especially below ∼200 km, to properly describe the MVIC observations. Above 200 km, where the atmosphere is isotropically scattering, a monodisperse, log-normal, or a bimodal/power law approximating a monodispersed population is able to fit the phase curves well. As compared to the results of previously published articles, we find that Pluto’s atmosphere must contain haze particle number densities an order of magnitude greater for small (∼10 nm) and large (∼1 μ m) radii, and relatively fewer intermediate sizes (∼100 nm). These conclusions support a lower aggregate aerosol growth rate than that found by Gao et al., indicating a higher charge-to-radius ratio, upwards of 60 e − μ m −1 . In order to generate large particles with a lower growth rate, the atmosphere must also have a lower sedimentation velocity (<∼0.01 m s −1 at 200 km), which is possible with a fractal dimension of less than 2.

Context.The potentially hazardous asteroid (85990) 1999 JV6 has been a target of previously published thermal-infrared observations and optical photometry. It has been identified as a promising candidate for possible Yarkovsky-O’Keefe-Radzievskii-Paddack (YORP) effect detection.Aims.The YORP effect is a small thermal-radiation torque considered to be a key factor in spin-state evolution of small Solar System bodies. In order to detect YORP on 1999 JV6 we developed a detailed shape model and analysed the spin-state using both optical and radar observations.Methods.For 1999 JV6, we collected optical photometry between 2007 and 2016. Additionally, we obtained radar echo-power spectra and imaging observations with Arecibo and Goldstone planetary radar facilities in 2015, 2016, and 2017. We combined our data with published optical photometry to develop a robust physical model.Results.We determine that the rotation pole resides at negative latitudes in an area with a 5° radius close to the south ecliptic pole. The refined sidereal rotation period is 6.536787 ± 0.000007 h. The radar images are best reproduced with a bilobed shape model. Both lobes of 1999 JV6 can be represented as oblate ellipsoids with a smaller, more spherical component resting at the end of a larger, more elongated component. While contact binaries appear to be abundant in the near-Earth population, there are only a few published shape models for asteroids in this particular configuration. By combining the radar-derived shape model with optical light curves we determine a constant-period solution that fits all available data well. Using light-curve data alone we determine an upper limit for YORP of 8.5 × 10−8rad day−2.Conclusions.The bifurcated shape of 1999 JV6 might be a result of two ellipsoidal components gently merging with each other, or a deformation of a rubble pile with a weak-tensile-strength core due to spin-up. The physical model of 1999 JV6 presented here will enable future studies of contact binary asteroid formation and evolution.

The New Horizons spacecraft extended the range in solar phase angle coverage for Pluto’s moon Charon from 1.°8—the maximum observable from Earth—to 170°. This extraordinary expansion in range has enabled photometric modeling and a robust determination of Charon’s phase integral and Bond albedo at visible wavelengths. Photometric modeling shows that Charon is similar in its photometric properties to other icy moons, except that its single particle phase function is more isotropic, suggesting the Kuiper Belt may represent a new regime for surface alteration processes. Charon’s phase integral is 0.70 ± 0.04 and its Bond albedo is 0.29 ± 0.05.

Context. The near-Earth asteroid (1917) Cuyo was subject to radar and light curve observations during a close approach in 1989, and observed up until 2008. It was selected as one of our ESO Large Programme targets, aimed at observational detections of the Yarkovsky-O’Keefe-Radzievskii-Paddack (YORP) effect through long-term light curve monitoring and physical modelling of near-Earth asteroids. Aims. We aim to constrain the physical properties of Cuyo: shape, spin-state, and spectroscopic and thermo-physical properties of the surface. Methods. We acquired photometric light curves of Cuyo spanning the period between 2010 and 2013, which we combined with published light curves from 1989 to 2008. Our thermal-infrared observations were obtained in 2011. Rotationally resolved optical spectroscopy data were acquired in 2011 and combined with all available published spectra to investigate any surface material variegation. Results. We developed a convex light curve-inversion shape of Cuyo that suggests the presence of an equatorial ridge, typical for an evolved system close to shedding mass due to fast rotation. We determine limits of YORP strength through light curve-based spin-state modelling, including both negative and positive acceleration values, between − 0.7 × 10 −8 and 1.7 × 10 −8 rad day −2 . Thermo-physical modelling with the ATPM provides constraints on the geometric albedo, p V = 0.24 ± 0.07, the effective diameter, D eff = 3.15 ± 0.08 km , the thermal inertia, Γ = 44 ± 9 J m −2 s −1∕2 K −1 , and a roughness fraction of 0.52 ± 0.26. This enabled a YORP strength prediction of ν = (−6.39 ± 0.96) × 10 −10 rad day −2 . We also see evidence of surface compositional variation. Conclusions. The low value of YORP predicted by means of thermo-physical analysis, consistent with the results of the light curve study, might be due to the self-limiting properties of rotational YORP, possibly involving movement of sub-surface and surface material. This may also be consistent with the surface compositional variation that we see. The physical model of Cuyo can be used to investigate cohesive forces as a way to explain why some targets survive rotation rates faster than the fission limit.

We are conducting a programme of optical lightcurve observations of NEAs to detect YORP-induced rotational accelerations [1-3]. For this we use optical photometry from a range of small to medium size telescopes. This is supplemented by thermal-IR observations and thermophysical modelling to ascertain expected YORP strengths for comparison with observations. For selected objects, we use radar data to determine shape models. We will present our latest results for one of the largest NEAs in our sample – NEA (1917) Cuyo [4]. We also report observations of excess emission at thermal-IR wavelengths interpreted as a surface mass shedding event in December 2005 [5]. We find that the size of the resulting dust cloud is consistent with theoretical predictions of frequent mass shedding events caused by radiation torques, and that material can be lost from Cuyo’s equator by centrifugal forces. 1. Observational Campaign Optical photometry and the search for YORP: The primary source for the optical lightcurve observations of Cuyo was from an ESO Large Programme at the NTT from April 2010 to April 2013 (11 lightcurves). Additional data were obtained from: the ESO 2.2 m telescope (Chile) (4 lightcurves, February 2-21, 2012); the JPL 0.6 m telescope at Table Mountain Observatory (USA) (6 lightcurves); the Palomar 5 m (USA) (1 lightcurve). We also used previouslypublished data when available (see [4] for full list), to provide a total lightcurve time span of 24 years (1989-2013) for our shape and spin-state analysis. Thermal observations and thermophysical modelling: Mid-IR observations provide valuable constraints on size, albedo and surface roughness, required to model the YORP effect, as well as thermophysical properties. We have a parallel programme to obtain mid-IR photometry for all targets that are sufficiently bright for detection with the VISIR instrument on ESO’s 8.2m VLT. Here, we also utilize archival Spitzer IRS data on Cuyo in our analysis. YORP effect predictions are made using the Advanced Thermophysical model (ATPM), e.g. [6], with shape models from our lightcurve analysis. Figure 1. (1917) Cuyo shape model indicating negative effective gravity near the equator [5]. This was produced using the nominal shape model of Cuyo [4] with a density of 2 g cm. Negative effective gravities indicate regions where surface material can be ejected by centrifugal forces. gE is 9.81 m s. EPSC Abstracts Vol. 13, EPSC-DPS2019-1561-1, 2019 EPSC-DPS Joint Meeting 2019 c © Author(s) 2019. CC Attribution 4.0 license.

We report measured spectral properties for more than 1000 NEOs, representing>5% of the currently discovered population. Thermal flux detected below 2.5 {\mu}m allows us to make albedo estimates for nearly 50 objects, including two comets. Additional spectral data are reported for more than 350 Mars-crossing asteroids. Most of these measurements were achieved through a collaboration between researchers at the Massachusetts Institute of Technology and the University of Hawaii, with full cooperation of the NASA Infrared Telescope Facility (IRTF) on Mauna Kea. We call this project the MIT-Hawaii Near-Earth Object Spectroscopic Survey (MITHNEOS; myth-neos).

We apply a new shape-based thermophysical model, SHERMAN, to the near-Earth asteroid (NEA) 8567 (1996 HW1) to derive surface properties. We use the detailed shape model of Magri et al. (2011) for this contact binary NEA to analyze spectral observations (2-4.1 microns) obtained at the NASA JRTF on several different dates to find thermal parameters that match all the data. Visible and near-infrared (0.8-2.5 microns) spectral observations are also utilized in a self-consistent way. We find that an average visible albedo of 0.33, thermal inertia of 70 (SI units) and surface roughness of 50% closely match the observations. The shape and orientation of the asteroid is very important to constrain the thermal parameters to be consistent with all the observations. Multiple viewing geometries are equally important to achieve a robust solution for small, non-spherical NEAs. We separate the infrared beaming effects of shape, viewing geometry and surface roughness for this asteroid and show how their effects combine. We compare the diameter and albedo that would be derived from the thermal observations assuming a spherical shape with those from the shape-based model. We also discuss how observations from limited viewing geometries compare to the solution from multiple observations. The size that would be derived from the individual observation dates varies by 20% from the best-fit solution, and can be either larger or smaller. If the surface properties are not homogeneous, many solutions are possible, but the average properties derived here are very tightly constrained by the multiple observations, and give important insights into the nature of small NEAs. (C) 2017 Elsevier Inc. All rights reserved.

NASA's New Horizons spacecraft's voyage through the Pluto system centered on 2015 July 14 provided images of Pluto's small satellites Nix and Hydra at viewing angles unattainable from Earth. Here, we present solar phase curves of the two largest of Pluto's small moons, Nix and Hydra, observed by the New Horizons LOng Range Reconnaissance Imager and Multi-spectral Visible Imaging Camera, which reveal the scattering properties of their icy surfaces in visible light. Construction of these solar phase curves enables comparisons between the photometric properties of Pluto's small moons and those of other icy satellites in the outer solar system. Nix and Hydra have higher visible albedos than those of other resonant Kuiper Belt objects and irregular satellites of the giant planets, but not as high as small satellites of Saturn interior to Titan. Both Nix and Hydra appear to scatter visible light preferentially in the forward direction, unlike most icy satellites in the outer solar system, which are typically backscattering.

The exploration of the Pluto-Charon system by the New Horizons spacecraft represents the first opportunity to understand the distribution of albedo and other photometric properties of the surfaces of objects in the Solar System's "Third Zone" of distant ice-rich bodies. Images of the entire illuminated surface of Pluto and Charon obtained by the Long Range Reconnaissance Imager (LORRI) camera provide a global map of Pluto that reveals surface albedo variegations larger than any other Solar System world except for Saturn's moon Iapetus. Normal reflectances on Pluto range from 0.08-1.0, and the low-albedo areas of Pluto are darker than any region of Charon. Charon exhibits a much blander surface with normal reflectances ranging from 0.20-0.73. Pluto's albedo features are well-correlated with geologic features, although some exogenous low-albedo dust may be responsible for features seen to the west of the area informally named Tombaugh Regio. The albedo patterns of both Pluto and Charon are latitudinally organized, with the exception of Tombaugh Regio, with darker regions concentrated at the Pluto's equator and Charon's northern pole The phase curve of Pluto is similar to that of Triton, the large moon of Neptune believed to be a captured Kuiper Belt Object (KBO), while Charon's is similar to that of the Moon. Preliminary Bond albedos are 0.25+/-0.03 for Charon and 0.72+/-0.07 for Pluto. Maps of an approximation to the Bond albedo for both Pluto and Charon are presented for the first time. Our work shows a connection between very high albedo (near unity) and planetary activity, a result that suggests the KBO Eris may be currently active.