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New Horizons Photometry of Pluto's Moon Charon
Abstract
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. © 2019. The American Astronomical Society. All rights reserved..
... The ISS images were obtained from the PDS and calibrated according to the procedures in the PDS with the COISS_0011 calibration. Disk-integrated photometry was performed on each image using the techniques we have developed for spacecraft images and described in Buratti & Veverka (1983), , Pitman et al. (2010), and Buratti et al. (2019). Figure 1 shows the opposition solar phase curves at all wavelengths studied for each of the six moons. ...
... Since the photons are multiply scattered, the effect should be most important for bright surfaces. Indeed, the effect was discovered first on icy bodies and these bodies all do seem to have huge coherent backscatter surges (Brown & Cruikshank 1983;Domingue et al. 1991;Verbiscer et al. 2005;Buratti et al. 2011Buratti et al. , 2019 Table 4. The functional form of the opposition surge is determined primarily by the compaction state of the surface, but the single-particle phase function P(α) and the single scattering albedo ϖ 0 can also be fit if observations are sufficiently dense in phase angle coverage, as they are for Rhea and Dione. ...
... However, certain trends appear. For example, the moons we analyzed are all backscattering, with a Henyey-Greenstein g similar to that of the Moon (Buratti 1985), Ganymede and Callisto (Buratti 1991) and numerous other bodies (see Table 2 in Buratti et al. 2019; note that Ciarniello et al. (2011) find that the Saturnian moons are forward scattering). Higher albedo moons tend to scatter more isotropically (i.e., a Henyey-Greenstein g closer to 0), mainly because multiplycattered photons are isotropic. ...
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 μ m, 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 μ m 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.
... We also made corrections to our observations of Pluto to account for changes in brightness due to rotational solar phase variations. Pluto, and to a lesser extent Charon, exhibit changes in brightness as a function of subobserver longitude (e.g., Buie et al., 2010;Buratti et al., 2015Buratti et al., , 2019. Our observations are not extensive enough to determine a rotational phase curve. ...
... We used the same numbers for 2018-2019, as these amplitudes change very slowly. For Charon, which does not have a published light curve in the near-IR, the rotational effect should be even smaller, as its lightcurve in the New Horizons MVIC NIR -filter has a half amplitude of about 0.03 magnitudes (Buratti et al., 2019). The center of this filter's bandpass is at ∼0.94 µm, so corrections to our longer-wavelength ground-based measurements cannot be made directly from data collected with it. ...
... The center of this filter's bandpass is at ∼0.94 µm, so corrections to our longer-wavelength ground-based measurements cannot be made directly from data collected with it. However, the visible rotational lightcurve is due mostly to differences in water ice abundance on the surface (Buratti et al., 2019;Howett et al. 2017 Figures 2D and 2E show there is no discernable color dependence to the opposition surge. Although the data are noisy, Charon also appears to exhibit a huge surge ( Figure 2F). ...
Near-infrared observations of the Pluto Charon system were captured with the Palomar High Angular Resolution Observer (PHARO) adaptive optics 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.
... The images were reduced to a solar phase curve in two different ways depending on whether the full disk of Pluto was captured. In the former case, we reduced the data with our usual analysis techniques (Buratti et al. 2015(Buratti et al. , 2019, in which a solar phase curve and rotational phase curve are simultaneously solved. Because a rotational light curve had already been determined at all LORRI and MVIC wavelengths during the approach phase of the New Horizons Mission , we corrected each data point at or below a solar phase angle of 50°in the integral curve with this light curve. ...
... Much of the range in solar phase angles-20°-70°occurred near the closest approach, when the spacecraft was turning rapidly to point to Pluto and when the images were not full disk, as required by the model in its current form. For these disk-resolved observations, we employed a technique described in Buratti & Veverka (1983) and fully implemented in Buratti et al. (2019). The technique computes the disk-integrated brightness of a spherical planetary body given the specific intensity or I/F (Chandrasekhar 1960) at a point on the surface at a known viewing geometry. ...
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.
... 7, while the Charon phase as viewed from Pluto is 88°.0 during the P_DEEPIM sequence. From the Charon phase curve measured by Buratti et al. (2019), the integrated flux decreases by 0.04 mag deg −1 at this part of the curve, giving I/F = 0.050 during the P_DEEPIM sequence. The ratio, R, of the Charon-light flux delivered to Pluto relative to the incident solar flux is then determined by the solid angle, Ω, that Charon subtends as seen from Pluto, such that R = (IΩ)/ (πF). ...
... 7, while the Charon phase as viewed from Pluto is 88°.0 during the P_DEEPIM sequence. From the Charon phase curve measured by Buratti et al. (2019), the integrated flux decreases by 0.04 mag deg −1 at this part of the curve, giving I/F = 0.050 during the P_DEEPIM sequence. The ratio, R, of the Charon-light flux delivered to Pluto relative to the incident solar flux is then determined by the solid angle, Ω, that Charon subtends as seen from Pluto, such that R = (IΩ)/ (πF). ...
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 disk–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 disk–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.
We present the first measurements of Charon’s far-ultraviolet (FUV) surface reflectance, obtained by the Alice spectrograph on New Horizons. We find no measurable flux shortward of 1650 Å, and Charon’s geometric albedo is <0.019 (3 σ ) at 1600 Å. From 1650 to 1725 Å, Charon’s geometric albedo increases to 0.166 ± 0.068 and remains nearly constant until 1850 Å. As this spectral shape is characteristic of H 2 O ice absorption, Charon is the first Kuiper Belt object with a H 2 O ice surface to be detected in the FUV. Charon’s geometric albedo is ∼3.7 times lower than Enceladus’s at these wavelengths but has a very similar spectral shape. We attribute this to similarities in their surface compositions and the difference in absolute reflectivity to a high concentration or more-absorbing contaminants on Charon’s surface. Finally, we find that Charon has different solar phase behavior in the FUV than Enceladus, Mimas, Tethys, and Dione, with a stronger opposition surge than Enceladus and a shallower decline at intermediate solar phase angles than any of these Saturnian satellites.
e Eridani , the fifth-closest Sun-like star, hosts at least three planets and could possibly harbor more. However, the veracity of the planet candidates in the system and its full planetary architecture remain unknown. Here we analyze the planetary architecture of e Eridani via DYNAMITE , a method providing an integrative assessment of the system architecture (and possibly yet-undetected planets) by combining statistical, exoplanet-population-level knowledge with incomplete but specific information available on the system. DYNAMITE predicts the most likely location of an additional planet in the system based on the Kepler population demographic information from more than 2000 planets. Additionally, we analyze the dynamical stability of e Eridani system via N -body simulations. Our DYNAMITE and dynamical stability analyses provide support for planet candidates g, c, and f, and also predict one additional planet candidate with an orbital period between 549–733 days, in the habitable zone of the system. We find that planet candidate f, if it exists, would also lie in the habitable zone. Our dynamical stability analysis also shows that the e Eridani planetary eccentricities, as reported, do not allow for a stable system, suggesting that they are lower. We introduce a new statistical approach for estimating the equilibrium and surface temperatures of exoplanets, based on a prior from the planetary albedo distribution. e Eridani is a rich planetary system with a possibility of containing two potentially habitable planets, and its vicinity to our solar system makes it an important target for future imaging studies and biosignature searches.
e Eridani, the fifth-closest Sun-like star, hosts at least three planets and could possibly harbor more. However, the veracity of the planet candidates in the system and its full planetary architecture remain unknown. Here we analyze the planetary architecture of e Eridani via DYNAMITE, a method providing an integrative assessment of the system architecture (and possibly yet-undetected planets) by combining statistical, exoplanet-population level knowledge with incomplete but specific information available on the system. DYNAMITE predicts the most likely location of an additional planet in the system based on the Kepler population demographic information from more than 2000 planets. Additionally, we analyze the dynamical stability of e Eridani system via N-body simulations. Our DYNAMITE and dynamical stability analyses provide support for planet candidates g, c, and f, and also predict one additional planet candidate with an orbital period between 549 -- 733 days, in the habitable zone of the system. We find that planet candidate f, if it exists, would also lie in the habitable zone. Our dynamical stability analysis also shows that the e Eridani planetary eccentricities, as reported, do not allow for a stable system, suggesting that they are lower. We introduce a new statistical approach for estimating the equilibrium and surface temperatures of exoplanets, based on a prior on the planetary albedo distribution. e Eridani is a rich planetary system with a possibility of containing two potentially habitable planets, and its vicinity to our Solar System makes it an important target for future imaging studies and biosignature searches.
A search for temporal changes on Pluto and Charon was motivated by (1) the discovery of young surfaces in the Pluto system that imply ongoing or recent geologic activity, (2) the detection of active plumes on Triton during the Voyager 2 flyby, and (3) the abundant and detailed information that observing geologic processes in action provides about the processes. A thorough search for temporal changes using New Horizons images was completed. Images that covered the same region were blinked and manually inspected for any differences in appearance. The search included full-disk images such that all illuminated regions of both bodies were investigated and higher resolution images such that parts of the encounter hemispheres were investigated at finer spatial scales. Changes of appearance between different images were observed but in all cases were attributed to variability of the imaging parameters (especially geometry) or artifacts. No differences of appearance that are strongly indicative of a temporal change were found on the surface or in the atmosphere of either Pluto or Charon. Limits on temporal changes as a function of spatial scale and temporal interval during the New Horizons encounter are determined. The longest time interval constraint is one Pluto/Charon rotation period (~6.4 Earth days). Contrast reversal and high-phase bright features that change in appearance with solar phase angle are identified. The change of appearance of these features is most likely due to the change in phase angle rather than a temporal change. Had active plumes analogous to the plumes discovered on Triton been present on the encounter hemispheres of either Pluto or Charon, they would have been detected. The absence of active plumes may be due to temporal variability (i.e., plumes do occur but none were active on the encounter hemispheres during the epoch of the New Horizons encounter ...
New Horizons unveils the Pluto system
In July 2015, the New Horizons spacecraft flew through the Pluto system at high speed, humanity's first close look at this enigmatic system on the outskirts of our solar system. In a series of papers, the New Horizons team present their analysis of the encounter data downloaded so far: Moore et al. present the complex surface features and geology of Pluto and its large moon Charon, including evidence of tectonics, glacial flow, and possible cryovolcanoes. Grundy et al. analyzed the colors and chemical compositions of their surfaces, with ices of H 2 O, CH 4 , CO, N 2 , and NH 3 and a reddish material which may be tholins. Gladstone et al. investigated the atmosphere of Pluto, which is colder and more compact than expected and hosts numerous extensive layers of haze. Weaver et al. examined the small moons Styx, Nix, Kerberos, and Hydra, which are irregularly shaped, fast-rotating, and have bright surfaces. Bagenal et al. report how Pluto modifies its space environment, including interactions with the solar wind and a lack of dust in the system. Together, these findings massively increase our understanding of the bodies in the outer solar system. They will underpin the analysis of New Horizons data, which will continue for years to come.
Science , this issue pp. 1284 , 10.1126/science.aad9189 , 10.1126/science.aad8866 , 10.1126/science.aae0030 , & 10.1126/science.aad9045
The measured volume-average single particle angular scattering functions of a large number of types of particle of interest for planetary regoliths in the visible-near-IR wavelength region can be represented to a reasonable approximation by two-parameter, double Henyey–Greenstein functions. When the two parameters of this function are plotted against one another they are found to be inversely correlated and lie within a restricted zone shaped like a hockey stick within the parameter space. The centroid of the zone is a curve that can be represented by a simple empirical equation. The wide variety of types of particles used to construct the plot implies that this equation may represent most of the particles found in regoliths. This means that when modeling the bidirectional reflectance of a regolith it may be possible to reduce the number of parameters necessary to specify the reflectance, and also to characterize the entire single particle phase function from observations at phase angles less than 90°. Even if the hockey stick relation has a finite width, rather than being a line, it restricts the parameter space that must be searched when fitting data. The curve should also be useful for forward modeling particle phase functions.
A mathematically rigorous formalism is derived by which an arbitrary photometric function for the bidirectional reflectance of a smooth surface may be corrected to include effects of general macroscopic roughness. The correction involves only one arbitrary parameter, the mean slope angle , and is applicable to surfaces of any albedo. Using physically reasonable assumptions and mathematical approximations the correction expressions are evaluated analytically to second order in . The correction is applied to the bidirectional reflectance function of B. Hapke (1981, J. Geophys. Res.86, 3039–3054). Expressions for both the differential and integral brightnesses are obtained. Photometric profiles on hypothetical smooth and rough planets of low and high albedo are shown to illustrate the effects of macroscopic roughness. The theory is applied to observations of Mercury and predicts the integral phase function, the apparent polar darkening, and the lack of limb brightness surge on the planet. The roughness-corrected bidirectional reflectance function is sufficiently simple that it can be conveniently evaluated on a programmable hand-held calculator.
We present new light-curve measurements of Pluto and Charon taken with the Advanced Camera for Surveys High-resolution Camera on the Hubble Space Telescope. The observations were collected from 2002 June to 2003 June at 12 distinct sub-Earth longitudes over a range of solar phase angle 036-174—a larger range than previously measured. The new measurements of Pluto show that the light-curve amplitude has decreased since the mutual event season in the late 1980s. We also show that the average brightness has increased in the F555W (Johnson V equivalent) passband while the brightness has decreased in the F435W (Johnson B equivalent) passband. These data thus indicate a substantial reddening of the reflected light from Pluto. We find a weighted mean (B – V) = 0.9540 ± 0.0010 that is considerably higher than the long-standing value of (B – V) = 0.868 ± 0.003 most recently measured in 1992-1993. This change in color cannot be explained by the evolving viewing geometry and provides the strongest evidence to date for temporal changes on the surface of Pluto that are expected to be linked to volatile transport processes. We also report on the discovery of a new rotational modulation of Pluto's hemispherical color that ranges from 0.92 to 0.98 with the least red color at the longitude of maximum light and most red at minimum light. The phase coefficient of Pluto is nearly the same as measured in 1992-1993 with a value of βB
= 0.0392 ± 0.0064 and βV
= 0.0355 ± 0.0045 mag deg–1 for the F435W and F555W data, respectively. The Pluto phase curve is still very close to linear but a small but significant nonlinearity is seen in the data. In contrast, the light curve of Charon is essentially the same as in 1992/1993, albeit with much less noise. We confirm that Charon's Pluto-facing hemisphere is 8% brighter than the hemisphere facing away from Pluto. The color of Charon is independent of longitude and has a mean weighted value of (B – V) = 0.7315 ± 0.0013. The phase curve for Charon is now shown to be strongly nonlinear and wavelength dependent. We present results for both Pluto and Charon that better constrain the single-particle scattering parameters from the Hapke scattering theory.
The radar properties of Europa, Ganymede, and Callisto are characterized by high reflectivities dominated by a diffuse component, and a large amount of polarization in the opposite sense of that expected if the waves were specularly reflected once. It is pointed out that these are the properties that might plausibly be expected when a collimated source illuminates a weakly absorbing, particulate medium in which wavelength-sized scatterers are separated by distances somewhat larger than the wavelength, such as a regolith consisting of voids and/or silicate rocks imbedded in an icy matrix. No specialized structures are required. Portions of the wave front that are multiply scattered within the medium and that traverse the same path in opposite directions combine coherently in the backscatter direction to produce an increased intensity. The enhancement is different for the two components of polarized reflected radiation, so that this model explains the observed linear polarization ratio and may be able to account for the observed circular polarization ratio. The coherent effects are confined to a peak centered on the backscatter direction of angular half-width , where λ is the wavelength and D is the photon diffusion length in the medium. Bistatic radar observations would test this model and give information on the structures of the regoliths. The same phenomenon may play a role in radar scattering from Saturn's rings and also in the sharp opposition effects observed on outer planet satellites at optical frequencies.
It is well known that the bidirectional reflectance of a particulate medium such as a planetary regolith depends on the porosity, in contrast to predictions of models based on the equation of radiative transfer as usually formulated. It is shown that this failure to predict porosity dependence arises from an incorrect treatment of the light that passes between the particles. In this paper a more physically correct treatment that takes account of the necessity of preventing particles from interpenetrating is used together with the two-stream approximation to solve the radiative transfer equation and derive improved expressions for the bidirectional and directional-hemispherical reflectances. It is found that increasing the filling factor (decreasing the porosity) increases the reflectance of low and medium albedo powders, but decreases it for ones with very high albedos. The model agrees qualitatively with measured data.
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. © 2018. The American Astronomical Society. All rights reserved.
The New Horizons flyby provided the first high-resolution color maps of Pluto. We present here, for the first time, an analysis of the color of the entire sunlit surface of Pluto and the first quantitative analysis of color and elevation on the encounter hemisphere. These maps show the color variation across the surface from the very red terrain in the equatorial region, to the more neutral colors of the volatile ices in Sputnik Planitia, the blue terrain of East Tombaugh Regio, and the yellow hue on Pluto's North Pole. There are two distinct color mixing lines in the color–color diagrams derived from images of Pluto. Both mixing lines have an apparent starting point in common: the relatively neutral-color volatile-ice covered terrain. One line extends to the dark red terrain exemplified by Cthulhu Regio and the other extends to the yellow hue in the northern latitudes. There is a latitudinal dependence of the predominant color mixing line with the most red terrain located near the equator, less red distributed at mid-latitudes and more neutral terrain at the North Pole. This is consistent with the seasonal cycle controlling the distribution of colors on Pluto. Additionally, the red color is consistent with tholins. The yellow terrain (in the false color images) located at the northern latitudes occurs at higher elevations.
We aim to explore the behavior of the opposition effect as an important tool in optical remote sensing on the nucleus of comet 67P/ Churyumov-Gerasimenko (67P), using Rosetta-OSIRIS images acquired in different filters during the approach phase, July-August 2014 and the close flyby images on 14 of February 2015, which contain the spacecraft shadow.
We based our investigation on the global and local brightness from the surface of 67P with respect to the phase angle, also known as phase curve. The local phase curve corresponds to a region that is located at the Imhotep-Ash boundary of 67P. Assuming that the region at the Imhotep-Ash boundary and the entire nucleus have similar albedo, we combined the global and local phase curves to study the opposition-surge morphology and constrain the structure and properties of 67P. The model parameters were furthermore
compared with other bodies in the solar system and existing laboratory study.
We found that the morphological parameters of the opposition surge decrease monotonically with wavelength, whereas in the case of coherent backscattering this behavior should be the reverse. The results from comparative analysis place 67P in the same category as the two Mars satellites, Phobos and Deimos, which are notably different from all airless bodies in the solar system. The similarity between the surface phase function of 67P and a carbon soot sample at extremely small angles is identified, introducing regolith at the boundary of the Imhotep-Ash region of 67P as a very dark and fluffy layer.
Light curves produced from color observations taken during New Horizons’ approach to the Pluto-system by its Multi-spectral Visible Imaging Camera (MVIC, part of the Ralph instrument) are analyzed. Fifty seven observations were analyzed, they were obtained between 9th April and 3rd July 2015, at a phase angle of 14.5° to 15.1°, sub-observer latitude of 51.2 °N to 51.5 °N, and a sub-solar latitude of 41.2°N. MVIC has four color channels; all are discussed for completeness but only two were found to produce reliable light curves: Blue (400–550 nm) and Red (540–700 nm). The other two channels, Near Infrared (780–975 nm) and Methane-Band (860–910 nm), were found to be potentially erroneous and too noisy respectively. The Blue and Red light curves show that Charon's surface is neutral in color, but slightly brighter on its Pluto-facing hemisphere. This is consistent with previous studies made with the Johnson B and V bands, which are at shorter wavelengths than that of the MVIC Blue and Red channel respectively.
A unique feature of Pluto's large satellite Charon is its dark red northern polar cap. Similar colours on Pluto's surface have been attributed to tholin-like organic macromolecules produced by energetic radiation processing of hydrocarbons. The polar location on Charon implicates the temperature extremes that result from Charon's high obliquity and long seasons in the production of this material. The escape of Pluto's atmosphere provides a potential feedstock for a complex chemistry. Gas from Pluto that is transiently cold-trapped and processed at Charon's winter pole was proposed as an explanation for the dark coloration on the basis of an image of Charon's northern hemisphere, but not modelled quantitatively. Here we report images of the southern hemisphere illuminated by Pluto-shine and also images taken during the approach phase that show the northern polar cap over a range of longitudes. We model the surface thermal environment on Charon and the supply and temporary cold-trapping of material escaping from Pluto, as well as the photolytic processing of this material into more complex and less volatile molecules while cold-trapped. The model results are consistent with the proposed mechanism for producing the observed colour pattern on Charon.
The exploration of the Pluto-Charon system by the New Horizons spacecraft represent 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" within the context of a geologic world. 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 revealed surface albedo variegations larger than any other world except for Saturn's moon Iapetus. Normal reflectances on Pluto range from 0.08-1.0. 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. The low-albedo areas of Pluto are darker than anything on Charon's surface. The phase curve of Pluto is similar to that of Triton, the large moon of Neptune, and a former KBO dwarf planet, 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 the Bond albedo for both Pluto and Charon are presented for the first time.
Approach images of Pluto and Charon taken by the LORRI imaging system during the New Horizons spacecraft encounter have been used to determine the mean radii and shape of Pluto and Charon. The primary observations are limb locations derived using three independent approaches. The resulting mean radii of Pluto and Charon are 1187.6 +/- 1.8 km and 605.4 +/- 1.2 km, respectively (2-sigma). The corresponding densities are 1857 +/- 6 kg m-3 and 1707 +/- 17 kg m-3 (1-sigma). The Charon radius value is consistent with previous Earth-based occultation estimates. Neither Pluto nor Charon show any evidence for tidal/rotational distortions; upper bounds on the oblateness are <0.6% and <0.5%, respectively.
The New Horizons spacecraft will encounter Pluto in 2015 July. As this fast flyby will yield a picture of Pluto frozen in time, ground-based observations are key to understanding this dwarf ice planet, especially with regard to the seasonal transport of surface volatiles. This paper reports on changes in Pluto's rotational light curve as evidence for this transport. Historical observations are consistent with a stable frost pattern, but since 2002, changes began to appear in both light curves and Hubble Space Telescope maps. Our BVR observations at Table Mountain Observatory from 2008 to 2014 show evidence for sustained and continued albedo and color changes on Pluto. The B and V albedos are stable, but Pluto is becoming redder in color, particularly on its low-albedo side. This view is consistent with the transport of a bright volatile (nitrogen) with the uncovering of a substrate of red material such as photolyzed methane. As Buie et al. reported a B - V of 0.96 in 2002-2003, and our B - V was higher in 2008-2012, Pluto may have experienced a transient reddening in the 1999-2012 period. We also discovered an opposition supersurge in all three colors at very small solar phase angles (~010). Explosive geysers have been observed on Triton and Mars, the two other celestial bodies with receding polar caps. Because the physical conditions existing on Pluto are similar to those on Triton, we predict that plume deposits and possibly active plumes will be found on its surface.
Dawn spacecraft orbited Vesta for more than one year and collected a huge
volume of multispectral, high-resolution data in the visible wavelengths with
the Framing Camera. We present a detailed disk-integrated and disk-resolved
photometric analysis using the Framing Camera images with the Minnaert model
and the Hapke model, and report our results about the global photometric
properties of Vesta. The photometric properties of Vesta show weak or no
dependence on wavelengths, except for the albedo. At 554 nm, the global average
geometric albedo of Vesta is 0.38+/-0.04, and the Bond albedo range is
0.20+/-0.02. The bolometric Bond albedo is 0.18+/-0.01. The phase function of
Vesta is similar to those of S-type asteroids. Vesta's surface shows a
single-peaked albedo distribution with a full-width-half-max ~17% relative to
the global average. This width is much smaller than the full range of albedos
(from ~0.55x to >2x global average) in localized bright and dark areas of a few
tens of km in sizes, and is probably a consequence of significant regolith
mixing on the global scale. Rheasilvia basin is about 10% brighter than the
global average. The phase reddening of Vesta measured from Dawn Framing Camera
images is comparable or slightly stronger than that of Eros as measured by the
Near Earth Asteroid Rendezvous mission, but weaker than previous measurements
based on ground-based observations of Vesta and laboratory measurements of HED
meteorites. The photometric behaviors of Vesta are best described by the Hapke
model and the Akimov disk- function, when compared with the Minnaert model,
Lommel-Seeliger model, and Lommel- Seeliger-Lambertian model. The traditional
approach for photometric correction is validated for Vesta for >99% of its
surface where reflectance is within +/-30% of global average.
Three weeks prior to the commencement of Cassini's 4 year tour of the saturnian system, the spacecraft executed a close flyby of the outer satellite Phoebe. The infrared channel of the Visual Infrared Mapping Spectrometer (VIMS) obtained images of reflected light over the 0.83 5.1 mum spectral range with an average spectral resolution of 16.5 nm, spatial resolution up to 2 km, and over a range of solar phase angles not observed before. These images have been analyzed to derive fundamental photometric parameters including the phase curve and phase integral, spectral geometric albedo, bolometric Bond albedo, and the single scattering albedo. Physical properties of the surface, including macroscopic roughness and the single particle phase function, have also been characterized. Maps of normal reflectance show the existence of two major albedo regimes in the infrared, with gradations between the two regimes and much terrain with substantially higher albedos. The phase integral of Phoebe is 0.29±0.03, with no significant wavelength dependence. The bolometric Bond albedo is 0.023±007. We find that the surface of Phoebe is rough, with a mean slope angle of 33°. The satellite's surface has a substantial forward scattering component, suggesting that its surface is dusty, perhaps from a history of outgassing. The spectrum of Phoebe is best matched by a composition including water ice, amorphous carbon, iron-bearing minerals, carbon dioxide, and Triton tholin. The characteristics of Phoebe suggest that it originated outside the saturnian system, perhaps in the Kuiper Belt, and was captured on its journey inward, as suggested by Johnson and Lunine (2005).
The theory of multiple scattering based on the equation of radiative transfer breaks down for directly backscattered radiation if the scattering particles are large enough and the medium dense enough for the particles to shadow one another. This ‘shadowing effect’ can be incorporated as a correction into the usual radiative transfer theory. The resultant theory may be applicable to Saturn's rings and the lunar surface.
Using clear-filter images from Voyager 2 (effective wavelength 0.48 μm), we have constructed the first-ever digital albedo map of Saturn's moon Phoebe. Most normal reflectances in this new map are between 0.07 and 0.11; the albedo histogram is largely bimodal, suggesting that the satellite is covered predominantly by two different types of surface materials. The highest albedos are confined to isolated, quasi-circular spots 40 to 100 km across, including three spots of varying albedo in a band immediately south of the equator and one especially bright spot at latitude 60°N (normal reflectance as high as 0.13, ≈50% brighter than the average surface). The bright northern spot and the brightest of the southern spots occur at approximately the same longitude, an alignment that gives Phoebe its significant rotational lightcurve. The low resolution of the Voyager images does not permit interpretation of the bright spots' origin.Phoebe's global-average photometric function was determined by combining the satellite's telescopic near-opposition phase curve (S. Kruseet al.1986,Icarus68, 168–175) with absolute disk-resolved reflectances measured from the Voyager images (triaxial-ellipsoid shape assumed with radii 115, 110, and 105 km). Modeling of the telescopic observations supports the presence of a significant opposition surge, although scatter in the data does not allow unambiguous determination of the surge's exact strength. The derived photometric function is consistent with the idea that Phoebe is a C-type object—probably a primitive, captured body related to Chiron, Pholus, and the inhabitants of the Kuiper Belt. The albedo map and photometric information will aid in the planning of high-resolution Cassini images of Phoebe, which will represent our first close look at this class of primitive outer Solar-System object.
The surface properties of the icy bodies in the saturnian system have been investigated by means of the Cassini-VIMS (Visual Infrared Mapping Spectrometer) hyperspectral imager which operates in the 0.35–5.1μm wavelength range. In particular, we have analyzed 111 full disk hyperspectral images of Rhea ranging in solar phase between 0.08° and 109.8°. These data have been previously analyzed by Filacchione et al. (Filacchione, G. et al. [2007]. Icarus 186, 259–290; Filacchione, G. et al. [2010]. Icarus 206, 507–523) to study, adopting various “spectral indicators” (such as spectral slopes, band depth, and continuum level), the relations among various saturnian satellites. As a further step we proceed in this paper to a quantitative evaluation of the physical parameters determining the spectrophotometric properties of Rhea’s surface. To do this we have applied Hapke (Hapke, B. [1993]. Theory of Reflectance and Emittance Spectroscopy, Topics in Remote Sensing: 3. Springer, Berlin) IMSA model (Isotropic Multiple Scattering Approximation) which allow us to model the phase function at VIS–IR (visible–infrared) wavelengths as well as the spectra taking into account various types of mixtures of surface materials. Thanks to this method we have been able to constrain the size of water ice particles covering the surface, the amount of organic contaminants, the large scale surface roughness and the opposition effect surge. From our analysis it appears that wavelength dependent parameters, e.g. opposition surge width (h) and single-particle phase function parameters (b,v), are strongly correlated to the estimated single-scattering albedo of particles. For Rhea the best fit solution is obtained by assuming: (1) an intraparticle mixture of crystalline water ice and a small amount (0.4%) of Triton tholin; (2) a monodisperse grain size distribution having a particle diameter am=38μm; and (3) a surface roughness parameter value of 33°. The study of phase function shows that both shadow hiding and coherent backscattering contribute to the opposition surge. This study represents the first attempt, in the case of Rhea, to join the spectral and the photometric analysis. The surface model we derived gives a good quantitative description of both spectrum and phase curve of the satellite. The same approach and model, with appropriate modifications, shall be applied to VIMS data of the other icy satellites of Saturn, in order to reveal similarities and differences in the surface characteristics to understand how these bodies interact with their environment.
Cassini observations of the surface of Titan offer unprecedented views of its surface through atmospheric windows in the 1–5 μm region. Images obtained in windows for which the haze opacity is low can be used to derive quantitative photometric parameters such as albedo and albedo distribution, and physical properties such as roughness and particle characteristics. Images from the early Titan flybys, particularly T0, Ta, and T5 have been analyzed to create albedo maps in the 2.01 and 2.73 μm windows. We find the average normal reflectance at these two wavelengths to be 0.15±0.02 and 0.035±0.003, respectively. Titan's surface is bifurcated into two albedo regimes, particularly at 2.01 μm. Analysis of these two regimes to understand the physical character of the surface was accomplished with a macroscopic roughness model. We find that the two types of surface have substantially different roughness, with the low-albedo surface exhibiting mean slope angles of not, vert, similar18°, and the high-albedo terrain having a much more substantial roughness with a mean slope angle of not, vert, similar34°. A single-scattering phase function approximated by a one-term Henyey–Greenstein equation was also fit to each unit. Titan's surface is back-scattering (gnot, vert, similar0.3–0.4), and does not exhibit substantially different backscattering behavior between the two terrains. Our results suggest that two distinct geophysical domains exist on Titan: a bright region cut by deep drainage channels and a relatively smooth surface. The two terrains are covered by a film or a coating of particles perhaps precipitated from the satellite's haze layer and transported by eolian processes. Our results are preliminary: more accurate values for the surface albedo and physical parameters will be derived as more data is gathered by the Cassini spacecraft and as a more complete radiative transfer model is developed from both Cassini orbiter and Huygens Lander measurements.
We present a multispectral photometric study of the Moon between solar phase angles of 0 and 85°. Using Clementine images obtained between 0.4 and 1.0 μm, we produce a comprehensive study of the lunar surface containing the following results: (1) empirical photometric functions for the spectral range and viewing and illumination geometries mentioned, (2) photometric modeling that derives the physical properties of the upper regolith and includes a detailed study of the causes for the lunar opposition surge, (3) an absolute calibration of the Clementine UV/Vis camera. The calibration procedure given on the Clementine calibration web site produces reflectances relative to a halon standard and further appear significantly higher than those seen in groundbased observations. By comparing Clementine observations with prior groundbased observations of 15 sites on the Moon we have determined a good absolute calibration of the Clementine UV/Vis camera. A correction factor of 0.532 has been determined to convert the web site (www.planetary.brown.edu/clementine/calibration.html) reflectances to absolute values. From the calibrated data, we calculate empirical phase functions useful for performing photometric corrections to observations of the Moon between solar phase angles of 0 and 85° and in the spectral range 0.4 to 1.0μm. Finally, the calibrated data is used to fit a version of Hapke's photometric model modified to incorporate a new formulation, developed in this paper, of the lunar opposition surge which includes coherent backscatter. Recent studies of the lunar opposition effect have yielded contradictory results as to the mechanism responsible: shadow hiding, coherent backscatter, or both. We find that most of the surge can be explained by shadow hiding with a halfwidth of ∼8°. However, for the brightest regions (the highlands at 0.75–1.0μm) a small additional narrow component (halfwidth of <2°) of total amplitude ∼1/6 to 1/4 that of the shadow hiding surge is observed, which may be attributed to coherent backscatter. Interestingly, no evidence for the narrow component is seen in the maria or in the highlands at 0.415μm. A natural explanation for this is that these regions are too dark to exhibit enough multiple scattering for the effects of coherent backscatter to be seen. Finally, because the Moon is the only celestial body for which we have “ground truth” measurements, our results provide an important test for the robustness of photometric models of remote sensing observations.
Ninety voyager images ranging in phase angle from 3 to 143° and covering the spectral range from 0.34 to 0.58 μm were analyzed to derive the photometric properties of Europa. At small phase angles the disk-integrated phase curve is remarkable in that it shows little or no evidence of an opposition effect (in agreement with earlier Earth-based observations by Millis and Thompson, Icarus26, 408, 1975). The phase integral determined in the Voyager clear filter (centered near 0.47 μm) is 1.09 ± 0.11, in good agreement with previous estimates based on radiometry. The bolometric Bond albedo is 0.62 ± 0.14. The scattering properties of Europa in general, and of the two major terrain types (bright plains and darker mottled terrain) in particular, cannot be represented by a lunar-like photometric law. However, an equation which is a linear superposition of a lunar-like scattering law and a Lambert component provides an adequate simple representation of the scattering properties. The plains are photometrically more homogeneous than the darker mottled terrain. In the Voyager clear filter, the average normal reflectance is 0.71 for the plains on both the leading and trailing hemispheres; for the darker mottled terrain the values are 0.60 on the leading hemisphere, and 0.48 on the trailing one.
Photometric analysis of Voyager images of the medium-sized icy satellites of Saturn shows that their surfaces exhibit a wide range of scattering properties. At low phase angles, Rhea and Dione closely follow lunar behavior with almost no limb darkening. Mimas, Tethys, and especially Enceladus shiw significant limb darkening at low phase angles, which suggests multiple scattering is important for their surfaces. A simple photometric function of the form I/F = f(α)Aμ0/(μ + μ0) + (1 − A)μ0 has been fit to the observations. For normal reflectances <0.6, we find lunar-like scattering properties (A = 1). No satellite's surface can be described by Lambert's Law (A = 0). Dione exhibits the widest albedo variations (about 50%). A longitudinal dark stripe which represents a 15% decrease in albedo is situated near the center of the trailing side of Tethys. A correlation is found between the albedo and color of the satellites: the darker objects are redder. Similarly, darker areas of each satellite are redder. Spectral reflectances of Mimas and Enceladus can be derived for the first time. After the proper calibrations to the Voyager color images are made, it is found that both satellites have remarkably flat spectra into the ultraviolet.
Voyager imaging observations provide new photometric data on Saturn's satellites at large phase angles (up to 133° in the case of Mimas) not observable from Earth. Significant new results include the determination of phase integrals ranging from 0.7 in the case of Rhea to 0.9 for Enceladus. For Enceladus we find an average geometric albedo pv = 1.04 ± 0.15 and Bond albedo of 0.9 ± 0.1. The data indicate an orbital lightcurve with an amplitude of 0.2 mag, the trailing side being the brighter. For Mimas, the lightcurve amplitude is probably less than 0.1 mag. The value of the geometric albedo of Mimas reported here, pv = 0.77 ± 0.15 (corresponding to a mean opposition magnitude V0 = +12.5) is definitely higher than the currently accepted value of about 0.5. For Dione, the Voyager data show a well-defined orbital lightcurve of amplitude about 0.6 mag, with the leading hemisphere brighter than the trailing one.
Hapke's photometric theory is fitted to Voyager whole disk observatios of Miranda, Ariel, Umbriel, Titania, and Oberon to obtain better determinations of the satellites' geometric albedos and phase integrals. Our analysis demonstrates that in terms of large-scale roughness, Umbriel, Titania, and Oberon are comparable to our Moon, but Ariel is significantly rougher. The most conspicuous difference among regolith parameters is the particle single scattering albedo (values range from 0.3 for Umbriel up to 0.7 for Ariel). On the other hand, the degree of backscattering of regolith particles is found to be very similar (values of the Henyey-Greenstein parameter g are all close to −0.3). Umbriel's opposition surge appears to be marginally detectable in Voyager observations. Our tentative results suggest that Umbriel's opposition surge is lunar-like and distinctly broader than Titania's. This would imply that Umbriel's regolith is less porous than Titania's and its solid particles are less strongly backscattering (g = −0.2).
Six nights of R-band CCD observations of the classical Kuiper Belt Object (KBO) 20000 Varuna (2000 WR106) were obtained at the Palomar Mountain 60- and 200-in telescopes. The observations were scheduled to take advantage of a particularly favorable apparition which allowed us to sample down to extremely small solar phase angle (α=0.036°). After rotational lightcurve subtraction, we found that the KBO exhibited a strong opposition surge of ∼0.1 mag at phase angles α<0.1°. We modeled our composite solar phase curve of Varuna using both H–G parameterization and Hapke theory and concluded that similar opposition surges may be wide spread among KBOs and that the regolith of Varuna may be significantly more porous than a typical main-belt C-type asteroid. Wide-spread opposition surges lead to higher albedos than derived assuming linear phase behavior: on the whole KBOs may be brighter than previously assumed.
A radiative transfer model, derived largely from the work of B.W. Hapke (1981, J. Geophys. Res.86, 3039–3054) and J.D. Goguen (1981, Ph.D. thesis, Cornell University, Ithaca, N.Y.), is fit to Voyager imaging observations of Europa, Mimas, Enceladus, and Rhea. It is possible to place constraints on the single-scattering albedo, the porosity of the optically active upper regolith, the single-particle phase functions, and, in the cases of Europa and Mimas, the mean slope angle of macroscopic surface features. The texture of the surfaces of the Saturnian satellites appears to be similar to the Earth's moon. However, Europa is found to have a distinctly more compact regolith and a more forward-scattering single-particle phase function.
The Hapke (1986) model has been well fitted to both full-disk and disk-resolved Voyager observations. The low phase angle data indicate a substantial opposition effect, and the Hapke analysis results show that while the regolith compaction parameter for Rhea is definitely larger than for Titania, it is comparable to that of the moon. Photometric differences other than albedo are noted between the leading and trailing hemispheres of the satellite. The albedo map of Rhea presented reproduces the observed lightcurve and demonstrates that no terrain or feature in the trailing hemisphere is as bright as any in the leading hemisphere. A quasi-circular low albedo region near the antiapex of motion is discovered.
An analytical model is developed for the opposition effect (heiligenshein) in the case of light scattering from a semi-infinite, particulate medium with particles that are large relative to the wavelength. The effect is common for natural materials, and comprises a brightness surge in light diffusively reflected from a surface at near zero phase. A generalized expression is devised for the extinction coefficient of a particulate medium. Models are developed for step function and hyperbolic tangent distributions of light scattered from a stratified medium and exhibiting the opposition effect. A maximum brightness amplitude increase of 0.753 is projected for the effect. Greater values must have other causes.
An approximate analytic solution is derived for the radiative transfer equation describing particulate surface light scattering, taking into account multiple scattering and mutual shadowing. Analytical expressions for the following quantities are found: bidirectional reflectance, radiance coefficient and factor, the normal, Bond, hemispherical, and physical albedos, integral phase function and phase integral, and limb-darkening profile. Scattering functions for mixtures can be calculated, as well as corrections for comparisons of experimental transmission or reflection spectra with observational planetary spectra. The theory should be useful for the interpretation of reflectance spectroscopy of laboratory surfaces and the photometry of solar system objects.
Photoelectric observations of the entire lunar disk made in 1964-1965 over phase angles from 6 to 12 deg in nine narrow bands from 0.35 to 1.0 microns and in UBV are reviewed. Phase curves are presented as a function of wavelength. The results confirm a reddening with increasing phase angle found by previous investigators for particular areas.
The surface of the Jovian satellite Europa is characterized on the basis of an analysis of ground photoelectric photometry at 470 and 550 nm and Voyager images. The data are presented in extensive tables and graphs and discussed in detail. At 550 nm, Europa has single-scattering albedo 0.964, opposition-effect amplitude 0.5, opposition-effect width 0.0016, double-lobed Henyey-Greenstein factors b = -0.429 and c = 0.113, and mean roughness angle 10 deg (much lower than on other solar-system objects). From the small roughness and the 96-percent porosity implied by the narrow opposition peak, it is concluded that the surface was formed mainly by endogenic processes. It is also noted that only one of three observational criteria for preferential ion bombardment of the trailing hemisphere are met in Europa.
Pluto and its moon, Charon, are the most prominent members of the Kuiper belt, and their existence holds clues to outer solar
system formation processes. Here, hydrodynamic simulations are used to demonstrate that the formation of Pluto-Charon by means
of a large collision is quite plausible. I show that such an impact probably produced an intact Charon, although it is possible
that a disk of material orbited Pluto from which Charon later accumulated. These findings suggest that collisions between
1000-kilometer-class objects occurred in the early inner Kuiper belt.
As one of the most geologically active bodies in the solar system, Saturn's moon Enceladus not only coats itself with water ice particles, it accounts for the unusually high albedos of the other satellites orbiting within Saturn's vast, tenuous E ring. This effect is evident in Hubble Space Telescope observations obtained at true opposition on 13 and 14 January 2005 that reveal that the mean geometric albedos of satellites embedded within the E ring approximate or exceed unity.
We have measured the solar phase curves in B, V, and I for 18 Trans-Neptunian Objects, 7 Centaurs, and Nereid and determined the rotation curves for 10 of these targets. For each body, we have made ~100 observations uniformly spread over the entire visible range. We find that all the targets except Nereid have linear phase curves at small phase angles (< 2 deg) with widely varying phase coefficients (0.0 to 0.4 mag/deg). At phase angles > 3 deg, the Centaurs (54598) Bienor and (32532) Thereus have phase curves that flatten. The recently discovered Pluto-scale bodies (2003 UB313, 2005 FY9, and 2003 EL61), like Pluto, have neutral colors compared to most TNOs and small phase coefficients (< 0.1 mag/deg). Together these two properties are a likely indication for large TNOs of high-albedo, freshly coated icy surfaces. We find several bodies with significantly wavelength-dependent phase curves. The TNOs (50000) Quaoar, (120348) 2004 TY364 (47932), and 2000 GN171 have unusually high I-band phase coefficients (0.290+/-0.038, 0.413+/-0.064, 0.281+/-0.033 mag/deg, respectively) and much lower coefficients in the B and V bands. Their phase coefficients increase in proportion to wavelength by 0.5 - 0.8 mag/deg/um. The phase curves for TNOs with small B-band phase coefficients (< 0.1 mag/deg) have a similar but weaker wavelength dependence. Coherent backscatter is the likely cause for the wavelength dependence for all these bodies. We see no such dependence for the Centaurs, which have visual albedos ~0.05.
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