Photometric properties of comet 67P/Churyumov-Gerasimenko from VIRTIS-M onboard Rosetta

Abstract

Aims. We investigate the nucleus photometric properties of the comet 67P/Churyumov-Gerasimenko as observed by the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) onboard the Rosetta spacecraft. Both full-disk and disk-resolved images of the comet have been analyzed, deriving light and phase curves as well as a photometric reduction of the radiance factor (I/F) to single scattering albedo (SSA) in the 0.4-3.5 μm range. Methods. Hyperspectral cubes from VIRTIS were calibrated and corrected for instrumental artifacts. We computed integrated fluxes from full-disk acquisitions to derive nucleus light curves and phase curves at low phase angles (1.2• < α < 14.9• ). Disk-resolved observations in the phase angle range 27.2• < α < 111.5• were reduced to SSA by means of a simplified Hapke model, deriving average spectrophotometric properties of the surface and producing SSA maps at different wavelengths. Spectral phase reddening in the visible (VIS) and infrared (IR) ranges was measured. Finally, full-disk and disk-resolved data were used together to derive a phase curve of the nucleus in the 1.2• < α < 111.5• range. Results. We measure an asymmetric double-peaked light curve that is due to the elongated shape of the nucleus. The average SSA albedo shows a reddish spectrum with a strong absorption feature centered at 3.2 μm, while the surface exhibits a backscattering behavior. The derived geometric albedo is A geo = 0.062 ± 0.002 at 0.55 μm, indicating a very dark surface. Phase reddening is significant both in the VIS and IR ranges, and we report spectral slopes of 0.20/kÅ and 0.033/kÅ, respectively, after applying photometric reduction. SSA maps indicate that Hapi and Imothep regions are the brightest in the VIS, with the former showing a bluer spectrum with respect to the rest of the surface. The phase curve of the nucleus shows a strong opposition effect, with β = 0.077 ± 0.002 for α < 15• .

Spectrophotometric properties of dwarf planet Ceres from the VIR spectrometer on board the Dawn mission

“…This is justified because CBOE is significant at very small phase angles (<2 • ) that are beyond the range investigated in this work, while as discussed in Li et al (2013a) and Ciarniello et al (2015), the effect of porosity cannot be decoupled by SSA for low-albedo objects such as Ceres. According to this, the free parameters in the model are w, b, v, B 0 , and h, all depending on the wavelength, andθ, which, being related to surface morphology, has no spectral dependence.…”

“…Furthermore, the derivation of a photometric model from a set of observations of a given target permits us to perform a photometric correction of the dataset itself Ciarniello et al 2015), thus decoupling intrinsic surface albedo variability from effects related to the observing geometry. To achieve this twofold aim, we take advantage of VIR observations acquired from December 2014 up to June 2015 to study the global spectrophotometric properties of the Ceres surface.…”

GIADA: shining a light on the monitoring of the comet dust production from the nucleus of 67P/Churyumov-Gerasimenko

“…VIRTIS onboard Rosetta describes the surface of 67P as coated by a complex mixture of organics that are generally dehydrated except for in active areas showing a small amount of water ice . Ciarniello et al (2015) attributed the slightly brighter aspect of the neck area with respect to the rest of the nucleus to enrichment in water ice. OSIRIS, the Rosetta camera, imaged meter-sized bright spots in the neck area, for example, that were interpreted as due to water ice exposure: dark mantle fragmentation exposes the icy material beneath it, which, triggered by insulation, might be responsible for the activation of new dust-emitting areas (Pommerol et al 2015).…”

Far-UV phase dependence and surface characteristics of comet 67P/Churyumov-Gerasimenko as observed with Rosetta Alice

“…Some small bright icy outcrops exist on the surface in higher resolution OSIRIS data, possibly recently uncovered or deposited at the surface (Pommerol et al 2015). Similarly, large-scale compositional homogeneity and a low albedo of 6.0% ± 0.3% at 550 nm, with localized variability mainly in the neck, is evident in the Rosetta Visual IR Thermal Imaging Spectrometer (VIRTIS) spectral data Ciarniello et al 2015) with spectral detections of water ice absorptions in select regions near shadowed areas. These data suggest that a dark refractory layer is widespread across the surface of the nucleus with very little ice content.…”

“…In this paper, we will focus on the FUV surface properties derived from data in which the comet was spatially resolved, acquired up through the week of the Philae lander delivery (November 12, 2014). The observing geometry around lander delivery makes it possible to study the phase dependence of the bidirectional reflectance of the surface material in the FUV, a trend never measured before in the FUV but determined in the visible and near-infrared regimes for a sampling of comets (Li et al 2013b;Fornasier et al 2015;Ciarniello et al 2015). With a photometric correction for the bidirectional reflectance at hand, a geometric albedo for 67P will be calculated and regional surface spectra of the nucleus corrected for viewing geometry will be compared to search for differences in reflectivity across the nucleus.…”