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

Abstract

Aims. The Alice far-ultraviolet (FUV) spectrograph onboard Rosetta has, for the first time, imaged the surface of a comet, 67P/Churyumov-Gerasimenko (67P), in the FUV. With spatially resolved data, the nucleus properties are characterized in the FUV, including phase dependence, albedo, and spectral slope. Regional measurements across the nucleus are compared to discern any compositional variations. Methods. Hapke theory was utilized to model the phase dependence of the material on the surface of 67P. The phase dependence of 67P was derived from a subset of data acquired at various phase angles in November 2014, within 50 km of the comet such that the nucleus was spatially resolved. The derived photometric correction was then applied to a different subset of spatially resolved data sampling several distinct geographical regions on the nucleus acquired in August−November 2014 under similar viewing geometries. Results. In the FUV, the surface of 67P is dark, blue sloped, has an average geometric albedo of 0.054 ± 0.008 at 1475 Å near the center of the Alice bandpass, and is mostly uniform from region to region, with the exception of the Hatmehit region, which is slightly more reflective. These results are consistent with the suggestion made by the Rosetta OSIRIS and VIRTIS teams that the surface of 67P is covered with a homogeneous layer of material and that surface ice is not ubiquitous in large abundances. The modeled Hapke parameters, specifically the single scattering albedo (w) and the asymmetry factor (ζ), are determined to be 0.031 ± 0.003 and −0.530 ± 0.025 near the center of the Alice bandpass at 1475 Å. These parameters are consistent with measurements of other comet nuclei that have been observed by flyby missions in the visible and the near-infrared regimes.

The Far Ultraviolet Wavelength Dependence of the Lunar Phase Curve as Seen by LRO LAMP

“…These formal 1-sigma errors in each parameter are computed as the square root of the diagonal elements of the covariance matrix returned by MPFIT, a program using the Levenberg-Marquart technique to solve the least squares problem. The derived w for the Moon is generally larger than that of comet 67P/Churyumov-Gerasimenko as observed with Rosetta Alice (Feaga et al, 2015), indicating a higher FUV albedo of the Moon than this comet, whereas the derived asymmetry factor b for the two bodies is very similar (Feaga et al, 2015). Figure 4 shows w and b as a function of wavelengths.…”

Far‐Ultraviolet Photometric Response of Apollo Soil 10084

“…The LAMP phase curves are fitted with a single term (Hapke et al, ), rather than the two‐term phase function as adopted here (Liu et al, , ). Similar phase curve analysis was also applied to comet C‐G/67P, as measured by Rosetta Alice (Feaga et al, ). The negative values of the LAMP asymmetry parameter for sample mare region indicate backscattering, consistent with our laboratory measurements.…”

Evidence of sub-surface energy storage in comet 67P from the outburst of 2016 July 03

“…Alice is a far-ultraviolet (70-205 nm) imaging spectrograph onboard Rosetta that observed emissions from various atomic and molecular species in the coma of comet 67P/Churyumov-Gerasimenko (Feldman et al 2015) as well as reflected solar radiation from both the nucleus and the dust coma (Feaga et al 2015). Alice employed a two-dimensional photon counting detector that accumulated counts over an interval, usually 5 or 10 min, into a histogram array of wavelength versus spatial position along the 5.5 • slit.…”