VIRTIS-H observations of the dust coma of comet 67P/Churyumov-Gerasimenko: spectral properties and color temperature variability with phase and elevation

F Andrieu,P Drossart,M Ciarniello,G Arnold,S Erard,F Capaccioni,J Crovisier,F Taylor,G Rinaldi,A Longobardo,P H Hasselmann,G Filacchione,M.-T Capria,D Kappel,D Bockelée-Morvan,M C De Sanctis,C Leyrat

doi:10.1051/0004-6361/201834764

Abstract

We analyze 2–5μm spectroscopic observations of the dust coma of comet 67P/Churyumov-Gerasimenko obtained with the Visible InfraRed Thermal Imaging Spectrometer (VIRTIS-H) instrument on board Rosetta from 3 June to 29 October 2015 at heliocentric distancesrh= 1.24–1.55 AU. The 2–2.5μm color, bolometric albedo, and color temperature were measured using spectral fitting. Data obtained atα= 90° solar phase angle show an increase in bolometric albedo (0.05–0.14) with increasing altitude (0.5–8 km), accompanied by a possible marginal decrease in color and color temperature. Possible explanations include dark particles on ballistic trajectories in the inner coma and radial changes in particle composition. In the phase angle range 50°–120°, phase reddening is significant (0.031%/100 nm deg−1) for a mean color of 2%/100 nm atα= 90°, which might be related to the roughness of the dust particles. Moreover, a decrease in color temperature with decreasing phase angle is also observed at a rate of ~0.3 K deg−1, consistent with the presence of large porous particles, with low thermal inertia, and showing a significant day-to-night temperature contrast. Comparing data acquired at fixed phase angle (α= 90°), a 20% increase in bolometric albedo is observed near perihelion. Heliocentric variations in dust color are not significant in the time period we analyzed. The measured color temperatures vary from 260 to 320 K, and follow arh−0.6variation in therh= 1.24–1.5 AU range, which is close to the expectedrh−0.5value.

Highlights

The Rosetta mission of the European Space Agency accompanied comet 67P/Churyumov-Gerasimenko between 2014 and 2016 as it approached perihelion (13 August 2015) and receded from the Sun
Data obtained at α = 90◦ solar phase angle show an increase in bolometric albedo (0.05–0.14) with increasing altitude (0.5–8 km), accompanied by a possible marginal decrease in color and color temperature
Comparing data acquired at fixed phase angle (α = 90◦), a 20% increase in bolometric albedo is observed near perihelion

Summary

Introduction

The Rosetta mission of the European Space Agency accompanied comet 67P/Churyumov-Gerasimenko (hereafter 67P) between 2014 and 2016 as it approached perihelion (13 August 2015) and receded from the Sun. The Micro-Imaging Dust Analysis System (MIDAS; Riedler et al 2007) acquired the 3D topography of 1–50 μm sized dust particles with resolutions down to a few nanometers, and showed that dust particles are agglomerates at all scales, with the smallest subunit sizes of less than 100 nm (Bentley et al 2016). The Cometary Secondary Ion Mass Analyzer (COSIMA; Kissel et al 2007) collected dust particles to image them at a resolution of 14 μm and performed secondary ion mass spectroscopy. Both porous aggregates and more compact particles were observed (Langevin et al 2016; Merouane et al 2016). The chemical analysis indicates that these particles are made of

Results

Discussion

Conclusion