Bidirectional reflectance and VIS-NIR spectroscopy of cometary analogues under simulated space conditions

Abstract

International audienceThis work is intended to be the second publication in a series of papers reporting on the spectro-photometric properties of cometary analogues measured in the laboratory. Herein, we provide in-situ hyperspectral imaging data in the 0.40–2.35 μm range from three sublimation experiments under simulated space conditions in thermal vacuum from samples made of water ice, carbonaceous compounds and complex organic molecules. The dataset is complemented by measurements of the bidirectional reflectance in the visible (750 nm) spectral range before and after sublimation. A qualitative characterization of surface evolution processes is provided as well as a description of morphological changes during the simulation experiment.The aim of these experiments is to mimic the spectrum of comet 67P/Churyumov-Gerasimenko (67P) as acquired by the Rosetta mission by applying sublimation experiments on the mixtures of water ice with a complex organic material (tholins) and carbonaceous compounds (carbon black; activated charcoal) studied in our companion publication (Jost et al., submitted). Sublimation experiments are needed to develop the particular texture (high porosity), expected on the nucleus' surface, which might have a strong influence on spectro-photometric properties. The spectrally best matching mixtures of non volatile organic molecules from Jost et al. (submitted) are mixed with fine grained water ice particles and evolved in a thermal vacuum chamber, in order to monitor the influence of the sublimation process on their spectro-photometric properties.We demonstrate that the way the water ice and the non-volatile constituents are mixed, plays a major role in the formation and evolution of a surface residue mantle as well as having influence on the consolidation processes of the underlying ice. Additionally it results in different activity patterns under simulated insolation cycles. Further we show that the phase curves of samples having a porous surface mantle layer display higher coincidence with data of 67P than the phase curves of the samples having a more compact surface layer with smooth texture.The analysis of spectral absorption bands of water ice in the near-infrared (NIR) range, similar to those acquired by the VIRTIS instrument onboard Rosetta, allows to link compositional considerations with surface activity and texture

Comet 67P/CG Nucleus Composition and Comparison to Other Comets

“…Various laboratory experiments have reproduced the darkening of analogs for cometary matter, as the ice initially mixed with dark refractory dust sublimates, resulting in the formation and thickening of a porous sublimation lag (Oehler and Neukum 1991). The red spectral slope of these analogs also increases as the albedo decreases, resulting in the negative correlation between these parameters systematically observed at the surface of the nucleus of 67P Poch et al 2016b,a;Jost et al 2017a).…”

Pronounced morphological changes in a southern active zone on comet 67P/Churyumov-Gerasimenko

“…This bluest corner clings to the shadow line and shows an VIS slope of −1 ± 3%/100 nm and A 0 > 56 ± 3%. When we consider that coarse water ice has A 0 = 99% and the neutral VIS spectrum, this corner must bear a component of dark comet-average material of no higher than 0.1% to preserve its flat spectral slope according to ice+carbon+tholin laboratory experiments (Jost et al 2017) and bright spot studies with VIRTIS-M (Raponi et al 2016).…”

The opposition effect of 67P/Churyumov–Gerasimenko on post-perihelion Rosetta images

“…6) may already point to the presence of this mechanism. Jost et al (2017) used the PHIRE-2 radiogoniometer to measure the scattering curve before and after sublimation of inter/intramixture samples of 0.7 wt per cent 67 ± 31 µm ice particles, 0.2 wt per cent carbon black and 0.1 wt per cent tholins at 750 nm under 5 • < α < 180 • . The samples were fabricated on the SCITEAS simulation chamber (Pommerol et al 2011(Pommerol et al , 2015a.…”

“…In the intra-mixture preparations the mineral grains are contained as inclusions within water-ice particles. Jost et al (2017) also measured the radiometry of a pure iceless mixture of tholins (33 per cent) and carbon black (66 per cent). Details of the preparation methods and the physicochemical properties of the mixtures are found in Poch et al (2016a).…”