Refractory and semi-volatile organics at the surface of comet 67P/Churyumov-Gerasimenko: Insights from the VIRTIS/Rosetta imaging spectrometer

Abstract

Laboratory spectral measurements of relevant analogue materials were performed in the framework of the Rosetta mission in order to explain the surface spectral properties of comet 67P. Fine powders of coal, iron sulphides, silicates and their mixtures were prepared and their spectra measured in the Vis-IR range. These spectra are compared to a reference spectrum of 67P nucleus obtained with the VIRTIS/Rosetta instrument up to 2.7 μm, excluding the organics band centred at 3.2 μm. The species used are known to be chemical analogues for cometary materials which could be present at the surface of 67P. Grain sizes of the powders range from tens of nanometres to hundreds of micrometres. Some of the mixtures studied here actually reach the very low reflectance level observed by VIRTIS on 67P. The best match is provided by a mixture of sub-micron coal, pyrrhotite, and silicates. Grain sizes are in agreement with the sizes of the dust particles detected by the GIADA, MIDAS and COSIMA instruments on board Rosetta. The coal used in the experiment is responsible for the spectral slope in the visible and infrared ranges. Pyrrhotite, which is strongly absorbing, is responsible for the low albedo observed in the NIR. The darkest components dominate the spectra, especially within intimate mixtures. Depending on sample preparation, pyrrhotite can coat the coal and silicate aggregates. Such coating effects can affect the spectra as much as particle size. In contrast, silicates seem to play a minor role.

Infrared Spectroscopy of Large, Low‐Albedo Asteroids: Are Ceres and Themis Archetypes or Outliers?

“…The absorption band seen at 3.20 μm in the March 2002 spectrum of Bamberga is likely due to some other source. The broad absorption on 67P centered near 3.25 μm is not attributed to a single species, with Quirico et al (2016) finding aromatic C-H, COOH groups, OH groups, and NH 4 + as plausible carriers of the band and favoring the carboxylic group as most plausible. These may also be present and responsible for some of the features in Themis-type spectra in addition to or rather than ice frost.…”

Detection of exposed H2O ice on the nucleus of comet 67P/Churyumov-Gerasimenko

“…Unfortunately we are still not able to infer the real composition of the organic-rich dark terrain present on the comet surface. The broad absorption band centered at 3.2 µm, and the difficulty of its interpretation, have been largely discussed by Quirico et al (2016) on the basis of present knowledge of the composition of cometary grains and all the components available in laboratory data. The mixture presented in this paper was consequently modeled by means of two spectral end members: crystalline water ice, simulated by using optical constants measured at T=160 K between 1 and 4 µm (Warren et al, 1984, Mastrapa et al, 2008, Mastrapa et al, 2009, Clark et al, 2012) and a Dark Terrain unit corresponding to the average spectrum of the comet's surface after the application of photometric correction , as shown in Fig.…”

New Constraints on the Abundance and Composition of Organic Matter on Ceres

“…One exception is 24 Themis, whose NIR spectral features have been interpreted to represent water ice and aliphatic organic compounds (Campins et al, 2010;Rivkin & Emery, 2010). Reflectance spectra of comet 67P/Churyumov-Gerasimenko also exhibited organic absorptions when observed by the Rosetta spacecraft (Capaccioni et al, 2015;Quirico et al, 2016), and other comets (e.g., Halley) and interplanetary dust particles (IDPs) that may come from comets are enriched in refractory organic matter compared with primitive meteorites Kissel & Krueger, 1987;Thomas et al, 1993).…”