Organic Matter in Cometary Environments

Abstract

Comets contain primitive material leftover from the formation of the Solar System, making studies of their composition important for understanding the formation of volatile material in the early Solar System. This includes organic molecules, which, for the purpose of this review, we define as compounds with C-H and/or C-C bonds. In this review, we discuss the history and recent breakthroughs of the study of organic matter in comets, from simple organic molecules and photodissociation fragments to large macromolecular structures. We summarize results both from Earth-based studies as well as spacecraft missions to comets, highlighted by the Rosetta mission, which orbited comet 67P/Churyumov-Gerasimenko for two years, providing unprecedented insights into the nature of comets. We conclude with future prospects for the study of organic matter in comets.

Keywords: astrobiology; comet; organics; volatiles.

Figure 1

Depiction of a protoplanetary disk and the relevant physical processes. As remote observations of other protoplanetary disks often cannot penetrate deep into the disk, comets serve as our best probes for the physics and chemistry occurring in the cold midplane. Image Credit: Geronimo Villanueva, priv. communication.

Figure 2

Optical spectrum of comet 122P/De Vico showing typical emissions observed in cometary spectra. Many of these species are likely released via the photodestruction of simple organic matter or carbon-rich dust grains. Figure from [29].

Figure 3

IR spectra of comet 45P/Honda-Mrkos-Pajdušáková obtained with iSHELL on the NASA Infrared Telescope Facility (IRTF) showing spectral regions containing typical emissions observed in cometary spectra. These include CH${}_4$ (a and b), C${}_2$H${}_6$ (b), CH${}_3$OH (b and c), H${}_2$O (d), H${}_2$CO (f and g), HCN, C${}_2$H${}_2$, and NH${}_3$ (e). In particular, high spectral resolution IR spectroscopy is the only method to remotely observe symmetric hydrocarbons like C${}_2$H${}_6$ and CH${}_4$ in comets due to their lack of pure rotational transitions. The observed spectra are in black, with fluorescence models overplotted in red and fits for individual species offset below the observed spectrum. The “*” after OH in the model labels indicates these emissions are prompt emission, while all other emissions are fluorescence. Figure from [52].

Figure 4

ALMA maps of the spatial distribution of HCN (a,b), HNC (c,d), and H${}_2$CO (e,f) in two comets: C/2012 F6 (Lemmon) (a,c,e) and C/2012 S1 (ISON) (b,d,f). The contours represent the line emission of the respective gas species, while the color scale shows the continuum emission from the dust coma. Figure from [48].

Figure 5

A graphic representation of all gases detected by the Rosetta mission, depicted here as the “Cometary Zoo”. Many of these species are organic molecules and were detected for the first time in a cometary coma by Rosetta. Image Credit: ESA.

Figure 6

Spectra of several regions on the surface of 67P showing strong absorption at 3.0–3.5 $\mathsf{\mu }$m, which is attributable in part to the presence of organic matter. Figure from [63].

Figure 7

Spatial profiles for various species observed in comet C/2007 W1 (Boattini) at IR wavelengths. Panel (a) shows H${}_2$O, CH${}_3$OH, OH (a tracer for H${}_2$O), and the dust continuum, while panel (b) shows C${}_2$H${}_6$, HCN, CH${}_4$, and CO. The “*” after the OH label indicates the emissions used to measure the spatial distribution are prompt emission, while all other emissions are fluorescence. Panel (c) shows the average profile of the species in each panel, and the grey shading highlights the difference between the two profiles. The species in panel (a) are spatially extended in the antisunward direction (positive x values), while the species in panel (b) are symmetric. This indicates different modes of release for the different volatiles. Figure from [100].

Figure 8

IR spectra showing CH${}_4$ emissions in comets C/1996 B2 (Hyakutake) (top six panels) and C/1995 O1 (Hale–Bopp) (bottom two panels). The measured spectra (complete with continuum) are shown above, whereas the continuum removed spectra are shown below in each panel, labeled as residual. For Hale-Bopp, the continuum removed spectra are multiplied by a factor of two, indicated by “residual*2”. The strong absorptions are due to telluric CH${}_4$, illustrating the need for cometary emissions to be Doppler-shifted away from the center of these features by sufficient geocentric velocity of the comet. Figure from [128].

Figure 9

Plot showing the abundances compared to H${}_2$O of various hydrocarbons detected by ROSINA in the coma of 67P at a heliocentric distance of 1.52 AU in May 2015. While butane and pentane were not detected in May 2015, they were detected in May 2016, when all hydrocarbons abundances relative to water were significantly higher than in May 2015. To account for this, the values plotted are their May 2016 measurements scaled by their mixing ratios compared to methane. Blue points represent species previously detected in comets, while red points denote species not previously detected, demonstrating the dramatic increase in known hydrocarbons provided by Rosetta. Figure created using data from [127] and based on Figure 8 from [65].

Figure 10

High spectral resolution optical spectrum showing individual CN lines, both of the main isotopologue as well as ${}^{13}$CN and C${}^{15}$N. The data are shown in black, with model fits to the observed emission for the main isotopologue (green), ${}^{13}$CN (blue), and C${}^{15}$N (red) overplotted. The R8, R6, and R4 labels indicate the rotational designation for the observed lines. This demonstrates the neccessity and power of high spectral resolution for isotopic studies of CN at optical wavelengths. Figure from [134].

Figure 11

Abundances of C${}_2$H${}_6$ plotted versus HCN abundances in the sample of comets measured at IR wavelengths. The abundances show a fairly strong positive correlation, suggestive of a common release mechanism. Figure from [8].