Investigation on the origins of comets as revealed through IR high resolution spectroscopy

We present an updated statistical analysis on molecular abundances retrieved from infrared spectra of 20 comets, observed with NIRSPEC-KECK since 1999. Using these results, we investigate the chemical diversity among comets, and we try to correlate them to the chemical and physical processes present during the formation of our planetary system.

Introduction: Comets are considered the remnants of the solar system formation. According to recent dynamical models [1], objects that formed between 5 and 17 AU likely scattered into the Oort cloud (OC), the primary source of long period and dynamically-new comets, while those that formed in the outer proto-planetary disk (beyond 17 AU) entered both the Oort cloud and Kuiper belt (KB) reservoirs. Investigating the chemical diversity in comets may unveil the physical and chemical conditions present during the formation and early evolution of our planetary system (e.g. hydrogenation on dust grains in cold environments or photo-dissociation processes due to UV/X-rays/Cosmic-rays radiation), as well as the processes that may have changed the nucleus composition after its formation (e.g. cosmic rays impacting the outer layer of the nucleus or successive surface warming on repeated passages through the inner solar system).

Since 1985, more than 60 comets have been investigated using ground based high resolution infrared spectrometers, and many efforts have been made to create a classification of these bodies [2,3]. However, some infrared results published before 2011 may contain systematic inaccuracies related to then-incomplete molecular models used to interpret the fluorescence excitation in comets, to a non-properly described atmospheric transmittance models and/or to the use of immature reduction algorithms. These inaccuracies impact mostly comets that were observed before 2011, and they need to be removed [4]. Here, we present a statistical analysis on our revised data for twenty comets and we investigate their possible connections to processes in the proto-planetary disks and/or the natal cloud.

Results and discussion: We have examined the distribution of molecular species among the comet population making use of boxplots (Figure 1). The amount of dispersion that we observe for individual species is expected and could be partially related to the temperature gradient in the proto-planetary disk and/or to the intensity of the radiation field, if present. For instance, the high dispersion range for CO may be related to efficient formation of CH₃OH through hydrogenation processes on dust grains at low temperatures (T < 20 K), or formation of CO₂ at higher temperatures (T > 20 K) [5]. The differential loss of highly volatile species (e.g. CO and CH₄) after the comet formation may also be relevant.

In Figure 2, we show two selected scatterplots for some of the analyzed molecules: these trends should reflect, at least in part, the conditions that were present during the formation of comets. We notice for example that CO shows a high and positive correlation with CH₄, and a much lower but still positive correlation with CH₃OH (an anti-correlation is expected if hydrogenation converted significant CO to CH₃OH). Aspects of the interpretation of the scatterplots will be discussed.

We compared our results with recent disk models [6,7,8], where the relative amounts of CH₃OH, CO, CH₄ and C₂H₆, are expected to depend on what chemical processes and how much radiation field (UV/X/CR) were present at different heliocentric distances from the proto-sun. Considering the combination of these four species (Figure 3), we notice that most of the sampled comets fall in the third and first quadrants and follow an (almost) linear relationship. Depending on the position of the comet with respect to this line, we can define a factor K that can be associated with an increasing processing of the cometary material, and try to reconstruct the chemical and physical history of each analyzed comet.

Conclusions: We report an updated statistical analysis on molecular abundances observed in 20 comets and their possible connections with protoplanetary disk models. The results reveal a diversity in comet composition, and different correlations among the observed chemical species, ultimately giving important hints about the physical and chemical condition present during the formation and evolution of our Solar System.

 

Figure 1. Boxplot statistic for the chemical species that we observed in 20 comets; for each box we report the interquartile range (IQR), the median (Med), the skewness (Skw), and the whiskers (error bars of the boxplots). Comets characterized by outlier values are highlighted.

 

Figure 2. Selected scatterplots showing the measured mixing ratios (% with respect to water). For each graphic, the correlation factor is indicated in the upper left corner. Jupiter family and Oort Clouds comets are shown with squares and circles, respectively.

 

   

Figure 3: Relationship between relative abundances of selected species and possible connections with the formation and evolution of comets in protoplanetary disks. Jupiter family and Oort Clouds comets are shown with squares and circles, respectively. Colors indicate a possible degree of evolution of the cometary material (violet =  low degree to red = high degree) due to different processes at different life stages of the comet.

 

This work is supported by the NASA Emerging Worlds Program EW15-57 and the NASA Astrobiology Program 13-13NAI7-0032

References: [1] Morbidelli, A. et al. 2007, Astron. J., 134, 1790; [2] Mumma & Charnley, Ann. Rev A&A, 49, 471 2011; [3] Dello Russo, N. et al. 2016, Icarus, 278, 301; [4] Lippi et al 2020, Astron. J., 159, 157; [5] Tielens, A. G. G. M., Rev. Of Modern Physics, 85, 1021, 2013; [6] Bosman et al. 2018, 618, A182; [7] Walsh 2010 ApJ, 722, 1607; [8] Eistrup et al. 2018, A&A, 613, A14.