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Project Progress

It has been suggested that icy bodies in the two main cometary reservoirs (Kuiper Belt (KB), Oort Cloud (OC)) formed in different regions of the proto-planetary disk. Whether the two families of comets have the same chemical composition has important implications for the amount of radial mixing in the proto-planetary disk. Fundamental clues to natal conditions can be provided by meas-uring native molecule abundances within an individual comet, and by building a comet classification or taxonomy through compositional comparisons among comets from different populations. Come-tary composition is usually derived from observations of volatile species (known as parent mole-cules), e.g., HCN, CH₃OH, CO, and CS₂, in the cometary comae that sublimate directly from the nu-clei. Likewise, thermal models can be used to predict the comet brightness as a function of helio-centric distance and infer the presence of major volatiles (H₂O, CO and CO₂) since they sublimate at very different temperatures.

5.1.3.1 – Radio Observations

Compared to other direct measurement techniques, radio spectroscopy offers two major ad-vantages, namely the pure rotational transition mechanism and high spectral resolution that cannot be achieved in other wavelength regimes. Using these techniques, we observed the comet C/2009 P1 Garradd in four observing runs between July 2011 and January 2012, covering the heliocentric distance ranging from 2.49 AU to 1.56 AU. Three parent volatiles, namely HCN, CH₃OH, and CO, as well as CS – originating from the short-lived CS₂ – were observed in comet Garradd before and after its perihelion. We monitored HCN J(4-3) line frequently to search for sporadic outbursts and rota-tional modulation. Although no short-term variability of spectral lines was observed on an hourly basis, the strength and the line shape of HCN varied with heliocentric and geocentric distances. Ro-tational temperature was measured from simultaneous observations of several CH₃OH lines, follow-ing the standard rotational diagram technique. Our observations show that CO in comet C/2009 is somewhat enriched, consistent with the observations in the infrared. The relative abundances of HCN, CH₃OH, and CS of comet C/2009 are typical for other Oort Cloud comets.

The sub-millimeter observation of comet C/2011 L4 was first made in April 2012, when the comet was 5 AU from the Sun, using the CSO telescope. We searched for HCN and CO in the sub-millimeter spectra of the comet. The CSO observation showed that the coma of C/2011 L4 mostly consists of cometary dust. No gas emission was detected. The second observation of C/2011 L4 was made in August 2012, when the comet was 3.6 AU from the Sun, using the JCMT telescope. No HCN or CO lines were observed in August. The infrared observations of C/2011 L4 using the Gemini-North tele-scope detected strong water ice absorption features. The sub-millimeter and infrared observations indicate that the comet’s ice sublimation rate remains weak when the comet is beyond 3 AU from the Sun. These observations are consistent with observations of other comets at similar heliocentric distances.

5.1.3.1 – Near Infrared Spectra

In a collaboration lead by the Goddard NAI lead team, we also obtained high-resolution near IR spectra of comet C/2009 P1 Garradd using the Keck II telescope both pre- and post-perihelion to assess the relative abundances of primary parent volatiles and compare them to other Oort Cloud comets. Our group found that CO was enriched, some parent organic volatiles were depleted, and others were at levels typical of other Oort cloud comets. These abundances may indicate surface oxidation on pre-cometary grains in addition to reduction reactions. The overall gas production was lower post-perihelion.

5.1.3.1 – Optical Imaging and Thermal Sublimation Models

Comet missions are changing the paradigm for understanding comet activity, composition, and the formation of planetesimals in the protoplanetary disk. Each encounter has shown the diversity of surface morphologies and new insights into comet chemistry, formation scenarios, activity mecha-nisms and geology. Prior to the comet 103P/Hartley 2 EPOXI encounter, the prevailing view was that H₂O-ice sublimation controlled most comet activity. Differences in the amounts of minor par-ent/daughter photo-dissociation species are attributed to differences in formation location, tem-perature and disk chemistry. However, reconstructing the protoplanetary disk dynamics and chem-istry consistent with observations hasn’t yet been achieved.

The EPOXI mission flew past the nucleus of comet 103P/Hartley 2 on 11/4/2010. This small nu-cleus was known to be exceptionally active prior to the encounter, by virtue of a very large water production rate relative to its surface area. EPOXI provided stunning images of a small nucleus with strong chemical heterogeneity and a swarm of large icy chunks driven from the nucleus by CO₂ jets. The EPOXI ground-based campaign provided a long-term baseline of observations of the pre-perihelion brightening of the comet which also showed that comet Hartley 2’s perihelion activity was dominated by sub-surface CO₂ outgassing.

Because the Earth’s atmosphere is opaque at the wavelengths for CO₂ emission, there is only a little information about CO₂ abundance in comets (primarily from space missions), yet CO, CO₂ and H₂O are likely key tracers of the chemistry in the protoplanetary disk. EPOXI has shown the crucial role that CO₂ plays in comet activity. We have now been applying thermal sublimation models to comets with well-observed light curves and for which we can get information about CO₂ production from the Akari and WISE missions. We now have the tools to explore the three most abundant vol-atile species (H₂O, CO and CO₂) in a large number of comets. This will be a necessary key compo-nent to understanding the connection between today’s observables, future resolved protoplanetary disk chemical observations, and chemical / dynamical models of disk formation.