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2013 Annual Science Report

NASA Goddard Space Flight Center Reporting  |  SEP 2012 – AUG 2013

Volatile Composition of Comets: Emphasis on Oxidized Carbon

Project Summary

DiSanti’s research emphasizes the chemistry of volatile oxidized carbon in comets, in particular the efficiency of converting CO to H2CO and CH3OH through reduction reactions on the surfaces of icy grains prior to their incorporation into the cometary nucleus. Additionally, oxidation reactions on grains can play a significant role, particularly for CO-enriched, C2H2-depleted comets such as C/2009 P1 (Garradd; see item 2 under Section 3 below). Such processes produce precursor molecules that (if delivered to Earth through impact of comet nuclei) could have enabled the emergence of life, and so are highly relevant to Astrobiology.

4 Institutions
3 Teams
1 Publication
0 Field Sites
Field Sites

Project Progress

M. DiSanti led two studies that were completed in this reporting period:
(1) A new empirical fluorescence model was developed for the ν2 vibrational band of CH3OH in comets. A paper describing the model and its application to short period comet 21P/Giacobini-Zinner was published in the Astrophysical Journal (DiSanti et al. 2013). A comparison of model and measurement is shown in Figure 1.

A. Comet 8P/Tuttle. Top traces: Continuum-subtracted residuals along with our empirical fluorescence model convolved to spectral resolving power λ/Δλ = 24,000. Subtracting modeled emissions for C2H6, OH*, and CH4 isolates emissions dominated by ν2 lines (labeled “CH3OH residuals”, lower trace), with our model superimposed. We identified 18 spectral intervals for sampling ν2 emissions. Regions marked ‘x’ contain excess intensity, perhaps due to CH3OH ν9 emissions (omitted from our analysis). The fully resolved model is also shown (“stick spectrum”). B. Application of our empirical model to CH3OH residuals in comet 21 P/Giacobini-Zinner. After DiSanti et al. (2013).

(2) We measured the chemical composition of comet C/2009 P1 (Garradd) based on observations with the high-resolution infrared spectrograph (NIRSPEC) on the Keck 2 telescope (DiSanti et al. 2014, Icarus). This paper reported pre- and post-perihelion abundances for several molecules, and (pre-perihelion) quantified the fractions of H2O that were released directly from the nucleus and from a distributed source in the coma (Figure 2). The paper also suggested a means of testing the role of “redox” reactions in forming volatiles on pre-cometary icy grains. Several volatiles detected in comets (e.g., H2CO) are species that play important roles in Astrobiology.

Graphical illustration showing our method for estimating the fraction of H2O released in the coma of C/2009 P1 (Garradd) on UT 2011 October 13. In each of two instrument settings (KL2 in panel ‘a’, KL1 in panel ‘b’, mean of KL2 and KL1 in panel ‘c’), the average spatial profiles for C2H6 and HCN (red trace) were scaled to the intensity of the H2O profile (blue trace) in the projected anti-sunward hemisphere (over the range 0 to -4000 km, as indicated by the horizontal lines in each panel). Because C2H6 and HCN profiles were consistent with release solely from the nucleus, the difference profile for water (H2O profile minus scaled C2H6 or HCN profile) provided a measure of the amount of H2O released in the coma. This difference is indicated by the dashed green trace in each panel, and was estimated to be 25 – 30 % of the total H2O released. After DiSanti et al. (2014).

Comets observed in this reporting period, using high-resolution infrared echelle spectrographs:
(1) C/2011 L4 PanSTARRS (IRTF/CSHELL): Two observing runs were awarded, one pre-perihelion (Feb. 26 – 28) and one post-perihelion (Mar. 31, Apr. 1); the latter dates were at the end of 8 consecutive days split between 3 proposals (PI’s Bonev, Villanueva, DiSanti). In February, the comet was found to be extremely dusty. Normally C2H6 is among the strongest organic signatures in comets but it was not detected in L4 PanSTARRS, and we achieved only a tentative detection of H2O (Figure 3). We subsequently switched targets to C/2012 F6 (Lemmon) for the later runs (see item 2 below).

Spectra targeting two Q-branches of the ν7 band of C2H6 in several comets, but not detected in Comet L4 PanSTARRS due to its extremely high dust-to-gas ratio.
Tentative H2O detection in Comet L4 PanSTARRS with CSHELL, resulting in a formal production rate of 1.2x1029 molecules s-1 and a rotational temperature Trot = 105+20/-13 K (1σ uncertainties); the corresponding convolved H2O fluorescence model (at 105 K; red trace) is compared with the residual emission spectrum in panel ‘g’.

(2) C/2012 F6 Lemmon (IRTF/CSHELL): M. DiSanti led CSHELL observations on 2013 March 31 & April 1/2, and the ensuing analysis of the CSHELL data set (Figure 4). The CSHELL results were included in a refereed publication (Paganini et al. 2014). This paper presents the first study combining cometary spectra obtained with all three existing ground-based high-resolution infrared spectrographs (VLT/CRIRES, IRTF/CSHELL, and Keck/NIRSPEC), and was a collaborative effort between the Goddard and Univ. Hawai’i NAI Teams.

Daytime measurement of H2O and CO emissions in C/2012 F6 (Lemmon) with CSHELL, at which time the comet was almost directly south of the Sun. Top traces: Spectral extract (solid black trace) representing the signal within a 2x3 arc-second aperture centered on the peak emission intensity. Subtracting the modeled cometary continuum (dashed red trace) leaves the net emission spectrum (second black trace), shown with (superimposed) molecular fluorescence models synthesized at the measured rotational temperature (100 K). The bottom trace represents the residual spectrum (after subtracting the fluorescence model) and the ±1σ stochastic noise envelope. The abundance ratio CO/H2O in Comet F6 Lemmon was 4% (Paganini et al. 2014), close to its median abundance among approximately two-dozen comets measured to date at infrared wavelengths. See also the report by L. Paganini.

(3) Comet 29P/Schwassmann-Wachmann 1 (Keck/NIRSPEC): One full night of observing time was allocated to the Univ. Hawai’i NAI Team (PI, K. Meech). J. Keane (UH) operated NIRSPEC, and members of the GCA (including M. DiSanti) participated in acquiring spectra of 29P and F6 Lemmon. The F6 results are included in Paganini et al. 2014. The new observations of 29P revealed prominent emissions from CO, and preliminary results were presented at the 2013 Division for Planetary Sciences meeting (Keane et al. 2013). L. Paganini describes the initial detections of multiple lines of CO in 29P and interpretations, in his report (see also Paganini et al. 2013). Work is in progress to characterize opacity effects in the recently observed CO lines. This effort represents an additional example of collaborative work among teams within the NAI.

(4) Comet C/2012 S1 (ISON). The combination of being dynamically new and also a sun-grazing comet led NASA HQ to call for a worldwide observing campaign. M. DiSanti was the sole representative from NASA-Goddard Space Flight Center to serve on the Comet ISON Observing Campaign team, devoted to facilitating communication among observers with the goal of maximizing the overall science returned. M. DiSanti and others within the GCA were instrumental in various aspects of preparation for the campaign, for example writing and submitting observing proposals, and detailed planning of observations. Members of the GCA were awarded time with NIRSPEC/Keck (M. Mumma, PI) and CSHELL/IRTF (B. Bonev and M. DiSanti, PIs), and UH colleagues also won time (J. Keane, PI). Observing occurred in the following reporting period and results will be included in our next report.