Notice: This is an archived and unmaintained page. For current information, please browse

2014 Annual Science Report

NASA Goddard Space Flight Center Reporting  |  SEP 2013 – DEC 2014

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 1 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
3 Publications
1 Field Site
Field Sites

Project Progress

Two studies were completed in this reporting period:
(1) C/2009 P1 (Garradd): We measured the chemical composition of comet based on observations with the high-resolution infrared spectrograph (NIRSPEC) on the Keck 2 telescope (DiSanti et al. 2014, Icarus 228:167-180). 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 1). The paper also suggested a means of testing the role of “redox” reactions in forming volatiles on precometary icy grains. Several volatiles detected in comets (e.g., H2CO) are species that play important roles in Astrobiology.

Figure 1. 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 KL2, 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 re-leased. After DiSanti et al. (2014).

(2) C/2012 S1 (ISON): We measured the evolution with heliocentric distance (Rh) of H2O production, and of abundance ratios relative to H2O for eight molecules generally considered to be primary volatiles. We used NIRSPEC on four dates (a mini-campaign led by Mumma, with participation by Univ. Hawai’i, CalTech, and GSFC) and subsequently CSHELL on six dates (GSFC, led by DiSanti), as part of the world-wide observing campaign. The production rates of H2O obeyed a relatively steep heliocentric power law (Rh 3.2) between 1.2 and 0.35 AU from the Sun (Figure 2). The abundances of CO, C2H6, CH4 and CH3OH remained relatively constant with Rh, while those of HCN, H2CO, and NH3 increased dramatically, and that of C2H2 showed a modest increase within 0.5 AU from the Sun (Figure 3).

Figure 2. Evolution of water production in C/2012 S1 (ISON) spanning 2013 October 22 (Rh = 1.21 AU) through November 22 (Rh = 0.35 AU). The NIRSPEC results were obtained simulta-neously on each date; the points on November 7 are separated horizontally only to visualize them more clearly. Of the 32 measurements, seven (shown darkened) were associated with periods of outburst; a heliocentric fit excluding these points (dot-dashed line) results in a power law Q(H2O) α Rh(-3.10±0.09), steeper than the canonical insolation-limited Rh-2 slope (DiSanti et al. 2015, in preparation).

Figure 3. Evolution of molecular abundance ratios relative to H2O in C/2012 S1 (ISON), as dis-cussed in Section 3, Item 2. Points shown in red were measured during periods of enhanced gas production (i.e., during “outbursts”); no discernable dependence of composition on activity level is seen (DiSanti et al. 2015, in preparation).

(3) C/2012 S1 (ISON): B. Bonev led an initial paper measuring the excitation of H2O vapor in the coma at Rh = 0.53 and 0.35 AU (Bonev et al. 2014, ApJ (Lett.) 796:L6). This demonstrated that high rotational temperatures were maintained away from the nucleus, and represents an important driver of complex thermal models of cometary atmospheres.

New observations acquired in this reporting period, using high-resolution infrared echelle spectrographs:
(4) C/2012 K1 PanSTARRS (Keck/NIRSPEC): Two half nights were awarded in May 2014 to observe Jupiter Family Comet 209P/LINEAR during its very close pass to the Earth, however the comet was much fainter than expected. Instead, spectra of C/2012 K1 were obtained. Multiple molecules were detected, and CO was constrained to about 2% or less. This was particularly significant for interpreting contemporaneous observations with the Spitzer Space Observatory.

(5) C/2014 E2 Jacques (Keck/NIRSPEC): M. DiSanti led observations on 2014 September 5 & 6. The comet was available for only 1 hour per night, but was bright enough to test the full suite of molecules except CO, which requires separate observations at longer wave-lengths. J. Keane led observations on September 16, as part of our collaborative effort between the Goddard and Univ. Hawai’i CAN-5 Teams. These emphasized simultaneous measurement of CO and H2O.

(6) C/2013 V5 Oukaimeden (Keck/NIRSPEC & IRTF/CSHELL): M. DiSanti led observations on 2014 September 5 & 6 (NIRSPEC), and September 10, 12, & 13 (CSHELL). For the NIRSPEC observations the comet was available for only the last 1.5 hours of the night, nonetheless the full suite of molecules (including CO) was sampled with NIR-SPEC. The CSHELL observations emphasized simultaneous measurement of CO and H2O.