2013 Annual Science Report
NASA Goddard Space Flight Center Reporting | SEP 2012 – AUG 2013
Fundamental Properties Revealed by Parent Volatiles in Comets
We detected prebiotic molecules in the atmosphere of the distant comet C/2006 W3 (Christensen), which has spent its entire orbital period well outside the zone of active water sublimation. We also integrated our spectral-spatial measurements of H2O emission in comet 73P/ Schwassmann-Wachmann 3B with state-of-the-art 3D physical models of the inner cometary atmosphere, leading to new insights on a previously unidentified heating of coma gas from vaporizing icy grain mantles. And, we published a fluorescence model needed to interpret emission from deuterated methane released from cometary nuclei. These projects aim at improved understanding of cometary chemistry – a test bed for the contribution of comets to the delivery of exogenous prebiotic organics and water to early Earth, hypothesized as a precursor event to the emergence of the biosphere.
We opened a new chapter in understanding the chemical diversity among comets. The abundances of molecules released from ices stored in cometary nuclei have been studied predominantly within 2 astronomical units (AU) from the Sun, where sublimation by water – the most abundant volatile in the cometary nucleus – is fully activated. However, at infrared wavelengths, we are now starting to successfully probe the composition of comets that spend their entire orbital period well outside the zone of active water sublimation, by targeting hypervolatiles that can sublimate (or even drive nucleus activity) beyond 2 AU. Comet C/2006 W3 Christensen (perihelion at 3.13 AU) is one such comet, which we observed with high-resolution infrared spectroscopy in 2009-2010. During this reporting period, we extracted the production rates for three “hypervolatiles”: methane (CH4), ethane (C2H6), and carbon monoxide (CO). These molecules are plausible chemical precursors of organic compounds that are of keen interest for understanding the emergence of life. Prebiotic chemical models link C2H6 and CH4 as the precursors of ethylamine and methylamine respectively, while CO is linked to formaldehyde (H2CO) and to sugars. Each of these three molecules was detected via multiple emission lines (e.g., Figure 1).
Although CO was detected in comets Hale-Bopp at 4.09 AU and in 29P/Schwassmann-Wachmann-1 at 6.26 AU (see the report by L. Paganini), the infrared detections of CH4 and C2H6 at 3.25 AU from the Sun are unique. These species can be sensed ONLY through vibrational fluorescence at infrared wavelengths because pure rotational transitions are forbidden for symmetric species, precluding detections at mm or sub-mm wavelengths. The gas release rate (“production rate”) of carbon monoxide in Christensen significantly exceeded those measured in a number of Oort Cloud comets (C/2004 Q2 Machholz, C/2007 N4 Lulin, etc.) when much closer to the Sun. These drastic differences in CO production are not caused by distinct orbital histories or by the very different gas rotational temperatures – exceeding 70K in comets Q2/Machholz and N4/Lulin versus less than 20K in Christensen. Instead, they likely reflect different histories for ices frozen in the nuclei of comets Machholz, Lulin, and Christensen during the formative stages of our solar system. Such extreme differences in volatile abundances among comets encode the range of physical environments in the solar nebula favorable to synthesis of prebiotic organic matter.
During this reporting period, we successfully integrated our long-term studies of spatially resolved measurements of H2O rotational temperature and molecular abundance (see report of 2012) with state-of-the-art 3D physical models of the inner cometary atmosphere (“coma”). We published a pioneering study of this type for comet 73P/Schwassmann-Wachmann-3B (Fougere et al. 2013). The synergy of our spectral-spatial measurements with coma kinetic models constrains the gas temperature, density, and outflow, as well as the nature of volatile release – directly from the nucleus versus from a source of icy grains in the coma. It reveals a previously unrecognized source of heating in the inner coma introduced by water released from icy grain mantles. This insight provided a giant leap in our quantitative understanding of the near-nucleus environment – critical for obtaining the most accurate abundances of water and prebiotic molecules.
We published an advanced solar fluorescence emission model for the ν4 band of mono-deuterated methane (CH3D). The primary application of this model is to constrain the abundance ratio CH3D/CH4 in cometary ices, considered a key cosmogonic parameter, but presently unknown. We derived the upper limit for CH3D/CH4 in comet C/2007 N3 (Lulin) using our model and illustrated the ability of current infrared spectrometers to stringently test various astrochemical model predictions for CH3D/CH4 in the early solar system.
PROJECT MEMBERS:Boncho Bonev
RELATED OBJECTIVES:Objective 2.2
Outer Solar System exploration
Sources of prebiotic materials and catalysts
Effects of extraterrestrial events upon the biosphere