Icarus 188 (2007) 224–232 www.elsevier.com/locate/icarus The organic composition of C/2001 A2 (LINEAR) II. Search for heterogeneity within a comet nucleus Erika L. Gibb a,∗ , Michael A. DiSanti b , Karen Magee-Sauer c , Neil Dello Russo d , Boncho P. Bonev e,f , Michael J. Mumma f a Department of Physics & Astronomy, University of Missouri–St. Louis, St. Louis, MO 63121, USA b Planetary Systems Laboratory, Solar System Exploration Division, NASA Goddard Space Flight Center, Code 693, Greenbelt, MD 20771, USA c Department of Chemistry and Physics, Rowan University, Glassboro, NJ 08028-1701, USA d Space Department, Planetary Exploration Group, Applied Physics Laboratory/Johns Hopkins University, 11100 Johns Hopkins Road, Laurel, MD 20723-6099, USA e Department of Physics, The Catholic University of America, Washington, DC 20064, USA f Solar System Exploration Division, NASA Goddard Space Flight Center, Code 690, Greenbelt, MD 20771, USA Received 17 January 2006; revised 27 September 2006 Available online 26 December 2006 Abstract The nucleus of Comet C/2001 A2 (LINEAR) split several times during its recent apparition, presenting an unusual opportunity to search for chemical differences in freshly exposed material. We conducted this search using NIRSPEC at the W.M. Keck Observatory on four dates in 2001: 9.5 and 10.5 July and 4.4 and 10.5 August. We detected the R0 and R1 lines of the ν 3 vibrational band of CH 4 near 3.3 μm on all dates. The R2 line was detected on 4.4 and 10.5 August. When we compare production rates of CH 4 to H 2 O, we find evidence of a significant enhancement in August relative to that found in July. H 2 CO was securely detected via its ν 1 and ν 5 bands on 9.5 July. On 10.5 July, H 2 CO emission was much weaker, and its mixing ratio had dropped by a factor of about four. The mixing ratios for other detected volatile species did not change significantly over the course of the observations. We discuss the implications of this evidence for chemical heterogeneity in the nucleus of Comet C/2001 A2.  2006 Elsevier Inc. All rights reserved. Keywords: Comets, composition; Infrared observations; Spectroscopy 1. Introduction Comets are arguably considered the most pristine bodies formed in the outer Solar System (beyond ∼5 AU), and there- fore studies of the chemical composition of comet nuclei have cosmogonic implications. Much work has been done in recent years to characterize the composition of nuclear ices in comets. One important question in cometary science is whether comet nuclei have heterogeneous compositions. There is increasing evidence of various processes that may have altered the primor- dial composition of comets since the time of their formation (cf. Stern, 2003; Weissman et al., 2004). The majority of these processes are believed primarily to affect a thin layer at the sur- * Corresponding author. Fax: +1 (314) 516 6152. E-mail address: gibbe@umsl.edu (E.L. Gibb). face of the nucleus. For example, the outer surface (to a depth of perhaps a few meters) may have been modified by cosmic rays during a comet’s long storage in the Oort cloud or by ther- mal processing from repeated passes through the inner Solar System [see Stern (2003) for a discussion of evolution mecha- nisms in the Oort cloud]. As deeper layers are exposed through evaporation or disruption of the nucleus, the composition may change. Also, repeated passes through the inner Solar System or the passing of nearby hot (O or B) stars or supernovae can cause thermal modification of the top several meters of mater- ial, as can mechanical sputtering by interstellar grain collisions. Another possible source of chemical heterogeneity may be bulk radial transport mechanisms in the early Solar System by, for example, gas drag, turbulence, or gravitational inter- actions with migrating planets/protoplanets (Tsiganis et al., 2005; Goldreich et al., 2004; Levison and Morbidelli, 2003; 0019-1035/$ – see front matter  2006 Elsevier Inc. All rights reserved. doi:10.1016/j.icarus.2006.11.009