Evidence for two modes of water release in Comet 103P/Hartley 2: Distributions of column density, rotational temperature, and ortho–para ratio

Abstract

This paper presents long-slit spectra of H2O emission from the inner coma of Comet 103P/Hartley 2, acquired with NIRSPEC/Keck 2 during the comet’s close approach to Earth in 2010. On UT 19.6 October 2010 the slit was oriented nearly orthogonal to the projected (in the plane of the sky) Sun–comet line, and the H2O rotational temperature and column density showed similar spatial distributions as a function of projected distance from the nucleus. On UT 22.5 October, the slit was oriented along the Sun–comet line, and the rotational temperatures revealed pronounced asymmetry while the column densities were nearly symmetric about the nucleus. We suggest this dichotomy reflects two qualitatively different mechanisms of volatile release, which introduce distinct rotational distributions in the sublimated material. Future modeling can test this hypothesis.We also report new retrievals of water nuclear spin species (ortho, para) in this comet, and we present the ortho-to-para ratio (OPR) for various projected nucleocentric distances. Our most precise individual measurement is OPR=2.59±0.13, corresponding to a nuclear spin temperature (Tspin) of 31±3K. A weighted mean of five independent measurements provides OPR=2.79±0.13 (Tspin=37-4+8K). Hartley 2 is the first comet for which the OPR has been measured in multiple apparitions. Our values (in 2010) are in good agreement with those obtained two apparitions earlier by the Infrared Space Observatory. Since the comet lost a substantial amount of material between 1998 and 2010, we see no evidence for variation of the OPR with depth in the nucleus. Further discussion of the advantages, assumptions, and biases introduced by various approaches when quantifying nuclear spin species (observing techniques, models and model parameters, sources of uncertainty) would likely aid in interpreting the OPRs measured in cometary volatiles.