The Sources of HCN and CH3OH and the Rotational Temperature in Comet 103P/Hartley 2 from Time-Resolved Millimeter Spectroscopy

TL;DR

This study uses millimeter spectroscopy to analyze the sources of HCN and CH3OH in comet 103P/Hartley 2, revealing chemical heterogeneity and nucleus rotation effects on molecular emissions.

Contribution

It identifies the main outgassing sources and demonstrates large-scale chemical heterogeneity linked to nucleus features and thermal evolution.

Findings

Short-term variability in spectral lines due to nucleus rotation.

Identification of two jets and icy grains as main outgassing sources.

Significant chemical heterogeneity between jets.

Abstract

One of the least understood properties of comets is the compositional structure of their nuclei, which can either be homogeneous or heterogeneous. The nucleus structure can be conveniently studied at millimeter wavelengths, using velocity-resolved spectral time series of the emission lines, obtained simultaneously for multiple molecules as the body rotates. Using this technique, we investigated the sources of CH3OH and HCN in comet 103P/Hartley 2, the target of NASA's EPOXI mission, which had an exceptionally favorable apparition in late 2010. Our monitoring with the IRAM 30 m telescope shows short-term variability of the spectral lines caused by nucleus rotation. The varying production rates generate changes in brightness by a factor of 4 for HCN and by a factor of 2 for CH3OH, and they are remarkably well correlated in time. With the addition of the velocity information from the line profiles, we identify the main sources of outgassing: two jets, oppositely directed in a radial sense, and icy grains, injected into the coma primarily through one of the jets. The mixing ratio of CH3OH and HCN is dramatically different in the two jets, which evidently shows large-scale chemical heterogeneity of the nucleus. We propose a network of identities linking the two jets with morphological features reported elsewhere, and postulate that the chemical heterogeneity may result from thermal evolution. The model-dependent average production rates are 3.5x10**26 molec/s for CH3OH and 1.25x10**25 molec/s for HCN, and their ratio of 28 is rather high but not abnormal. The rotational temperature from CH3OH varied strongly, presumably due to nucleus rotation, with the average value being 47 K.