Michal Drahus, David Jewitt, Aurelie Guilbert-Lepoutre, Waclaw Waniak, Albrecht Sievers
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 at IRAM 30-m shows short-term
variability of the spectral lines caused by nucleus rotation. The varying
production rates generate changes in brightness by a factor of 5 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
2.10x10**26 molec/s for CH3OH and 1.25x10**25 molec/s for HCN and their ratio
is typical of comets. The rotational temperature from CH3OH varied strongly,
presumably due to nucleus rotation, with the average value 47 K.
View original:
http://arxiv.org/abs/1202.3194
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