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Asymmetry of Gaseous CO2 and H2O in the Inner Coma of Comet Tempel 1

Introduction: Deep Impact (DI) encountered comet 9P/Tempel 1 on July 4, 2005, continuously monitoring the coma for 15 days prior to impact with the HRI-IR spectrometer. Infrared spectra of the coma, 1-5μm, were acquired every 4 hours during the approach phase and more frequently leading up to impact. Gaseous CO2 and H2O emission bands at 4.26μm and 2.66μm, respectively, are detected in these spectra [1]. The distribution of the gas in the inner coma was mapped and an asymmetry was found in both the CO2 and H2O. The CO2 is more prevalent in the southern coma, while the H2O is more prevalent in the sunward direction. The ratio of CO2 compared to H2O is over a factor of 2 larger in the southern, anti-sunward direction than in any other region. Further study of this asymmetry will give insight to the chemistry of the near-nucleus ambient coma as well as the anisotropic outgassing of the nucleus. Observations: The pre-impact coma observations include data with spatial resolution of 120 km/pixel improving to better than 160 m/pixel minutes before impact. The infrared data were typically acquired as spatial scans across the comet nucleus, with each spatial pixel having a unique spectrum. The data highlighted in this abstract are part of a 64 frame north-tosouth scan of the comet 10 minutes prior to impact. The data include signal from the northern and southern coma. (The anti-saturation filter in the spectrometer, required to limit the thermal contribution of the nucleus, introduces artifacts during the data analysis and is to be studied in more detail in the future.) A spatial image of the data can be made at any of the observed wavelengths. Figure 1 shows the data in the 2μm band, oriented such that ecliptic north is up and the sun is to the right.