... Looking for molecules
When the 360 kg impactor of the Deep Impact spacecraft will be hit by Comet 9P/Tempel 1, it will most probably create an artificial crater, leading to the release of gas and dust from beneath the surface. This will allow astronomers to study in detail, and for the first time, volatiles ice from the pristine interior of a comet. Observations of the fresh evaporating ices opens the opportunity to study material unchanged probably since the formation of the solar system.
The spectroscopic observations of the gas component of the cometary coma, will allow an international team of astronomers  using ESO telescopes to study in great detail its chemical composition. Special attention will be paid to compare the light emitted (spectra) before impact, when the material evaporates solely from the surface, and after the impact event, when fresh material from the interior will contribute to the coma.
A high quality spectrum covering the full optical range has recently been obtained with the UV-Visual Echelle Spectrograph (UVES) at the Very Large Telescope (VLT) KUEYEN telescope at Paranal and will serve as a reference to which post impact data will be compared.
At the time of these observations in early June, Comet 9P/Tempel 1 was about 230 million kilometres from the Sun (1.53 AU) and 115 million km from the Earth (0.77 AU), moving inwards to the Sun. The comet had a magnitude of about 10.5, i.e. 15,000 times fainter than the famous Hale-Bopp comet at maximum. In fact, it is one of the faintest comets for which such detailed spectra have been obtained.
This high quality spectrum contains information about the composition of the material coming from the crust of the nucleus and will serve as a reference to which post impact data will be compared. The new crater created by the Deep Impact mission may become the main active region of the comet. During an observing campaign of 10 nights around the date of the impact the new spectra collected by the UVES observers will then reveal the composition of the new fresh material which has never been exposed to the sunlight before. This will allow the instrument experts  to search for new species usually trapped inside the nucleus, or species produced under the very hot conditions of the impact.
The relative abundance of the various sorts ("isotopes") of Carbon and Nitrogen will be measured , providing new clues on the solar system formation and comets evolution. In addition, the study of the molecule NH2 will permit to determine the temperature of the solar nebula from which comets, and all the planets, formed.
A few days before the UVES observations, two lower resolution spectra were also obtained with EMMI, the spectrograph of the NTT at the La Silla Observatory (Chile) . Similiar long-slit spectra will be obtained during the six nights following the date of the impact using the instrument FORS2 mounted on the VLT ANTU telescope at Paranal Observatory.
Those spectra will reveal the principal gaseous components of the coma emitting at visible wavelengths (the light molecules CN, C2, C3, NH2, and CH) and allow the astronomers to measure the total amount of each species released by the impact, study their evolution with time as well as to study their spatial distribution. This will provide unique information about the parent molecules (HCN, C2H2, C2H6, NH3 for example), directly released by the nucleus ices, from which the observed small molecules are coming.
The spectra of comets consist of a reflected solar spectrum from the dusty clouds surrounding the comet nucleus, and it contains also sharp emission lines emitted by the gas component of the comet's coma. The molecules in the coma absorb some light from the Sun and re-emit it at specific wavelengths, producing the typical signature. This physical process is known as "resonance-fluorescence" and it is the same process that is used in neon bulb lights.
Both solar continuum (which currently dominates the spectrum of Tempel 1) and emission lines are well visible in the low and high resolution spectra. Most of the emission lines visible in the red part of the low resolution spectrum are coming from the Earth's atmosphere (which is glowing) and are called "sky lines" (see ESO PR Photo 13a/03).
Leading scientists of the ESO DI campaign: H. Boehnhardt, O. Hainaut, H.U. Kaufl, H. Rauer.
: Responsible scientist for the UVES program: E. Jehin (see also the page of the Liège team at http://vela.astro.ulg.ac.be/themes/solar/Comets/index_e.html).