Comets are thought to be pristine relics from the birth of our Solar System and they have experienced the least thermal alteration. Comets, in particular long-period ones, thus keep the information of the time when they were formed. In spring 2004, the apparitions of two long-period comets C/2001 Q4 (NEAT) and C/2002 T7 (LINEAR), at a distance of 0.3 and 0.27 AU respectively from the Earth at perigee, provided us a rare opportunity to have a detailed study cometary inner coma where photodissociation is ineffective and most cometary parent molecules remain.
We successfully imaged the spatial distribution of CS and CH3OH of C/2002 T7 (LINEAR) using the Submillimeter Array (SMA). Detailed analysis of SMA images of Comet/T7 suggests strongly that methanol molecules are sublimated directly from the cometary nucleus. Earlier studies implied that CS is a daughter molecule of CS2. Even with our SMA observations at 1500×900 km high-linear resolution, however, we were not able to discriminate whether CS is a parent or a daughter molecule, i.e., whether the CS molecule seen in the inner coma was directly sublimated from the cometary nucleus or it was produced via photodissociation of CS2 molecule.
Single-dish observations using the Kitt Peak 12m radio telescope (KP12M) were also carried out. Molecular emission of CS, HCN, H2CO in Comet C/2001 Q4 (NEAT), and HCN, H2CO, HDCO and CH3OH in Comet C/2002 T7 (LINEAR) were detected. HDCO has never been detected in any comet previously. The D/H ratio of Comet C/2002 T7 derived is 0.28 based on our measurements of HDCO and H2CO. The high D/H ratio observed is analogous to the values of Class 0 solar-type protostellar sources reported, and therefore implies that the Solar Nebula may have gone through evolutionary stages similar to that of these protostellar sources.

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