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Neptune's atmosphere is characterized by the presence of abundant gas phase carbon monoxide (CO) (e.g., Rosenqvist et al. (1992)). Recently, detailed analyses of the CO line shape have shown that Neptune's upper stratosphere has a larger CO mole fraction than its lower region (Lellouch et al. (2005), Hesman et al. (2007), Fletcher et al. (2010) and Luszcz-Cook and de Pater (2013)). Such a bimodal vertical distribution implies downward transportation of CO, which originates from an external source. In other words, Neptune's stratosphere is likely to have an external origin. Among other candidates, comets are possible sources of CO. In fact, the collision of comet Shoemaker-Levy 9(SL9) on Jupiter in 1994 produced abundant CO, with total mass was similar to that observed in Neptune's stratosphere (Lellouch et al., 2005). Recently, the cometary origin of CO has been suggested on both Saturn and Uranus (Cavalie et al. (2010), Cavalie et al. (2014)). Although their derived mixing ratio were smaller than Neptune's case, these new discoveries may suggest that the stratospheres of gaseous planets have some species of cometary origin. For a more comprehensive understanding of the cometary origin and its aftermath on the planetary atmosphere, a further study of Neptune's case is important because its atmosphere is likely to contain a larger amount of cometary originated gases than other planets.
After a large cometary impact, vaporized cometary nuclei deposit various kinds of species in the planetary atmosphere. For the SL9 event, CO, OCS, S2, CS2, H2S, HCN, CS and H2O were detected in the Jovian stratosphere (e.g., Noll et al. (1995), Lellouch et al. (1995), Sprague et al. (1996), Moreno et al. (2001)). A similar discovery may be expected in Neptune's case. In particular, we have focused on the sulfur-bearing species in Neptune's stratosphere. In case of the SL9 event, both the S and O atoms in the produced gases were supplied mainly by the comet (Moreno et al., 2003). New findings of S-bearing species whose distribution is limited within Neptune's CO-rich region can support not only the presence of recent cometary impact but also give us new clues to study the sulfur chemistry induced by the cometary impact.
Following the arguments above, we conducted new observations of S-bearing species on Neptune in 2010 and 2013. The first target here was CS, which was the main sulfur reservoir among the remnant gases of the SL9 event (Moreno et al., 2003) Neptune's CS was searched in 2010. We also selected SO2, H2S, H2CS, C3S, SO and OCS as other sulfur reservoir candidates and searched for them in 2013.