A retrieved upper limit of CS in Neptune's atmosphere

We present our new result of CS(J=7-6), CO(J=3-2) observations of Neptune's atmosphere carried out with 10-m ASTE sub-mm waveband telescope on August 2010. As a result, while CS line was not detected with 6.4 mK 1-sigma r.m.s. noise level, CO line was detected as 282 mK with 9.7 mK noise level in antenna temperature scale. All of the observations were carried out with 512 MHz bandwidth and 500 kHz resolution, the total integration time for CS and CO were 23 m 40 s and 11 m 00 s, respectively. Abundances have been obtained from the comparison between the intensity and the synthesis spectra modeled by plane parallel 1-D radiative transfer code assuming various mixing ratio of each gas. The retrieved upper limit of CS mixing ratio was 0.03 ppb throughout tropopause to stratosphere. CO mixing ratio have been retrieved 1.0 ppm with errors +0.3 and -0.2 ppm, and the result was consistent with previous observation [1]. The origin of abundant CO in Neptune's atmosphere has been long discussed since its mixing ratio is 30 - 500 times higher than the value of other gas giants [2][3][4]. Assuming that all of CO is produced by thermochemical equilibrium process in deep interior of Neptune, required O/H value in interior is 440 times higher than the solar value [5]. For this reason, it is claimed that the external CO supply source, such as the impact of comet or asteroid, is also the possible candidates of the origin of CO along with the internal supply source [6]. In this observation, we searched the remnant gas of cometary impact in Neptune's atmosphere. Along with CO and HCN, CS could be one of the possible candidate of the remnant gas of cometary impact since CS was largely produced after the impact of comet SL/9 on Jupiter while many other major sulfur compounds have not been detected. Actually, derived < 0.00003 [CS]/[CO] value from our observations is 1000 times more smaller than the value of Jupiter of 0.037 [7]. Our observation result shows the depletion of CS in comparison with the case of Jupiter represents the two implications to external supply scenario: (1)From the numerical simulation, it is suggested that the sulfur compounds produced after the impact of comet changes with the abundance of oxygen in both comet and air [8]. Considering Neptune's water vapor rich environment [9], it is possible that other sulfur compounds such as SO or SO2 would have been produced after the impact. We are preparing the new line survey observation of sulfur compounds from these chemical aspects. (2): In contrary to (1), in the case that CS was produced along with CO by the impact, photo-dissociation process distinguishes the total amount of both CO and CS. Considering the difference of photo-dissociation lifetime of CO and CS, we estimated the required time elapsed after the impact to achieve the derived [CS]/[CO] value and was longer than 30 years. This value is consistent with previous study estimated by vertical transportation of CO [5]. References: [1]Marten et al., 2005. A&A 429, 1097-1105. [2]Lellouch et al., 2004. A&A 309, L91-94 [4]Noll et al., 1988. ApJ Part 1, 324, 1210-1218. [5]Lodders et al., 1994. Icarus 112, 368-375 [6]Lellouch et al., 2005. A&A 430, L37-40. [7]Moreno et al., 2003. Planetary and Space Sciences 51, 591-611 [8]Zahnle et al.,1995. GRL 22, 1593-1596 [9]Feuchtgruber et al., 1999. Proceeding of the conference "The universe as seen by ISO"