Search of SO2 in the Martian atmosphere by ground-based submillimeter observation

We searched sulfur oxide (SO2 and SO) in the Martian atmosphere by the Atacama Submillimeter Telescope Experiment (ASTE). Sulfur oxide is one of the most evident species in terrestrial volcanic gases. Although it has not yet been detected at Mars, this detection can constraint the Martian crustal and volcanic activities. We observed northern winter of Mars on 26 December 2007 (Ls=8.1°) in 346 GHz range with ∼1 h integration. We get the upper limit of the SO2 mixing ratio, 2 ppb. This is mostly equivalent to the value observed in northern summer (Ls=205°) (Krasnopolsky, 2005). We concluded that the crustal or volcanic gas produced into the atmosphere is tenuous in northern winter. Even at Mars, it can be expected that the crustal or volcanic gas includes non-negligible amount of SO2 produced as degasification from the magma. Our result and recent detections of CH4 suggest a possibility of some kind of carbon hydride sources under the ground reservoir, independent of usual crustal and volcanic activities.