Detectability of molecular species in planetary and satellite atmospheres from their rotational transitions
High-resolution spectroscopy in the millimeter and submillimeter range has made significant contributions to the study of chemical composition and thermal structure of planetary and satellite atmospheres. This field of research is expected to make considerable progress in the future, from both ground-based and space experiments, with the availability of heterodyne receivers at frequencies up to 1 THz, and with the development of space missions devoted to the exploration of the far-infrared and submillimeter range. A compilation of guidelines for searching molecular species in planetary atmospheres through their rotational transitions, from the millimeter to the far-infrared range (100 μm) is presented. These transitions are specified, and the corresponding synthetic spectra are shown, or detectability limits are estimated. Several observational scenarios are considered: (1) ground-based heterodyne observations (such as those with IRAM, CSO, JCMT); (2) space heterodyne observations with a 3 m antenna (such as that on the European FIRST space mission); (3) Fabry-Pérot spectroscopic observations from the ground and from space (such as the ISO and FIRST space missions). All planets (except Mercury) and the three satellites surrounded by an atmosphere, Titan, Triton and Io, are considered. The main results of this compilation can be summarized as follows. In addition to previously detected molecules, several species appear as promising candidates for detection in the far-infrared to millimeter spectral range. For ground-based heterodyne observations, they include: HCl, O 3 and SO on Venus; HCl, O 3, H 2O 2 and NO on Mars; HCl in the giant planets; SO on Io. For space observations, they are: O 2 on Venus and Mars (provided Venus is observable with FIRST), H 2O in Saturn and Titan (with ISO and FIRST); HD, HCl and other halides in the giant planets (ISO). In the case of heterodyne observations, detectability limits are also indicated. They typically correspond to mixing ratios in the range 10 -8-10 -10, depending upon the strength of the observed transition. It should be noted that a 30 m antenna operating at 230 GHz remains very competitive, especially for small objects (e.g. Io) with large dilution factors. Heterodyne observations of Triton and Pluto appear beyond the limit of detection.