Invited review: Infrared spectroscopy of planetary atmospheres: Searching for insights into their past and present histories

This article reflects my personal experience and illustrates some developments in the field of planetary spectroscopy, achieved within the Planetology Group of Paris Observatory over the past fifty years. Over these decades, planetary spectroscopy has led to the identification of minor atmospheric species with mixing ratios as low as a few parts per billion (ppbv). Since the early 1970s, we have been measuring infrared spectra of giant planets and their satellites, in order to search for new minor species and to determine elemental and isotopic ratios, for better constraining their formation and evolution processes. In particular, observations of Jupiter at the time of the Shoemaker-Levy 9 collision, in 1994, allowed us to monitor the thermal sequence and the formation of stratospheric water vapor; a few years later, spectra taken by the Infrared Space Observatory confirmed the presence of an external oxygen source in all giant planets and Titan. More recently, the advent of bi-dimensional infrared arrays has allowed us to map the distribution of these components over planetary disks, and to use them as tracers of dynamical and photochemical processes. In complement to in-orbit observations recorded by space missions, high-resolution spectral mapping from the ground allows us to obtain instantaneous global maps of the planets, and thus to trace transient phenomena or temporal variations of minor atmospheric species over short and long timescales. Over the past twenty years, we have been monitoring the behavior of minor atmospheric species on Mars and Venus, using the TEXES (Texas Echelon Cross Echelle Spectrograph) at the NASA IRTF (InfraRed Telescope Facility) at Maunakea Observatory, and the EXES (Echelon Cross Echelle Spectrograph) aboard SOFIA (Stratospheric Observatory For Infrared Astronomy): H2O2 and H2O on Mars, D/H on Mars, SO2 and H2O at the cloud top of Venus. In addition, in an attempt to build a 3-D image of the sulfur and water cycles on Venus, we have obtained maps of SO, SO2 and HDO in its upper mesosphere (about 20 km above the cloud top) using the ALMA (Atacama Large Millimeter/submillimeter Array) facility in Chile. These observations are presented and discussed in the light of global dynamical and photochemical models, and interpreted in the context of the past and present history of these planets. In the conclusion, I present the perspectives of this work for the forthcoming development of exoplanetary spectroscopy. •This article illus