“Cassini-Huygens at Titan” with Athena Coustenis (Paris Observatory, CNRS, PSL Univ., Sorbonne Univ., Univ. Paris, France)

The NASA/ESA/ASI Flagship-class Cassini-Huygens mission was launched on October 15, 1997 and entered into orbit around Saturn in July 2004. It carried the Huygens probe which was the first ESA planetary lander and landed on Titan on January 14, 2005. The lander transmitted data for 90 minutes during descent and after landing. The mission was an outstanding success. Most of what we know today about Saturn, its largest moon Titan and the Saturn system of rings and satellites comes from this mission which was named after Giovanni Cassini and Christiaan Huygens who discovered main features of the ring system and several satellites, including Titan.

The Cassini orbiter carried a large suite of instruments including optical and mass spectrometers, an imaging system, as well as a magnetometer, a cosmic dust analyzer and other fields, particles and waves and microwave remote sensing instruments. The Huygens probe had an atmospheric structure instrument, a doppler wind experiment, a descent imager and spectral radiometer, a gas chromatograph mass spectrometer, an aerosol collector and pyrolyser, and a surface science package.  The objectives of the mission included the exploration of the planet, its atmosphere and magnetosphere, and its moons as well as the prominent ring system. For Titan, the objectives included the study of the atmosphere, the properties of the surface and the interior. What we knew about Titan before Cassini-Huygens came from the Voyager missions in the 80s from one fly-by, but the Cassini-Huygens large number of close fly-bys and the in-situ exploration revolutionized our understanding of the satellite.   

Titan is the only moon in the solar system that has a substantial and optically thick atmosphere dominated by dinitrogen (N2) with traces of methane and hydrogen leading to an evolved organic chemistry, but very little oxygen and low temperatures of about -180°C. Instead of water, methane is at the center of a methanological cycle (a weather system in which methane takes the role of water) creating features like haze, precipitation, lakes and rivers or drainage systems of liquid hydrocarbons on Titan. Evidence was also found for the presence of an undersurface liquid water ocean on Titan, similar to Enceladus, as revealed by Cassini gravity data. Thus, the atmosphere and surface on Titan are similar to the Earth’s but different at the same time in terms of materials, creating the opportunity to study a unique world with a strong astrobiological potential.

The mission ended almost three years to the date of the present seminar on September 15th, 2017, when the Cassini spacecraft plunged and disintegrated in Saturn’s atmosphere sending more valuable data to the end. In addition to the exploration of Titan, which was a major target, another highlight of the mission was the discovery of active cryovolcanism on Enceladus, a satellite of only roughly 500km diameter, spurting water vapor geysers to space.  

Prof. Athena Coustenis is director of research at the Laboratoire d’etudes spatiales and d’instrumentation en astrophysique (LESIA) at the Paris Observatory, in Meudon, France. She is involved in several NASA and ESA space missions and has served on a large number of advisory and managing committees for the agencies, COSPAR, the IAU, ISSI, IUGG, EGU, DPS and EPSC. Athena Coustenis is a highly cited and respected expert for the planets and moons of the outer solar system, and in particular, for Titan.

This seminar was recorded on September 17, 2020.