The 2017 Jean Dominique Cassini Medal & Honorary Membership is awarded to Luciano Iess for his command of the Doppler radio technique leading to important discoveries about the interiors of giant planets and their moons, and for performing high accuracy tests of the theory of General Relativity.
Luciano Iess is a world leader in the use of Doppler data to determine the gravity fields of planets and satellites. He developed a technique to completely remove the Sun’s plasma noise, which in turn enabled vastly improved Doppler data quality. From these data, we obtained our best and most quantitative understanding of interior structures, including the discovery of interior oceans in Titan and Enceladus. Moreover, Iess used Cassini instruments to perform the highest precision tests of General Relativity ever made, exceeding previous measurement precision by a factor of 50. On several occasions, he has gone beyond the obvious analysis of tracking (Doppler) data to derive spectacular results. Specifically, he led the planning, advocacy, analysis and interpretation of Cassini Radio Science Subsystem’s (RSS) observations that detected time-variable tidal distortions in Titan’s shape correlated with orbital phase. The results were definitive in requiring the presence of an ocean, most likely salty (substantially higher density than pure water). Iess also overcame limitations in previous analysis techniques that impeded the combining of individual flybys, enabling him to use multiple flybys over five years to obtain a single consistent gravitational solution. This proved crucial not only for Titan but also for the much smaller Saturnian moon Enceladus, where Iess initiated and led the planning, advocacy, analysis and interpretation of the RSS observations to show convincingly that a large internal liquid water reservoir exists within that moon as well. The discovery of internal oceans on two satellites of Saturn by Iess and his team, stand as extraordinary scientific achievements of the Cassini mission to Saturn. The exploration programmes currently developed by space agencies like NASA and ESA, in which habitability conditions will be explored in multiple worlds in the outer Solar System, was in a large measure galvanised by these results. Iess has led the development of the detailed radio science plan for determining the Jovian gravity field with the Juno spacecraft now in orbit around Jupiter. He was the first to realise and show quantitatively that Cassini could obtain data of similar quality for Saturn. He led the Ulysses gravity wave experiment and did precision Doppler tracking of the SMART-1 satellite in the lunar orbit. He is the Principal Investigator of the radio science package for ESA’s JUICE mission, which promises to obtain gravity data on some of the Galilean moons of quality similar to Cassini at Saturn. He is also the Principal Investigator of the radio science experiment aboard BepiColombo that will revolutionise our understanding of the gravity field of Mercury. Iess is also very active in transmitting his expertise to a group of postdocs and graduate students, the largest university-based research group in planetary science in Italy today. The discipline, rigour and extraordinary precision of Iess’s approach to interplanetary radio science has revolutionised our understanding of the Solar System while testing one of the great accomplishments of 20th century physics. He is a most worthy recipient of the EGU Jean Dominique Cassini Medal.