Titan Science Questions at the End of Cassini-Huygens

Planetary Science Vision 2050 Workshop 2017 (LPI Contrib. No. 1989) 8156.pdf

RIDDLES OF THE SPHINX: TITAN SCIENCE QUESTIONS AT THE END OF CASSINI-HUYGENS. C.A. Nixon1, R. K. Achterberg2, A. Buch3, R. N. Clark4, P. Coll5, F. M. Flasar1, A. G. Hayes6, L. Iess7, R. D. Lo- renz8, R. Lopes9, M. Mastrogiuseppe7, F. Raulin5, T. Smith8, A. Solomidou9, C. Sotin9, D. F. Strobel10, E. P. Turtle8, V. Vuitton11, R. West9, R. Yelle12. 1Planetary Systems Laboratory, NASA Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, USA, [email protected]; 2Dept. of Astronomy, Univ. Maryland, Col- lege Park, MD 20742, USA; 3LGPM, Ecole Centrale Paris, Chatenay-Malabry, France; 4U.S. Geological Survey, Denver Federal Center, Denver, Colorado, USA; 5LISA, UPEC-UPD/CNRS/IPSL, Paris-Créteil, France; 6Dept. of Astronomy, Cornell Univ., Ithaca, New York, USA; 7Dipartimento di Ingegneria Meccanica e Aerospaziale, Univer- sità La Sapienza, via Eudossiana 18, 00184 Rome, Italy; 8Johns Hopkins Applied Physics Lab., Laurel, MD 20723, 9 10 USA; Caltech/Jet Propulsion Lab., 4800 Oak Grove Drive, Pasadena, CA 91109, USA; Dept. of Earth and Plane- tary Sciences, 3400 N. Charles Street, Johns Hopkins Univ., Baltimore, MD 21218, USA; 11UJF-Grenoble 1/CNRS- INSU, Institut de Planétologie et d’Astrophysique de Grenoble (IPAG) UMR 5274, Grenoble F-38041 ; 12Lunar and Planetary Lab., Dept. of Planetary Sciences, Univ. of Arizona, Tucson, Arizona, USA.

Introduction: Since July 2004, the Cassini space- (1B) Why does Titan have an atmosphere, and how has craft has been orbiting Saturn, making repeated close it changed over time? encounters with Titan, its largest moon and the only (1C) What is the age of the features we see on Titan’s moon in the solar system to possess a dense atmos- surface? phere. By the end of the mission in September 2017, Interior and surface: Cassini will have amassed a vast wealth of scientific (2A) How thick is the icy shell and how deep is the ocean? data about Titan collected by 12 instruments during (2B) Is Titan geologically active today? 127 close flybys. These include: images and spectra (2C) What materials are exposed on the surface? from four remote sensing devices; particles and fields (2D) What factors influence the distribution of the measurements from six further instruments; and radar lakes and seas? and radio measurements of the atmosphere and surface (2E) What is the composition of the lakes and seas? at multiple wavelengths. (2F) What degree of prebiotic chemical complexity has In January 2005 the Huygens probe descended by been reached in surface environments? parachute to the surface, making the first in-situ measurements of the neutral atmosphere and surface with Lower and middle atmosphere: six on-board instruments, including winds, atmospher- (3A) How can we explain the observed variations in ic composition, temperature-pressure structure, hazes, Titan’s hydrogen profile? and surface characteristics. (3B) How can atmospheric models be reconciled with The international Cassini-Huygens mission has observations of clouds in the lower atmosphere? made huge advances in our understanding of this en- (3C) Why is the stratospheric axis tilted with respect to igmatic world, the greatest stride forward since the Titan’s solid body? Voyager 1 encounter of 1980. However it is clear in (3D) What is the reason for the detached haze layer? the closing stages of the mission that many mysteries Upper atmosphere and exosphere: remain. During a meeting of the Cassini-Huygens sci- (4A) What chemical processes lead to the formation of ence team in October 2016, Titan scientists were invit- complex molecules, including aromatics? ed to consider how many of the high-level questions (4B) How rapidly is Titan’s methane escaping into from the start of the mission have now been answered. space? The team concluded that while some of the pre-Cassini (4C) What gives rise to the observed electron density mysteries are now largely “solved”, some of the most profile? perplexing questions remain only partially understood, (4D) Do we see the expected nitrogen torus? while new riddles have also emerged from the data.

Key Questions: the questions are divided into four Conclusions: In this paper we present a viewpoint broad thematic areas, while recognizing that in practice on the most important questions remaining for Titan these subject areas are all tied together: (1) history; (2) science at the end of the Cassini-Huygens mission. interior and surface; (3) lower and middle atmosphere; New data will be accumulated by powerful ground and (4) upper atmosphere and exosphere. space-based observatories in the years after Cassini; History: however, some of the most important questions will (1A) How did Titan form, and was this early or late in await new, focused spacecraft missions to Titan in the the solar system? decades from 2020 to 2050 to be more fully answered.