Athena Coustenis Discusses Her Work on Saturn’s Moon Titan
Special Topic of Planetary Exploration Interview, October 2011
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What did you learn from the images?
We distinguished bright and dark regions implying a complex surface. We also saw a huge storm raging in Titan's southern hemisphere and we were able to monitor it over several years. When Cassini arrived the storm had blown itself out but we were able to monitor its after effects for several years.
I was the first to discover the dewpoint effect: that is the difference between morning and night-time on Titan where you have changes in the atmospheric composition due to the switch between day and night; the cycle lasts a total of 16 days. That's what we got from a new method for processing the images, all of it very important.
Two years later, in 2003, you and your
colleagues published a paper on Titan's atmosphere based on observations
made by the orbiting Infrared Space Observatory (ISO), launched in 1995
(Coustenis A, et al., "Titan's atmosphere from ISO mid-infrared
spectroscopy," Icarus 161[2]: 383-403, February 2003).
ISO was a wonderful opportunity. It was actually scheduled as an astronomy mission, but then planetary scientists, such as my colleague Thérèse Encrenaz, managed to get on board, metaphorically speaking, so we got information on planetary bodies, including Titan. I scheduled some of the observations. ISO had very high spectral resolution that allowed us to look at the chemical make-up of Titan's atmosphere with high precision. The new discoveries of molecules were of water vapor and benzene. The latter is the most complex hydrocarbon to date found there. The ISO spectroscopy paper is highly cited because it announced really exceptional discoveries.
What's the main content of your top three
papers in this survey?
These three papers are based around results from the three Cassini-Huygens instruments with which I have been involved.
The first paper, for which Marty Tomasko was principal investigator (Tomasko M, et al., "Rain, winds and haze during the Huygens probe's descent to Titan's surface," Nature 438[7069]: 765-78, 8 December 2005), highlights the results from the descent imager and spectrometer on Huygens. This was a wonderful opportunity to get information on the atmosphere, mainly the aerosol content and this was the first time that we felt that the aerosol content was not negligible, even at the surface.
A. Coustenis (middle) in front of the replica of the
Huygens probe with Robert Mitchell (left) and Jean-Pierre Lebreton
(right).
But some of the most exciting results with this instrument came from the surface images. The features and the panoramas were fantastic: a surface carved with channels. It is important to remember that what we found were traces of once-flowing liquid. The landing site was near a methane river strewn with water ice pebbles and a dry lake. We were not prepared for any of this.
Paper #2 had Marcello Fulchignoni as principal investigator (Fulchignoni M, et al., "In situ measurements of the physical characteristics of Titan's environment," Nature 438[7069]: 785-91, 8 December 2005) and it reports on data from the Huygens Atmospheric Structure Instrument, which monitored temperature and density profiles from a height of 1,400 km right down to the ground and allowed us to calibrate all other data from the mission. These data are compared to those from Cassini infrared spectrometer, which is still operating.
The first science results from Cassini Composite Infrared Spectrometer (CIRS) on Titan's atmospheric content are reported in paper #3, with F.M. Flasar of the NASA Ames Research Center as principal investigator (Flasar FM, et al., "Titan's atmospheric temperatures, winds, and composition," Science 308[5724]: 975-8, 13 May 2005). This is ongoing research and will provide us with insights until 2017.
Are planetary scientists such as you keen to
get back to Titan?
It has been highly rewarding for me because throughout my career I have been looking at all kinds of different aspects of Titan's atmosphere and surface with a range of different instruments and techniques. But I believe that in situ exploration is irreplaceable. Yes! It would be wonderful to have another mission.
I spent two years, in 2008 and 2009, putting together a proposal and co-leading a study for a future mission to Titan: The Titan Saturn System Mission. This was a joint effort of NASA and the European Space Agency for the exploration of Saturn's moons Titan and Enceladus, another fascinating object that involved an orbiter, a hot-air balloon to float in the moon's atmosphere for six months, as well as a lake lander. A strong global collaboration of more than 200 scientists, all enthusiastic Titanophiles, was developed around this mission and that's a measure of how strongly the community, but also the public, supports a return to Titan.
Since then, and while waiting for the possibility of such a large and ambitious mission to be developed, other mission concepts to Titan have been proposed, all of them very exciting. A Titan lake lander is currently under study in the framework of the NASA Discovery program and I'm looking forward to seeing any or all of these concepts fly one day…
Meanwhile, I am continuing to use the instruments on Cassini, which is
funded until 2017. And I'm also involved in the ESA Laplace-JUICE Mission,
planned to study the Jovian system and its moon Ganymede around
2030.
Athena Coustenis
LESIA
Observatoire de Meudon
Meudon, France
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ATHENA COUSTENIS'S MOST CURRENT MOST-CITED PAPER IN ESSENTIAL SCIENCE INDICATORS:
Tomasko MG, et al., “Rain, winds and haze during the Huygens probe’s descent to Titan’s surface,” Nature 438(7069): 765-78, 8 December 2005 with 195 cites. Source: Essential Science Indicators from Clarivate.
KEYWORDS: PLANETARY SCIENCE, SATURN, TITAN, CASSINI-HUYGENS, URANUS, VOYAGER, LESIA, THERMOMETER-BAROMETER, DESCENT IMAGER/SPECTRAL RADIOMETER CAMERA, COMPOSITE INFRARED SPECTROMETER, ATMOSPHERE, ORGANIC CHEMISTRY, TITAN’S DISK, VERY LARGE TELESCOPE, METHANE, STORM, SOUTHERN HEMISPHERE, DEWPOINT EFFECT, INFRARED SPACE OBSERVATORY, WATER VAPOR, BENZENE, AEROSOL CONTENT, SURFACE IMAGES, HUYGENS ATMOSPHERIC STRUCTURE INSTRUMENT, TEMPERATURE, DENSITY, TITAN SATURN SYSTEM MISSION, ENCELADUS, EUROPA JUPITER SYTEM MISSION.
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