Athena Coustenis Discusses Her Work on Saturn’s Moon Titan

Special Topic of Planetary Exploration Interview, October 2011

Athena Coustenis

Titan is Saturn's largest moon and it is the only natural satellite in our solar system that has an extended nitrogen atmosphere. In that and many other ways Titan is similar to the early Earth, although it's much colder. It has intrigued planetary scientists from the early days of its discovery in 1665. In July 2004 the Cassini-Huygens spacecraft arrived at Saturn and commenced observations. The Huygens probe touched down on Titan's icy surface in January 2005, and the Cassini orbiter has made several fly-bys. Titan is a fascinating world with a dynamic atmosphere which has created surface features similar to those on Earth, albeit with different materials.

Our Special Topics analysis of papers on planetary exploration, published 2001–2011, has identified Dr. Athena Coustenis as a highly cited author. This decadal survey shows she has contributed to 72 papers related to Cassini-Huygens that have received 1,945 total citations, which ranks her at #9 by total cites and #13 by cites per paper among scientists working on planetary exploration. Five of these papers are among the top 20 papers over the past decade or the past two years.

Coustenis is Director of Research at the Centre National de la Recherche Scientifique (CNRS) and an astrophysicist with the Laboratoire d'Etudes Spatiales et d'Instrumentation en Astrophysique (LESIA) of the Paris-Meudon Observatory, France.

From her lab in Meudon she explained the highlights of her special interest, Titan, to European correspondent, Dr. Simon Mitton.

SW: What stimulated you to take an interest in astronomy and planetary exploration?

It started when I was about 15, and the main sources of inspiration were books by Carl Sagan and Isaac Asimov. Carl Sagan was the major figure for me, and I used to watch the Cosmos television series at home in Greece. Asimov's Foundation series really engaged my imagination. In Greece I attended a French school, and that enabled me to attend university here in Paris where I studied astrophysics and space science, plus English literature, which my father insisted on as an insurance against not being able to get a job in astronomy!

SW: Our Special Topics analysis shows that you've had a very productive career as a planetary scientist. Who guided you into planetary research?

Towards the end of my university studies I met Daniel Gautier, a planetary scientist at the Observatoire de Paris who was working on Voyager infrared data, 10 years into the mission. First I did a small project with him on the data from the Uranus fly-by, and then he proposed that I should work on Titan. My doctoral thesis was devoted to Voyager Titan infrared data.

At the professional level, I was always interested in joining a research field where I could test theories or models relating to objects that could be reached. That's why I preferred to work on solar system science rather than stars or remote galaxies. I like space missions because they involve us in going places. As it turned out, my whole career has been spent in the same lab—it's now known as LESIA—at the Meudon site campus of the Paris observatory.

Athena Coustenis
Athena Coustenis.

Athena Coustenis
Coustenis at the Paris Observatory Telescope.

SW: I can remember vividly the impact that the Cassini-Huygens mission had on arrival at Saturn and Titan. How did you come to work on the instrumentation and data analysis for such an exciting mission?

I got involved in the mission at an early stage because of my lab's expertise in hardware and Titan science. Because I had worked on Titan for my doctoral training I was active in devising several instrument proposals, and I became a co-investigator for three of the finally selected instruments. Those were: a thermometer-barometer on the Huygens probe (HASI), the Descent Imager/Spectral Radiometer (DISR)—that's a spectro-imager—on Huygens again, and the Composite Infrared Spectrometer (CIRS) on Cassini, the orbiter.

It was very exciting from the beginning: putting together the mission, writing the proposal for each instrument, then seeing the mission fly, and then being thrilled with how amazingly successful it was. It was a huge endeavor, and the orbiter has been delivering data since 2004. After the first stage of development, where actually I contributed in defining the science we were going to get back from some of the instruments, we can now harvest the results of the data analysis.

SW: Your highly cited papers suggest that this mission generated a great deal of information on the atmosphere and surface of Titan. Is your research on Titan still a work in progress?

Yes, it certainly is. Since 2004 I have been working on data from the orbiter spectrometer and the Huygens probe investigating the atmosphere of Titan, and also the images from the descent imager camera, trying to figure out what we were looking at on the surface.

Emotionally and scientifically Huygens was enjoyable because it was a European mission, on which our involvement was perhaps more tangible than is the case with Cassini, which is a NASA-led mission. But in all cases, the collaboration within the team and the sharing of understanding and expertise was most rewarding. I had spent 10 years working on Titan before the probe landed, and then suddenly we were looking at a completely new world, beyond anything anyone had imagined or expected. I was also doing new research that I hadn't tried my hand at before.

To this day, we are still plowing through the fantastic amount of data that Cassini is returning to build a continuing story. The infrared spectrometer on the orbiter is still working. The mission is now extended to 2017. My particular field with the Cassini spectrometer has been to define the composition of the atmosphere of Titan and to search for new molecules; we are seeing a very complex organic chemistry which is also evolving in time over a whole Titan year now, since the 1980 Voyager encounter.

SW: I'd like know more about your two papers from 2001, one in Planetary and Space Science (McKay CP, et al., "Physical properties of the organic aerosols and clouds on Titan," 49[1]: 79-99, January 2001) and the other in Icarus (Coustenis A, et al., "Images of Titan at 1.3 and 1.6 mu m with adaptive optics at the CFHT," 154[2]: 501-15, December 2001). These of course pre-date the arrival of Cassini-Huygens and they deal with observations from ground-based telescopes as a preparation for the space mission.

The aerosols paper was a thorough review of our understanding at the time of the complex problem of aerosols and clouds on Titan.

For the other one, we did imaging of Titan using large telescopes on the ground, notably the Very Large Telescope in Chile, and a new techniques, adaptive optics, that my lab here developed so we could resolve Titan's disk. That was a huge breakthrough because we had spatial resolution to see the surface in the methane windows, which had not been possible with Voyager. This allowed us to get some of the very first maps from the ground. We had several methane windows that gave us maps at different wavelengths, and we retrieved information about the surface not being homogeneous, and possibly covered with a mixture of ice and tholins, certainly not, as we previously thought, covered with a hydrocarbon ocean.

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