Titan’s Greenhouse Effect and Climate: Lessons from the Earth’s Cooler CousinA White Paper Submission to the NRC Planetary Science Decadal Survey

Conor A. Nixon1,*, Athena Coustenis2, Jonathan I. Lunine3, Ralph Lorenz4, Carrie M. Anderson5, F. Michael Flasar5, Christophe Sotin6, J. Hunter Waite Jr.7, V. Malathy Devi8, Olivier Mousis9, Kim R. Reh6, Konstantinos Kalogerakis10, A. James Friedson6, Henry Roe11, Yuk L. Yung12, Valeria Cottini1, Giorgos Bampasidis13, Richard K. Achterberg1, Nicholas A. Teanby14, Gordon L. Bjoraker5, Eric H.

Wilson6, Tilak Hewagama1, Mark A. Gurwell15, Roger Yelle3, Mark Allen6, Nathan J. Strange6, Linda J. Spilker6, Glenn Orton6, Candice J. Hansen6, Jason W. Barnes16, Jason M. Soderblom3, Vladimir B. Zivkovic17, Anezina Solomonidou13, David L. Huestis10, Mark A. Smith3, David H. Atkinson18, Patrick G. J. Irwin14, Mathieu Hirtzig2, Simon B. Calcutt14, Timothy A. Livengood5, Sandrine Vinatier5,

Theodore Kostiuk5, Antoine Jolly19, Nasser Moazzen-Ahmadi20, Darrell F. Strobel21, Mao-Chang Liang22, Patricia M. Beauchamp6, Remco de Kok23, Robert Pappalardo6, Imke de Pater24, Véronique Vuitton25, Paul N. Romani5, Robert A. West6, Lucy H. Norman26, Mary Ann H. Smith27, Kathleen Mandt7, Sebastien Rodriguez28, Máté Ádámkovics24, Jean-Marie Flaud29, Kurt K. Klaus30, Michael

Wong31, Jean-Pierre Lebreton32, Neil Bowles14, Marina Galand33, Linda R. Brown6, F. Javier Martin-Torres12, Brook Lakew5, Shahid Aslam34.

1Univ. Maryland, *conor.a.nixon@nasa.gov, 2Obs. Paris, 3Univ. Arizona, 4APL, 5NASA GSFC, 6Caltech/JPL, 7SWRI, 8Coll. Wm. and Mary, 9Obs. Besançon, 10SRI, 11Lowell Obs., 12Cal. Inst. Tech, 13Univ. Athens, 14Univ. Oxford, 15Harvard-Smithsonian, 16Univ. Idaho (Physics), 17Univ. N. Dakota, 18Univ Idaho (E.Eng.), 19LISA Univ. Paris, 20Univ. Calgary, 21JHU, 22Academica Sinica, 23SRON, 24UC Berkeley, 25Lab. Plan. Grenoble, 26UCL, 27NASA LRC,

28CEA/Univ. Paris, 29LISA/CNRS, 30Boeing, 31STScI, 32ESA/ESTEC, 33Imperial Coll. London, 34MEI Technologies.

ABSTRACT

1. GREENHOUSE EFFECT: HOW DOES IT WORK?

4. THE FATE OF THE ATMOSPHERE

OUR RECOMMENDATIONS:• Key molecules in Titan’s atmosphere are more transparent to visible light than to infrared radiation.• When sunlight reaches Titan’s surface, some re-radiated thermal energy is trapped, warming the lower atmosphere and surface.• A feedback loop also exists, whereby small increases in H2, which is not limited by saturation, causes more CH4 to be retained in the atmosphere, increasing and amplifying the warming.

• The Decadal Survey for Planetary Science conducted by the Space Studies Board of the US National Academies, recently issued a call for white papers, to inform prioritization of future funding for research, including missions. This poster summarizes our submitted paper.• We show here that Titan is an atmospheric ‘greenhouse

world’, like the Earth, Mars and Venus.• Study of Earth’s planetary ‘cousins’, including Titan, has

great potential to inform us about the nature of the greenhouse effect and long-term climate change on our world.• See the final box for our recommendations on how best

to address this important topic.

Titan Atmosphere Schematic. Credit: ESA

2. ANTI-GREENHOUSE EFFECT• An opposing anti-greenhouse effect cools the atmosphere.• This is mainly due to stratospheric haze particles that are transparent to infrared but absorb visible light.• The net result of the positive (+23 K) and negative (-11 K) greenhouse effects is +12 K, raising the surface temperature from 84 K to 94 K.• Compare to the Earth (+30 K), Venus (+500 K) and Mars (+5 K).• MESSAGE: Titan’s greenhouse effect resembles the Earth’s. By studying Titan we can better understand the Earth’s atmospheric processes.

At least 12 haze layers are seen on this Cassini ISS image at 10°S. (NASA/JPL/SSI)

3. SEASONAL CHANGE: SMILE OR FROWN? • Titan experiences an ~30 yr seasonal cycle due to its orbital inclination.• Imaging in 1992 showed a ‘smile’: a bright up-turned arc in the southern hemisphere at red wavelengths. Blue images showed the opposite: a bright northern hemisphere.• In 2002 the trend was reversed, with a ‘frowning’ bright (red) north.• This seasonal ‘migration’ of haze from south to north and back is caused by a summer-pole-to-winter-pole circulation in the stratosphere.• MESSAGE: Titan experiences seasons with parallels to the Earth. Modeling these changes is a useful test of terrestrial models: including physical, chemical and dynamical processes.

Image: R. Lorenz/STScI

• Titan’s upper atmosphere functions as a vast chemical factory, turning the raw materials (N2, CH4, H2O) into more complex molecules and haze.

• These condense and fall from the atmosphere, causing an irreversible depletion of methane. The CH4 inventory will last just ~107 Myr, unless resupplied: perhaps by volcanism, outgassing, or cometary impacts.• If all CH4 is removed, then the atmosphere may periodically collapse and freeze out on the surface.• MESSAGE: Titan is a case study in climate change. By investigating Titan’s fate, we can better appreciate how the Earth may undergo natural or anthropogenic climate change. Graphic: NASA TSSM Final Report/J. Lunine.

For more information, including electronic downloads of this poster and the full white paper, please visit: http://www.astro.umd.edu/~nixon/titanclimate.html

To advance critical research into Titan's climatology - simultaneously advancing our understanding of the Earth’s atmosphere - we advocate to the NRC Decadal Survey for Planetary Science the following steps:

1. Endorse: the strong positive findings of the recent Senior Review of the Cassini Solstice Mission, to continue the mission until 2017.

2. Urge that a successor Titan-focused mission be given very high priority for near-term development and launch.

3. Recommend continued funding for strong ground-based, airborne and space-based observing campaigns for continuous, long-term Titan monitoring.

4. Support continued funding for applicable NASA R&A programs and the NSF Planetary Astronomy Program; and for associated laboratory experiments, modeling and theoretical calculations.

5. Propose that a dedicated NASA outer planetary flagship mission program be initiated, analogous to the Mars and lunar programs, to encompass the continued operations of Cassini, and follow-on flagship missions.