The Cassini Ultraviolet Imaging Spectrometer (UVIS) occultation measurements for the
atmosphere of Titan identified and measured eight species. Seven gaseous species, which were
obtained previously, are nitrogen, methane, acetylene, ethylene, ethane, diacetylene, and
hydrogen cyanide. However, to reproduce the UVIS spectra requires broad band absorption
features. The introduction of tholin particles with radius 25 A provides a satisfactory fit to the
spectra. This represents the first spectral identification of the aerosol in the mesosphere. Upper
limits for benzene and dicyanodiacetylene are obtained. Photochemical modeling for the
atmosphere is performed. The model results are in good agreement with the observations. We
find that dicyanodiacetylene is condensible at 700 km, where the atmospheric temperature
minimum is located. This species is by far the simplest molecule identified to be condensible.
New observations are needed to confirm the existence of dicyanodiacetylene. [A paper has been
submitted to Ap. J. Lett.]
The recent Cassini discovery of water vapor plumes ejected from the south pole of the Saturnian
satellite, Enceladus, presents a unique window of opportunity for the detection of extant life in
our solar system. With its significant geothermal energy source propelling these plumes >80 km
from the surface of the moon and the ensuing large temperature gradient with the surrounding
environment, it is possible to have the weathering of rocks by liquid water at the rock/liquid
interface. For the cases of the putatively detected salt-water oceans beneath the ice crusts of
Europa and Callisto, an isolated subsurface ocean without photosynthesis or contact with an
oxidizing atmosphere will approach chemical equilibrium and annihilate any ecosystems
dependent on redox gradients unless there is a substantial alternative energy source. This
thermodynamic tendency imposes severe constraints on any biota that is based on chemical
energy. On Enceladus, the weathering of rocks by liquid water and any concomitant radioactive
emissions are possible incipient conditions for life. If there is CO, CO2 and NH3 present in the
spectra obtained from the plume, then this is possible evidence that amino acids could be formed
at the rock/liquid interface of Enceladus. The combination of a hydrological cycle, chemical
redox gradient and geochemical cycle give favorable conditions for life. We discuss the search
for signatures of these species and organics in the Cassini UVIS spectra of the plume and
implications for the possible detection of life. [A paper is in press in Astron. & Astrophys. ]
Publications supported by this grant
Parkinson, C. D., A. S. Wong, A. I. F. Stewart, and Y. L. Yung. (2006). "Enhanced Transport in the
Polar Mesosphere of Jupiter: Evidence from Cassini UVIS Helium 584 A Airglow." J. Geophys.
Res. 111 E02002, doi:10.1029/2005JE002539.
Parkinson, C. D., M. C. Liang, H. Hartman, C. J. Hansen, G. Tinetti, V. Meadows, J. V. Kirschvink
and Y. L. Yung (2006). “Enceladus: Cassini Observations and Implications for the Search for
Life.” Astron. Astrophys. In Press.
Liang, M. C., Y. L. Yung, and D. Shemansky (2007) “Detection of photochemical aerosols in the
mesosphere of Titan”, Astrophys. J. Lett. Submitted.
Conference presentations
R. West, K. Larsen, Y. L. Yung, E. Wilson, D. Tice, I. Stewart, L. Esposito, C. Porco, 2005,
Constraints on Titans high haze from Cassini UVIS and ISS observations. DPS meeting. Bull.
Am. Astron. Soc., 37, 708.
J. E. Colwell, L. W. Esposito, K. Larsen, A. I. F. Stewart, W. E. McClintock, M. Sremcevic, D.
E. Shemansky, J. T. Hallett, C. J. Hansen, A. R. Hendrix, R. A. West, J. A. Ajello, W. R. Pryor,
Y. L. Yung, 2005, UV spectroscopy of the Saturn system. DPS meeting. Bull. Am. Astron. Soc.,
37, 631.
C. D. Parkinson, A. I. F. Stewart, A.-S. Wong, J. M. Ajello, Y. L. Yung, 2005, Enhanced
transport in the polar mesosphere of Jupiter: evidence from Cassini UVIS Helium 584 Å
airglow. DPS meeting. Bull. Am. Astron. Soc., 37, 680.
Y. L. Yung, M. C. Liang, G. Tinetti, D. Shemansky, R. A. West, D. Tice, 2005, Titan:
Photochemical modeling and production of aerosols. DPS meeting. Bull. Am. Astron. Soc., 37,
734.
M. Liang, Y. L. Yung, D. Shemansky, 2006, Thiolin Particles in the Atmosphere of Titan. DPS
meeting. Bull. Am. Astron. Soc., 38, 27.15.
A. Wong, M. Liang, Y. L. Yung, J. I. Moses, K. A. Boering, 2006, Hydrocarbon
Photochemistry: Evaluation Of Chemical Kinetics Using Laboratory Measurements. DPS
meeting. Bull. Am. Astron. Soc., 38, 27.07.
R. A. West, P. Dumont, D. Tice, B. Cassler, J. Snow, R. Cleaver, D. Shemansky, Y. Yung, K.
Larsen,
M.
Evans,
L.
Esposito,
C.
Porco,
2006,
Titan's Haze Structure from Cassini ISS and UVIS Observations. DPS meeting. Bull. Am.
Astron. Soc., 38, 22.02.
Christopher, C. C., Y. L. Yung, M. C. Liang, H. Hartman, C. J. Hansen, G. Tinetti, V. Meadows,
J. L. Kirschvink (2006), Enceladus: Cassini Observations and Implications for the Search for
Life. EOS Transactions American Geophysical Union, Vol. 87, No. 52, Fall Meeting
Supplement, Abstract P13B-0176
Orzechowska, G. E., R. P. Hodyss, P. V. Johnson, J. D. Goguen, A. L. Lane, C. S. Boxe, J. L.
Kirschvink, Y. L. Yung, I. Kanik (2006), Investigation of Organic Chemical Evolution on
Enceladus. EOS Transactions American Geophysical Union, Vol. 87, No. 52, Fall Meeting
Supplement, Abstract P13B-0175