Cassini Encounters Titan
The Cassini/Huygens mission to Saturn is one of the most exciting planetary exploration projects ever undertaken. Launched in 1997, Cassini went into Saturn orbit on June 30, 2004, beginning its four-year tour of the planet's complex systems of rings, moons, radiation belts, and atmospheres. The major highlight will take place on Jan. 14, 2005, when the Huygens probe will drop into the smoggy atmosphere of Saturn's mysterious moon Titan, conducting a two-and-a-half hour survey of its cryogenic environment. Seeking to understand Titan's atmosphere, GISS scientists Anthony Del Genio and Michael Allison are involved in three of Cassini/Huygens instrument teams: the Imaging Science Subsystem and Cassini Science Radar Team on the orbiter and the Doppler Wind Experiment on the Huygens probe.
Titan's atmosphere is mostly molecular nitrogen, as is Earth's, but it is about 50% thicker. The surface is hidden by a stratospheric haze (see Fig. 1), believed to form when sunlight breaks apart methane molecules and forms hydrocarbon particles that settle to the surface. This would have easily depleted all methane in Titan's atmosphere over the life of the solar system, so the presence of atmospheric methane today implies a methane source. One possibility is that, similar to Earth's water cycle, lakes or shallow seas of liquid methane-ethane feed a "hydrologic" cycle of methane evaporation, cloud formation, and rain.
Pictures of Titan taken in visible light are almost featureless, showing only its hydrocarbon haze, but at other wavelengths surface details are revealed. In the infrared, wavelengths longer than the human eye can see, the haze is more transparent, and Cassini's cameras can obtain beautiful global maps of dark and bright surface regions (see Fig. 2). It is tempting to interpret the bright areas as water ice "continents" and dark areas as methane/ethane "seas" or surfaces coated with hydrocarbons, but we do not yet know what the regions are made of. Regions of enhanced reflection, called sunglint, would be expected from a smooth liquid surface. Ground-based radar observations made several years ago did show evidence of enhanced reflection, but so far the Cassini imaging team has not.
We do know that bright patches near the south pole in these images are clouds. Clouds cluster near the pole because it is summer there, and constant sunlight has probably warmed the surface enough to cause methane rainstorms. Outside the polar region, clouds are scarce. The reason for this is a mystery. Perhaps there are no methane seas on Titan. Instead, occasional geysers from a subsurface methane reservoir might be the only methane source for the atmosphere, and the resulting low relative humidity of methane gas prevents cloudiness in most places. Or perhaps there is plenty of methane to make clouds but few aerosol particles to act as seeds for cloud formation, as sulfate pollution and sea salt do on Earth.
Venus, Titan's slowly rotating cousin, has strong "super-rotating" winds of up to 200 miles per hour planetwide despite the solid planet's own slow rotation. Climate model simulations made a decade ago by GISS scientists led by Del Genio predicted this should also be true of Titan. To demonstrate the effect actually occurs we need to see clouds drifting with the winds, and that required Cassini.
Outside the polar region, looking for clouds on Titan is like looking for a needle in a haystack, but every once in a while you find a needle. In late May, as Cassini approached the Saturn system, a cloud was detected by Del Genio and John Barbara in distant images of Titan at 38°S (see Fig. 3). Over two and a half hours the cloud moved eastward at a speed of 76 mph. Images from the October flyby of Titan yielded several other examples of midlatitude clouds, all moving eastward. These represent the first direct evidence of super-rotation on Titan and confirm at least one prediction about the mysterious moon.
Surface detail is revealed at wavelengths beyond the infrared, in the microwave and radar range. Cassini's first Titan radar maps acquired during its low-altitute flyby on Oct. 26 — within 750 miles of the surface — revealed a variety of distinct features and surface properties (see Fig. 4). Although these maps showed little definitive indication of impact craters, a few especially smooth regions may reflect small areas of hydrocarbon liquids or sludge. Radar altimetry measurements showed only small variations in elevation, less than 500 ft. over a ground track 250 miles long. Active radar mapping of Titan during a single flyby covers only a tiny fraction of the surface, so many more close passes will be required to build up a global picture of its topography and roughness.
The Cassini radar instrument also operates in a passive "listening" mode, providing large-scale but crudely resolved maps of the satellite's brightness at microwave and radio frequencies. The contoured image in Fig. 5 shows Cassini's first Titan radiometry scan. The contours and colors show observed variations in Titan's radio temperature at 2 cm wavelengths, while the light and dark shading show the structure in the near infrared.
On Christmas Eve, 2004, Cassini will release the wok-shaped Huygens probe on the start of its inbound approach to an intimate date with Titan. On Jan. 14, at 4 a.m. EST, Huygens will enter Titan's atmosphere at a speed of 12,000 mph, rapidly decelerate, then deploy its parachute at an altitude above 90 miles. For the next two and half hours, Huygens will measure the temperature, pressure, and chemistry of Titan's atmosphere and observe the surface with a downward looking camera. Precise measurements of the Doppler-shifted frequency of the probe-to-orbiter radio relay will measure Huygens' wind drift and therefore Titan's atmospheric circulation. (The Doppler measurement works on the same principle as the apparent shift in the sound-pitch of a passing train's whistle.)
The expected wind-blown drift of Huygens will likely carry it some 10-20° east of its original entry point. Tracking the variation of the apparent position of the Sun in the Titan sky as seen from the probe by its imager should provide an independent estimate of its wind-carried motion. If the probe and its radio relay survive surface impact, another specially designed instrument package will provide further information on the surface's solid or liquid character. All these measurements will represent an essential "ground truth" check on the continued survey of Titan by Cassini's remote sensing instruments for years to come.
Allison, M., D.H. Atkinson, M.K. Bird, and M.G. Tomasko 2004. Titan zonal wind corroboration via the Huygens DISR solar zenith angle measurement. In International Workshop on Planetary Probe Atmospheric Entry and Descent Trajectory Analysis and Science (A. Wilson, Ed.). ESA SP-544, pp. 125-130. ESA Publications Division, ESTEC. Noordwijk, The Netherlands.
Bird, M.K., R. Dutta-Roy, M. Heyl, M. Allison, S.W. Asmar, W.M. Folkner, R.A. Preston, D.H. Atkinson, P. Edenhofer, D. Plettemeier, R. Wohlmuth, L. Iess, and G.L. Tyler 2002. The Huygens Doppler Wind Experiment: Titan winds derived from probe radio frequency measurements. Space Sci. Rev. 104, 611-638.
Del Genio, A.D., and W. Zhou 1996. Simulations of superrotation of slowly rotating planets: Sensitivity to rotation and initial condition. Icarus 120, 332-343, doi:10.1006/icar.1996.0054.
Elachi, C., M.D. Allison, L. Borgarelli, E. Encrenaz, E. Im, M.A. Janssen, W.T.K. Johnson, R.L. Kirk, R.D. Lorenz, J.I. Lunine, D.O. Muhleman, S.J. Ostro, G. Picardi, F. Posa, C.G. Rapley, L.E. Roth, R. Seu, L.A. Soderblom, S. Vetrell, S.D. Wall, C.A. Wood, and H.A. Zebker 2004. RADAR: The Cassini Titan radar mapper. Space Sci. Rev. 115, 71-110, doi:10.1007/s11214-004-1438-9..
Porco, C.C., R.A. West, S. Squyres, A. McEwen, P. Thomas, C.D. Murray, A. Del Genio, A.P. Ingersoll, T.V. Johnson, G. Neukum, J. Veverka, L. Dones, A. Brahic, J.A. Burns, V. Haemmerle, B. Knowles, D. Dawson, T. Roatsch, K. Beurle, and W. Owen 2004. Cassini imaging science: Instrument characteristics and capabilities and anticipated scientific investigations at Saturn. Space Sci. Rev. 115, 363-497, doi:10.1007/s11214-004-1456-7.