Physics – Optics
Scientific paper
Dec 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agufm.u12b..09l&link_type=abstract
American Geophysical Union, Fall Meeting 2001, abstract #U12B-09
Physics
Optics
4560 Surface Waves And Tides (1255), 5409 Atmospheres: Structure And Dynamics, 5445 Meteorology (3346), 6207 Comparative Planetology, 6280 Saturnian Satellites
Scientific paper
Saturn's giant moon Titan has a thick (1.5 bar) nitrogen atmosphere, and quite probably large expanses of liquid hydrocarbons on its surface. The physical processes in these lakes and seas will open new vistas on oceanography and limnology. Although the Voyager-era paradigm of a deep, global ocean is ruled out by radar and infrared data showing that at least part of Titan's surface is icy, the photochemical arguments that originally led to the proposal of hydrocarbon oceans still apply. Even if the methane in the atmosphere is being resupplied by delivery from the interior, the ethane produced by photolysis would still accumulate to form large deposits on the surface. The near-infrared maps of Titan's surface from the Hubble Space Telescope and groundbased adaptive optics consistently show a number of dark (in fact, pitch-black!) regions that are strong candidates for hydrocarbon seas. These could be up to some 500km in extent. Titan promises to be a new laboratory for oceanography. Like in meteorology, many ocean processes are better parameterized than they are understood, and thus the different physical circumstances on Titan may shed new light on them. Titan has a lower gravity and its ocean fluids are of lower density, perhaps of lower viscosity (depending on solutes and suspended material) and probably rather more likely to cavitate. The ratio of atmospheric density to ocean density is much larger on Titan than on Earth, suggesting that liquid motions will be well-coupled to surface winds (although the distance from the sun is such that the energy in such winds is likely to be low.) Titan is also subject to strong tidal forces (the equilibrium tide due to Saturn's gravity is some 400x larger than that of the moon on Earth.) Although the 100m tidal bulge stays almost fixed because Titan rotates synchronously, the eccentricity of Titan's orbit leads to significant libration and variation in the tidal strength. The 500km seas allowed by the IR data may yet have a 2m tidal amplitude. The long period of tidal excitation, however, means that tidal resonances are unlikely to occur. The NASA-ESA Cassini/Huygens mission will arrive in late 2004, and deliver the parachute-borne Huygens probe to Titan's surface in early 2005, taking images during its descent. The Cassini orbiter during its 4 year tour will fly by Titan some 45 times, taking SAR and altimeter data with a multimode radar, and observing the surface with optical and near-IR sensors. Future missions to Titan are already being contemplated, and might involve such platforms as helicopters or blimps.
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