Titan 477
still haven’t exactly determined the nature of all the surface
constituents, the combination of the information retrieved
by all the observing teams will eventually force Titan to un-
cover its mysterious soil. Undoubtedly the signs of dried
lakes, volcanoes, and channels on Titan’s surface were un-
expected. They offer an even more amazing view of a land
much fantasized on.
3.1 Pre-CassiniGlimpses of an Exotic Ground
To theVoyagercamuas, the surface of Titan was obscured
by the dense haze in the atmosphere. Glimpses of what lay
below were revealed afterwards by ground-based radar and
infrared images fromHSTand ground-based observatories.
Theory argued that unless methane supersaturation
conditions prevailed on Titan, the organics present in the at-
mosphere should condense at some level in the lower strato-
sphere and precipitate out, ending up on Titan’s surface and
coat the ground in large proportions. Based on the surface
conditions believed to prevail on Titan, liquid methane—
and its principal by-product, ethane—is expected to exist
and could even form an ocean, and in the troposphere,
methane clouds (formed by saturation of methane gas)
might cause rains. The degree of saturation in the lower
atmosphere, however, was unknown, so the methane abun-
dance was difficult to determine.
On the other hand, much of the outer part of the solid
body of the satellite must, to be consistent with the observed
mean density, consist of a thick layer of ice. The ethane
ocean model, developed in 1983, was aesthetically appeal-
ing and compatible with all theVoyager-era data. It has since
then long been abandoned in view of the spectroscopic and
imaging evidence for a heterogeneous surface and the radar
echoes indicating the presence of solid material.
Indeed, a shallow, global ocean was shown to be inconsis-
tent with the constraints imposed by Titan’s orbital charac-
teristics. The tidal action on an ocean less than 100 m deep
would have dissipated Titan’seccentricityof 0.03 (where 0
is circular and 1 is parabolic) long ago. Furthermore, the first
remote-sensing technique to be used for sounding Titan’s
surface, radar, indicated that the surface may be nonuni-
form but mostly solid with at most small lakes. Indeed, the
radar echos obtained in 1990 using the National Radio As-
tronomy Observatory’s Very Large Array in New Mexico
combined as a receiver of the signal transmitted to Titan
by the NASA Goldstone radio telescope in California were
among the first evidence against the global ocean model
of the surface. Radar measurements from Arecibo Obser-
vatory in Puerto Rico in 2003, however, revealed a specu-
lar component at 75% (12 of 16) of the regions observed
(globally distributed in longitude at about 26◦S), which was
interpreted as indicative of the existence of dark, liquid
hydrocarbon on Titan’s surface. The idea of a widespread
surface liquid was challenged in more recent observations
from the ground, which failed to find any such signatures
and proposed instead that very flat solid surfaces could be
causing the radar evidence. The nature and extent of the
exchange of condensable species between the atmosphere
and the surface and the equilibrium which exists between
the two is a key science topic.
More compelling evidence against a global hydrocarbon
ocean on Titan came from spectroscopic data in the near-
infrared (0.8–5μm). This part of Titan’s spectrum, like that
of the giant planets, is dominated by the methane absorp-
tion bands. At short (blue) wavelengths, light is strongly
absorbed by the reddish haze particles. At red wavelengths,
light is scattered by the haze, although the column optical
depth is still high. In the near-infrared, the haze becomes
increasingly more transparent (since the haze particles
are smaller than the wavelength), although absorption by
methane in a number of bands is very strong. Where the
methane absorption is weak, clear regions or “windows,”
situated near 4.8, 2.9, 2.0, 1.6, 1.28, 1.07, 0.94 and 0.83μm,
permit the sounding of the deep atmosphere and perhaps
of the surface (Fig. 6). In between these windows, con-
trary to the giant planets, solar flux is not totally absorbed
but scattered back through the atmosphere by stratospheric
aerosols, especially at short wavelengths. The near-infrared
spectrum is thus potentially extremely rich in information
on the atmosphere and surface of Titan.
Titan’s near-infrared spectrum was used to investigate
Titan’s surface in terms of detailed radiative transfer models
of the near-infrared spectrum. This study indicated a sur-
facealbedoinconsistent with a global ocean and a surface
reflectivity that showed a change in Titan’s albedo precisely
correlated with Titan’s rotation.
The observations all agreed: Thegeometric albedoof
Titan, measured over one orbit (16 days), shows significantFIGURE 6 Titan’s albedo observed from ground-based
observatories such as the Very Large Telescope in Chile and the
Keck Telescope in Hawaii, as well as with the satellite ISO (in
the 2.75 micron window, where the terrestrial turbulence doesn’t
allow us to observe Titan from the ground). The spectrum
exhibits several strong methane absorption bands, but also
“windows” where the methane absorption is weak enough to
allow for the lower atmosphere and surface to be probed.