Titan 481
be caused by rainfall creating drainage channels, implying
a liquid source somewhere or at some times on Titan’s sur-
face. The former stubby channels are wider and rectilinear.
They often start or end in dark circular areas suggesting
dried lakes or pits. No obvious crater features were ob-
served.
Stereoscopic analysis was performed on the DISR im-
ages indicating that the bright area cut with the dendritic
systems is 50–200 m higher than the large darker plane to
the south. If the latter feature is a dried lakebed, it seems too
large by Earth standards to have been created by the creeks
and channels seen on the images and could be due to larger
rivers or a catastrophic event in the past. The dark channels
visible in Fig. 9 could be due to liquid methane irrigating
the bright elevated terrains before being carried through
the channels to the region offshore in southeasterly flows.
This migration toward the lower regions probably leads to
water ice being exposed along the upstream faces of the
ridges. The slopes are generally on the order of 30◦. Some
of the bright linear streaks seen on the images could be due
to icy flows from the interior of Titan emerging through
fissures.
The images taken after the probe had landed on Titan’s
surface show a dark riverbed strewn with brighter round
rocks. These “stones,” which are 15 cm in diameter at most,
could possibly be hydrocarbon-coated water ice pebbles
(Fig. 10).
The spectra acquired during the descent gave informa-
tion on the atmospheric properties (Table 1) and on the
surface properties. Indeed, it was shown from spectral re-
flectance data of the region seen from the probe that the
differences in albedo were related to differences in topogra-
phy, which in turn can be connected to the spectral behav-
ior of the ground constituents. Thus, the higher brighter
regions were also found to be redder than the lowland
lakebeds. The regions near the mouths of the rivers are
also redder than the lake regions. The spectra taken by
DISR are compatible with the presence of water ice on Ti-
tan’s surface, something that had already been suggested
from ground-based observations. The most intriguing fea-
ture found in the spectra was, however, the featureless
quasi-linear unidentified blue slope observed between 830
and 1420 nm. No combination of any ice and organic ma-
terial from laboratory measurements has been adequate in
reproducing this characteristic. The jury is still out on the
constituent(s) that create(s) this signature.
Although many questions still remain about the se-
quence of flooding and the formation of all the complex
structures observed by DISR, these data tend to clear the
picture we have of Titan today and at the same time enhance
the impression that by studying Saturn’s satellite we’re look-
ing at an environment resembling the Earth more closely
than any other place in our solar system.
No “little orange men” were photographed on Titan. The
public is very interested about a possible past, present, or
future life on Titan. One of the elements in the negative
FIGURE 10 Titan’s surface after the landing of theHuygens
probe. The icy pebbles are at most 15 cm in diameter, and the
darker riverbed is thought to be methane-wet sand (Tomasko
et al., 2005;Nature8 Dec. 2005. Image Credit: ESA/JPL
University of Arizona.)response (at least so far as the present or past life is con-
cerned) was found by the GCMS in the^13 C/^14 C isotopic
ratio (around 82), which showed that no active biota exist
on Titan and that the methane on Titan is not produced
by life (a biological origin would have required the isotopic
ratio to be in the 92–96 range).
The reality pictured by theCassini–Huygensinstru-
ments went beyond anything that has been speculated about
Titan’s surface. The diversity of the terrain includes impact