480 Encyclopedia of the Solar System
FIGURE 8 CassiniRADAR images (P.I. Elachi) of Titan’s
surface in synthetic aperture mode taken on July 21, 2006, and
showing the highly contrasted terrain with a variety of geological
features like the dark areas which are most probably
hydrocarbon lakes. The top radar image is centered at 80◦N,
92 ◦W and measures about 420 km by 150 km. The lower one is
centered at 78◦N, 18◦W and measures about 475 km by 150 km.
The most resolved features in these images are about 500 m
across. (Image Credit: NASA/JPL/Space Science Institute.)
The missing reservoir of liquid methane or ethane, which
scientists have speculated on for a long time, may indeed—
at least partly—be found in such areas.
3.3 In Situ Data: Landing on Titan
On January 14, 2005, theHuygensprobe manufactured
by ESA landed at 10.3◦S and 192.3◦W on Titan, providing
the “ground truth” for the orbital measurements in terms
of composition, structure, and geomorphology. The probe
flew over an icy surface and then floated down and drifted
eastward for about 160 km. Several of the instruments on
board contributed to our knowledge of Titan’s surface con-
ditions.
The HASI instrument measured the surface tempera-
ture and the pressure at the landing site to be 93. 65 ± 0 .25 K
and 1467±1 bar, respectively. The fact that the surface is
solid but unconsolidated was verified by all the data. The
first part of the probe to touch the surface was the Sur-
face Science Pachage (SSP) penetrometer whose data are
now interpreted as indicative of the probe first hitting one
of the icy pebbles littering the landing area before sinking
into the softer, darker ground material. The SSP detected
the ground from 88 m in altitude by acoustic sounding, re-
vealing a relatively smooth, but not flat surface for which
our best current hypothesis is gravel, wet sand, wet clay, or
lightly packed snow. With a landing speed of about 5 m/s
the front of the probe followed and penetrated the surface,
then slid slightly before settling to allow the DISR camera
to take several pictures of a Mars-like landscape, complete
with a dark riverbed and brighter pebbles.
No evidence for liquid was found at theHuygenslanding
site, but the surface is expected to be very humid because
methane evaporation (a 40% increase of the abundance)
was measured by the GCMS after landing. Thus, either
the methane liquid reservoir may not be so far below the
surface, but located instead in niches close to the exposed
ground, or perhapsHuygenslanded on Titan at a “dry”
season when the rivers and lakes that may exist near the
equator were empty but that could be flowing with hydro-
carbons at a different era. Also, the presence of hydrocarbon
lakes close to the North Pole, may also imply that there are
seasonal phenomena that distribute the liquid on the
ground. Nevertheless,Huygenslanded on an organic-rich
surface, with trace organic species such as cyanogens and
ethane detected on the ground.
In spite of some misadventures (loss of the sun sen-
sor measurements, of about half the images from Channel
B and the probe’s erratic motion), the DISR imager and
spectrometer gathered a precious set of data both in spec-
troscopy and imaging. Starting from the first surface image
at 49 km, down to the unprecedented-quality snapshots of
theHuygenslanding site, and through the lamp-on data
recorded below 700 m in altitude, this instrument played a
decisive part in untangling the enigma of Titan’s surface
morphology and lower atmospheric content. Panoramic
mosaics constructed from a set of images taken at different
altitudes show brighter regions separated by lanes or lin-
eaments of darker material, interpreted as channels, which
come in short stubby features or more complex ones with
many branches (Fig. 9). This latter dendritic network canFIGURE 9 Titan’s surface as viewed by theHuygens/DISR
cameras from a distance of 8 km in altitude. (Tomasko et al.,
2005;Nature, 438 , 465–778, 8 Dec. 2005. Image Credit:
ESA/JPL University of Arizona.)