| Strong evidence for a visible Huygens to
March 20, 2005
| For a comprehension
of the landscape details Cassini sees on Titan, it is extremly valuable
if an exact correlation with the detailed images of Titan´s
surface, obtained by Huygens, can be found. Here I argue that the
correlation I already supposed on the basis of the Huygens DISR images
and my Large Surface Coverage Mosaic ( "Where
did Huygens descend" ), can be
confirmed with help of higher resolved Cassini ISS images that have
been published recently.
image above shows two different attempts to find the correct
the Huygens Mosaic (bottom) and the Cassini ISS images (top). The
scaling of the mosaic in the upper right image bases on the calibration
of the Huygens images, the correlation in the upper left is a visual
best fit between both images, (Huygens mosaic and Cassini ISS images).
At first glance, this visual best fit seems to be plausible, but it
contains the problem that with the necessary image scaling the absolute
size of the Huygens frames as well as the height above ground
values are by a factor of roughly 2.5 different from what was
originally published with the corresponding Huygens data. Nevertheless,
this visual best fit still is my favorized Cassini to Huygens
and in the following I will give a detailed analysis of the accordances
In Figure 2, Cassini´s Titan Mosaic, as published by the ISS Team
at the Space Science Institute, is presented in a version with only the
outline of the Large Surface Coverage Huygens Mosaic (top) as well as
with the Mosaic as an inset (bottom) in the position and size I found
to fit Cassini´s ISS Titan images best. This representation is
useful for an attempt to correlate feeble details in the Cassini images
with landscape features visible in the Huygens Mosaic.
an interpretation of ESA scientists the bright surface structures may
be outcrops of water-ice bedrocks, and the medium dark regions are dry
or muddy, lower level areas, where dark hydrocarbon mixed soil has
accumulated, I will refer to as "highlands" for the bright areas and
"lowlands" for the dark ones.
coarse correspondence of the outline of the bright triangular shaped
"highland" with this portion of Titans surface was already presented in
the comparison with the Cassini VIMS
data, and is also present in the
In the following I will concentrate on smaller details that become
visible because of the higher resolution of the ISS images with respect
to the VIMS data.
In the figure above, several areas in the Huygens Mosaic that exhibit a
slightly different albedo than their neighboring regions are marked
with lines. At a close inspection of the Cassini ISS images,
corresponding regions can be identified. For all the following detailed
inspections it should be taken into account, that a sometimes
extensive, subjective photometric correction has been carried out on
the individual Huygens frames, that is expected to be not perfect.
The patches labeled a), b) and c) appear slightly brighter than the
this roughly triangular shaped "highland". Corresponding patches can be
found in the Cassini ISS albedo map. Especially for a), even some
details within this area seem
to be visible in the Cassini images. The lower (southern) part of a)
exhibits two patches that are surrounded by dark channels, at close
inspection corresponding features can be seen in the Cassini maps
(better seen in Figure 2 because of its higher resolution). The
darker area between a) and b) seems to be a little bit narrower in the
Cassini images, this may be an effect of an imperfect photometric
correction in my mosaic. I suppose the same for the area d), that
should be darker
when compared with the Cassini images. The large delta like structure
north of d) finds its correspondence in the Casini images trouble-free.
northern boundary of the "highland" at e) is also represented in the
ISS mosaic. The bright patch f) seems to be invisible in the Cassini
images; I suppose my photometric correction of this area to
overrepresent it with respect to the truth.
The dark "lowland" that is present in g, as
well as northwest, north, and east of g), is a strong argument that the
upper right correlation in Figure 1) following the Huygens size
calibration, is wrong. The shape boundary of the "highland" should be
clearly different in this area when compared with the Huygens
albedo maps. The fact that the area in g) is actually much darker than
the highlands is confirmed when looking at the other panoramas (see "Panorama of the Shoreline " and "Huygens Distant View") as well as
from close inspection of the corresponding
South of f) a brighter patch of the surface is visible in
the Cassini ISS image (h); this corresponds in shape and size to a
large brighter area protruding a little from the south into the Huygens
Mosaic. Huygens Distant View shows a little bit more of this patch
(frames 424-464); a slightly darker, elongated spot may also be visible
in the distant view (frames 525, 494, 473) as well as in the Cassini
ISS images (above label h).
Below i) the ISS albedo map shows another
striking bright patch; looking at Huygens Distant View in the version
that is not
flatfield corrected, a similar bright patch can be spotted at
the horizon (frames 605, 566, 509, 404, 436) that is in nice
correspondence concerning shape, size and
distance. With the Huygens size calibration (Figure 1 upper right) this
patch should be clearly much farther apart, invisble behind the hazy
horizon, and no other bright surface features of that size should be
present in that area of the panorama.
There are some more
correspondencies of feeble details you may find in the Cassini images
to structures visible in the Huygens mosaic. Many of them would be more
or less close to the limit of speculation, so I will
not mention them here.
To sum up the results,
this comparison of the Cassini ISS images with
Huygens Large Surface Coverage Mosaic, shows a lot of accordances,
but no strong discrepancy
for the Mosaic positioning and scaling following my "visual best fit"
(Figure 1 upper left). If the Huygens size calibration (Figure 1 upper
right) is maintained, very little correspondence, but many
Taking into account, that the Huygens altitude information above 40 km
height based mainly on a timing sequence, not on an independent
altimeter, an error as large as a factor of 2.5 may easily occur;
unexpected upcurrents for instance may have delayed Huygens descend.
the correlation presented here can be definitely approved, an
reliable scalebar for Huygens images and a calibration for all other
dependent data is found.