PRE-PRINT
High Resolution Enceladus Atlas derived from Cassini-ISS images
Th. Roatsch(1), M. Wahlisch(1), B. Giese(1), A. Hoffmeister(1), K.-D. Matz
(1), F. Scholten(1), A. Kuhn(1), R. Wagner(1), G. Neukum(2), P.
Helfenstein(3), and C. Porco(4).
(1) Institute of Planetary Research, German Aerospace Center (DLR), Berlin,
Germany; (2) Remote Sensing of the Earth and Planets, Freie Universitat
Berlin, Germany; (3) Department of Astronomy, Cornell University, Ithaca, NY,
(4) CICLOPS/Space Science Institute, Boulder, CO. (Thomas.Roatsch@dlr.de)
Abstract
The Cassini Imaging Science Subsystem (ISS) acquired 377 high-resolution
images (< 1 km/pixel) during three close flybys of Enceladus in 2005 (Porco
et al., 2006). We combined these images with lower-resolution Cassini images
and four others taken by Voyager cameras to produce a high-resolution global
controlled mosaic of Enceladus. This global mosaic is the baseline for a
high-resolution Enceladus atlas that consists of 15 tiles mapped at a scale of
1:500,000. The nomenclature used in this atlas was proposed by the Cassini
imaging team and was approved by the International Astronomical Union (IAU).
The whole atlas is available to the public through the Imaging Team's
website [http://ciclops.org/maps].
Keywords: Cassini, Icy Satellites, Planetary Mapping, Saturnian system
1. Introduction
The Cassini spacecraft started its tour through the Saturnian system in July
2004. The Imaging Science Subsystem onboard the orbiter consists of a high-
resolution Narrow Angle Camera (NAC) with a focal length of 2000 mm and a
Wide Angle Camera (WAC) with a focal length of 200 mm (Porco et al., 2004).
One of the main objectives of the Cassini mission is to investigate the icy
Saturnian satellites. Enceladus, the second innermost of the medium sized
satellites, was imaged by the Cassini spacecraft during three close flybys
(Table I, Porco et al., 2006). The images taken during these flybys together
with lower resolution frames allowed us to create a global mosaic of
Enceladus with a spatial- resolution of about 110 m/pixel. Unfortunately, the
Cassini ISS has not yet imaged the northern high latitude regions (> 67
degrees) because they are shrouded in seasonal darkness and will not be
illuminated by the Sun until later in the decade when the Cassini extended
mission begins. Fortunately, the Voyager camera was able to take images from
these regions during its flyby in the early 1980's. We thus used Voyager
images to fill the North Polar gaps in the global mosaic.
Details of the image processing will be described in Section 2. Section 3
summarizes the high-level cartographic work that produced our high-resolution
atlas, which consists of 15 maps of the different regions of Enceladus. Three
examples of these maps are shown. A brief overview of future work is given
in Section 4.
2. Data Processing
2.1. Image processing
The image data returned from the spacecraft are distributed to the Cassini
imaging team in VICAR (Video Image Communication and Retrieval) format
[http://wwwmipl.jpl.nasa.gov/external/vicar.html]. The first step of the
image processing is the radiometric calibration of the images using the ISS
Team's CISSCAL computer program (Porco et al., 2004). Next, the specific
subset of images that will be used to construct the global basemap is
selected. At the time of this writing, a total of 2353 images of Enceladus
are available. This total data set contains images obtained through a variety
of different ISS colour filters and at spatial resolutions ranging from 3
m/pixel up to 14 km/pixel. For our mosaic, we selected only those images
taken with the filters CL1, CL2 or GRN, as these images show comparable
albedo contrasts among different Enceladus terrains. 50 Cassini NAC images,
one Cassini WAC image (Table II), and four Voyager high-resolution images
were used to produce a 40 pixel/deg global mosaic. Figure 1 shows the
location of the individual Cassini images. The resolution of the selected
Cassini images varies between 0.064 and 8.8 km/pixel. The resolutions of the
Voyager images C4398347, C4400044, C4400412, C4400432 are 8.8, 1.85, 1.0
and 1.0 km/pixel, respectively.
The next step of the processing chain is to map project the images to the
proper scale and map format - a process that requires detailed information
about the global shape of Enceladus. The inner Saturnian satellites are best
described by tri-axial ellipsoids as derived from ISS images by Thomas et al.
(2006). The latest radii for Enceladus are 256.6, 251.4, and 248.3 km.
However, to facilitate comparison and interpretation of the maps, ellipsoids
were used only for the calculation of the ray intersection points, while the
map projection itself was done onto a sphere with the mean radius (252.1 km)
(Thomas et al., 2006). The Cassini orbit and attitude data used for the
calculation of the surface intersection points are provided as SPICE kernels
[http://naif.jpl.nasa.gov] and were improved using a limb-fitting technique
(Roatsch et al., 2006). We chose an equidistant map projection as our
reference format. The coordinate system adopted by the Cassini mission for
satellite mapping is the IAU 'planetographic' system, consisting of
planetographic latitude and positive West longitude. But because a spherical
reference surface is used for map projections of the satellites,
planetographic and planetocentric latitudes are numerically equal. The Hapke
photometric model (Hapke, 1993) was applied to adjust the brightness of each
map pixel so that it represents the reflectance that would be observed at the
nadir at 30-deg phase angle. Imaging data viewed at incidence and emission
angles greater than 80-deg were omitted from the map. After photometric
correction, mosaicking was the final step of the image processing (Roatsch
et al., 2006).
2.2. Least-squares adjustment of attitude data
A 3-D control net was set up to correct errors in the nominal camera pointing
data. Here, we applied least-squares adjustment techniques (Giese et al.,
2006). The image coordinates of 88 control points (Fig. 2) collected in 11
NAC images (Table III) were applied as observational data in the adjustment,
and the ground coordinates of the control points and the camera pointing
angles were treated as unknowns. The spacecraft orbit was fixed during the
calculations. In result we obtained improved camera pointing angles and a 3-D
control net with average 1¦Ò errors of 736 m, 335 m, 608 m for the x, y, z
coordinates, respectively. Unfortunately, the control points are not equally
distributed over Enceladus' surface due to missing stereo data around the
prime meridian (Fig. 2). The improved pointing data were used to calculate a
medium-resolution, controlled mosaic. Finally, the high-resolution mosaic
calculated as described in Section 2.1 was registered on the controlled
mosaic to improve its global accuracy and feature definition.
3. Enceladus map tiles
The Enceladus atlas was produced in a scale of 1: 500,000 and consists of 15
tiles that conform to the quadrangle scheme proposed by Greeley and Batson
(1990) and Kirk (1997, 2002, 2003) for large satellites (Fig. 3). A map scale
of 1: 500,000 guarantees a mapping at the highest available Cassini
resolution and results in an acceptable printing scale for the hardcopy map
of 4.5 pixel/mm. The individual tiles were extracted from the global mosaic
and reprojected, coordinate grids were superposed as graphic vectors and the
resulting composites were converted to the common PDF-format using software
that was originally developed for Mars maps, the Planetary Image Mapper
(PIMap) (Gehrke et al., 2000). The equatorial part of the map (-22 degrees to
22 degrees latitude) is in Mercator projection onto a secant cylinder using
standard parallels at -13 degrees and 13 degrees latitude. The regions
between the equator region and the poles (-66 degrees to -21 degrees and 21
degrees to 66 degrees latitude) are projected in Lambert conic projection
with two standard parallels at -30 degrees and -58 degrees (or 30 degrees and
58 degrees, respectively). The poles are projected in stereographic
projection (-90 degrees to -65 degrees latitude and 65 degrees to 90 degrees
latitude). The Mercator maps are 72 degrees in longitude dimension, the
Lambert maps 90 degrees, and the poles 360 degrees. The individual tiles
overlap in the North-South direction by one degree, however no overlapping
region is present in the East-West direction (see Fig. 3). We have produced
the maps using the same scaling factors in overlapping regions at the
matching parallels +- 21.34 degrees and +- 65.19 degrees latitude, 1.0461 and
1.0484 respectively (Snyder, 1987). Using this quadrangle scheme in the
1: 500,000 scale for Enceladus, we get the printed maps with the Mercator
and Lambert projection in the same user-friendly size of 1050 mm width by 750
mm height, and the dimensions of the polar maps are 900 mm by 700 mm. We also
added resolution maps and index maps for every individual tile, showing the
image resolution, the image numbers and the location of the images for every
map, respectively. Three map examples in different projections are shown in
the Figures 4, 5, and 6.
The Cassini imaging team proposed 38 names for geological features, in
addition to the 22 features already named by the Voyager team that are used
in the maps. By international agreement, the features must be named after
people or locations in the medieval Middle Eastern literary epic 'One
Thousand and One Nights'. The locations and dimensions of all previously
known features were measured again on the basis of the Cassini data and were
corrected when necessary. Table IV shows a comparison of the locations
measured on the basis of the Voyager data and the Cassini data for three
craters. The nomenclature proposed by the Cassini-ISS team was approved by
the IAU [http://planetarynames.wr.usgs.gov/]. The entire Enceladus atlas
consisting of 15 map tiles will be made available to the public through the
Imaging Team's website [http://ciclops.org/maps]. The map tiles will also be
archived as standard products in the Planetary Data System (PDS)
[http://pds.jpl.nasa.gov/].
4. Future Work
The Cassini spacecraft will continue its imaging campaign through the
Saturnian system. The next close flyby of Enceladus is scheduled for March
2008. Additional flyby opportunities are currently under investigation for
the extended Cassini mission, which is expected to last until 2010. These
upcoming flybys will help to replace the low-resolution parts of this atlas
with higher resolution image data and to enlarge the control point coverage.
The northern part of Enceladus will be illuminated during the extended
mission providing an opportunity to obtain high-resolution Cassini coverage
of high northern latitudes.
Acknowledgements:
The authors gratefully acknowledge helpful discussions with J. Blue and R.
Kirk (USGS) about the proposed nomenclature for Enceladus features and for
reviewing the Enceladus atlas.
The authors also would like to thank Dave Williams (ASU) and another
anonymous reviewer for their help to improve the paper.
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Figure 1: Global mosaic showing the location of the Cassini ISS images (see
Table II). Mosaic is in simple cylindrical projection with latitude=0 degrees,
longitude=180 degrees in the center.
Figure 2: Global mosaic showing the location of the control points. Mosaic
is in simple cylindrical projection with latitude=0 degrees, longitude=180
degrees in the center.
Figure 3: Quadrangle scheme filled with the 15 Enceladus tiles.
Figure 4: Enceladus map sheet 09: Ebony Dorsum
Figure 5: Enceladus maps sheet 12: Obtah
Figure 6: Enceladus map sheet 15: Damascus Sulcus
Table I: Cassini Enceladus flybys in 2005.
Table II: Cassini images used for the mosaic.
Table III: Images used for the least-squares adjustment of attitude data.
Table IV: Comparison of Voyager and Cassini crater location.