Topographic Modeling of Phoebe Using Cassini Images
Total Page：16
File Type：pdf, Size：1020Kb
ARTICLE IN PRESS Planetary and Space Science 54 (2006) 1156–1166 www.elsevier.com/locate/pss Topographic modeling of Phoebe using Cassini images Bernd Giesea,Ã, Gerhard Neukumb, Thomas Roatscha, Tilmann Denkb, Carolyn C. Porcoc aDLR-Institute of Planetary Research, Rutherfordst 2, 12489 Berlin, Germany bInstitut fu¨r Geologische Wissenschaften, Freie Universita¨t, 12249 Berlin, Germany cCICLOPS, Space Science Institute, 4750 Walnut Street, Boulder, CO 80301, USA Accepted 4 May 2006 Available online 22 August 2006 Abstract High-resolution Cassini stereo images of Saturn’s moon Phoebe have been used to derive a regional digital terrain model (DTM) and an orthoimage mosaic of the surface. For DTM-control a network of 130 points measured in 14 images (70–390 m/pixel resolution) was established which was simultaneously used to determine the orientation of the spin-axis. The J2000 spin-axis was found at Dec ¼ 78.0170.11 and RA ¼ 356.6170.31, substantially different from the former Voyager solution. The control points yield a mean figure radius of 107.2 km with RMS residuals of 6.2 km demonstrating the irregular shape of this body. The DTM was computed from densely spaced conjugate image points determined by methods of digital image correlation. It has a horizontal resolution of 1–2 km and vertical accuracies in the range 50–100 m. It is limited in coverage, but higher in resolution than the previously derived global shape model of Phoebe [Porco et al., 2005. Cassini imaging science: initial results on Phoebe and Iapetus. Science 307, 1237–1242] and allows us to study the morphology of the surface in more detail. There is evidence for unconsolidated material from a steep and smooth slope at the rim of a 100 km impact feature. There are several conically shaped craters on Phoebe, which may hint at highly porous and low compaction material on the surface. r 2006 Elsevier Ltd. All rights reserved. Keywords: Phoebe; Cassini; Topographic mapping; Digital terrain model; Craters 1. Introduction over the early Voyager data. In a first analysis of the images, we derived a new spin-pole solution as well as a Phoebe is the outermost large satellite of Saturn. It 21 Â 21 global shape model showing the major topographic moves in a retrograde orbit and is therefore believed to be a landforms of Phoebe (Porco et al., 2005). However, about captured object (Pollack et al., 1979; Cuk and Burns, 2004). one half of Phoebe’s surface was covered with high- Unlike its neighbors, Phoebe is not tidally locked in its resolution images to better than 200 m/pixel. These images orbit. Rather, it revolves about Saturn in 550 days and provide multi-stereo coverage suitable for topographic spins at a rate of 9.3 h (Bauer et al., 2004). From the mapping at substantially higher level of detail than analysis of Voyager images obtained early in 1981, a rough revealed by the global shape model. spin-axis orientation (Colvin et al., 1989) and global shape In this paper, we present the results of a photogram- parameters (diameters: 230 Â 220 Â 210 km, Thomas et al., metric analysis of the high-resolution stereo images 1986) could be derived. However, poor image resolutions involving a revisit of the spin-pole analysis, a regional (best resolution 22 km/lp) have prevented detailed mor- digital terrain model (DTM) of the surface, and a DTM- phologic studies of the surface. based orthoimage mosaic. With the DTM in hand, we During Cassini’s encounter with Phoebe on 11 June describe the morphology of craters and discuss implications. 2004, the imaging science subsystem (ISS) on board acquired a series of images with resolutions much improved 2. Data base ÃCorresponding author. Tel.: +49 30 67055 322; fax: +49 30 67055 402. The Cassini ISS consists of two framing cameras, a wide E-mail address: [email protected] (B. Giese). angle camera (WAC) and a narrow angle camera (NAC) 0032-0633/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.pss.2006.05.027 ARTICLE IN PRESS B. Giese et al. / Planetary and Space Science 54 (2006) 1156–1166 1157 (Table 1), each equipped with a large number of spectral to the image resolutions and the instantaneous field of view filters (Porco et al., 2004). The analysis of Phoebe images of a pixel, respectively. But usually the spacecraft orbit has carried out in this paper involved 12 NAC and 2 WAC offset errors in the order of a kilometer or less, and frames (Table 2) selected from imaging sequences taken nominal pointing errors are typically tens of pixels or even prior to and after closest approach (range ¼ 2175 km). hundreds of pixels. Therefore, these errors must be During the acquisition of these frames the sub-spacecraft corrected first. Moreover, the calculation of ground points longitude changed by 1191 (Table 2, col. #3), an indication is indirectly related to the body’s rotational parameters, for the high stereo potential of the data set. Unfortunately, which may also be incorrect. To handle these problems, the phase angles (Table 2, col. #4) reveal that for most of least-squares adjustment techniques using control points the images significant areas of Phoebe were in shadow and can be applied (Giese et al., 1998). therefore exempt from modeling. Moreover, the two WAC The observational data in the adjustment for Phoebe frames suffer from substantial over-exposure (Fig. 1) but consist of the image coordinates of 130 control points were nevertheless included in the analysis to maximize (shown for two of the frames in Fig. 1) and the nominal DTM coverage. The NAC frames are clear filter images camera pointing angles for the 14 images (see Table 2 for and the WAC frames are a filter combination of clear and point statistics). Measurements of Phoebe’s center of figure green. All frames were obtained within a lossless compres- (assumed to represent the body’s center of mass) were also sion mode. made. Image points measured in NAC frames were Spacecraft orbit and camera pointing data were taken extracted at an accuracy of 0.5 pixels and those measured from NAIF-SPICE kernels provided by the Cassini flight in the WAC frames were considered to be accurate within 1 project. pixel only. For measurements of the center of figure, which were carried out by limb fitting, we adopted errors of 2 pixels. The nominal camera pointing data were assumed to 3. Photogrammetric analysis be accurate within 1 mrad (Porco et al., 2004). We considered the ground coordinates of the control 3.1. Least-squares adjustment points and the camera pointing angles as unknowns in the adjustment model. The spacecraft orbit was fixed, as there Terrain modeling at the stage of ground point calcula- is not sufficient control information to correct the (minor) tions requires precise orbit- and camera pointing data close orbit errors and the pointing errors at the same time. While we made an attempt to estimate the rotational axis Table 1 orientation of Phoebe, we anticipate that its rotational ISS camera parameters used in the modeling rate (which was recently re-determined by Bauer et al. NAC WAC (2004)) cannot be improved by our measurements. In the technical realization of the least-squares adjust- Sensor CCD framing cameras ment software, the analysis is performed in the body-fixed (1024 Â 1024 pixels) frame. Thus, the rotational axis of Phoebe cannot be Pixel size (mm) 0.012 0.012 estimated directly but is incorporated in the conversion of Focal length (mm) 2003.44 200.77 FOV center (pixel) 512.5 512.5 the spacecraft positions and the camera pointing angles Radial distortion 0 0 from the inertial J2000 frame to the Phoebe-fixed frame. Hence, in order to obtain a spin-axis solution for Phoebe coordinate conversion and least-squares adjustment were Table 2 carried out repeatedly until a minimum misfit between the Phoebe-stereo images involved in the analysis observations and the predicted values (RMS of the Frame Res (m) s/c long (deg) Phase (deg) ctrl # residuals) was achieved. N1465664794 390 63 85 20 3.2. Results of the least-squares adjustment N1465668387 254 26 85 53 N1465669778 201 11 84 48 N1465669953 194 10 84 74 Normalized misfits for the north pole declination and N1465672687 91 345 80 20 right ascension of Phoebe are given in Fig. 2. It shows that N1465672842 85 344 79 19 the pole declination is well constrained at 78.0170.11 and N1465672905 83 343 79 29 differs from the former Voyager value by 11.31. The right N1465673021 78 342 79 43 17 1 N1465673138 74 342 78 28 ascension is less constrained at 356.6 0.3 and closer to W1465674502 243 348 59 51 the Voyager result. Within the error limits, this spin pole W1465674830 151 9 39 51 solution agrees with the solution we obtained earlier within N1465677386 89 97 85 16 the framework of global shape modeling (Dec ¼ 77.91, N1465677447 91 97 85 26 RA ¼ 356.61; Porco et al., 2005). N1465679337 162 80 89 35 Although the main purpose of the least-squares adjust- Res: image resolution; ctrl: measured # of control points. ment was to obtain improved camera pointing data for the ARTICLE IN PRESS 1158 B. Giese et al. / Planetary and Space Science 54 (2006) 1156–1166 Fig. 1. Two of the frames used in the modeling: NAC frame N1465668387 (left) and WAC frame W1465674830 (right). Significant surface portions of Phoebe are in shadow (left, phase angle ¼ 851), and large parts of the WAC frame are saturated. Dots mark the locations of control points measured in the images, and arrows (left image) show the location of the highest (left) and lowest (right) point within the modeled area (Chapter 3.2).