Lunar and Planetary Science XXX 1822.pdf

THE GLOBAL COLORS OF GANYMEDE AS SEEN BY GALILEO SSI. T. Denk1, K.K. Khurana2, R.T. Pappalardo3, G. Neukum1, J.W. Head3, T.V. Rosanova4, and the Galileo SSI Team, 1DLR, Institute of Planetary Exploration, 12484 Berlin, Germany, e-mail: [email protected], 2UCLA, Los Angeles, CA, 3Brown University, Providence, RI, 4USGS, Flagstaff, AZ.

Ganymede, as observed by the Galileo SSI Dark vs. bright and polar terrain. The bright camera, shows a banded, latitude-dependent color ("sulci") and dark ("regio") areas as well as the polar structure which is partly independent of geologic caps are the most obvious surface features on Ganyme- units. A correlation of the surface color with the de when seen at global scale from large distances. The magnetic field of Ganymede is reported, with areas albedo of the polar caps on the leading side is highest, exposed to the charged particles coming from the of the "regio" areas lowest, and of the "sulci" areas in Jovian environment often being redder than shielded between. (The polar caps of the trailing side will be terrain. The northern polar cap can be subdivided discussed below.) The bright polar caps are probably into a whitish area on the pole and the leading side, caused by water frost (e.g., Smith et al. 1981, Hillier and a darker, reddish area on the trailing side. The frost of the south-polar cap appears less opaque than in the north. The spectra of the dark "regio" areas are redder at the long SSI wavelength range than those of the brighter "sulci" terrains, but not significantly different at short SSI wavelengths. Some dark craters like Kittu are spectrally very different to the regular dark terrain. Data. During the primary mission and GEM, four global color observation sequences of Ganymede have been obtained. They completely cover the whole surface except for a larger gap from 210 to 230¼W and small gaps near the poles. The E6 and E14 data cover approximately the same area (jupiter-facing side/trailing side), with the location of the terminator being reversed. E6 has more filters, but E14 has better com- Fig. 1: Color ratio and albedo images of Ganymede pression. Numbers are given in Table 1. The spectral from G1 SSI data, showing the Galileo Regio and sensitivity of SSI covers the wavelength range from 0.4 Uruk Sulcus hemisphere (a-c), and from E14, showing to 1.0 um. Hereafter, the range from 0.4 to 0.6 um is the jupiter-facing part of the trailing side (d-f). See also referred to as the "short wavelength range", and the Table 1 for details. a,d: VLT/GRN ratio images. Low wavelengths from 0.6 to 1.0 um to as the "long wave- DNs indicate steep, positively sloped spectra, bright length range". The spectral slopes of measured areas at DNs indicate less reddish spectra. b,e: Enhanced color short wavelengths are expressed by the "violet-to-green images (filters 1MC, GRN, VLT). c,f: 1MC/GRN color ratio" (VLT/GRN), and at long wavelengths by ratio images. Dark areas have flat spectra, while bright the "1-micron-to-green color ratio" (1MC/GRN). areas show positively sloped spectra. Ñ Note that the dark area in the center of Fig. a is not correlated with Table 1: Galileo SSI global observation sequences Uruk Sulcus. ÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑ of Ganymede. ÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑ et al. 1996). The colors of all terrains on Ganymede Spatial Sub-S/C have always a positive spectral slope at the short SSI Orbit Filters resolution cen-lon. Phase wavelength range, and are flat or slightly red from 0.6 ÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑ to 1 um (wavelength range that was not observable by G1 VGrNMO 13.3 km/pxl 155¼W 30.4¼ Voyager). This positive slope at short wavelengths is E6 VGnmO 14.2 km/pxl 323¼W 37.0¼ found everywhere, even in the "iciest" areas. It indi- C10 VGNmo 33.7 km/pxl 17¼W 0.2¼ cates that a violet-absorbing material of so far unknown E14 VGO 18.4 km/pxl 306¼W 10.4¼ composition covers Ganymede globally, similar to the ÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑ situation on Europa (Denk et al. 1998a, 1999). V=violet (VLT, 414 nm); G=green (GRN, 559 The spectral slopes of the "regio" and "sulci" areas nm); R=red (RED, 664 nm); N=near-Infrared (NIR, differ systematically in the long SSI wavelength range, 757 nm); M=methane 2 (MT2, 888 nm); O=one with the dark "regio" areas showing redder spectra micron (1MC, 990 nm). Small letters indicate images (Fig. 1c,f). However, at short SSI wavelengths, the with gaps. "regio" and "sulci" areas cannot be distinguished anymore (Fig. 1a,d). The subdivision into dark "regios" and bright "sulci" is also commonly used for Lunar and Planetary Science XXX 1822.pdf

GANYMEDE GLOBAL COLORS: T. Denk et al.

geologic units. For long SSI wavelengths, Ganymede's However, the color/OCFLB correlation is poorer for color properties correlate close to these geologic units. the southern hemisphere. This might be due to The bright polar caps on the leading side show incomplete magnetic-field modelling, work is currently significantly flatter spectra than the "sulci" areas at in progress. Additionally, the best color/OCFLB fit is short SSI wavelengths, and have comparable spectral found at areas where the spacecraft passed over (all slopes at long wavelengths (Fig. 1). The relatively flat flybys were over the northern hemisphere), suggesting spectra of the polar caps and the "sulci" terrains at long that the color boundaries might even be used to wavelengths, compared to the "regio" terrains, are improve the models of Ganymede's magnetosphere. consistent with a higher amount of water ice or frost in High southern latitudes. The frost of the southern these areas, supporting the often stated idea that the polar cap is less opaque and presumably coarser albedos on Ganymede are roughly correlated with the grained than in the north, but it is also elongated ice content (e.g., T.V. Johnson et al. 1983). towards the leading side and shows the reddish High northern latitudes. The northern polar cap appearance on the trailing side. On the leading side, it can be subdivided into a "whitish", ellipse-shaped, reaches latitudes as low as 25¼S, the lowest latitude of frost-covered area close to the pole which is elongated any part of the polar caps on Ganymede. The towards the leading side, and a reddish-colored area appearance of the reddish terrain of the southern cap on located at mid-latitudes on the trailing side (Fig. 1b,e). the trailing side is similar to the northern one, with the The "whitish" part is completely located within the "whitish", polar area being somewhat smaller than in part of Ganymede's surface that is not shielded by the the north. The difference of the caps on the leading side satellite's own magnetic field against the Jovian plasma might be contributed to the underlying terrain: North environment. It extends down to about 40¼N in of the equator, the large Galileo Regio and Perrine Xibalba Sulcus at 90¼W (leading side), while it reaches Regio dominate the leading side, while the southern only latitudes down to about 60¼N on the trailing side. hemisphere virtually completely lacks dark terrain and Instead of "whitish" frost, as found on the leading side, should be slightly colder on the surface. This implies a reddish color has been observed at mid-latitudes on that thermal effects, beside magnetospheric effects, may the trailing side. Its relatively high VLT/GRN ratio influence the polar cap location. (Fig. 1a,d) appears consistent with a frost coverage, Leading/trailing side dichotomy. At short but its albedo is unusually low for frost (Fig. 1 b,e), wavelengths, an increase of the spectral slope is found and the long wavelength spectra (Fig. 1 c,f) are quite on the trailing side (Fig. 1d). This indicates that the steep; findings that are inconsistent with pure water ice violet-absorbing material is either more stable on the or frost. However, E6 spectra from the Galileo NIMS trailing side, or preferrably implanted by particles instrument show deep water ice absorptions at these swept away from the Jovian magnetosphere. The latter latitudes, suggesting that this reddish terrain might be is rather unlikely because the Ganymedian magneto- described as "unusually dark and red frost" rather than sphere shields the low-latitude parts of the surface "unusual regio terrain". against charged particles. Therefore, some kind of Banded global structure, magnetically shielded erosion on the leading side appears more likely. vs. unshielded. A key role for the global colors on Dark craters. Some dark craters like Kittu (left Ganymede should play the interaction of the magnetic from the center in Fig. 1e) have a very unusual field of this satellite with the Jovian field. At lower appearance for dark material. Their normalized spectra latitudes, Ganymede's surface is "magnetically shiel- are relatively flat, similar to the normalized spectra of ded" against charged particles coming from the Jovian the bright icy craters (for Voyager, this was already field environment, while at mid- and high-latitudes, it reported by Schenk and McKinnon, 1991). Note the is "unshielded" (Kivelson et al. 1998, Khurana et al. bright spot at the location of Kittu in Fig. 1d. This 1999). The boundary between these areas (the "open/ pronounced difference to other dark, but redder areas closed field-lines boundary" or "OCFLB") shows a suggests that the dark material of craters like Kittu correlation with a color boundary in SSI data (Denk et originates from the impacting body. Possibly, C-type al. 1998b, Pappalardo et al. 1998): The dark materials asteroids (with neutral spectra) caused this type of of the "unshielded" terrain measured so far have craters on Ganymede. "redder" spectra at 0.6 to 1 micron than the low- References. Denk, T., et al., LPSC XXIX, abstract 1684, latitude terrains, indicating that charged particles 1998a. Denk, T., et al., poster presented at Galileo/Cassini-Huygens symposium, Nantes, France, 11-15 May 1998. Abstract published should have caused a reddening of the dark material on in: Galileo/Cassini symposium, Abstracts book, p. 49, 1998b. Denk, Ganymede. This correlation is especially obvious T. et al., LPSC XXX, colors of icy Galilean satellites part 1, 1999. within Galileo Regio (Fig. 1). On the trailing side, the Hillier, J., et al., Icarus 124, 308-317, 1996. Johnson, T.V., et al., northern OCFLB is located very close to the low- JGR 88, B7, 5789-5805, 1983. Khurana, K.K., et al., Nature or Science 1999, in preparation. Kivelson, M.G., et al., JGR 103, E9, latitude boundary of the reddish polar cap. This 19963-19972, 1998. Pappalardo, R.T. et al., DPS abstract, BAAS 30, supports the hypothesis of Pappalardo et al. (1998) p.1120, 1998. Schenk, P.M. and W. B. McKinnon, Icarus 89, 318- that water ice is redistributed by sputtering entirely 346, 1991. Smith, B.A., et al., Science 206, 927-950, 1979. within the polar regions.