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Table of contents
List of Acronysm
2. Map production
3. Spectral indices and RGB map
The purpose of this deliverable is to describe the parameters defined as diagnostics of the spectral variability of the Hokusai Quadrangle. We introduced the meaning of the spectral parameters to summarize the spectral information, and the dataset used for the 8-color mosaic of Mercury surface. We report the procedure used to produce the mosaic, and the projection. We have defined spectral parameters and RGB, which are important to develop in the future the spectral units of the Hermean surface. In particular, because of the spectral properties of Mercury (featureless) and the multispectral dataset (between 400 and 1000 nm), we could not define spectral parameters related to absorption bands. Therefore, we defined two parameters related to the reflectance (value of reflectance at 750 nm, and the PC1) and two parameters related to the slopes (spectral slope between 966 and 433 nm, and the PC2). Moreover, we have produced three different RGBs, describing the color variability influenced by reflectance and slope variations, that are in turn indicative of differences in spectral properties associated with terrain composition and maturity.
List of Acronysm
Mercury Dual Imaging System
Wide Angle Camera
Narrow Angle Camera
Experiment Data Record
Digital Elevation Model
Integrated Software for Imagers and Spectrometers
Principal Component Analysis
Red - Green - Blue
U.S. Geological Survey
MErcury Surface, Space ENvironment, GEochmistry, and Ranging
Istituto Nazionale di AstroFisica
Istituto di Astrofisica e Planetologia Spaziali
The first step was to produce the 8-color (filter) mosaic of the Hokusai quadrangle, necessary to derive the spectral indices and the RGB maps required. Among the 11 WAC filters devoted to the scientific analysis, we considered the 8 filters with the higher number of images that cover the whole quadrangle at a spectral range between 400 and 1000 nm (see Table 1). MDIS-WAC was equipped with a multi-filter rotating wheel, and the images were acquired one by one for each filter, therefore images captured in different moments do not exactly overlap each other. Furthermore, an upstream filter selection has been done during the acquisition. Among the 10,373 images available for the Hokusai quadrangle only 5,024 are useful for our purpose. These images cover a large spatial resolution interval (103-171920 m/px) then a further selection on spatial resolution is needed. We selected images with a spatial resolution lower than 1000 m/pixel, choosing for the final mosaic an average spatial resolution of 450 m/pixel.
Fig.1: Mercury quadrangle scheme. Red square indicates the H-5 Hokusai quadrangle.
2. Map production
The steps applied to produce the 8-color mosaic of Hokusai quadrangle are the following:
The spectral indices and RGB color combination choice is helpful to enhance compositional variations and to understand the geology of a surface. Each surface is characterized by specific spectral features, therefore a proper spectral parameters selection is needed for each body. Mercury surface spectra are characterized by the lack of evident absorption bands in the visible and near-infrared. Only a shallow band centered at 600 nm has been identified in very localized regions, in correspondence of sublimation depressions called hollows (Vilas et al., 2016; Lucchetti et al., 2018, in press), or associated to low reflectance materials (Klima et al., 2018). At the global scale, the Hermean surface is characterized by reflectance and spectral slope variations. The main goal of this delivery is to identify spectral indices and RGB color combinations that may emphasize spectral variations across the Hokusai quadrangle. Below we report the spectral indices and RGB maps selected to better point out the Hokusai quadrangle spectral diversity (see Table 2 and following figures).
Table 2 - summary of calculated parameters/RGB images.
Reflectance at 750 nm (Fig.2)
PCA first channel, reflectance variation (Fig.3)
PCA second channel, color variation (Fig.4)
(R996- R430)/((996-430)*R433 (Fig.5)
RGB "Enhanced Color Mosaic"
R: PC2, G: PC1, B: 430/996nm (Fig.6)
RGB "False Color Mosaic"
R: 996 nm, G: 750nm, B: 430 nm (Fig.7)
RGB "Clementine Color Mosaic"
R: R(748 nm)/R(430 nm), G: R(748 nm)/R(828 nm), B: R(430 nm)/R(748 nm) (Fig.8)
Figure 2. R750: Reflectance at 750 nm is a standard wavelength useful to emphasize reflectance variation where often no absorption bands occur. The linear stretch applied is 0.060-0.215. This parameter permits to differentiate bright material to darker one, and it has been used to threshold different color units (see Murchie et al., 2015).
The PCA allows for converting a set of possibly correlated variables into a set of linearly uncorrelated variables called principal components by applying an orthogonal transformation. Almost all the information is contained in the first bands. We performed it using the PCA ISIS3 command.
Figure 3. PC1: Here, we show the PC1, with a linear stretch between 0.10 and 0.45. This parameter is principally able to enhance reflectance variations.
Figure 4. PC2: Here, we show the PC2, where the applied linear stretching is: 0.03-0.10. This parameter permits to discriminate color variations, which in the case Mercury is strongly correlated with slope variability.
Figure 7: RGB false color mosaic: Hokusai quadrangle, R: 996 nm, G: 750 nm, B: 430 nm. This RGB combination emphasizes the color variations on Mercury surface, showing regions where the reflectance at longer (red), intermediate (green) and shorter (blue) wavelengths dominates. The linear stretches are 996 nm: 0.040 - 0.130, 747 nm: 0.060-0.215, 430 nm: 0.080 - 0.245.
Figure 8 : RGB "Clementine Color Mosaic" color assignments similar to those used for the Clementine maps of the Moon (Pieters, C.M. et al., 1994): RED = R(748 nm)/R(430 nm), GREEN = R(748 nm)/R(828 nm), and BLUE = R(430 nm)/R(748 nm). This color combination can introduce information related to the maturity, mafic or glassic components, secondary elements locally concentrated. In Hokusai mosaic, this color composite highlights regions with differing spectral characteristics. Yellow areas typically represent high-reflectance material and blue areas indicate low-reflectance material and crater rays. Green areas represent regions with higher intermediate slope, and red areas have steeper visible slopes relative to bluer areas. The orange/red regions are those with the steepest visible slopes. The linear stretch applied are R(748 nm)/R(430 nm): 1.47 - 1.83, R(748 nm)/R(828 nm): 0.83 - 0.94, R(430 nm)/R(748 nm): 0.54 - 0.68.
J.N. Cuzzi et al. 2009. Ring particle composition and size distribution, in Saturn from Cassini-Huygens, edited by M.K. Dougherty, L.W. Esposito, and S.M. Krimigis, Springer, Berlin, 459-509, Doi:10.1007/978-1-4020-9217-6 15
F. Vilas et al. 2016. Mineralogical indicators of Mercury's hollows composition in MESSENGER color observations. Geoph. Res. Lett., Doi: 10.1002/2015GL067515
Map template used to project the images with ISIS3:
PixelResolution = 450.0 <meters/pixel>
N.B.: The map conversion from equirectangular to lambert conformal projection has been done by using QuantumGIS: