With reference to Fig. 1, the classical bright and dark regions are well visible in the red images ( = 0.8 µm), while the contrast almost vanish in the bluest. Condensates are visible on Hellas and North pole; limb clouds and equatorial haze are also visible. The lowering contrast of the Martian features at short wavelengths is due to the spectral characteristics of the iron oxides which dominate the Martian surface. As the wavelength becomes shorter and shorter, Syrtis Major tends to disappear while northern hemisphere becomes darker than the equatorial region and comparable in brightness to the south dark units. Any spatial detail in the northern hemisphere is completely lost, as can be seen by comparing the 0.41 µm image with the 0.8 µm, where a dark unit on Vastitas Borealis is visible. In summary, even a coarse examination of the data set in the form of multispectral images, shows the occurrence of contrast reversal in diverse regions of Mars. Let now focus our attention on the spectra. We have plotted in Fig. 2a spectra of the main albedo units of Mars visible in the red images. Each spectrum is an average of a 22 box centered on the area of interest; bright, intermediate and dark units have been sampled. Mean latitude, longitude, incidence and emission angles are shown in Table 2.
The spectra of low to intermediate albedo regions show a negative slope throughout the visible near infrared. In the 0.4-0.6 µm range, we clearly see the occurrence of crossover of the reflectance curves for some areas. In order to better identify the spectra at short wavelengths, another plot is shown in Fig. 2b, where spectra relative to Arabia are shown. For example, the spectrum of Elysium (brown) crosses the Arabia (yellow) at 0.519 µm, while the Elysium (dark green) crosses the Arabia (yellow) at 0.455 µm. The Utopia spectrum crosses the Syrtis Major (green) at 0.544 µm. Sinus Meridiani crosses Syrtis Major (green) at 0.519 µm and Acidalia-Vastitas at 0.485 µm. The two Elysium spectra cross themselves at 0.614 µm. Syrtis Major (green) crosses Arabia (yellow) at 0.425 µm. Other interesting findings are: 1) the coincidence of Sinus Meridiani and Syrtis Major (red) spectra in the 0.6-1 µm range while they diverge shortward of 0.598 µm; 2) the overlap of Hellas cloud-Syrtis Major (green) in the 0.6-1 µm range while they diverge shortward of 0.628 µm.
© European Southern Observatory (ESO) 2000
Online publication: October 30, 2000