Appendix A: Use of Multiple Sensors for Surface Observations

The detailed study of a planetary surface or atmosphere requires the simultaneous use of multiple sensors covering a large part of the electromagnetic spectrum. This is a result of the fact that any individual sensor covers only a small part of the spectrum in which the wave–matter interaction mechanisms are driven by a limited number of the medium properties. For example, in the case of solid surfaces, x-ray sensors provide information on the content of radioactive materials, visible and near-infrared sensors provide information about the surface chemical composition, thermal infrared sensors measure the near-surface thermal properties, and radar sensors are mainly sensitive to the surface physical properties (topography, roughness, moisture, and dielectric constant). Similarly, in the case of the atmosphere, in order to cover the wide range of possible chemical constituents, detect and characterize atmospheric particles (including rain), and sound the physical properties of the atmosphere, a suite of sensors covering selected bands in the visible, infrared, millimeter, and microwave spectral regions will be needed. To illustrate howmultiple sensors can be used collectively to enhance the ability of an interpreter in the study of a planetary surface, a set of data products covering the area of Death Valley in eastern California are presented. Figure A.1 shows three images of Death Valley acquired with three separate instruments in the visible/near IR (Figure A.1a), thermal IR (Figure A.1b), and radar (Figure A.1c) spectral bands. With the topography database, false illumination images can be generated to highlight the surface topography (Fig. A.2). This topography database can then be coregistered to the multispectral image data (Fig. A.1) and used to generate perspective images from a variety of observing directions, as illustrated in Figures A.3 and A.4. The observing direction, the vertical exaggeration, the spectral bands, and the color coding can be selected by the interpreter and displayed on a monitor instantaneously. This will effectively be equivalent to bringing the study site into the laboratory for detailed “dissection” and analysis. Of course, there will always be the need to do field work for direct surface observation, but the above-described database will go a long way in developing a basic understanding of the surface properties. 507