Atmospheric modeling. An accurate atmospheric model is essential to successful landing, scientific analysis, and the prediction of the possibility of indigenous life. The construction of an atmospheric model for Earth (including composition, structure and dynamics) has taken many years, an iterative process dictated by evolving technology plus the developing knowledge and expertise of investigators in a young field. To a large extent this emerging methodology has been driven by the measurability of accessible variables, which may or may not be optimal from a systems theoretical point of view. But given higher technology, observational freedom from Earth's atmosphere, and fresh unknown territory to explore, many more options become available with respect to what should be measured and in what order to define an atmosphere most efficiently and unambiguously. The process has not yet been adequately systematized to permit clear-cut rational choices.
Atmospheric modeling should begin early in the approach to an unknown planet since many mode-of- exploration decisions require information on the nature of the atmosphere. During the course of the mission the atmospheric model accumulates greater detail with continuous updating as higher sensor resolution is achieved and probes are deployed for direct measurements. The investigation of an atmosphere differs from studies of surface characteristics in that it involves the complex integration of many interrelated subhypotheses and measurements of numerous allied parameters. Studies of the surface are more a problem of deriving hypotheses from completed maps representing different measurements and then overlaying these maps as a final step.
Specific initial tasks related to atmospheric modeling include: ? Determination of the region of the spectrum in which most of the electromagnetic radiation is emitted. ? Determination of the sources of opacity for selection of optimum communications link frequency (for landers and probes) and for choosing wavelengths in which to perform infrared and millimeter radiometry. ? Search for unbroadened spectral lines above the atmosphere to provide information on the overall composition of the air. ? Observe where spectral lines interfere with blackbody temperature measurements and determine the wave- length(s) at which the atmosphere may be fully penetrated and planetary surface temperatures accurately recorded. ? Perform preliminary temperature and pressure measurements, to be updated once a comprehensive atmospheric model has been constructed. ? Begin atmospheric modeling with remote sensing at millimeter and infrared wavelengths.
Surface modeling. The best method for planetary surface structure hypothesis formation requires scanning of the body with sequentially increasing resolution in at least four distinct steps. The first step obtains global average values for temperature, surface structure, composition, etc., and establishes norms for keying future observations at higher resolution. Gross features such as lunar maria and highlands or the martian polar caps would appear in this type of survey.
The second observational phase exposes finer detail, identifying regions on the scale of the Tharsis Plain of Mars or the Caloris basin of Mercury. As the explorer approaches Titan, higher-resolution observations of the surface become possible and morphological changes can be observed in each succeeding frame. Recognition of features such as craters, mountains, rivers, and canyons may be accomplished by an advanced expert system which includes models of surface processes in its knowledge base, although present-day pattern recognition and vision systems will require significant refinement before this capability can be realized.
The third step is the recognition of sites with high potential for usefulness in the construction of world models. Such sites mainly include unusual features that are interesting because of their anomalous nature. Identification requires a stored concept of "usual," as for instance: "There is usually a sharp boundary between continents and oceans" and "Craters viewed from directly above usually are circular." An original supply of these simple concepts are programmed into the system by humans before the mission begins; however, additional and revised definitions of normality must be developed and refined as the mission study of a particular planetary body progresses, with self- developed concepts of "usualness" updated by the system as various stages and modes of multisensor investigation are completed. The recognition of that which is "unusual" is discussed at greater length below.
The fourth and final step includes detailed surveys at maximum resolution of selected sites and additional imaging of various undistinguished sites spaced along a grid to pick up interesting features missed by other searches at lower resolution.
Automated selection of interesting sites. It is desirable to minimize raw data storage in order to maximize the efficiency of onboard concurrent mission tasks and analyses. Some method must be found to deal with the information overload which might result from exhaustive exploratory surveys, particularly high-resolution topographic mapping.
Data preprocessing and compression are needed not only because of memory limitations but also to help reduce the complexity of information to be assimilated into world models. Without some way of narrowing the field of interest or of identifying "highlights," the task of converting multiple correlations of many detailed data sets into complete models is cumbersome and impractical. Simplification mass and power requirements. The major mission accomplishments expected of each
