Mission Objectives Overview  ===========================    Exploration of Mars, and the Viking Mission in particular, has    been part of a larger quest -- the search for better    understanding of the formation and history of the solar system.    For Mars, the specific objectives have included:         1) evolution and current structure of its interior;        2) characteristics of the surface, including its chemistry           and physical nature;        3) evolution and current composition and structure of its           atmosphere;        4) nature of the climate, including controls on both daily           and seasonal variations;        5) whether life is, or ever has been, present.     Although most Viking investigations could be defended on one or    more of the first four objectives, virtually all secondarily    addressed the fifth.  On the other hand, the investigations    which focused primarily on objective #5 barely scratched the    surface of that single question; the nature of life -- and    especially its expression on another planet -- is not well    understood.  According to [SOFFEN1977], 'It was finally decided    to send a set of biological tests that range in their    environmental setting from a totally aqueous milieu, rich in    organics, to a Marslike environment with no water or any other    additives.  Even so, only a very narrow set of all    possibilities could be tested on the small samples    acquired ...'.     The Viking investigations and their primary objectives are    summarized in the paragraphs below.  More information is    available in [SOFFEN1977], [SNYDER1977], [SNYDER1979], and    [SNYDER&MOROZ1992].      Orbiter Imaging    ---------------      An early objective was assisting in landing site selection      and certification.  Once the landers were safely in place,      the Orbiter imaging system was used to provide a geologic      context for the surface observations.  Globally, images were      collected to provide high-resolution mosaics and maps at      resolutions approaching 100 meters.  Stereo pairs of images      could be used to derive local topography; photoclinometry      could be used on single images to derive elevations and      slopes at lower accuracy.  Images were also used to infer      the origin and history of major terrain types, including      disruptive events such as apparent catastrophic floods.      Crater morphologies which suggest a permafrost layer pointed      toward complex interactions of regolith and atmosphere.      The Orbiter imaging system was also used to monitor      atmospheric changes including clouds, hazes, and suspended      particles.  Images of the satellites Phobos and Deimos      showed their surfaces from distances as close as 100 km.      Mars Atmospheric Water Detector (MAWD)    --------------------------------------      MAWD was designed to measure the water vapor content of the      atmosphere from orbit.  Patterns were sought as a function      of local time, season, latitude, and elevation.  Objectives      of the investigation included better understanding of both      diurnal and seasonal transport of water vapor as well as      location of sources and sinks.      Infrared Thermal Mapping (IRTM)    -------------------------------      IRTM measured reflections and emissions in several infrared      bands from orbit.  These data could be used to infer the      physical properties of surface materials including the      relative proportions of rock, sand, and dust.  Apparent      surface temperatures were used to infer the composition of      polar ices, assisting in development of atmospheric      circulation models.      Radio Science    -------------      Radio tracking of the Landers allowed determination of their      positions on the surface, the planetary rotation axis, the      spin rate, and moment of inertia.  Tracking of the Orbiters      allowed determination of a gravity field for Mars.  Radio      occultations yielded planetary radii and atmospheric      temperature-pressure profiles at dozens of locations.  Radio      observations were also conducted to measure structure in the      solar corona and to test a prediction of general relativity      associated with passage of the radio path through the Sun's      gravitational field.      Entry Science    -------------      During descent each landing module measured both the physical      structure and chemical composition of the atmosphere.  The      composition of the ionosphere allowed inference of dominant      reactions.  At lower altitudes isotopic ratios could be used      to infer age of the atmosphere and an earlier composition.      Measurements such as mean molecular weight, density profile,      and composition near the surface could be used to interpret      measurements from other instruments.  Measurements at      different altitudes could be used to determine how well the      atmosphere was mixed.      Lander Imaging    --------------      Lander images were used to select samples for testing in the      biology and physical properties investigations; they were also      used to select sites for experiments using the sampler arm.      Images recorded trenches that were dug, rocks that were      overturned, footpads that penetrated the surface, and magnets      that were covered by iron-bearing loose material.  Images were      used to determine the distribution and appearance of rocks      and other materials near the landing sites, leading to      improved understanding of both the local area and its      history.  Images of the atmosphere were used to estimate the      opacity due to suspended particles; images of materials at      the site were used to infer wind stress and rates of erosion.      One unfulfilled objective of Lander imaging was detection of      signs of life at each site.      Physical and Magnetic Properties    --------------------------------      The sampler arm and sample collector on each Lander were      used in conjunction with the Lander imaging system to      determine density, cohesion, and other physical properties      of the surface material.  Repeated failure to collect rocks      in the 1 cm size range suggested they are scarce, which has      implications for creation and destruction of material in      that size range.  Visual evidence that magnets were      saturated was important in estimating the concentration and      state of iron in surface particles.      Seismology    ----------      The objectives of the Lander seismology investigation were      to detect seismic events or to set limits on the activity      level of Mars compared with Earth.  One local event was      detected at VL2, allowing estimation of crustal thickness      and damping.  In practice the seismology investigation      supported the meteorology investigation since most seismic      signals turned out to be caused by wind.      Meteorology    -----------      The Lander meteorology investigation sought to characterize      local atmospheric conditions; those in turn would constrain      global models.  Diurnal and seasonal trends were sought;      effects of dust storms were also measured.      Inorganic Chemistry    -------------------      Elemental compositions of soils at each Lander site were      determined using X-Ray Flourescence Spectrometers.  Results      were to be compared with compositions of terrestrial analogs      but were found to be 'dissimilar to any single known mineral      or rock type' [TOULMINETAL1977].  With addition of physical      properties, the recent history of duricrust could be inferred.      The fact that these materials were very similar at the two      Landing sites can be used in modeling transport of dust and      other small diameter particles.      Molecular Analysis    ------------------      The Gas Chromatograph Mass Spectrometer gave composition of      the atmosphere at each landing site; the result was      consistent with the entry science composition.  Isotope ratios      were used to infer the amount of outgassing and, from that,      the volume of volatiles which may have been lost from Mars      over geologic time.  Surface samples were analyzed in an      attempt to detect organics and to measure the amount of water      present.  Both are questions important in the search for life.      Biology    -------      The original objectives of the Lander biology experiment were      to detect presumed Martian life forms by their release of      metabolic products upon addition of heat, water, a dilute      aqueous solution of simple nutrients, and a concentrated      mixture of many organic compounds.  After sudden and      surprising positive results, which were not consistent with      expectations or with other observations, the objectives were      expanded to include abiotic interpretations.