Among the most dramatic images returned from space over the past forty years were those transmitted by the Ranger spacecraft, and those filmed by the Apollo astronauts, during their descents to the lunar surface. These images provided not only impressive views of the Moon, they did so in a particularly memorable way. In addition to unambiguously telling where a spacecraft has landed, images acquired during descent to a planet's surface provide the public with a visual perspective of spectacular, often breathtaking, beauty and excitement. The process of acquiring such images is simple, and the results easily understood by all who see them. Descent imaging provides tangible results for early release to the public and engenders a sense of "being there" not usually available with planetary missions.
Descent imaging systems also provide a crucial link between orbiter and lander observations. They provide context for the lander data as a function of scale (resolution) and area. No other form of observation provides such context. Descent imaging addresses the common themes of the Mars Surveyor Program--volatiles, life, resources--not by measurement of a particular elemental isotope or molecular species, but by searching for landforms representative of surface processes that reflect the martian environment, by providing other instruments the ability to reject or accept their own measurements based on the general and specific geologic context of those observations, and by linking orbiter observations to those made by the lander. In the future, Mars descent images will also provide context for planning the traverses of rovers that roam around the martian landscape, looking for interesting geologic features to study.
The primary scientific contributions of descent images are likely to be through serendipitous observation of primary and/or secondary landforms that show the action of specific processes (e.g., channeling, patterned ground, etc.), and through the constraints placed on relative age relationships of features seen at the landing site (e.g., features imaged at resolutions between millimeters and meters are likely to reflect surface processes that operate on timescales of a few years to a few hundred million years).
The relative importance of descent imaging to science and engineering has evolved with time: the return on investment has, on occasion, been deemed less than for more highly-valued measurements (e.g., the aforementioned chemical analyses). However, the Mars Surveyor program has acknowledged its importance, selecting descent imaging first as a Principal Investigator experiment (for Mars Polar Lander), and then (for Mars Surveyor 2001) as government-furnished equipment to be used jointly by a competitively-selected facility science team (for science studies) and mission engineers (for mission planning and, potentially, landing hazard avoidance on future missions).