1990 Update to Strategy for Exploration of the Inner Planets

8

Program Balance

The question of the relative scientific priority for exploration of the inner planets is discussed in depth in the 1978 report. It emphasizes comparative planetology and the value of planetary studies in understanding the Earth. The report selects limited goals of high importance and focuses on selected planets. The three planets with atmospheres receive the highest priority because, as stated, the comparative planetology of these bodies is a key to understanding the formation and evolution of the Earth, and its atmosphere, oceans, and planetary processes.

The committee endorses the high priority of comparative planetology and believes that significant benefits may be derived from deeper study of Venus and Mars. Advances in our knowledge of the Earth and its environment for life are tied to progress in our understanding of the terrestrial planets as a class. The major environmental changes on Venus and Mars challenge our understanding of the terrestrial environment. Because of their similarities and valuable intercomparisons, the Earth, Venus, and Mars should be a special concern of solar system exploration within an overall balanced program.

Nevertheless, for a full understanding of the planets we need increased knowledge about the Moon and Mercury. These two objects provide unique information about planetary evolution and processes. They record on their surfaces the bombardment history in the inner solar system and give direct information on the processes of impact cratering. They also contain a record of their initial differentiation and magmatic activity prior to about 3.8 billion years ago—information mostly missing on Venus, Earth, and Mars. The history of planetary accretion and differentation is a major unsolved problem for all the inner planets. The magnetic field of Mercury is the example most similar to that of our own planet, and consequently, for a better understanding of the Earth's internal geodynamics, the interior of Mercury might provide valuable comparisons. Similarly, the Moon's small core may have generated a magnetic field for about a billion years; determining the size of the lunar core will help scientists understand how this field was generated. The low priority of Mercury in the 1978 report was partly based on the technical statement that orbiting missions to that planet were prohibitively difficult. This technical barrier has disappeared with the discovery of mission scenarios that use successive gravity assists from Venus and Mercury itself to reach orbit.

Thus, evolving scientific and technical understanding along with new discoveries has led to a more important status for the terrestrial bodies without atmospheres. Each object offers fruitful comparisons.

The scientific strategy for the inner planets may focus on one planet or several, but not to the complete exclusion of any one. This requirement holds even if NASA’s commitment to manned exploration requires extensive scientific precursor studies of Mars and the Moon. The committee realizes that the broad goals of planetary science require a balanced program of basic science and exploration, which includes studies of all planets, the Moon, and select asteroids and comets in our solar system. The committee therefore endorses the statement from the 1978 report that on a time scale of two decades the general level of exploration for all the planets of the inner solar system should be brought into balance. The committee further recommends that exploration of the inner planets in the next two decades include further exploration of Mercury and Venus because a program of planetary exploration that includes only Mars and the Moon is scientifically inadequate.