Space Studies Board

Our solar system consists of nine known planets orbiting the Sun, and a large number of other objects: moons, asteroids, planetary rings, and comets. Among the mysteries that have preoccupied human thought throughout history are the mechanisms by which the solar system came into existence, the laws and physical processes that shape the evolution and behavior of planets, and the relationship of the solar system to the wider cosmos. The same questions continue to preoccupy modern planetary science as well.

Planetary studies illuminate some of the deepest and longest-standing scientific questions. Moreover, from a human perspective, planetary studies have additional significance. Planets are likely to be the only bodies in the universe capable of supporting advanced life. Among its other objectives, planetary science seeks to understand the formation of life-supporting planets and the conditions under which life arises and develops. The answers to these questions will shape our perceptions about our origins and our situation in the universe.

To understand how the solar system originated;
To understand how the planets evolved, including Earth and the planetary satellites, and to understand their present states;
To learn what conditions led to the origin of life;
To learn how physical laws work in large systems.

To understand how the solar system originated. Research aimed at understanding the origin of the solar system focuses largely on those objects thought to retain clues about the primordial conditions and processes that attended the system's formation. The most detailed clues come from investigations of comets, asteroids, and meteorites—small primitive objects that have changed little since their formation in the protoplanetary nebula.

The cold, volatile-rich matter of comets is thought to contain the most faithfully preserved samples of condensed protoplanetary material remaining in the solar system. The asteroids form an ordered assemblage of protoplanetary fragments that seem to remain near the locations of their original formation. They are thought to reflect the radial variation of conditions in the protoplanetary nebula. Laboratory analyses of meteorite fragments of asteroids and comets show the importance of the information these objects can provide. Detailed study of comets and asteroids is expected to fundamentally advance our understanding of the solar system's formation.

Planetary systems are believed to occur commonly in the universe as a result of the same processes that formed our own solar system. Failure to find such systems would force a fundamental revision of our theories about the origin of this planetary system and about star formation. Studies of star-forming regions and the discovery and study of other planetary systems will likely precipitate important advances in our understanding of the formation of the solar system, and in our understanding of planetary systems as a class.

To understand how the planets evolved. Because we live on Earth, a terrestrial planet, the evolution and environment of terrestrial planets is of special interest. Substantial advances in understanding can be realized by investigating, as a class, the terrestrial planets Mercury, Venus, Earth, and Mars, and other close analogs. In addition, studies of many of the outer-planet satellites and of the largest asteroids should reveal important information about solid planet evolution. Much of what we know of the terrestrial planets derives from ideas and concepts that originated in studies of Earth. Conversely, planetary investigations of objects that evolved under conditions far different from those on Earth may prod us to seek a deeper grasp of natural terrestrial phenomena, as well as a more complete understanding of Earth's history. By exposing circumstances in which concepts based on terrestrial analogs fail, planetary investigations help us define the limits of applicability of these Earth-based ideas.

As the world population increases and stresses the ability of our environment to accommodate it, terrestrial scientists will be called upon to model environmental impacts and to help develop tradeoffs between urgent resource needs and the consequences of meeting those needs. Models that can predict the properties of the widely varying atmospheres of the terrestrial planets will make this job much easier and enhance the credibility of scientists' pronouncements about this planet.

To learn what conditions led to the origin of life. Earth remains the only realm in which we know life has arisen. Our search to understand the origin of life involves several planetary questions: what are the physical conditions under which life arose, and have living organisms, either incipient or well-developed, arisen in other places where they can be studied? Presumably, living organisms arose out of an organic, prebiotic medium and were preceded by an interval of chemical evolution, which led more or less continuously into biological evolution. By understanding the formation of the planets, we will come to know the circumstances under which life arose on Earth. Many objects in the solar system seem not to have undergone substantial evolution since their formation. Some—Saturn's moon Titan, for example—probably carry important clues about the early material in which life arose. These and other objects in the solar system—including Mars—may have had prebiotic chemical species or harbored forms of incipient life, leaving evidence we can still collect.

Investigations of the composition of cosmic matter and primitive solar system matter show that the basic building blocks of terrestrial life, including amino acids, occur naturally, at least in trace amounts. One of the greatest challenges in understanding the origin and distribution of life is to determine just how widespread biological evolution may be in the cosmos. An important aspect of this question is the degree to which special terrestrial conditions were involved in prebiotic chemical- evolution. Detailed chemical assays of comets, asteroids, and other primitive objects will reveal the extent to which life could have arisen directly from preplanetary matter without an interval of special processing to condition the chemical mix. This will provide important clues as to the possible ubiquity of biological evolution. (For a discussion on the origin of life from an exobiology perspective, see Chapter 7.)

To learn how physical laws work in large systems. Various phenomena are the unique result of the large scale of natural systems or arise from the very long times over which slow processes work. Investigation of large-scale physical processes involves virtually all of the objects in the solar system. The giant planets provide clues about properties of matter under high pressures; planetary interiors and magnetospheres demonstrate the curious behaviors of magnetized fluids and plasmas; and planetary atmospheres and surfaces present puzzles about the long-term evolution of complex interacting systems that constitute planetary environments and interiors.

Because these phenomena do not occur under normal laboratory conditions, it is only through direct observations in the solar system that we can understand them. Our current theories of planetary tectonism and cosmic plasma processes, for example, have developed in this manner. Since there is little prospect that in the foreseeable future we will make in situ measurements in other planetary systems, detailed investigations within our own solar system will continue to be the foundation upon which we build much of our understanding of natural phenomena throughout the universe.

In the past 20 years or so most of the planets have been visited and several have been explored in some detail. A few highlights will be mentioned here.

The same physical processes operate on all the planets, but different starting conditions have led to a remarkable diversity of present states. This diversity is also influenced by violent collisions. The first recognized effect was cratering, prominent on the Moon, Mercury, and parts of Mars, where the largest basins and craters represent effects produced until accretion ended about 3.7 billion years ago. Craters are prominent on many of the moons of the outer planets, and a few of these moons seem to have been shattered by even larger impacts and then reformed. On Earth, geological processes have obliterated all but a few relatively recent craters, including one from an impact that may have caused mass extinctions at the end of the Cretaceous era some 65 million years ago. Recent theories explain the anomalous densities of Moon (low) and Mercury (high) in terms of enormous collisions with molten protoplanets in which the iron had already sunk to the center.

Great climatic change has been inferred for Mars, where abundant water once flowed, and for Venus, which may once have had the equivalent of a terrestrial ocean. Abundances of noble gases are remarkably different on Venus, Earth, and Mars, and different again on the parent bodies of meteorites. Ring systems are now known around all four of the giant planets, Jupiter, Saturn, Uranus, and Neptune; each ring system is totally different, and evidence is accumulating that some, at least, are transient. Jupiter's moon Io is the seat of many simultaneous volcanoes that shoot sulfur dioxide far above the surface. The sulfur and oxygen reappear as ions in a plasma torus enveloping Io's orbit. There they emit enormous amounts of ultraviolet radiation. More energetic ions populate the entire jovian magnetosphere and dominate much of its behavior. Saturn's large moon Titan is a terrestrial planet in many ways, but made of materials characteristic of the outer solar system. The dense nitrogen atmosphere contains methane clouds and a dark organic haze. A global ocean of liquid ethane is predicted. This variety of organic matter gives an environment analogous to what may have existed on a prebiotic Earth.

There is little doubt that more surprises and new concepts are still awaiting us and that the future of solar system exploration will be as rich as its past.

Progress toward realizing these goals requires a balanced program of basic science and exploration that includes studies of all planets, the Moon, and select asteroids and comets in our solar system. Concurrently, astronomical observations of star-forming regions and other planetary systems should be made. It is not safe to exclude parts of the system from study; experience has taught us to expect the unexpected. The initial exploration of all the planets, except Pluto, will be complete when Voyager flies by Neptune in August 1989. The next step is a more detailed examination of the planets to dissect the processes at work there.

So far, we have intensively studied only the Moon, Venus, Mars, and Jupiter. Beyond 1995, planetary exploration will shift increasingly toward orbiters, atmospheric probes, landers, sample returns, and perhaps manned exploration—the type of research required for a more complete understanding of the solar system. To complement these in situ investigations we will require laboratory experimentation and theoretical analysis as well. Data from spacecraft are largely responsible for the rapid advance in our view of the solar system. The steering group envisions that spacecraft investigations will continue to play this primary role.

Several projects that are now ready for launch or under development will set the stage for the vigorous solar system science and exploration program in the early years of the twenty-first century. Magellan will carry a radar system to map almost all the surface structure of Venus at a resolution of 300 m. Resolution of this quality will provide information key to comprehending the variety of evolutionary histories and processes undergone by the terrestrial planets. The Mars Observer mission will carry out the first global geochemical analysis of the martian surface and will investigate some properties of the planet's atmosphere. The Lunar Geoscience Orbiter will carry out a similar survey of the Moon.

The Galileo probe will carry instruments deep into Jupiter's atmosphere to measure its composition and physical structure. The orbiter will also perform synoptic observations of the atmospheric dynamics, conduct a detailed investigation of the magnetosphere, and obtain detailed images and spectra of numerous jovian satellites. The Comet Rendezvous Asteroid Flyby will conduct detailed in situ investigations of a comet nucleus and will probe the physics of the comet-solar wind interaction. The detailed comet nucleus measurement will, hopefully, provide information about the conditions under which protoplanetary matter accumulated in the solar nebula.

The Soviet Union is expected to carry out major investigations of Mars and its satellite Phobos. This ambitious project is to include a new generation of analytical instruments for analysis of the surface composition of Phobos. In addition, the USSR is expected to carry out an asteroid rendezvous mission, involving a flyby of either Mars or Venus (probably the former. The Soviets are also considering a lunar polar orbiter mission, which would investigate the Moon's global chemical and mineral composition, magnetic fields, and temperatures.

Over the 20-year interval from 1995 to 2015, the recommended program encompasses investigations of all of the major planetary bodies in the solar system along with selected satellites and primitive objects.

Landers, rovers, selected sample returns, and networks of automated observation stations on planetary surfaces will be the primary systems used to study the terrestrial planets. Specialized surface landers and rovers would allow the exploration of varied terrains and the surface material analyses that are necessary to ascertain the evolutionary histories of the planets. Analysis of selected samples returned to Earth will help us to determine both r the character and the absolute dates of many of the major evolutionary events on the terrestrial planets.

Each of the major outer planets—Jupiter, Saturn, Uranus, and Neptune—presents a complex, ordered system including the planets themselves, a magnetosphere, and a family of satellites and rings. Studies of the outer planets should include orbiting spacecraft to investigate all of these aspects. Atmospheric entry probes will reveal information critical to determining the composition and evolution of those planets. In situ studies of selected satellites will collect information pertaining to the primordial state of the volatile and organic matter in the solar system, and may yield clues about prebiotic chemical evolution.

In situ studies of the primitive bodies began with the missions to Halley's Comet. We will need rendezvous missions to other comets and asteroids to select the objects and the instrumentation to be used in later detailed studies. Investigations following rendezvous of a selected set of asteroids will allow us to determine their compositions and structures, as well as the variation of these properties in the main asteroid belt. We will thus obtain insights into processes in the protoplanetary nebula and explore early evolutionary mechanisms. In situ studies of comets and small outer solar system objects will permit analysis of the most complete and best-preserved samples of primitive matter, yielding clues to the origin of the solar system and of life. Return of samples from selected primitive bodies will allow the in-depth laboratory analysis possible only on Earth to contribute to this effort.

Discovering and studying other planetary systems require the use of advanced telescopes in space. Planets disturb the motion of their central stars, and the evidence of these disturbances can be found by measuring the positions and motions of stars. In addition, such measurements can give information about the masses and orbits of the surrounding planets—information that can provide critical tests of our ideas about the formation of planetary systems and stars. The steering group recommends the development of specialized telescopes capable of detecting planets at least as small as Uranus and Neptune around a large number of nearby stars. The technology on which these telescopes depend is now within reach for use in space; the use of such telescopes in association with the Space Station would permit an observing program sufficiently long to allow the search for and study of planetary systems around a large number of nearby stars. Once other planetary systems have been discovered, there will be strong incentive to develop more sensitive instruments for further studies.

A Mars-focused program is recommended in parallel with the general program outlined above. However, this Mars-based program is not a substitute for a broader, balanced program of planetary exploration. There is no reason to expect that studying one or two planets in depth will allow us to understand how the entire solar system originated and evolved.

Planets and their environments exhibit behavior that, for fundamental reasons, cannot be predicted from first principles. The complexity. of, planetary environments is such that a planet can, in principle, exist in a large variety of states with the same conditions imposed from outside. The possible presence of living organisms further extends the variety of states in which a planet can persist. Finally, accidents of evolution can affect the state of a planet profoundly. A major challenge of planetary science is to trace the evolution of terrestrial planets, and enumerate the possible varieties and causes of their diverse environments. Meeting this challenge will require comparative studies of the terrestrial planets, including detailed studies of the changes that individual planetary environments undergo.

Of particular interest in the comparison of terrestrial planets is the puzzle posed by the triad of planets with atmospheres: Venus, Earth, and Mars. The differences in their present environments and in their styles of evolution seem large in comparison with the differences in their sizes, locations, and overall compositions. Solving this puzzle is important to us because the differences between these planets occur in those aspects of their environments key to sustaining life.

Spacecraft investigations of Mars during the past 15 years reveal that the planet has undergone perplexing changes throughout its history. Although its surface is now dry and cold, there is clear evidence of a sustained, abundant flow of water during times past. Such changes in the martian environment directly pertain to long-term concerns about the behavior of Earth's environment. The prior presence of water on Mars raises important questions about its early, if temporary, suitability for life.

Images returned to Earth by Mariner and Viking spacecraft reveal spectacular geographical formations and deep-cut relief. It is evident that detailed study of the martian surface will yield information about the character of the planet's early environments, their arrangement in time, and perhaps clues as to the influences that produced such marked environmental change.

Of all the planets beyond Earth, Mars is the one most accessible to detailed study. It is relatively easy to reach with scientific spacecraft, and the surface is the most conducive to sustained operation of scientific instruments on mobile platforms. Furthermore, Mars is the only planet outside of the Earth-Moon system that we can currently consider for manned exploration and settlement.

4. Possible Human Exploration (The issue of the role of humans is discussed in a separate section of this report.)

The recommendations put forward here, if implemented, will advance our understanding of the solar system on the broad front that is needed to progress toward answering some of mankind's long-standing questions about the cosmos. A recommended Mars focus within that broad-based program will further our understanding of the terrestrial planets, including Earth, and will address pressing questions about planetary environments and their stability. The recommended investigations will also provide the information needed for proper planning of later manned exploratory missions to the Moon and planets.