Homes Away From Home: Map I

Homes Away from Home: Map I

The exploration of our solar system is founded upon the pursuit of three simple yet profound questions:Where do we come from? Where are we going? Are we alone?

This three-part series, "Homes Away from Home," highlights what is known today, and hoped to be discovered, about our solar neighborhood. The story of life elsewhere is told beginning with the very large and distant places, as they might have seeded what today is very small and near. Our solar system began to take shape about 4.6 billion years ago, as the primordial solar nebula of dust and gas began to coalesce around the infant Sun. Within the first billion years or so, the planets formed and life began to emerge on Earth — and perhaps elsewhere.

Protoplanets: What and How?

The process of planet formation proceeds according to physical principles that are generally well understood. Less well known, however, are the ingredients and initial conditions that resulted in the solar system we know today. Beyond Neptune, the material that became the building blocks of the planets never accreted into major bodies, and it remains relatively unaltered even now. This region, known as the Kuiper Belt, is believed to represent the best available record of the original interstellar materials that formed the solar nebula.

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Saturn’s mysterious rings.
Credit:JPL/SSE Roadmap

Pluto and its moon Charon are the largest known Kuiper objects and the only ones for which we have significant telescopic observations. This region is also the birthplace of the short-period comets, still smaller bodies that have been gravitationally dislodged from the Kuiper Belt. As the comets enter the inner solar system, they not only become visible from Earth but they also become accessible targets for intensive robotic exploration.

Determination of the chemical composition and physical characteristics of Pluto, other Kuiper objects, and short-period comets will give us unique insight into the materials and processes that dominated the initial stages of planet and satellite formation.

Time Travel to the Moon

Unfortunately, the very early geological history of Earth has been nearly completely obliterated by the actions of tectonics, weathering, and biology; on our home planet the earliest rock records date back about 3.8 billion years but no further.

The Moon, however, still retains some of the earliest records of the formation of the Earth-Moon system. Leading models suggest a very early origin of the Moon as a result of the collision of a Mars-sized body with the newly formed Earth. Samples from the Apollo and Luna programs elucidated some of this history, but the nature of these samples, limited to equatorial regions of the lunar near side, leaves many key questions unanswered.

The Moon’s South Pole-Aitken Basin, one of the largest impact structures known within the solar system, exposes material from deep within the crust and possibly even the upper mantle that was excavated by the impact. In addition, the floor of this basin probably retains impact melt rocks created by the giant impact. Apollo experience shows that such melt rocks provide insight into the average composition of the basin, and that by dating such samples we can infer the age of the basin itself. This will help to resolve questions that are raised by the observed cratering record of the lunar highlands, with important implications for the early history of the Moon and all of the terrestrial planets, including Earth. Analysis of ancient lunar material will thus provide critical insights into the processes that occurred on Earth and the other terrestrial planets during their early history.

Giant Planetary Protection

The formation of the giant planets had a major effect on the events and processes at work in the early solar system. Informed and constrained by the recent Galileo mission’s probe and orbiter investigations, today’s models indicate that critical clues to giant planet formation can be found in the structure and masses of their rock-ice cores, and in the composition of their deep atmospheres and interiors. Characterization of the gravitational and magnetic fields of Jupiter, and measurement of its deep atmospheric composition and water abundance, will enable us to determine the processes and timing of Jupiter’s formation. In the longer term, comprehensive exploration of the ice giant Neptune will permit direct comparison with Jupiter and more complete modeling of giant planet formation and its effects on the inner solar system.

Shaping the Hospitable Planet

Planetary interiors, surfaces, atmospheres, and magnetospheres are now known to be highly interdependent. Earth’s magnetic field, for example, which is generated by processes within the planet’s molten core, shields us from fatal high-energy radiation. Recent observations suggest that Mars may have had a similar protective magnetosphere early in its history. Io’s eccentric orbit causes tidal flexing, which drives volcanoes that feed charged particles into Jupiter’s magnetosphere, producing lethal radiation; by contrast, Europa’s eccentric orbit and tidal flexing may keep an ocean from freezing, which may provide a habitable environment.

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Ice cracks on Europa.
Credit:JPL/SSE Roadmap

Impact processes clearly played a crucial role in bringing the solar system to its present state. The young solar system contained a significant amount of non-accreted material left over from the formation process. A collisional environment very different from today’s probably dominated the period following planet formation, as the gravitational influence of the newly formed giant planets cleared out the surviving smaller debris from vast volumes of space.

This solar system-wide rain of projectiles had a profound effect on all of the planets, delivering volatiles and organic material from the colder outer solar system to the inner planets while at the same time causing frequent, catastrophic impacts. This impact environment must have had a major effect on the emergence of life on Earth, perhaps delaying its expansion or "resetting" the evolutionary clock with periodic global extinctions.

The geologic record of this period has long since vanished from Earth, but important links to this era still exist on the Moon and in the outer reaches of the solar system. While the record of cratering deduced from lunar samples shows a precipitous decline in the impact rate starting about 3.5 billion years ago, we have as yet no direct data relating to the flux in the preceding billion years. Thus a critical step is to determine how the impactor flux varied in the early solar system. Competing models have vastly different implications for the conditions under which life might have emerged on Earth. The study of material from the lunar South Pole-Aitken Basin will provide a vital reference point for constraining models of the early impact history, while comparative studies of the cratering records on Pluto, Charon, and other Kuiper objects will allow determination of the impact flux that emanated from that region.

Home Away From Home

More than hundred extrasolar planets have been discovered so far. The apparent abundance of large gas giant planets, in orbits very close to their stars, has led to new theories of how the forming planets in our system may have migrated or become frozen in their present locations. Models also suggest that giant planet formation is a critical feature of planetary systems in general, and may govern the formation and early evolution of rocky inner planets that can possess habitable environments. Since our current understanding of extrasolar planetary systems depends in large part on our observations of the largest planets within them, study of the gas giants represents an important tie point between those systems and our own.

Map II for "Homes Away from Home" will address the requirements for sustained life on other planets.


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