How did the Sun's family of planets and minor bodies originate? Scientists have a solid understanding of how planets are formed, but it the actual ingredients and conditions that resulted in the creation of our solar system remain unclear. This makes the Kuiper Belt - the vast region of icy-rocky bodies beyond Neptune - a prime target for exploration. Pluto is the best known object in the Kuiper Belt. This region of our solar system is also believe to be the birthplace of short-period comets - such as comet Halley - that pass through the inner solar system.
Meteorites and asteroids also are time capsules that preserve information about the chemical and physical processes that operated at microscopic to planetary scales in the early solar system. Earth's geologic history has been mostly obliterated by tectonic activity. But the Moon's South Pole Aitken Basin, one of the largest known impact structures, retains some of the earliest records of the formation of the Earth-Moon system. Additional evidence and different perspectives may exist in the highlands of Mars and Venus.
The four gas giant planets - especially Jupiter - played a major role in shaping our solar system. 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. Scientists have targeted Jupiter and Saturn as critical areas of exploration, but Neptune and Uranus also can provide crucial information.
NASA has developed a comprehensive plan to explore these diverse science targets with a series of planetary spacecraft that will each contribute key pieces to the larger puzzle.
How did the solar system evolve to its current diverse state? Our solar system is exceedingly dynamic. Virtually everywhere we look we find continual change - predictable or chaotic, physical or chemical, subtle or catastrophic. Only by observing solar system bodies under different conditions and from a variety of vantage points can we begin to understand the processes by which they evolved from their initial formative states to the wide diversity we see today.
Planetary processes such as impacts, volcanism, tectonics, climate change, and greenhouse gas warming are difficult to comprehend when their study is confined to just one body - Earth, for example - but by comparing how these processes operate and interact in a variety of planetary settings, we can gain insight into their variations and effects.
For example, Earth's magnetic field, generated by processes in its molten core, shields the planet from the damaging solar wind. Recent evidence suggests Mars may have once had a similar protective magnetic field. At Jupiter, Io's tidal flexing drives volcanoes which feed deadly radiation into Jupiter's magnetosphere while a similar effect on nearby Europa may keep an ocean from freezing, making the small moon a prime candidate for the discovery of life beyond Earth.
Impacts may have delivered the key ingredients for life on Earth - and caused devastating extinctions. Studies of impact cratering on a wide variety of bodies - from Earth's Moon to Pluto and beyond - will tell a story of planetary evolution that has long since been erased here on Earth.
Comparative studies also will help to reveal why Earth teems with life while Mars and Venus - which formed about the same time under similar conditions - are so radically different. Understanding the evolutionary pathways of Earth's planetary neighbors is a critical step in forecasting the future habitability of our home world. This knowledge will also help guide the search for habitable worlds in other solar systems.