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.
What are the characteristics of our solar system that lead to the origins of life?
The possibility of finding life elsewhere is for many people the most compelling reason for humankind to explore beyond Earth. We believe that liquid water and carbon are required for life to arise and thrive, as well as a source of energy. Many places in our solar system provide these, at least for a time; not only planets, but also some moons and even certain comets. But for life to arise we presume that a hospitable environment must be more than just transient.
Earth is in the continuously habitable zone, meaning at our size and at our distance from the Sun water has been stable at the surface even though the brightness of the Sun has varied.
Not all planets are so lucky.
We now know that there once was liquid water on the surface of Mars, but was it there long enough for life to develop? We are not sure, but its possible and if so then life might still linger beneath the surface, perhaps in a place where sub-surface heat meets the surface permafrost. Venus too shows signs it lost the equivalent of Earth's oceans into space. Did life have a chance to evolve before the planet became the dry, superheated world we know today?
There are other places where there has been liquid water for as long as on Earth. Jupiter's icy moon Europa almost certainly has a liquid water ocean beneath the surface even though its five times further from the Sun than we are. If there are hydrothermal vents at the bottom of Europa's ocean, then that would seem a very hospitable place for life, but that doesn't mean its there. The only way we are going to find out is by going there. Other moons that may have liquid water deep below the surface include Jupiter's moons Callisto and Ganymede and perhaps Saturn's moons Titan and Enceladus.
How did life begin and evolve on Earth and has it evolved elsewhere?
Microbial life forms have been discovered on Earth that can survive and even thrive at extremes of high and low temperature and pressure, and in conditions of acidity, salinity, alkalinity, and concentrations of heavy metals that would have been regarded as lethal just a few years ago. These discoveries include the wide diversity of life near sea-floor hydrothermal vent systems, where some organisms live essentially on chemical energy in the absence of sunlight. Similar environments may be present elsewhere in our solar system.
Understanding the processes that lead to life, however, is complicated by the actions of biology itself. Earth's atmosphere today bears little resemblance to the atmosphere of the early Earth, in which life developed; it has been nearly reconstituted by the bacteria, vegetation, and other life forms that have acted upon it over the eons.
Fortunately, our solar system has preserved for us an array of natural laboratories in which we can study life's raw ingredients - volatiles and organics - as well as their delivery mechanisms and the prebiotic chemical processes that lead to life. Comets, for example, are believed to have delivered many of life ingredients to Earth after the planet cooled. The ones we see now in our night sky contain a record of the earliest days of our solar system, which makes them an important target for robotic explorers.
Mars and Venus -- now so different from Earth even though they appear to share a prime, hospitable location in our solar system -- also provide platforms to hunt for signs of life and clarify how planets evolve. Did life evolve on those worlds earliy in their devopment? If so, what happened?
In the outer solar system, the moons Europa, Ganymede and Callisto at Jupiter and Titan and Enceladus at Saturn all harbor some of the key ingredients for life and have been targeted for detailed study by robotic spacecraft.
We can also find on Earth direct evidence of the interactions of life with its environments, and the dramatic changes that life has undergone as the planet evolved. This can tell us much about the adaptability of life and the prospects that it might survive upheavals on other planets.
What are the hazards and resources in the solar system that will affect the extension of human presence in space?
Our home planet is continuously bombarded by energetic particles, cosmic rays, dust and - occasionally - larger objects, all of which can be hazardous to human life. This risks increase as robotic and human explorers venture farther beyond the protection of Earth's atmosphere and magnetic field.
While they were once a source of life-giving organics and water, comets and asteroids also have the potential to wreak widespread destruction. Evidence continues to mount that the so-called Cretaceous-Tertiary mass extinction event 65 million years ago was caused by the impact of an extraterrestrial body about 10 kilometers (about 6.2 miles) in diameter.
Efforts are currently underway to inventory the objects that might pose an impact hazard to Earth. Scientists believe they have catalogued about half of the potentially hazardous objects larger than 1 kilometer (.62 miles) in diameter. Future focus will zero in on objects with orbits that could bring them dangerously close to Earth.
Evidence of the effects of impacts is plentiful in our solar system - especially on Earth's Moon - but further study is needed. While NASA's Deep Impact spacecraft provided a unique change to witness a hypervelocity impact with a comet, there have been no direct observations of the formation of planetary impact craters.
One way to avoid the fate of the dinosaurs is to extend human life beyond Earth.
NASA is already working on plans to return humans to the Moon and begin inventorying resources throughout our solar system that humans will need to travel to Mars and beyond. Those resources include water, essential for drinking and creating fuel and air; rare metals for use on Earth and construction on space colonies; and other resources to protect and foster life beyond the protection of our home world.
NASA 2006 Solar System Exploration Roadmap