Introduction. Our solar system was born about 4.6 billion years ago when the collapse of a cloud of gas and dust resulted in the formation of a nascent Sun surrounded by an accretion disk. Subsequently, condensation and coalescing of materials in the disk formed solid aggregates that became the bu
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1. GOAL #1: LEARN HOW THE SUNíS FAMILY OF PLANETS AND MINOR BODIES ORIGINATED. M. Wadhwa
2. Introduction Our solar system was born about 4.6 billion years ago when the collapse of a cloud of gas and dust resulted in the formation of a nascent Sun surrounded by an accretion disk. Subsequently, condensation and coalescing of materials in the disk formed solid aggregates that became the building blocks of the major and minor solar system bodies including the planets and their moons, asteroids, Kuiper-belt objects and comets. Many of the characteristics of our solar system, and the bodies within it, were established during the first billion years or so if its history. This is also the period during which life emerged on Earth, and possibly in other places in the solar system. However, processes active since this earliest epoch have either overprinted or erased much of the record of conditions and processes in the early solar system, making it extremely challenging to decipher this record. Nevertheless, this record is still well preserved in the physical and chemical characteristics of some solar system materials, such as the oldest rocks on the Earth, Moon and Mars, primitive asteroidal meteorites, and comets. Moreover, novel approaches involving theoretical modeling, computer simulations and experiments, as well as astronomical observations of other newly born star systems, allow us to probe through the depths of time to better understand the conditions and processes occurring in the earliest history of our own solar system.
3. OBJECTIVE #1: Understand conditions in the solar accretion disk and processes marking the initial stages of planet formation Investigation 1a: Chemical and isotopic compositions of primitive meteorites and their components [Blake].
4. OBJECTIVE #1: Understand conditions in the solar accretion disk and processes marking the initial stages of planet formation Investigation 1b: Physical, chemical and isotopic characteristics of comets and KBOs [Blake].
5. OBJECTIVE #1: Understand conditions in the solar accretion disk and processes marking the initial stages of planet formation Investigation 1c: Theoretical modeling of nebular dynamics to account for the physical and chemical characteristics of primitive solar system materials and astronomical observations of accretion disks [Lodders].
[Lodders] The history of the solar nebula - the gaseous accretion disk surrounding the Sun during its birth Ė plays a central role in understanding the origin of the solid and gaseous planets as well as the other small bodies in the solar system. Accretion disks observed around other young stars can provide snapshots of the physics and chemistry that also must have been ongoing when our solar system formed. Although currently disk observations may not yet have the spatial resolution necessary for detailed studies, such observations will give valuable tests for dynamical models of planet formation around young stars.
The gas and dust from an interstellar molecular cloud went through various processes in the solar nebula and resulted in the formation of the Sun and planets. Several questions about these processes are still lingering unanswered. What are the time scales for photochemical and thermochemical reactions involving the compounds which influenced the opacity and hence the energy transport and mixing processes within the solar nebula? What is the influence of magnetic field driven turbulence on dynamical mixing of gas and dust in accretion disks? How much unprocessed interstellar cloud material remains in primitive objects such as comets and Kuiper-Belt Objects located in the outer solar system, and how much was preserved in the asteroid belt from where most primitive meteorites originate? What compounds of the life-sustaining elements carbon, oxygen and nitrogen were inherited by the solar nebula from the interstellar medium? The questions about the carbon and oxygen containing compounds are of renewed interest because recent spectroscopic analyses of the solar photosphere indicate that these elements are almost a factor of two less abundant in the Sun and in the overall solar system than previously thought.
The relatively recent observations of gas giant planets quite close to a central star in other solar systems suggest that planets may form at some distance from the central star and then migrate inwards toward the star. By analogy, such processes may also have played a role in the solar system. If so, the gravitational tug from the migration of large, massive gas-giant planets could have influenced the accretion history (e.g., late accretion of volatile-rich planetesimals) of the terrestrial planets.
6. OBJECTIVE #1: Understand conditions in the solar accretion disk and processes marking the initial stages of planet formation Investigation 1d: Theoretical modeling and experimental obervations of the processes involved in the initial stages of planet formation [Abbas].
[Chambers; modifications by Hubbard and Abbas] The formation of planets involves a number of steps with different physical and chemical processes occurring at each stage. For the rocky planets, early stages involved interactions between dust grains and diffuse, turbulent gas in a microgravity environment. Later stages involved high speed collisions between large solid bodies and gravitational interactions during near misses. Giant planets such as Jupiter are mostly composed of gas but a large solid core may have been necessary to trigger their formation. Such cores would have formed in the same way as the rocky planets. The ice-rich giant planets Uranus and Neptune may be the incomplete cores of hydrogen-rich giant planets similar to Jupiter and Saturn, suggesting that the Sunís primordial gas nebula was too tenuous or already lost when Uranus and Neptune formed. Gravitational interactions between growing planets and the Sun's protoplanetary nebula played a big role in determining the current configuration of the planetary system. Theoretical simulations of these processes and of the rate of migration of primordial giant planets, as affected by the nebula, will help us to understand the present and past architecture of our solar system and extrasolar planetary systems. However, theoretical models need to be based on observations and experimental data.
Interpretation of observations of emissions from dust grains as well as modeling of the protoplanetary disk processes is based on radiative transfer models that involve experimental measurements of the optical properties. These measurements include complex refractive indices, and extinction properties or opacities of the analogs of dust grains in the protoplanetary disk, in particular in the infrared and the UV spectral regions. The dust grains in the disk are generally charged, and the grain charge influences the grain dynamics, grain-grain and grain-gas interactions, grain coagulation and evolution. Experimental investigations of grain charging processes by photoemission (photoelectric yields of micron to submicron size dust grains), collisions with gas phase electrons, and by triboelectric and contact charging processes are needed to provide more realistic information to understand and model the processes involved. In addition, experimental investigations of the growth and sticking efficiencies of dust grains by studying condensation processes of volatile gases on dust grains will provide valuable information for studies of the growth of dust grains in the early stages. Thus, studying dust grain sticking and collisions in a turbulent, low pressure gas and in microgravity will provide an important foundation for our understanding of the early stages of planetary growth and essential ground truth for computational models of planet formation. The processes involved in collisions between large solid bodies and the dispersal or reaccumulation of fragments can best be understood by observing these processes and their outcome in the modern asteroid belt and planetary ring systems. The asteroid belt bears the scars of many energetic collisions in its history. Flybys, orbiting spacecraft and sample return missions will tell us much about the outcome of violent collisions between asteroids as well as the gentler collision that allowed these bodies to accumulate from dust grains and small solids. Our limited understanding of planetary migration can be enhanced using constraints based on the modern orbital arrangement and chemical makeup of the main-belt, Trojan and Kuiper-belt asteroids. Current theories of planet formation infer that chance played an important role in determining the shape of the Solar System. For this reason, continuing efforts to detect and characterize extrasolar planets will improve our understanding of the processes involved by determining the range of possible outcomes.
7. OBJECTIVE #2: Learn about the earliest processes occurring on the surfaces and interiors of planets and minor bodies Investigation 2a: Physical and chemical characteristics of asteroids and their relationship to meteorites [?].
8. OBJECTIVE #2: Learn about the earliest processes occurring on the surfaces and interiors of planets and minor bodies Investigation 2b: Studies of ancient rocks on the Earth, Moon and Mars [Wadhwa].
[Wadhwa] The earliest processes occurring in the inner solar system have left their imprint on the rock record on the terrestrial planets. Unfortunately, rocks on the Earth older than about 3.5 billion years have been almost completely eradicated by processes such as impact, weathering, tectonics and biological activity. Nevertheless, although rare, there are still localities where rocks and minerals that preserve a record of the first billion years of Earth history may be found and petrologic, chemical and isotopic investigations of these materials can help us to understand the environment on the early Earth and the processes that shaped it. Unlike the Earth, the Moon still retains a substantial record of the formation of the Earth-Moon system. The latest computational models indicate that the Moon was formed by the energetic impact of a Mars-sized body into the early Earth. The Apollo and Luna samples and lunar meteorites are helping to elucidate some of this early history, but the limited regions of the Moon sampled by these materials restricts their ability to address some fundamental questions. For example, we still have only limited constraints on how the impact flux varied in the region of the Earth-Moon system during the first billion years of solar system history, even though this issue has important implications on questions relating to the environment on the early Earth and the timing of emergence of life on this planet. The South Pole-Aitken basin on the Moon, one of the largest impact structures in solar system, exposes materials from the deep crust and possibly the upper mantle, and provides an opportunity to sample materials unlike those that have been previously available. Moreover, impact melts from this structure would provide the opportunity to obtain a precise age of the basin-forming event. The ancient highlands of Mars also preserve a record of the earliest processes occurring on that planet. Remote analyses by spacecraft and detailed studies in state-of-the-art laboratories on Earth of returned samples of ancient Mars rocks will be invaluable towards a better understanding the earliest conditions and processes on the terrestrial planets.
9. OBJECTIVE #2: Learn about the earliest processes occurring on the surfaces and interiors of planets and minor bodies Investigation 2c: Interior structure and chemical-isotopic compositions of the deep atmospheres of the giant planets and comparison with characteristics of exoplanets [Hubbard].
[Lodders] In our solar system, most of the planetary mass is in Jupiter, Saturn, Uranus, and Neptune. Still, their deep atmospheric composition as well as the interior structure remains poorly known. How much water do they contain? What is the cloud-layer structure in the gas-giant planets? How big are their deep cores and if their cores indeed exist, how and when did these cores form? Information on the isotopic composition of C, N, O, and the noble gases is another desired diagnostic tool to understand giant planet formation and evolution. Self-consistent models for the formation and evolution of all giant planets require better observational data of the chemical and physical properties that only can be provided by spacecraft missions. The Galileo Mission to Jupiter showed us that that entry-probe measurements can lead to valuable results but reliable in-situ measurements of the abundances of methane, ammonia, water, the noble gases, compounds containing elements such as e.g., sulfur and phosphorus, are required for all outer planets to build a solid base for models of giant planet formation in the outer solar system.
[Hubbard] We now have our first measurements of atmospheric compositions in giant planets orbiting other stars. Interpretation of these measurements depends on many poorly understood processes such as cloud formation, deep convection and local ďweatherĒ, and effects of irradiation from the parent star. The same processes are at work in the atmospheres of our own giant planets. Some hot exoplanets may even have observable silicate clouds analogous to those thought to be buried deep in the atmospheres of our own giant planets, together with more easily observable water vapor. Definitive measurement of Jupiterís deep water abundance is needed to understand the formation processes for giant planets, and will soon be needed for comparison with exoplanet measurements. While the highly successful Galileo probe gave us our first look at Jupiterís atmospheric chemistry, it did not reach high enough pressures to definitively measure water, a key tracer of Jupiterís formation. Interpretation of the probe data was complicated by local meteorology, and at the deepest levels, the probeís interior temperature was far higher than planned. The available Jupiter probe abundance results are a puzzling combination of some values that are higher than expected (e.g., noble gases that are indicative of low-temperature enrichment processes) and some that are much lower than expected (e.g., the noble gas neon). However, the probe confirmed an important result from interior modeling studies, namely that Jupiterís envelope is enhanced in elements heavier than hydrogen and helium. The next step in experimentally understanding these results is to probe Jupiterís atmosphere again, preferably at locations that have varying meteorology, as well as to deeper levels, say to about 100 bars. Similarly, it is essential to make comparable measurements in the atmospheres of our other three giant planets. New revisions of abundances of the elements carbon and oxygen in the Sun itself are upsetting the paradigm for the interior structure of the Sun and its helium abundance, and it is certain that comparable measurements in our giant planets and extrasolar giant planets will lead to far-reaching and revolutionary changes in our understanding of planetary formation and evolution.