U43B-01 INVITED 13:40h
The Origin, Evolution, and Composition of the Universe
Modern cosmology -- the study of the universe as a whole, its origin, evolution, and structure -- is undergoing a scientific revolution. New ground- and space-based telescopes can now observe every bright galaxy in the universe, and see back in time to the cosmic dark ages before galaxies formed. We can read the history of the early universe in the ripples of heat radiation still arriving from the Big Bang. We now know that everything that we can see makes up only about half a percent of what is there, and that most of the universe is made of invisible stuff called "dark matter" and "dark energy." The Cold Dark Matter theory based on this appears to be able to account for all the main features of the observable universe, including the heat radiation and the large scale distribution of galaxies. There are possible problems, of course -- for example, some details of the structure of galaxies may not agree with theory -- and we still do not know what happened before the Big Bang. Nevertheless, modern cosmology is developing humanity's first story of the origin and nature of the universe that might actually be true, meaning that it will still be true in the distant future, although new knowledge will no doubt expand beyond our current understanding.
U43B-02 INVITED 14:10h
How to Build a Planetary System
Current observational evidence suggests that planetary systems are common. We do not yet know whether Earthlike planets are common. Many aspects of planetary system construction are easily understood, almost unavoidable. Other aspects are less obvious, even somewhat mysterious. In all likelihood, the diversity of outcomes is large and non-deterministic and our particular outcome is correspondingly uncommon despite the abundance of systems. Formation of a disk during star formation is almost unavoidable and provides the natural source of planet-forming material. Presence of solid particulate matter in the disk is assured. Overall abundances of "gas", "ice" and "rock" are also likely to be very similar from system to system. However, the aggregation of solid matter into planetesimals is not assured and remains poorly understood. Their formation in our solar system is not in doubt because of meteoritical evidence. Aggregation from planetesimals to larger solid bodies is unavoidable except at high (and improbable) relative encounter velocities. This process may proceed rapidly to bodies of Moon or Mars sized (or even larger in the giant planet zone). Perhaps surprisingly, the least well understood aspect of planet formation is the bodies that are evidently quite common: giant planets (including Uranus and Neptune ice giants). Direct gravitational instability from the disk might work but may fail to explain our giant planets, especially Uranus and Neptune. The more popular process of solid accumulation followed by gas inflow is potentially too slow relative to the time scale of gas loss from the nebula. Theory suggests that inward spiral of planets imbedded in gaseous disks is expected and in that sense the extrasolar systems found thus far are understandable. However, the range of possible outcomes is large, our knowledge of these is still quite primitive and there may be many surprises ahead. It's an exciting time in the planet system game!
U43B-03 INVITED 14:40h
Accretion, Core Formation, and Redox State of the Primordial Earth
Elemental and isotopic differences between the Earth, Mars, and meteorites indicate that the bulk of each terrestrial planet formed from very narrow (<0.5 AU) feeding zones. The formation of Earth's core, the crystallization of its magma ocean, the formation of early crust, and the outgassing of Earth's oceans and atmosphere are natural consequences of planetary accretion. The environment within which primordial differentiation of the Earth occurred appears to have been a deep magma ocean. This conclusion derives from the observation that the abundances of moderately siderophile elements in Earth's mantle appear to be set by equilibrium with metal at the base of a deep magma ocean. Highly siderophile elements are in chondritic relative abundances and may point to their delivery in an oxidized "late veneer" followed by very efficient mixing into a by now metal-free (the metal had segregated into Earth's core) magma ocean. It has generally been thought that the accretion disk was too hot at 1 AU for hydrous minerals to be stable. Paradoxically, the Earth appears to have accreted "wet", a conclusion that derives from modeling, D/H observations of comets, and Os isotope measurements of meteorites. However, dust in the accretion disk was bathed for some time in a sea of H and O, and some amount of water must have formed. Stimpfl et al. (2004) MAPS 39, A99 show that several Earth oceans of water could be adsorbed onto grains in the inner solar system and subsequently accreted into the terrestrial planets. This conclusion suggests that H is dissolved in Earth's core, as water dissociates in the presence of Fe metal at even modest pressures and temperatures. The implication is the progressive oxidation of the magma ocean as Earth grows, H is segregated into the metal phase, and OH is liberated, thus accounting for the observation that the upper mantle of Earth is about 3 log units more oxidizing than required for equilibrium with metal. Thus the "late veneer" might have contained Fe metal, which would have oxidized because of the overwhelming excess of oxygen in the magma ocean. Different Xe isotopic reservoirs in the Earth can most readily be explained by separation of I from Xe through the medium of liquid water, implying the presence of seas or oceans within 100 Ma of nucleosynthesis of 129I.
U43B-04 INVITED 15:10h
Planetary Differentiation and the Emergence of Life
Organisms use energy from light, chemical reactions, or both. The use of chemical energy apparently pre-dates the use of light, and forms the foundation of all subsurface and high-temperature ecosystems. Therefore, the emergence of life seems linked to the availability of chemical energy in the Earth's crust. Chemical energy sources exist because many of the materials composing the habitable volume of the Earth formed at higher temperatures, and are out of oxidation/reduction equilibrium with their cooler surroundings. At extreme temperatures, such as those that prevail in the mantle or in magmatic systems, many processes occur in response to shifting equilibria, with little to no inhibition from kinetic barriers. However, differentiation processes allow materials to migrate and to become segregated, with direct consequences for the development of disequilibrium states. As an example, recently-formed melt in the upper mantle is far closer to equilibrium with its surroundings than is the resulting solidified basaltic lava at the surface. One reason is the steep temperature dependence of the rates of oxidation/reduction reactions. Redox disequilibria develop as materials cool through the 400 to $200\deg$C range, depending on the chemical system. As temperatures drop, redox reaction rates become so slow that they converge on geologic time. These redox disequilibria form the chemical energy supplies used by life, which catalyzes favorable but sluggish reactions to drive metabolic processes, lowering the energy state of the total system. It follows that life continues the release of energy begun by planetary differentiation but stalled by sluggish oxidation/reduction reactions. In this sense, life is inevitable, as it greatly enhances the efficiency of reactions that release energy, which would otherwise not proceed in its absence. Therefore, understanding the emergence of life relies on our ability to conceptualize the initiation of catalysis in systems that are rich in redox disequilibria. Processes that focus and juxtapose redox disequilibria are likely crucibles for the emergence of catalysis and life. In particular, the flow of aqueous fluids through diverse products of differentiation may engender the necessary gradients.