2010 Annual Research Report

O'Brien has continued his work on the collisional and dynamical evolution of the main-belt and near-Earth asteroid populations.  He is currently working on several projects that fall under this area of research, including studies of the origin and collisional evolution of the Trojan asteroids (with Alessandro Morbidelli), modeling how the asteroid size distribution can evolve differently in different regions of the asteroid belt (with Patrick Michel), and exploring how variations in collision velocity in different parts of the asteroid belt may affect the histories of different meteorite parent bodies.  He is currently collaborating with a group, including Fred Ciesla and Gareth Collins, to couple models of the dynamical and collisional environment in the primordial asteroid belt to hydrocode models of impacts between planetesimals, in order to estimate the degree of shock heating that can occur in those bodies.  That work may provide an explanation for the early thermal alteration observed in numerous meteorite classes.  He has also been selected as a participating scientist for NASA's Dawn mission, and will be working to help put Vesta's evolution and cratering history in the context of the evolution of the asteroid belt as a whole.

The other major focus of his work is the numerical simulation of terrestrial planet accretion in our Solar System as well as in systems around other stars.  He is working with Alessandro Morbidelli, Kevin Walsh, Sean Raymond and Avi Mandell to reconcile multiple lines of chemical and dynamical evidence for the formation of terrestrial planets in our Solar System. Recent work has explored a new paradigm for the formation of the terrestrial planets, in which the inward-then-outward migration of Jupiter early in the history of the Solar System truncates the disk in the terrestrial planet zone and scatters material into the asteroid belt from a large range of semimajor axis.  This helps to explain both the small mass of Mars (which has been difficult to reproduce in past simulations), as well as the diversity of bodies found in the asteroid belt. Several presentations have been given on this work, and a paper is currently in review.

He is working with postdoc Jade Bond to couple numerical simulations of terrestrial planet formation in the Solar System and other planetary systems with models of the chemical evolution and condensation of solids in protoplanetary nebulae. Several papers on that work (with Jade as first author) have been published this year, one that focuses on matching the composition of the terrestrial planets in our Solar System, and the second extending that work to extrasolar planetary systems to estimate the bulk composition of terrestrial planets that may exist in those systems.  This work is currently being expanded to include more realistic simulations with larger numbers of particles, gas drag, and the effects of giant planet migration, and is being applied to planets around low-mass stars.

This work is also being extended to better understand the detailed chemical evolution of individual planets.  O'Brien is working with Francis Nimmo and Thorsten Kleine to understand how the Hafnium-Tungsten isotopic system evolves as planets grow in his simulations, which has important implications for understanding the formation and differentiation of the Earth and Moon.  He is also working with David Rubie and others to couple models for the chemical evolution of the Earth during core formation with models of terrestrial planet accretion.

Papers

J. Chambers, D. P. O'Brien and A. M. Davis (2010).  Accretion of Planetesimals and the Formation of Rocky Planets. Protoplanetary Dust: The Astrochemical and Cosmochemical Perspectives, D. Apai and D. S. Lauretta, eds., Cambridge University Press.

J. C. Bond, D. P. O'Brien and D. S. Lauretta (2010).  The Compositional Diversity of Extrasolar Terrestrial Planets: I. In-Situ Simulations.  The Astrophysical Journal 715, pp. 1050-1070.

F. Nimmo, D. P. O'Brien and T. Kleine (2010).  Tungsten Isotopic Evolution During Late-Stage Accretion: Constraints on Earth-Moon equilibration.  Earth and Planetary Science Letters 292, pp. 363-370.

J. C. Bond, D. S. Lauretta and D. P. O'Brien (2010).  Making the Earth: Combining Dynamics and Chemistry in the Solar System.  Icarus 205, pp. 321-337.

J. C. Bond, D. S. Lauretta and D. P. O'Brien (2010). The Diversity of Extrasolar Terrestrial Planets.  Proceedings of IAU Symposium 265: Chemical Abundances in the Universe--Connecting First Stars to Planets, K. Cunha, M. Spite and B. Barbuy, eds., Cambridge University Press, pp. 399-402.

Abstracts

D. P. O'Brien, K. J. Walsh, A. Morbidelli, S. N. Raymond, A. M. Mandell and J. C. Bond (2010).  Early Giant Planet Migration in the Solar System: Geochemical and Cosmochemical Implications for Terrestrial Planet Formation.  American Astronomical Society, DPS meeting 42, Pasadena, CA, abstract no. 04.03.

D. P. O'Brien (2010).  Collision Velocities in the Asteroid Belt: Implications for Melt Production on Meteorite Parent Bodies.  72nd Meteoritical Society Meeting, New York, NY.

J. C. Bond, D. P. O'Brien and D. S. Lauretta (2010).  The Diversity of Extrasolar Terrestrial Planet Compositions: Adding Migration into the mix.  6th Astrobiology Science Conference, League City, TX.

J. C. Bond, D. P. O'Brien, D. S. Lauretta and J. I. Lunine (2010).  Extrasolar Planets: From Silicates to Carbon. Gordon Research Conference on Environmental Bioinorganic Chemistry, Salve Regina University, Newport, RI.

J. C. Bond, D. P. O'Brien and D. S. Lauretta (2010).  The Diversity of Extrasolar Terrestrial Planets.  ESF-FWF Conference in Partnership with LFUI, Putting our Solar System in Context: Origin, Dynamical and Physical Evolution of Multiple Planet Systems, Universitatszentrum Obergurgl, Austria.

K. J. Walsh, A. Morbidelli, S. N. Raymond, D. P. O'Brien and A. M. Mandell (2010).  Origin of the Asteroid Belt and Mars' Small Mass.  American Astronomical Society, DPS meeting 42, Pasadena, CA, abstract no. 04.02.

N. Haghighipour, E. R. D. Scott, G. J. Taylor and D. P. O'Brien (2010).  Reconciling Giant Planet Formation With the Origin and Impact History of the Parent Bodies of Differentiated Meteorites.  American Astronomical Society, DPS meeting 42, Pasadena, CA, abstract no. 02.06.

D. C. Rubie, D. J. Frost, F. Nimmo, D. P. O'Brien, Y. Nakajima, A. Vogel, U. Mann and H. Palme (2010). Heterogeneous Acretion and Core-Mantle Differentiation of the Terrestrial Planets.  Paneth Kolloquium, Nordlingen, Germany.

T. M. Davison, G. S. Collins, F. Ciesla and D. P. O'Brien (2010).  Cumulative Impact Heating of Planetesimals.  72nd Meteoritical Society Meeting, New York, NY.

C. A. Dwyer, F. Nimmo, E. Asphaug and D. P. O'Brien (2010).  Erosion During Accretion: Consequences for Iron-Silicate Ratios and Tungsten Isotope Anomalies.  Goldschmidt Conference, Knoxville, Tennessee.

D. C. Rubie, D. J. Frost, F. Nimmo, D. P. O'Brien, U. Mann and H. Palme (2010). Accretion of Volatile Elements to the Earth and Moon.  LPSC 40, abstract no. 1134. 

C. A. Dwyer, F. Nimmo, E. Asphaug and D. P. O'Brien (2010). Erosion During Accretion: Consequences for Planetary Iron-Silicate Ratios and Tungsten

Isotope Anomalies.  LPSC 40, abstract no. 2271.

T. M. Davison, G. S. Collins, F. J. Ciesla and D. P. O'Brien (2010).  Heating Planetesimals by Impacts.  LPSC 40, abstract no. 2230.