NASA Postdoctoral Program Opportunities

Space Science: Dust Properties and Temporal Activity of Comets and Protoplanetary Disks

Opportunity ID	17536
Opportunity URL	https://npp.usra.edu/opportunities/details/?ro=17536
Location	Ames Research Center Moffett Field, CA 94035
Field of Science	Planetary Science
Advisor	Diane H. Wooden 650-604-5522 Diane.H.Wooden@nasa.gov
Citizenship Requirement	U.S. Citizens Accepted Lawful Permanent Residents Accepted Foreign nationals who are in the U.S. at the time of application and on a valid J1 visa Accepted; Foreign nationals, asylees or refugees in the U.S. at the time of application with a valid EAD card and pending I-485 or I-589 forms Accepted. Applications with citizens from Designated Countries will not be accepted at this time, unless they are Legal Permanent Residents of the United States. A complete list of Designated Countries can be found at: https://www.nasa.gov/oiir/export-control.
Description	A great challenge in modern astrophysics is to understand protoplanetary disk conditions during the epoch of planetesimal formation and subsequent planet aggregation. Clues to planet-forming processes are provided by the properties of the dust grains in protoplanetary disks and in cometary nuclei in our own solar system. Comets are the `deep freezers' of ices and dust grains extant in our protoplanetary disk at the time that the planetesimals were forming. The crystalline silicate fraction in comets has become a touchstone for radial transport models of protoplanetary disks, since crystals formed in the inner disk and were then radially transported to the outer disk where comets accreted volatile ices. The current work effort includes analysis of Spitzer observations of comets. Our comet team is working on modeling mixed mineral aggregate grains with crystalline inclusions on the NAS supercomputers to fit Spitzer IRS spectra of comets. To date, our constraints on the crystalline fraction in comets is limited to assessing the sub-micron to micron-sized portion of the grain size distribution. Prior to this work, only crystals computed as discrete particles of this size range fit the data. If there are only micron-sized and smaller crystals along with larger particles of amorphous silicates and amorphous carbon, then the crystalline fraction could be a very small mass fraction, in contrast to the high crystalline fraction that we deduce from analyzing only the sub-micron to micron-portion of the comet coma grain size distribution. Based on Stardust samples and interplanetary dust particles, we expect that crystals are components of porous aggregate particles, and perhaps up to 30-50 % of the mass. Computing crystals as parts of aggregates is the next step in constraining crystalline fraction in comet comae for a size distribution that extends to 20--100 microns. We use DDSCAT on the NAS computers for these computations.

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