2010 Annual Science Report
VPL at University of Washington Reporting | SEP 2009 – AUG 2010
Delivery of Volatiles to Terrestrial Planets
Habitable planets are too small to trap gases from the planet-forming disk. Their oceans and atmospheres must originate in the planetesimals from which the planet is built. In this task, we explore how, when, and from where Earth, Mars and habitable worlds around other stars can accumulate water and organic carbon. The main challenge is that water and organic carbon are relatively volatile elements (compared to rock and metal). Therefore, during the period of time in which solids condensed at the current position of Earth, water and carbon would have been mainly in the gas phase. Getting these materials to the habitable zone requires that material from further out in the disk would be transported inward. Another challenge is that upon reaching the Earth, both large and small suffer severe heating during atmospheric entry. We also have investigated the fate of these compounds upon release into the atmosphere.
This project models the physical and chemical processes involved in the formation of habitable planets with atmospheres and oceans. An understanding of how planets form allows us to understand the composition of these planets, which in turn allows us to evaluate whether they may be habitable. An important new result is that PAHs (polycyclic aromatic hydrocarbons, a major component of soot) are relatively short-lived in the inner regions of disks where the temperatures exceed 1000 K. These conditions are easily met at 1 AU; thus the 'soot line’ (like the snow line) in the protoplanetary disk is well beyond the region to be occupied by forming terrestrial planets. This result is in agreement with earlier results that much of the Earth’s water must have come from beyond the inner solar system, and probably from the vicinity of the asteroid belt.
We previously discovered that PAHs would have been efficient UV absorbers in the prebiotic atmospheres of habitable planets. As a follow-on from this work we investigated the fate of meteoritic organic compounds upon atmospheric entry. We had previously found that as exogenous material enters Earth’s atmosphere, organic compounds will be released at altitudes of 85-110 km, including methane, phenol and styrene. Methane released at this altitude will be rapidly dissociated by solar UV. A major new result is that complex organics containing aromatic bonds can undergo further reaction in the atmosphere. Specifically, we found that phenol styrene can undergo reaction under upper atmosphere conditions to form a much more complex molecule that contains a para-disubstituted aromatic ring.
The Brownlee group discovered evidence in Stardust samples for large-scale transport of inner disk materials to the Kuiper belt, and from there, Kuiper belt objects migrate inward to the inner solar system, where they are more likely to be delivered to habitable planets.
PROJECT INVESTIGATORS:Monika Kress
PROJECT MEMBERS:Rory Barnes
RELATED OBJECTIVES:Objective 1.1
Formation and evolution of habitable planets.
Sources of prebiotic materials and catalysts
Earth's early biosphere.
Effects of extraterrestrial events upon the biosphere