"What has the sea level elevation of the Pacific Ocean been at varied periods in the last 4.5 billion years."
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Photochemistry and impact-induced shock chemistry on Earth, Titan, comets, and beyond
PI: Zahnle, Kevin
This research proposal addresses disequilibrium chemistry in possible planetary atmospheres of exobiological interest. The broad context is to quantitatively examine the sorts of disequilibria we might expect to see on living and dead planets, especially those in other solar systems. The specific problems we propose to address include (1) the abundance of CO in planetary atmospheres, with emphasis on its value as a biomarker, as an anti-biomarker, and as a prebiotic molecule; (2) the photochemical consequences of the origin of oxygenic photosynthesis, with emphasis on the details of the oxic transition; and (3) the products of impact shock chemistry over a wide range of energies and over a wide range of planets, ranging from Earth to Titan to comets and beyond.
(1) I propose that CO can be both an effective biomarker and an effective anti-biomarker. Which it is depends on context. In particular, CO may be the most accessible indicator of biological activity on exoplanets for ground-based observers. The proposed work consists of numerical experiments using a 1-D photochemistry code to generate steadystate atmospheric compositions. In addition the model will address the fate of chemical products of impacts.
(2) I propose that, for plausible biogenic fluxes stemming from oxygenic photosynthesis, there can be multiple solutions for the amount of O 2 in the atmosphere. Moreover the high O 2 solution appears to be unstable. Thus it is possible that O 2 runaways can occur without any change in the redox budget of the atmosphere. This task also uses the 1-D photochemistry code. Stability will be tested using the code in its time-dependent mode. Near the oxic transition O 2 can have an atmospheric time constant of less than a day. In general it makes sense to study these atmospheres using a time-dependent code that accounts for diurnal and perhaps annual cycles. To my knowledge nothing of this sort has been attempted.
(3) I propose to develop a more capable model for computing the products of impact shock chemistry and apply it to modern Titan, early Earth, and the composition of comets, beginning with SL9 and extending to analysis of Deep Impact results (if any). The proposed work assembles several preexisting computer codes. The major ones are a time-dependent thermochemistry code built to model the chemical consequences of the 1994 SL9 impact events, and a code built to compute NOx chemistry for Earth over a full range of impact energies from meteors to planetary catastrophes. Recent revisions of the solar composition have lead to a reconsideration of what the overall reduced character of D/Shoemaker-Levy 9’s impact debris might mean. It would appear that cometary “ices” may often have C/O ratios greater than unity, so that when shocked during impacts at cosmic velocities the resulting chemical debris are mostly reduced. This has implications for volaitles of early Earth.
