Atmospheric Oxidation Chemistry of Meteor Ablated Phosphorus
Abstract
Phosphorus, P, is a key biological element with major roles in replication, information transfer, and metabolism. Interplanetary dust particles contain ∼0.1% P by weight, and meteoric ablation in the 1 μbar region of a planetary upper atmosphere can generate significant amounts of atomic P, which will then undergo atmospheric processing before deposition at the surface. Orthophosphate (oxidation state +5) is the dominant form of inorganic P at the Earth's surface; however, due to the low water solubility and reactivity of such P(V) salts, they have a poor bio-availability. In contrast, less oxidised forms of P (oxidation state ≤+3) are far more bio-available. It has been suggested that these reduced forms of P may have originated from extra-terrestrial material that fell to Earth during the heavy bombardment period. Previous studies have focused on the direct delivery of P to the surface in meteorites, to undergo processing through aqueous phase chemistry. In contrast, the atmospheric chemistry of P has so far been ignored. The vaporized P atoms entering the upper atmospheres of the terrestrial planets will undergo chemical processing to form a variety of compounds in which P may exist in different oxidation states due to the presence of both oxidizing and reducing agents. Initial oxidation of P is likely to proceed via reactions 1-4 to produce PO2. From PO2, an exothermic route to phosphoric acid exists via the formation of HOPO2 (R5-6); however, the bio-available compound phosphonic acid (H3PO3) should also form via HPO2 (R7-8): P + O2 → PO + O (R1) P + CO2 → PO + CO (R2) PO + O2 → PO2 + O (R3) PO + CO2 → PO2 + CO (R4) PO2 + OH → HOPO2 (R5) HOPO2 + H2O → H3PO4 (R6) PO2 + H → HPO2 (R7) HPO2 + H2O → H3PO3 (R8) Using a pulsed laser photolysis (PLP)-laser induced fluorescence (LIF) technique, reactions R1-4 have been studied as a function of temperature for the first time. P atoms were generated in the presence of O2 and CO2 by PLP of PCl3, and the subsequent growth and decay of the PO radical monitored by LIF. Initial investigations into the reaction of P + O2 (R1) indicated an inverse pressure dependence, with the rate decreasing with increasing pressure. We attribute this pressure dependence to the interference of two reactive low-lying metastable states of P (the 2D and 2P states), which are quenched at higher bath gas pressures. Thus by conducting experiments at high bath gas pressures (∼20 torr) rate coefficients for the reaction of ground state P(4S) could be measured. The removal rates of both excited states of P with O2 and CO2 have also been investigated, and studies of the reactions of PO2 (R5 and R7) are currently underway. In addition to understanding the reaction kinetics, the ablation process is also under investigation. Using a meteoric ablation simulator, the temperature at which P and PO ablate from apatite, a phosphorus rich mineral, has been measured, and compared with a computer model of the process.
- Publication:
-
EGU General Assembly Conference Abstracts
- Pub Date:
- April 2019
- Bibcode:
- 2019EGUGA..2115488D