Atmospheric Oxidation Chemistry of Meteor Ablated Phosphorus

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 atmosphere can generate significant amounts of atomic P, which will then undergo atmospheric processing before deposition at the surface. Orthophosphate, P(V), 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.

Vaporized P atoms entering the upper atmosphere of a terrestrial planet 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 R1+2 to produce PO₂. From PO₂, an exothermic route to phosphoric acid exists via the formation of HOPO₂ (R3-4); however, the bio-available compound phosphonic acid (H₃PO₃) should also form via HPO₂ (R5-6):

Using a pulsed laser photolysis laser induced fluorescence technique, reactions R1+2 have been studied as a function of temperature for the first time. Initial investigations into the reaction of P + O₂ 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. By conducting experiments at high bath gas pressures 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 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.