METEOROID ABLATION IN NEPTUNE ATMOSPHERE

Meteoroid ablation rates in Neptune's atmosphere are calculated for certain assumptions concerning the mass and velocity distribution of meteoroids in the outer Solar System. Silicate meteoroids lose most of their mass in the 1- to 100-μbar region of Neptune's atmosphere while water-ice meteoroids ablate at even higher altitudes, i.e., pressures {less-than or approximate}1 μbar. Although the ablation of both silicate and water ice occurs at altitudes that are too low to significantly affect ionospheric chemistry, meteors might contribute to ionization in Neptune's lower "bottomside" ionosphere. In addition, the interaction of water dissociation products with neutral atmospheric constituents can generate CO molecules in Neptune's upper atmosphere. The total column influx of water molecules due to meteoroid ablation on Neptune is found to range from 7 × 105 to 2 × 108 cm-2 sec-1 (an amount that could lead to stratospheric column abundances of CO of 5 × 1015 to 1× 1018 molecules cm-2); the exact value depends on the characteristics of the meteoroid population. Even the upper value of this range appears to be too small to explain the recent observations of CO on Neptune; however, given the uncertainty is these calculations, meteoroid ablation cannot be ruled out as the source of CO on Neptune. The total column influx of silicate material ranges from 2 × 106 to 3 × 108 cm-2 sec-1. Even the lower value of this range is sufficient for providing a source of dust particles to initiate nucleation and condensation of hydrocarbons in Neptune's lower stratosphere. © 1992.