Photochemistry of Saturn's atmosphere II. Effects of an influx of externaloxygen

We use a one-dimensional diurnally averaged model of photochemistry and dif fusion in Saturn's stratosphere to investigate the influence of extraplanet ary debris on atmospheric chemistry, In particular, we consider the effects of an influx of oxygen from micrometeoroid ablation or from ring-particle diffusion; the contribution from cometary impacts, satellite debris, or rin g vapor is deemed to be less important. The photochemical model results are compared directly with Infrared Space Observatory (ISO) observations to co nstrain the influx of extraplanetary oxygen to Saturn. From the ISO observa tions, we determine that the column densities of CO2 and H2O above 10 mbar in Saturn's atmosphere are (6.3 +/- 1) x 10(14) and (1.4 +/- 0.4) x 10(15) cm(-2), respectively; our models indicate that a globally averaged oxygen i nflux of (4 +/- 2) x 10(6) O atoms cm(-2) s(-1) is required to explain thes e observations. Models with a locally enhanced influx of H2O operating over a small fraction of the projected area do not provide as good a fit to the ISO H2O observations. If volatile oxygen compounds comprise one-third to o ne-half of the exogenic source by mass, then Saturn is currently being bomb arded with (3 +/- 2) x 10(-16) g cm(-2) s(-1) of extraplanetary material. T o reproduce the observed CO2/H2O ratio in Saturn's stratosphere, some of th e exogenic oxygen must arrive in the form of a carbon-oxygen bonded species such as CO or CO2. An influx consistent with the composition of cometary i ces fails to reproduce the high observed CO2/H2O ratio, suggesting that (i) the material has ices that are slightly more carbon-rich than is typical f or comets, (ii) a contribution from an organic-rich component is required, or (iii) some of the hydrogen-oxygen bonded material is converted to carbon -oxygen bonded material without photochemistry (e.g., during the ablation p rocess). We have also reanalyzed the 5-mu m CO observations of Noll and Lar son (Icarus 89, 168-189, 1990) and determine that the CO lines are most sen sitive to the 100- to 400-mbar column density for which we derive a range o f (0.7-1.5)x 10(17) cm(-2); the CO observations do not allow us to distingu ish between an external or internal source of CO on Saturn. If we assume that all the extraplanetary oxygen derives from a micrometeoro id source, then the unfocused dust flux at 9.5 AU must be (i) (1 +/- 0.7) x 10(-16) g cm(-2) s(-1) if interstellar grains are the source of the extern al oxygen on Saturn, (ii) (4 +/- 3) x 10(-17) g cm(-2) s-' if IDPs on rando mly inclined, highly eccentric orbits (e.g., Halley-type comet grains) are the source of the external oxygen, or (iii) (2 i 1.4) x 10(-18) g cm(-2) s( -1) if IDPs on low inclination, low eccentricity orbits (e.g., Kuiper-belt grains) are the source of the external oxygen. These estimates can be used in combination with future Cassini dust detection data to determine the ult imate source of the dust at Saturn's distance from the Sun. (C) 2000 Academ ic Press.