Latitude variations of the abundances of ammonia, acetylene, and phosphine and vertical mixing in the atmospheres of Jupiter and Saturn.

Abstract

Ultraviolet observations of Jupiter and Saturn performed in the past have typically suffered from poor spatial resolution and have therefore focused on either globally-averaged values or north-south asymmetries. Spectra taken with the Hubble Space Telescope Faint Object Spectrograph have a spatial resolution of 1 arc second, making it possible to sample the disk at several latitudes. The observations presented here were taken at latitudes along the Central Meridian of both Jupiter (65$\sp\circ$S, 48$\sp\circ$S, 33$\sp\circ$S, 25$\sp\circ$S, 12$\sp\circ$S, 6$\sp\circ$S, 0$\sp\circ$, 6$\sp\circ$N, 15$\sp\circ$N, 20$\sp\circ$N, 25$\sp\circ$N, 48$\sp\circ$N, 65$\sp\circ$N, along with the Great Red Spot and the South Equatorial Belt) and Saturn (4$\sp\circ$S, 4$\sp\circ$N, 12$\sp\circ$N, 20$\sp\circ$N, 40$\sp\circ$N, 48$\sp\circ$N, 60$\sp\circ$N, 79$\sp\circ$N, 90$\sp\circ$N). The wavelength interval covers a range from 180 nm to 240 nm. Albedos have been calculated by dividing the observations by intensity of the Sun at each planet. Removal of solar features reveals the presence of ammonia, acetylene, and continuum absorbers (e.g. phosphine and aerosols). With the use of a photochemical model which includes a multiple scattering radiative transfer code, the altitude profiles of these species have been generated by using the eddy diffusion coefficient as a free parameter. From the altitude profiles, synthetic spectra were calculated and compared with the observations at each latitude. This work reveals that a variation of ammonia, acetylene, phosphine, and the vertical mixing with altitude exists in both atmospheres. The presence of a strong ammonia signature in the Jovian spectra allows it to be used as a tracer for the dynamics near the tropopause. The value of the eddy mixing coefficient at 6$\sp\circ$N increases rapidly with depth from a value of $5\times10\sp2$ cm$\rm\sp2s\sp{-1}$ to 10$\sp4$ cm$\rm\sp2s\sp{-1}$ between 90-150 mbar. Above the 90 mbar level, the value increases reaching a value of $8\times10\sp2$ cm$\rm\sp2s\sp{-1}$ at 60 mbar. A latitudinal variation in the ammonia mixing ratio and eddy mixing profile is also derived from the FOS observations. In the northern hemisphere of Jupiter, the ammonia mixing ratio above the tropopause and the eddy mixing are found to decrease as one moves further north. In the southern hemisphere, however, there is more variation with latitude. The strongest vertical mixing and the largest abundance of ammonia is in the Great Red Spot region. On Saturn, phosphine can be assumed to provide the source of opacity for the continuum. Based on this assumption, the vertical distribution of phosphine and eddy mixing coefficients have been derived. The largest mixing ratio of phosphine and the strongest mixing ($\rm 9\times 10\sp3\ cm\sp2s\sp{-1}$) are found near 12$\sp\circ$N, with both decreasing monotonically away from 12$\sp\circ$N.