| Popis: |
We present our findings on the photometric variability of the impact spots of comet Shoemaker-Levy 9 (SL 9) in the Jovian atmosphere. Extensive imaging and differential photometry of impact spots in the methane band (8950/90 A) have enabled us to conservatively extract their photometric contribution from that of Jupiter. In this methane band, the SL 9 spots appear brighter than the surrounding Jovian surface because they lie above the main concentrations of methane that overlie the cloud tops. Our observations of two well-observed and isolated spots, H and Q1, indicate that spots experience a photometric growth that initially approximates a power law in time of index 0.3 and then appears to level off in the case of the H spot. We consider two explanations for the brightening of spots: (1) the dispersal of high optical depth core regions and (2) the coagulation of dust from the atomized or fragmented cometary ejecta. The former has been rejected because one spot, H, which was disrupted by a cyclonic storm, exhibited no discontinuity in its rate of photometric growth that would correspond to the disruption. We adopt the second as a working hypothesis because the growth of grains is consistent with our observations and has been cited as the cause for the observed changes in the integrated optical depth in the near-IR and the UV. We propose a simple model in which sites for grain nucleation are provided by the chemical interaction of cometary material and the shocked Jovian atmosphere and in which grain growth is restrained by the diminishing availability of raw materials for grain formation on timescales inversely proportional to the original aerosol density. This model, applied to a volumetric power-law distribution of aerosols, can produce an integrated scattering amplitude growth rate closely resembling our observations. |
