THE PHOTOMETRIC GROWTH OF 2 SHOEMAKER-LEVY-9 IMPACT SITES ON JUPITER

We present our findings on the photometric variability of the impact s pots of comet Shoemaker-Levy 9 (SL 9) in the Jovian atmosphere. Extens ive imaging and differential photometry of impact spots in the methane band (8950/90 Angstrom) have enabled us to conservatively extract the ir photometric contribution from that of Jupiter. In this methane band , the SL 9 spots appear brighter than the surrounding Jovian surface b ecause they he above the main concentrations of methane that overlie t he cloud tops. Our observations of two well-observed and isolated spot s, H and Q1, indicate that spots experience a photometric growth that initially approximates a power law in time of index 0.3 and then appea rs to level off in the case of the H spot. We consider two explanation s for the brightening of spots: (1) the dispersal of high optical dept h core regions and (2) the coagulation of dust from the atomized or fr agmented cometary ejecta. The former has been rejected because one spo t, H, which was disrupted by a cyclonic storm, exhibited no discontinu ity in its rate of photometric growth that would correspond to the dis ruption. We adopt the second as a working hypothesis because the growt h 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 g rain nucleation are provided by the chemical interaction of cometary m aterial 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 aeroso l density. This model, applied to a volumetric power-law distribution of aerosols, can produce an integrated scattering amplitude growth rat e closely resembling our observations.