The temporal evolution of the July 2009 Jupiter impact cloud

Clouds formed on Jupiter by the impact of July 19, 2009 were observed from the IRTF with the same instrument and near-infrared filters during four observing runs over 50 days, beginning one day after impact, providing comprehensive diagnostics of cloudtop altitude, particle size and opacity and yielding quantitative information on the temporal evolution of the impact cloud (IC). The IC evolved relatively rapidly during the first 26 days after impact (Period I) and relatively slowly for the next 23 days (Period II). For the column volume density of the IC core, analyzed over a range of models with varying Mie-scattering particle radii over 0.1-1.1 mu m and imaginary indices of refraction (n(i)) from 0.001 to the limiting model-constrained value of 0.03, the Period I e-folding timescale is 4-16 times less than for Period II, with a best-fit timescale of of 23 days (Period I) increasing to 117 days (Period II) for the nominal best-fit case of large (0.7-1.1 mu m) dark (n(i)=0.01) particles consistent with previous determinations of the size and near-infrared brightness of impact cloud particles (de Pater et al. Icarus 210, 722-741, 2010). Over the entire period, the nominal model mean particle radius ranges from similar to 0.85 mu m one day after impact to 0.89-1.06 mu m 49 days later, considerably larger than the 0.21-0.28 mu m particles determined for the Shoemaker Levy 9 impact (SL9; West et al., Science 267, 1296-1301, 1995). The 1.69 and 2.12 mu m nominal model IC core opacities show timescales averaged over the entire seven-week period of similar to 33 and similar to 35 days, respectively, similar to 60% longer than the 18-23 day timescales of small-particle models (-0.36 mu m radius) which are more consistent with the similar to 15-day visible timescale reported by Sanchez-Lavega et al. (2011, Icarus 214,462-476). Including the area of the entire IC, we find that the total particle volume over the 49 days changes from similar to 0.036 to similar to 0.022 km(3) for the nominal model, corresponding to a variation of the diameter of an equivalent sphere from similar to 0.41 to similar to 035 km, close to the similar to 10% diameter change over one month reported for SL9 clouds (West et al, ibid). Nominal Period II timescales for opacities and total cloud volume - 50-300 days - are 2-11 times longer than nominal Period I timescales; indeed, values of infinity are consistent with the uncertainties. The relatively long timescales found for total cloud dissipation are consistent with (1) material dispersal by wind shears, and (2) relatively weak sedimentation/coagulation, consistent with SL9 results (West et al., ibid). Finally, the IC thickness of similar to 1 scale-height permits a vertical windshear of similar to 1 m/s, inconsistent with the derived similar to 7 m/s cloud spreading, thus implying that the meridional shear is the dominant cloud shear component, as reported for visible measurements (Sanchez-Lavega et al. ibid). (C) 2012 Elsevier Ltd. All rights reserved.