LONG-TERM RESPONSE OF JUPITERS THERMAL STRUCTURE TO THE SL9 IMPACTS

Information on the thermal structure prevailing over the SL9 impact si tes hours to months after the collisions is available from measurement s of thermal emission at selected wavelengths. Spectral measurements o f CH4 and C2H2 emission lines indicate that the stratospheric heating over the impact sites was primarily confined to the pressure levels af fected by the fallback of the ejecta plumes (p < 500 mu bar for L, p < 20 mu bar for E). Perturbations amounting to several tens of kelvins around 10 mu bar were found over areas 15 000-20 000 km wide centered on the K and L sites within a day following impact. A large fraction o f the kinetic energy of the plumes appeared to have been used to heat the jovian atmosphere rather than being rapidly radiated away. Existin g millimeter and thermal infrared measurements give no evidence for a significant heating of the lower stratosphere or upper troposphere, be neath the region of the plume fallbacks. Enhanced brightness observed over many sites in the H-2-He continuum very likely results from strat ospheric emission of dust particles and not from tropospheric heating. Temperature perturbations from large impactors (G, K, L) were no long er detectable a week after the impacts. Months later, millimeter measu rements of shock-produced compounds consistently indicate a temperatur e of 150-160 K around 100 mu bar, likely representative of the nominal state of Jupiter's atmosphere The E site was still approximate to 40 K warmer than nominal around 3 mu bar, 2.6 days after impact. Small te mperature elevations in this region(approximate to 10 K) were still ma rginally visible over the W and Q(1) sites, 8-10 days after impact. Gl obally, measurements from various observers consistently indicate that the Large sites cooled rapidly with a timescale of 1-2 days, much sma ller than the radiative relaxation time in normal quiescent conditions . Enhanced cooling from the gases manufactured and deposited by the im pact processes cannot explain this behavior. On the other hand, the ma ss of silicate particles simultaneously deposited by the plumes is eno ugh to produce the large cooling rate observed. Detailed time-dependen t modelling of the radiative balance over the impact sites shows that silicate particles can reproduce the observed evolution of temperature s, provided that their radius is smaller than approximate to 0.05 mu m to delay sedimentation. Mixing with ambient air was probably signific ant to further reduce the temperature perturbations after several days . A detailed investigation of two particular events, E and H, strongly suggests that the two plumes had vastly different dust-to-gas ratios, a caveat to remember when synthesizing the characteristics of a ''gen eric'' event. Further analyses of existing data and further modeling e fforts should help refining our view of the perturbations caused by th e SL9 collision. (C) 1997 Elsevier Science Ltd. All rights reserved.