Temperature, Size, and Energy of the Shoemaker-Levy 9 G-Impact Fireball

Abstract The fortunate position of the Galileo spacecraft provided us with a unique opportunity to directly observe the Shoemaker–Levy 9 impacts as they occurred on the far side of Jupiter, and we present observations of the G fireball obtained by the Near Infrared Mapping Spectrometer (NIMS). These measurements were performed using 10 spectral bands, 4 representing continua and spanning the wavelength range 1.84 to 4.38 μ. Fireball signals were evident for up to 80 sec, with the time of intensity maxima and duration being greater for longer wavelengths. Color temperatures and effective emitting areas were estimated by fitting blackbody functions at the four continuum wavelengths. Good blackbody fits were found, and their intensities at shorter wavelengths show excellent agreement with the Galileo Photopolarimeter/Radiometer measurements. Temperatures near the beginning are above 3000 K, decreasing to ∼1000 K after 1 min. The corresponding areas range from 400 to 20,000 km 2 . The effective diameter of the luminous fireball shows approximately linear time variation, at least for the first 45 sec. From the temperature–effective diameter relation we find an adiabatic coefficient of γ = 1.2 ± 0.1, much as expected from theoretical considerations. The luminosity, when integrated over the period of observations and assuming a Stephan—Boltzmann radiator, gives an above-cloud radiative energy loss of 0.48 ± 0.13 × 10 25 erg. As a conceptual aid, we developed a simple, heuristic theory of the fireball phenomenon, considering the penetrating fragment's wake (termed debris channel) to consist of high-temperature jovian and cometary material, which undergoes radial expansion and acceleration back along the wake axis. The outer layer of the material in this debris channel is presumed optically thick, radiating as a blackbody to produce the observed emissions (we speculate that the opacity is produced by condensed refractories such as MgO and SiO 2 , probably containing impurities). One-dimensional, variable-area axial flow of a radiating, compressible, inviscid gas is concurrently solved with the radial shock motion occurring in the non-axisymmetric atmosphere. We calculate debris surface radii and velocities using Sedov's theory for line explosions. The assumed initial debris surface temperature is consistent with entry shock heating. Our simple model shows good agreement with the observations, for both the temperature and luminous area, and suggests that the diameter of the G fragment (assumed spherical and of unit density) was 300 ± 100 m, with a nominal energy of 2.5 × 10 26 erg. The measured luminous energy is within a factor of 2 of that predicted for the nominal impactor size, whereas the amount of water in the splashback, as measured by G. L. Bjoraker et al . (1996, Icarus 121, 411–421) and Th. Encrenaz et al . , (1997, Planet. Space Sci. ) agrees to a factor of 3 with the model results; however, the large CO abundance obtained by E. Lellouch et al . (1995, Nature 373, 592–595) is inconsistent with the suggested size. This diameter estimate must be considered provisional and probably a lower limit; precise estimates require comparison of the measurements with comprehensive numerical simulations, which we encourage.