Computer Science – Numerical Analysis
Scientific paper
Jul 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994apj...429..863c&link_type=abstract
The Astrophysical Journal, vol. 429, no. 2, pt. 1, p. 863-875
Computer Science
Numerical Analysis
26
Comets, Explosions, Impact Prediction, Jupiter Atmosphere, Luminosity, Shock Waves, Ablation, Fragmentation, Local Thermodynamic Equilibrium, Numerical Analysis
Scientific paper
In view of the expected collision of comet Shoemaker-Levy 9 (1993e) with Jupiter in 1994 July, we calculate basic properties of the initial interaction for a simplified Jovian atmosphere. The comet is expected to impact Jupiter at 60 km/sec and at an angle of 45 deg to the zenith. The shock wave generated by the bolide should be optically thick once it has penetrated to an atmospheric density approximately 10-6 gr/cm3, and we calculate the post-shock conditions assuming local thermodynamic equilibrium (LTE) for shock velocities vsh in the range 10 to 60 km/sec and preshock densities rhoa = 10-6 to 10-2 gr/cm3. Our shock calculations include molecular hydrogen, atomic hydrogen, ionized hydrogen, neutral helium, and singly ionized helium. Even at the highest shock velocity, the gas is only partially ionized and the postshock temperature rises with preshock density in order to maintain the ionization. The value of the effective shock adiabatic index gammash varies from 1.17 (at low vsh and rhoa to 1.40 (at high vsh and rhoa. The ablation rate is limited by the radiative flux that reaches the bolide surface. We argue that the ablated gas does not efficiently transfer its kinetic energy to the atmosphere, and it ultimately slows in a similar fashion to the comet material. As the bolide initially falls through the atmosphere, the character of the shock emission changes. At rhoa approximately 10-8 gr/cm 3, the gas is optically thin and we expect line emission; in the optical spectrum, Balmer emission is expected from the shocked atmosphere and low-ionization metal lines from ablated cometary material. At rhoa approximately 10-6 gr/m3, the shocked gas is optically thick and the shock front near the bolide produces a blackbody spectrum. The temperature is favorable for ultraviolet (1000 to 3000 A) emission and the luminosity may be approximately 5 x 1023 ergs/sec for approximately 0.6 sec for a bolide 1 km in radius. At rhoa approximately 10-4 gr/cm3, the bolide has passed below the ultraviolet photosphere. The shock front emits considerable ionizing radiation, but it is absorbed in a narrow preshock region. The bolometric correction for the optical luminosity is large and we expect a 3000 to 8000 A luminosity of approximately 3 x 1023 ergs/sec for approximately 1 sec. The optical emission is strongly peaked in the vicinity of the bolide. The bolide does have a somewhat less luminous, optically thick trail extending greater than or equal to 10 km, but the radiation is characterized by a temperature of 4000 to 5000 K. From the fragmentation model of Chyba, Thomas, & Zahnle (1993), the bolide deposits most of its kinetic energy at rhoa approximately 10-3 gr/cm3 and this is the effective explosion site. The shock wave from such an explosion can move up about one density scale height.
Chevalier Roger A.
Sarazin Craig L.
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