Toshiko Takata

Impact of comet Shoemaker‐Levy 9 on Jupiter

Three-dimensional numerical simulations of the impact of Comet Shoemaker-Levy 9 on Jupiter and the resulting vapor plume expansion were conducted using the Smoothed Particle Hydrodynamics (SPH) method. An icy body with a diameter of 2 km can penetrate to an altitude of -350 km (0 km = 1 bar) and most of the incident kinetic energy is transferred to the atmosphere between -100 km to -250 km. This energy is converted to potential energy of the resulting gas plume. The unconfined plume expands vertically and has a peak radiative power approximately equal to the total radiation from Jupiters disc. The plume rises a few tens of atmospheric scale heights in ∼10² seconds. The rising plume reaches the altitude of ∼3000 km, but no atmospheric gas is accelerated to the escape velocity (∼60 km/s).

Radiative signatures from impact of comet Shoemaker‐Levy‐9 on Jupiter

The visible optical power emitted from the expansion plumes from 0.4 and 2 km diameter fragments of Shoemaker-Levy are expected to be, ∼25% and comparable to, the visible solar flux reflected from Jupiter, respectively, for several minutes, and could be easily observed by sensors on the Galileo spacecraft. Earth-based observers can detect these plumes as these expand over the SW limb of Jupiter and come into earth view some minutes after impact!

Atmospheric effects on cratering on Venus

A paraboloidal bow shock model is developed in order to estimate the surface distribution of gas shock-induced modifications surrounding Venusian impact craters. We apply two-dimensional oblique shock dynamics to describe a three-dimensional paraboloidal-shaped bow shock impinging upon an assumed incompressible Venusian surface. The effects of the hypersonic atmospheric shock acting on the Venusian surface are considered in terms of induced maximum gas pressure, density, particle velocity, and temperature, for varying angles and velocities of impact. The maximum boulder size that can be saltated by the shock wave induced gas flow and the degree of mutual collision of the surface materials are also considered. The present calculations quantitatively predict the areal extent of the gas shock perturbed surface for normal and oblique impact as a function of impact angle and velocity, and radii of impactors. For a 1-km radius stony meteorite impacting normally at 20 km/s, the radius of the disturbed area extends ∼10–17 times the 3–5 km crater radius. The perturbed surface affects the surface radar properties, and the present results can provide an explanation of the wide “dark/bright halos” surrounding some of the Venusian impact craters observed via Magellan imagery. For example, a ∼50-km radius bright halo surrounding a ∼20-km dark halo is observed around the 3.1-km radius crater located at 16.5° north latitude and 334.4° longitude. The average value of the radar backscatter cross section of the ∼20-km radius dark halo indicates that ∼50-cm-thick layer of porous lithologic material is superimposed upon an assumed undisturbed basement rock surface. The bright halo indicates that the surface roughness in this region is ∼30 % greater than that of the surrounding original surface. These features can be induced by atmospheric shock waves. The present model can relate the observed crater halo radii to the impact parameters, such as projectile radius and density, and the impact velocity and angle.

Comet Shoemaker-Levy 9: Fragment and progenitor impact energy

Initial observational data from the impact of fragments of Comet Shoemaker-Levy 9 (SL9) are compared with smoothed particle hydrodynamic (SPH) calculations to determine their pre-impact diameters and the equivalent diameter of the SL9 progenitor. Diameters (solid ice) of 2.0±0.1, 2.0±0.05, 2.1±0.04 and 1.9±0.05 km for fragments A, E, G1, and W are obtained from impact-induced plume heights from the Hubble Space Telescope (HST) data. Applying these values to scale apparent diameters for the balance of 18 SL fragments in Weaver et al.s [1995] catalog of 22 objects yields a SL9 progenitor diameter of 5.0±1.8 km. This corresponds to total impact energy of 1.2 (+1.8−0.8) × 10^(30) erg. Such an energetic event occurs on Jupiter and Earth at least every 4,900 +4,700, −2,700, and ∼0.5 × 10^8 years, respectively.