Showing papers by "Andrew P. Ingersoll published in 1994"

First Images of Asteroid 243 Ida

Abstract: The first images of the asteroid 243 Ida from Galileo show an irregular object measuring 56-kilometers by 24 kilometers by 21 kilometers. Its surface is rich in geologic features, including systems of grooves, blocks, chutes, albedo features, crater chains, and a full range of crater morphologies. The largest blocks may be distributed nonuniformly across the surface; lineaments and dark-floored craters also have preferential locations. Ida is interpreted to have a substantial regolith. The high crater density and size-frequency distribution (–3 differential power-law index) indicate a surface in equilibrium with saturated cratering. A minimum model crater age for Ida—and therefore for the Koronis family to which Ida belongs—is estimated at 1 billion years, older than expected.

Galileo multispectral imaging of the north polar and eastern limb regions of the moon

Abstract: Multispectral images obtained during the Galileo probe's second encounter with the moon reveal the compositional nature of the north polar regions and the northeastern limb. Mare deposits in these regions are found to be primarily low to medium titanium lavas and, as on the western limb, show only slight spectral heterogeneity. The northern light plains are found to have the spectral characteristics of highlands materials, show little evidence for the presence of cryptomaria, and were most likely emplaced by impact processes regardless of their age.

Atmospheric gravity waves from the impact of comet Shoemaker‐Levy 9 with Jupiter

Abstract: We study the effect of the Jovian water cloud on internal gravity waves generated by the impact of comet SL9. Vertical structure follows Voyager data to the 1-bar level, a moist adiabat from 1 to 5 bars, and a dry adiabat below the 5-bar level. The waves are trapped in the moist layer and propagate horizontally. Their speed is related to the vertical integral of the Brunt-Vaisala frequency, and varies as the square root of the water abundance (130 m/s for solar composition). The amplitudes are large, e.g., ±1 K at a distance of 8000 km for an energy of 10^(27) ergs. The circular ripples should be detectable one or two days after the impact in thermal infrared and visible images.

Numerical Simulation of Baroclinic Jovian Vortices

Abstract: This thesis consists of two papers on the dynamics of Jovian planet atmospheres. The first paper discusses the uses of a normal-mode expansion in the vertical for modeling the dynamics of Jupiter's atmosphere. The second paper uses a non-linear numerical model based on the normal-mode expansion of the first paper to study the dynamics of baroclinic vortices. The abstracts for the two papers are reproduced below. Paper 1: We propose a non-linear, quasi-geostrophic, baroclinic model of Jovian atmospheric dynamics, in which vertical variations of velocity are represented by a truncated sum over a complete set of orthogonal functions obtained by a separation of variables of the linearized quasi-geostrophic potential vorticity equation. A set of equations for the time variation of the mode amplitudes in the non-linear case is then derived. We show that for a planet with a neutrally stable, fluid interior instead of a solid lower boundary, the barotropic mode represents motions in the interior, and is not affected by the baroclinic modes. One consequence of this is that a normal mode model with one baroclinic mode is dynamically equivalent to a one layer model with solid lower topography. We also show that for motions in Jupiter's cloudy lower troposphere, the stratosphere behaves nearly as a rigid lid, so that the normal-mode model is applicable to Jupiter. We test the accuracy of the normal-mode model for Jupiter using two simple problems: forced, vertically propagating Rossby waves, using two and three baroclinic modes, and baroclinic instability, using two baroclinic modes. We find that the normal-mode model provides qualitatively correct results, even with only a very limited number of vertical degrees of freedom. Paper 2: We examine the evolution of baroclinic vortices in a time dependent, nonlinear numerical model of a Jovian atmosphere. The model uses a normal-mode expansion in the vertical, using the barotropic and first two baroclinic modes (Achterberg and Ingersoll 1989). Our results for the stability of baroclinic vortices on an ƒ-plane in the absence of a mean zonal flow are consistent with previous results in the literature, although the presence of the deep fluid interior on the Jovian planets appears to shift the stability boundaries to smaller length scales. The presence of a mean zonal shear flow acts to stabilize vortices against instability, significantly modifies the finite amplitude form of baroclinic instabilities, and combined with internal barotropic instability (Gent and McWilliams 1986) produces periodic oscillations in the latitude and longitude of the vortex as observed at the level of the cloud tops. This instability may explain some, but not all, observations of longitudinal oscillations of vortices on the outer planets. Oscillations in aspect ratio and orientation of stable elliptical vortices in a zonal shear flow are observed in this baroclinic model, as in simpler two-dimensional models (Kida 1981). The meridional propagation and decay of vortices on a β-plane is inhibited by the presence of a mean zonal flow. The direction of propagation of a vortex relative to the mean zonal flow depends upon the sign of the meridional potential vorticity gradient; combined with observations of vortex drift rates, this may provide a constraint on model assumption for the flow in the deep interior of Jupiter.