Shoemaker-Levy Research Articles

Comet Shoemaker–Levy 9 Dust Size and Velocity Distributions

Pre-impact observations of Comet Shoemaker–Levy 9 (S-L9) obtained with the Hubble Space Telescope are examined, and a model of an active, dust-producing comet is fitted to images of fragments G, H, K, and L. The model assumes steady isotropic dust emission from each fragment's sunlit hemisphere. Best-fit results indicate that the dominant light-scatterers in these fragments' comae were relatively large dust grains of radii 10 μm ≲ R ≲ 3 mm. The fragments' dust size distributions were rather flat in comparison to other comets, d N ( R ) ∝ R −2.3±0.1, and the dust ejection speeds were ∼0.5–1.5 m/s. The S-L9 fragments themselves were not detected directly, and upper limits on their radii are 1.0–1.5 km assuming an albedo a=0.04. However, these fragments' vigorous production of dust, which ranges from 6 to 22 kg/s, places a lower limit of ∼100 m on their radii at the moment of tidal breakup. Any fragments smaller than this limit, yet experiencing similar mass loss rates, would have dissipated prior to impact. Such bodies would fail to leave an impact scar at Jupiter's atmosphere, as was realized by fragments F, J, P 1, P 2, T, and U.

Wave Disturbances from the Comet SL-9 Impacts into Jupiter's Atmosphere

Wave disturbances due to the Shoemaker–Levy 9 (SL-9) cometary impacts into Jupiter's atmosphere have been simulated with a fully compressible (nonhydrostatic), time-dependent, nonlinear, axisymmetric, f-plane, finite difference computational scheme. Energy is released in a cylindrical region with a radius of 250 to 1000 km as suggested by models of the reentry of impact ejecta following the initial explosion. The model produces outward moving gravity waves at stratospheric altitudes with speeds and relative amplitudes in agreement with observations. The waves emerge from a cylindrical region of alternating inflow and outflow that extends high into the atmosphere in the main region of energy release. The disturbances originate as horizontally propagating waves at the periphery of this region, thereby providing an explanation for the observed large initial radius (∼450–700 km) of the main ring. The model results suggest that the waves are made visible by the inflow of particulate impact debris into outward moving rings of wave horizontal convergence. The inner edge of the extensive clear zone outside of the main dark ring may be the divergence phase of the leading fast wave. The results of this study remove the necessity to invoke a stable, water-rich, wave-trapping layer in Jupiter's atmosphere in order to understand the Comet SL-9 observations of dark wave-like rings expanding radially away from the impact sites.

Presidents, Experts, and Asteroids

Presidents, Experts, and Asteroids

Extracting sulfur from Shoemaker-Levy 9

Extracting sulfur from Shoemaker-Levy 9

Entry Flashes of Cometary Fragments in Jovian Upper Atmosphere

Short lasting flashes, called as First Precursor (PC1), were observed by some ground-based near-infrared observations for the impacts of large-sized fragments of comet Shoemaker-Levy 9 (SL9) in July 1994. The impact detections by the spacecraft Galileo [2, 7] about 10 seconds after the PC1 detections by ground based telescopes, combined with the far-side impacts of SL9 fragments as viewed from Earth, suggest that the source of the PC1 should be located in the Jovian upper atmosphere above the limb, at which the atmospheric pressure is extremely low. Thus, an important problem on the PC1 is how does the falling cometary fragment, which is a huge meteorite, emit near-infrared in the extremely thin atmosphere. The ablation model, which is usually used for an impact bolide, can only estimate flux from the bolide in a dense atmosphere at visible wavelength. In this paper, we assume that the PC1s are thermal radiation from the fragments and attempt quantitative estimations of the PCI fluxes using a simple entry flash model.

Tidal Splitting of Comets in Earth’s Vicinity

AbstractWe present a study of the influence of tidal splitting In order to clarify the role of comets as Earth impactors and bringers of organics to the young planet. Based on low values for the density and material strength, as indicated by Comet Shoemaker-Levy 9, we have modeled the evolution of a comet in an orbit leading to Earth impact. We find that cometary nuclei impacting the Earth with radii ≥ 0.1 - 0.3 km will split just before doing so. Nuclei of several km radius will split when passing within a distance ≤, 5 - 8 Earth radii. Cometary nuclei with radii ≥ 1km stand a much larger chance of being tidally split than of hitting the Earth. Consequently, streams of fragments and debris form in Earth-crossing orbits more often than large nuclei cause impacts. The mass distribution of impacting material is shifted toward sub-km objects, whereby the survivability of organics may be significantly increased. If the cometary impactors move in orbits where ≥ 5 passages within Earth’s Roche zone are expected, we estimate that most of the material reaching Earth’s surface will appear in the form of split fragments.

Comet Shoemaker-Levy 9: an active comet

The important elements of the debate over the activity versus dormancy of comet Shoemaker-Levy 9 (S-L-9) are reviewed. It is argued that the circularity of the isophotes in the inner comae of S-L 9 as well as the spatial dependencies of the comae brightness profiles are indicators of sustained dust production by S-L 9. It is also shown that the westward tail orientations, which were formerly interpreted as a sign of the comet's dormancy, are not a good indicator of either activity or dormancy. Rather, the tail orientations simply place constraints on the dust production rate for grains smaller than ≈ 5 μm. All the available evidence points to S-L 9 as having been an active, dust-producing comet. Synthetic images of an active comet are fitted to Hubble Space Telescope images of the S-L 9 fragment K, and its grain size and outflow velocity distributions are extracted. These findings show that the appearance of the dust coma was dominated by large grains having radii between ≈ 30 μm and ≈ 3 mm, produced at a rate of M ≈ 22 kg s −1, and ejected at outflow velocities of ≈ 0.5 m s −1. Only upper limits on the production rates of smaller grains are obtained. The nucleus of fragment K was not observed directly but its size is restricted to lie within a rather narrow interval 0.4 ≲ R f ≲ 1.2 km.

Magnetic mapping of auroral signatures of comet SL9 in the Jovian magnetosphere

The electrodynamic interaction of comet Shoemaker-Levy 9 (SL9) with the Jovian magnetosphere gave rise to the detection of several unique phenomena in the UV, X-ray and radio wavelength ranges. Among them, the detection of an unusual FUV bright spot in Hubble Space Telescope images of the southern polar cap on July 20, just before P2 collision, may be attributed to auroral-like processes triggered by the charged environment of the comet fragments. We model here in detail the time-varying morphology of the instantaneous magnetic field lines passing through the comet fragments during their crossing of the magnetosphere, with special focus on the location of the magnetic footprint and the nature of the field line. We show that the FUV bright spot, not corotating with the planet, is likely to be related with a fragment still in the magnetosphere, and that fragment Q is the most presumable source of the interaction, as its foot-print can easily be resolved from fragment P2's, and also, although less easily, from the more distant fragment R to W's ones. We show also that Q, as well as the other fragments, was on an open magnetic field line at the time of the observations, in agreement with the absence of observable conjugate emission in the north. But, the deformation of the magnetic field line passing through Q during the following few hours is such that it presumably became closed to the northern hemisphere during two separate periods between the observations under study and fragment Q's collision. A series of X-ray bursts detected in the north precisely during the first of these periods could be related to the same process and strengthen our identification. A second FUV set of data was taken during the same period of closed field lines, but due to an unfavourable viewing geometry, the identification of observed bright spots with fragment Q footprint is more ambiguous. Finally, we estimate crudely the energy of the particles precipitating in the FUV spot, and discuss briefly possible plasma processes.

McDonald Observatory data on the comet Shoemaker-Levy 9 impacts on Jupiter and the resulting haze particles

Simultaneous light curves of the Comet Shoemaker-Levy 9 fragment R and V impacts were measured with the McDonald Observatory 2.7 m and 0.8 m telescopes, at wavelengths of 2.12 μm and 0.893 μm, respectively. The R impact was clearly detected at 2.12 μm, but not at 0.893 μm, leading to an upper limit on the “main event” temperature of 1300 K. The V impact was not detected. A possible detection of the U impact at 2.12 μm was recorded. CCD spectrophotometry of Jupiter was obtained with the McDonald 2.1 m telescope during the Shoemaker-Levy 9 impact period (17 July–23 July 1994 UT; Barker et al. (1994) Bull. Am. Astron. Soc. 26, 1569). Spectra of Jupiter from 0.55 to 1.08 μm (10 Å resolution) were obtained using a long-slit CCD spectrograph (ES2), with a seeing-limited spatial resolution of about 1–2″. Impact regions were darker than neighboring non-impact regions by some 5–10% at all wavelengths except in the strong methane absorption bands (0.727, 0.864, and 0.890 μm) where the impact sites are brighter. Aerosol models of this spectral change are examined. The grey absorption observed is not typical of a population of particles much smaller than the wavelength. Neither is the similar haze brightening seen in two methane bands of similar strength at 0.727 and 0.864 μm. Our radiative transfer models of the H and E impact aites favor broad particle size distributions, with a mean particle radius 〈 r〉 = 0.25 μm and a log-normal size distribution of width b ≈ 0.6. A constant imaginary refractive index of 0.006 in this wavelength range provides an adequate spectral fit to the H data, while an index of 0.012 better fits the E data. Additional ES2 Jupiter spectra from June 1995 with both “blue” (0.31–0.58 μm) and “red” (0.55–1.08 μm) gratings were obtained. Blue spectra and images still show some evidence for residual impact haze opacity near the impact latitudes; red spectra and images do not. Apparently sedimentation of the larger particles in a broad initial size distribution has removed the red opacity.

Eugene M. Shoemaker (1928-1997)

Eugene M. Shoemaker (1928-1997)

Thermal Infrared Imaging Spectroscopy of Shoemaker–Levy 9 Impact Sites: Spatial and Vertical Distributions of NH3, C2H4, and 10-μm Dust Emission

Spatially resolved spectroscopy of the Shoemaker–Levy 9 (SL9) sites traces the dynamical evolution of cometary material, upwelled tropospheric gas, and compounds produced when the plume splashed back upon the atmosphere. The emissions of impact-produced stratospheric NH3, C2H4, and dust were imaged at NASA's Infrared Telescope Facility, with Irshell (the U. Texas mid-IR echelle spectrometer) 21 hr and 6, 11, and 12 days following the K impact. The images covered a ∼7 × 17″ region generally centered on the K site and were composed of 0.8 × 1 arcsec pixels, each containing a spectrum of resolution ∼15,000. We find evidence for two sources of NH3. Most of the stratospheric NH3resided at ∼20 mbar. A second reservoir existed above 1 mbar, with a column abundance ∼120lower than that of the deeper source (2 ± 1 × 1017molecules cm−2above 40 mbar for the K site). The position of the high altitude NH3suggests that it rose and was quenched within the fireball and survived the splash. The 2 ± 1 × 1013g of low altitude NH3indicates that the K impact upwelled at least ∼2 × 1016g of jovian gas from Jupiter's troposphere. Its altitude coincides with the level where static stability is maximum. The NH3lineshape 12 days following impact indicates a gradual depletion of the high altitude source, which suggests that NH3was partially shielded from UV radiation. Enhanced continuum emission observed around 908 and 948 cm−1and not at wavenumbers outside the silicate feature is consistent with 8 ± 4 × 1012g of cometary dust residing in the plume fallback region. The total mass of C2H4was found to be 1 ± 0.3 × 1012g and remained constant within error limits throughout the observations. The compounds above 1 mbar displayed differing horizontal coverages consistent with each molecule's role in a ballistic plume, having a range of temperatures. Ammonia at 20 mbar spread out with time; however, its coverage was never as extensive as that of the dark material seen in HST images. In contrast, the dust, C2H4, and HCN (B. Bézardet al.1997,Icarus125,94–120), observed at significantly lower pressures than NH3, covered a broader spatial extent, similar to the coverage of the ejecta blanket observed by HST. Six days following impact, the dust and C2H4spread 7° eastward of NH3, similar to the dark particulates. The quiescent behavior of the NH3at 20 mbar in contrast to the zonal drift of the dust indicates the presence of winds above 1 mbar that are disconnected from those in the lower stratosphere.

Impact of Comet Shoemaker–Levy 9—Size, Origin, and Plumes: Comparison of Numerical Analysis with Observations

Numerical results of impact of Comet Shoemaker–Levy 9 (SL9) on Jupiter are compared with observational phenomena to investigate the size and origin of SL9 and impact features. Plume evolution is simulated numerically for 0.4- and 2.0-km-diameter (D) impactors. Analytical estimates indicate that the maximum plume heights are proportional to (D/lnD)3. This relation is also confirmed by the extrapolation of numerical simulations. From the correlation between maximum plume heights and impactor diameters, the diameters of large SL9 fragments and the parent body are estimated as ∼2 and 4–5 km, respectively. The plume induced by the impact of fragments of SL9 consists of atmospheric gas and impactor material. The chemical abundances of the plumes, especially their high CO abundances, indicate that they contain primitive materials from fragments and thus SL9 is probably of cometary origin. The lateral expansion of plume is also examined. Our simulations suggest that the energy source required to produce the observable waves should be located in the stratospheric region, rather than in the deep troposphere. Finally, the ejecta pattern observed by Hubble Space Telescope [Hammelet al.(1995)Science267, 1288–1296] is simulated by a simple particle model. The results suggest that the ejection of atmospheric gas and cometary materials from a narrow ∼30° cone region along the trajectory spreads out and yields the ejecta patterns observed by Hubble Space Telescope overlying the clouds.

Evolution of vortices formed in Jupiter’s atmosphere after the collision of the planet with Comet Shoemaker-Levy 9

The evolution of disturbances formed in the Jovian atmosphere after the collision with large fragments of Comet Shoemaker-Levy is investigated. Simplified equations for the evolution of large vortices in a shallow, horizontally inhomogeneous atmosphere are derived taking account of the latitudinal nonuniformity of the zonal wind and also taking account of viscosity and heat conduction. The results of the numerical simulation of the evolution of the vortices are in good agreement with observations from the Hubble platform. It is shown that the evolution of the vortex structure depends strongly on the zonal wind velocity field in the fall region of a comet fragment. The threshold energy of the initial disturbance at which large, long-lived vortices, such as the Great Red Spot, can form is estimated to be E∼ 1024 ergs.

Size, Density, and Structure of Comet Shoemaker–Levy 9 Inferred from the Physics of Tidal Breakup

Detailed consideration of possible fragmentation mechanisms shows that Comet Shoemaker–Levy 9 (SL9) had negligible effective strength, even in comparison with tide and self-gravity, by the time it attained perijove in 1992. This reduces the tidal physics to a computable basis: we model the elongation of this “rubble-pile,” and the onset of the instability which created a chain of gravitationally bound clumps, using anN-body code with self-gravity and simple collisions. Gravitational clumping depends only on the density ρcof the progenitor (for a given orbit), and chain length then scales linearly with initial diameterdc. We thus constrain from chain morphology that ρc≈ 0.6 g cm−3and from chain length thatdc≈ 1.5 km.Our numerical results accurately match analytical derivations for the threshold of tidal breakup, and lead to general relations for erosion and disruption of strengthless or regolith-covered bodies. For a given random encounter, we show that a rubble-pile comet, or one mantled in deep regolith, is half as likely to be destroyed by tides as it is to impact an outer planet. Because of their similar density ratio, the same holds true for rubble-pile asteroids encountering terrestrial planets. Split comets such as SL9 near Saturn are unlikely: the required periapses intersect the planet.

Influence of Mass-Loss on the Orbit of Comet Shoemaker-Levy 9

Using the Jeans problem and the Fermi problem of two bodies with variable masses, the influence of solar and cometary mass-loss on the orbit of comet Shoemaker-Levy 9 (SL9) is discussed in this paper. It is shown that solar and cometary mass-loss both will cause the secular and periodic terms in the semimajor axis a of comet SL9. The influence of solar mass-loss on the orbit of comet SL9 is very small and it may be omitted. But the influence of mass-loss of comet SL9 itself on its orbit is large and it may cause the secular variation in a to be about 104 km every year. The cometary mass-loss will obviously influence the time and place of the comet SL9 crashing into Jupiter.

Observational constraints on the composition and nature of comet D/Shoemaker-Levy 9

What did the break-up of comet Shoemaker-Levy 9 (SL9) and its subsequent impact on Jupiter teach us about the nature and constitution of this comet? The break-up of the comet apparently triggered activity of the fragments. Although a dust coma was continuously present around the fragments that orbited Jupiter, spectroscopic observations did not reveal any sign of gas. The impact itself was so energetic that most molecules of the impactor were dissociated and that any chemical memory was lost. Ultraviolet and visible spectroscopy of the impact sites revealed emission lines from several atoms, giving potential information on elemental abundances. However, the fact that both neutral and ionized atoms are emitting, and that both fundamental and inter-system lines are present, suggest that the medium is out-of-equilibrium and that emitting mechanisms other than simple resonance fluorescence are at work. Ultraviolet, infrared, and radio spectroscopy revealed lines of several molecular species, in emission and/or absorption, that are not normally present in Jupiter's upper atmosphere. In the visible, dark spots due to aerosols developed at the impact sites. It is not clear at the present time which part of this material is coming from preserved impactor material, from the recombination of the dissociated impactor material, from reactions between the impactor's and Jupiter's material, or from material coming from the lower layers of Jupiter's atmosphere. Realistic modelling of the impacts and of the following chemical reactions will be necessary to address all these issues. In this chapter, we will review the observational clues to the composition and structure of SL9 in the context of our knowledge of the composition and structure of comets and asteroids.

Waves from the Shoemaker-Levy 9 impacts

Images of Jupiter taken by the Hubble Space Telescope (HST) reveal two concentric circular rings surrounding five of the impact sites from comet Shoemaker-Levy 9 (SL9). The rings are visible 1.0 to 2.5 hours after the impacts. The outer ring expands at a constant rate of 450 ms-1. The inner ring expands at about half that speed. The rings appear to be waves. Other features (diffuse rings and crescent) further out appear to be debris thrown out by the impact. Sound waves (p-modes), internal gravity waves (g-modes), surface gravity waves (f-modes), and rotational waves (r-modes) all are excited by the impacts. Most of these waves do not match the slow speed, relatively large amplitude, and narrow width of the observed rings. Ingersoll and Kanamori have argued that internal gravity waves trapped in a stable layer within the putative water cloud are the only waves that can match the observations. If they are correct, and if moist convection in the water cloud is producing the stable layer, then the O/H ratio on Jupiter is roughly ten times that on the Sun.

On a possible mechanism of the formation of large-scale disturbances in Jupiter’s atmosphere as a result of the falling of fragments of Comet Schoemaker-Levy 9

A mechanism explaining the sizes and structure of large-scale disturbances produced in Jupiter’s atmosphere by the impact of the fragments of Comet Shoemaker-Levy is proposed.

Determination of the impact times and locations of Comet 1993e with Jupiter: Wu Guang-jieYunnan Observatory, Kunming 650011

Comet shoemaker-Levy 9 collided with Jupiter in July 1994. A lot of reports and news about the impact times, combined with predictions, have been systematically analysed in this paper.

Collision of a comet with Jupiter: Determination of fragment penetration depths from the molecular spectra

After the fragments of comet Shoemaker-Levy 9 fell onto Jupiter, some molecules which were formed as a result of chemical reactions in the shock-compressed gas during the explosion of the comet fragments and ejected into the upper atmosphere by the shock wave were observed in the upper atmosphere. As the initially hot gas expanded, these molecules were quenched. Therefore they represent a kind of “memory” of the shock processes which occurred in the region of generation of the strong shock wave. The radiation from such molecules was registered by both ground-based observers and the Hubble space telescope. In the present paper we estimate the penetration depth and energy of the largest comet fragments on the basis of data on the content of shock-synthesized molecules in Jupiter’s upper atmosphere.