Donald Korycansky

NUMERICAL MODELING OF THE DISRUPTION OF COMET D/1993 F2 SHOEMAKER-LEVY 9 REPRESENTING THE PROGENITOR BY A GRAVITATIONALLY BOUND ASSEMBLAGE OF RANDOMLY SHAPED POLYHEDRA

We advance the modeling of rubble-pile solid bodies by re-examining the tidal breakup of comet Shoemaker-Levy 9, an event that occurred during a encounter with Jupiter in 1992 July. Tidal disruption of the comet nucleus led to a chain of sub-nuclei ~100-1000 m diameter; these went on to collide with the planet two years later. They were intensively studied prior to and during the collisions, making SL9 the best natural benchmark for physical models of small-body disruption. For the first time in the study of this event, we use numerical codes treating rubble piles as collections of polyhedra. This introduces forces of dilatation and friction, and inelastic response. As in our previous studies we conclude that the progenitor must have been a rubble pile, and we obtain approximately the same pre-breakup diameter (~1.5 km) in our best fits to the data. We find that the inclusion of realistic fragment shapes leads to grain locking and dilatancy, so that even in the absence of friction or other dissipation we find that disruption is overall more difficult than in our spheres-based simulations. We constrain the comets bulk density at ρbulk ~ 300-400 kg m–3, half that of our spheres-based predictions and consistent with recent estimates derived from spacecraft observations.

Impact flux on Jupiter: From superbolides to large-scale collisions

Context. Regular observations of Jupiter by a large number of amateur astronomers have resulted in the serendipitous discovery of short bright flashes in its atmosphere, which have been proposed as being caused by impacts of small objects. Three flashes were detected: one on June 3, 2010, one on August 20, 2010, and one on September 10, 2012. Aims. We show that the flashes are caused by impacting objects that we characterize in terms of their size, and we study the flux of small impacts on Jupiter. Methods. We measured the light curves of these atmospheric airbursts to extract their luminous energy and computed the masses and sizes of the objects. We ran simulations of impacts and compared them with the light curves. We analyzed the statistical significance of these events in the large pool of Jupiter observations. Results. All three objects are in the 5−20 m size category depending on their density, and they released energy comparable to the recent Chelyabinsk airburst. Model simulations approximately agree with the interpretation of the limited observations. Biases in observations of Jupiter suggest a rate of 12−60 similar impacts per year and we provide software tools for amateurs to examine the faint signature of impacts in their data to increase the number of detected collisions. Conclusions. The impact rate agrees with dynamical models of comets. More massive objects (a few 100 m) should impact with Jupiter every few years leaving atmospheric dark debris features that could be detectable about once per decade.

Shoemaker-Levy 9 Impact Modeling. I. High-Resolution Three-dimensional Bolides

We have run high-resolution, three-dimensional, hydrodynamic simulations of the impact of comet ShoemakerLevy 9into the atmosphereof Jupiter.Wefindthat the energydeposition profile islargely similar to theprevious twodimensional calculations of Mac Low & Zahnle, although perhaps somewhat broader in the range of height over which the energy is deposited. As with similar calculations for impacts into the Venusian atmosphere, there is considerablesensitivityintheresultstosmallchangesintheinitialconditions,indicatingdynamicalchaos.Wecalculated the median depth of energy deposition (the height z at which 50% of the bolide’s energy has been released) per run. The mean value among runs is � 70 km below the 1 bar level, for a 1 km diameter impactor of porous ice of density � ¼ 0: 6gc m � 3 . The standard deviation among these runs is 14 km. We find little evidence of a trend in these results with the resolution of the calculations (up to 57 cells across the impactor radius, or 8.8 m resolution), suggesting that resolutions as low as 16 grid cells across the radius of the bolide may yield good results for this particular quantity. Visualization of the bolide breakup shows that the ice impactors were shredded and/or compressed in a complicated manner but evidently did not fragment into separate, coherent masses, unlike calculations for basalt impactors. The processes that destroy the impactor take place at significantly shallower levels in the atmosphere (�� 40 km for a 1 km diameterbolide), but the shreddedremains haveenoughinertia to carry themdownanother scale heightor more before they lose their kinetic energy. Comparison of basalt impactor models shows that energy deposition curves for these objects have much less sensitivity to initial conditions than do ice impactors, which may reflect differences in the equation of state forthedifferentkinds of objects, or a scale-dependentbreakup phenomenology, with the preferred scaledependingonimpactordensity.Modelsofimpactorscoveringa � 600-foldrangeofmass(m)showthatlarger impactors descend slightly deeper than expected from scaling the intercepted atmospheric column mass by the impactor mass. Instead, the intercepted column mass scales as m 1.2 . Subject headingg comets: individual (Shoemaker-Levy 9) — hydrodynamics

PLUME DEVELOPMENT OF THE SHOEMAKER-LEVY 9 COMET IMPACT

We have studied the plume formation after a Jovian comet impact using the ZEUS-MP 2 hydrodynamics code. The three-dimensional models followed objects with 500, 750, and 1000 m diameters. Our simulations show the development of a fast, upward-moving component of the plume in the wake of the impacting comet that pinches off from the bulk of the cometary material ~50 km below the 1 bar pressure level, ~100 km above the depth of the greatest mass and energy deposition. The fast-moving component contains about twice the mass of the initial comet, but consists almost entirely (>99.9%) of Jovian atmosphere rather than cometary material. The ejecta rise mainly along the impact trajectory, but an additional vertical velocity component due to buoyancy establishes itself within seconds of impact, leading to an asymmetry in the ejecta with respect to the entry trajectory. The mass of the upward-moving component follows a velocity distribution M(>v) approximately proportional to v ?1.4 (v ?1.6 for the 750 m and 500 m cases) in the velocity range 0.1 km s?1 < v < 10 km s?1.

Disruption and reaccretion of midsized moons during an outer solar system Late Heavy Bombardment

We investigate the problem of satellite survival during a hypothetical late heavy bombardment in the outer solar system, as predicted by the Nice Model (Tsiganis, Gomes, Morbidelli, & Levison 2005, Nature 435). Using a Monte-Carlo approach we calculate, for satellites of Jupiter, Saturn, and Uranus, the probability of experiencing a catastrophic collision during the LHB. We find that Mimas, Enceladus, Tethys, and Miranda experience at least one catastrophic impact in every simulation. Because re-accretion is expected to be rapid, these bodies will have emerged as scrambled mixtures of rock and ice. Tidal heating may have subsequently modified the latter three, but in the nominal LHB model Mimas should be a largely undifferentiated, homo geneous body. A differentiated Mimas would imply either that this body formed late, or that the Nice model requires significant modification.

Exploring Ocean Waves from Asteroid Impacts

An asteroid the size of the Roman Coliseum crashes into the ocean somewhere on Earth, producing a transient cavity miles across. The spectacular collapse launches tsunamis in all directions, casting the asteroids impact energy far and wide. Everyone living on these shores may be in for a bad day. Based partly on the assumption that relatively minor asteroid impacts spawn dangerous tsunamis, the National Research Council has recommended that NASA and the National Science Foundation design and build a Large Synoptic Survey Telescope (LSST) to image the entire sky every 7 days and detect thousands of asteroids to the 24th magnitude, especially those down to 300-m diameter on near-Earth orbits. The LSST will have many astronomical uses apart from asteroid hunting; nevertheless, before

Validation of numerical codes for impact and explosion cratering: Impacts on strengthless and metal targets

100 million to

Volatile retention from cometary impacts on the Moon

200 million gets spent, one must examine the justifications. Are sub-kilometer asteroids really a significant threat? If not, then existing survey telescopes are adequate to catalog −98% of the 1100±100 near-Earth asteroids larger than 1 km diameter in the next few decades. Of these, half have been detected so far, and most of the rest have orbits that are harder to discover from Earth. Perhaps the LSST money should instead be dedicated to a space-borne observatory for bagging “holdouts” larger than a kilometer, those capable of causing global catastrophe.