Engineering topics

MacLow, M.-M.

Publications and source records attributed to MacLow, M.-M..

4 records

NASA NTRS1995

Nonlinear Propagating Features in the Stratosphere of Jupiter Generated by the Impact of SL-9

Among the more intriguing features observed by HST after the impacts of the fragments of Shoemaker-Levy 9 were rings, encircling several of the impact sites, that expanded radially at constant velocities of order 450-500 m/s. One credible suggestion has been that the rings are stratospheric manifestations of linear gravity waves seated in the troposphere (Ingersoll and Kanamori, 1995). In this scenario the waves are made visible by a condensible material of unknown composition. Based on simulations of the comet impact by Zahnle and Mac Low (1995), we suggest an alternative hypothesis, namely that the rings are nonlinear stratospheric gravity waves which break and entrain impact debris material in the breaking wave front. The computations produce just such a feature, with the nonlinear breaking wave front in the stratosphere propagating radially away from the impact site with the correct speed. The computed ring of material is located in an altitude region where the static stability profile has a local maximum and therefore forms a waveguide. Because entrained impact debris forms the ring feature in the model, a condensate of some unknown composition is not required in order to view the wave front. However, there is one severe problem with the computed nonlinear wave: in contrast to a linear wave, it slows down. The computed feature stops within 1000 seconds, while the observed feature lasted at least 10,000 seconds. Since the nonlinear breaking wave front is a natural product of our impact simulations, we are now investigating the conditions that will enable the breaking wave front to propagate further, in accord with the observations. The results of these computations will be presented.

Young, R. E.↗

NASA NTRS1995

Why the SL9 Plumes Were All About the Same Height

Several of the SL9 ejecta plumes were observed by the HST to reach approximately the same height, about 3000 km above the jovian cloud tops. The duration of the infrared events, which were produced by the plume falling back on the atmosphere, measures time aloft and hence provides a second, more sensitive measure of plume height; the light curves indicate that the largest impacts produced modestly higher plumes. Evidently these plumes were launched with about the same vertical velocity, roughly 10-13 kilometers per second. As the impactors themselves were not all the same, nor the impacts equally luminous, nor the plumes equally opaque, the similar plume heights has been seen as a puzzle needing explanation. A second, closely related matter that needs to addressed quantitatively is the popular contention that a big plume requires a big impact. This view is misleading at best, yet plume heights can be used to constrain impact parameters. Dimensional analysis indicates that plume height goes as z alpha v (sup 2) (sub ej) alpha E/pH (sup 2), where v (sub ej) is the ejection velocity, E the explosion energy, and p and H the ambient pressure and scale height at termination. Using a semi-analytic model for the deceleration, disintegration, and destruction of intruding bodies by an ever-vigilant atmosphere, we find that the ratio E/pH(sup 2) is roughly constant for fragments with diameters of order 100 m to 1000 m. Constancy of v(sub ej) is in part due to the greater role of radiative ablation on the flight of smaller objects. We conclude that similar plume heights is a direct consequence of smaller impactors exploding at higher altitudes, in such a way that the different explosions were geometrically similar.

Zahnle, K.↗

NASA NTRS1994

More Consequences of the Collision of a Comet and Jupiter

The impending collision of P/Shoemaker-Levy 9 with Jupiter has excited a wide spectrum of large amplitude predictions. Although the prediction waves themselves have been easily detected propagating through diverse media here on Earth, their future manifestation at Jupiter may prove mostly undetectable. In this talk we may or may not attempt to explain what may or may not have been observed. We will doubtless emphasize our most successful predictions, if any.

Zahnle, Kevin↗

NASA NTRS1994

The End of Comet Shoemaker-Levy 9

The pieces of the former Comet Shoemaker-Levy 9 struck Jupiter during the week of July 16-22 1994. We divide the general impact phenomena into four phases: (1) Atmospheric entry should have been accompanied by an entry flash, probably only detectable by the Galileo spacecraft. (2) Aerodynamic forces cause the impactor to explode. We show that the apparent altitude of the explosions is consistent with the analytical model proposed by Zahnle and Chyba et al, with numerical models calculated by Mac Low and Zahnle and Yabe et al., and with dynamically-based estimates of fragment masses Suggested by Scotti and Melosh and Asphaug and Benz. (3) The explosion produces a fireball that propagates most quickly up the wake, as predicted by Takata et al. and Crawford et al. The fireball's luminosity fades over a timescale of 30-60 seconds. (4) Each explosion produced a plume that reached thousands of kilometers above the cloud-tops. The plumes appear to have been dusty and SO illuminated when they reached sunlight. The plumes rose and fell over a period of ten or twenty minutes, governed by the simple laws of ballistics. When plume material (dust) fell back on the atmosphere it was reheated and glowed brightly in the thermal infrared for a period of order ten to twenty minutes; a large fraction of the impact energy was spent thus. Geometry introduces additional factors that complicate interpretation of the observed light curves.

Zahnle, K. J.↗