Dynamical Lifetimes and Final Fates of Small Bodies: Orbit Integrations vs Öpik Calculations
ABSTRACT The dynamical lifetimes of small bodies against ejection from the Solar System or collision with the Sun or a planet are often estimated by Monte Carlo codes based on the equations of Öpik and using a method implemented by Arnold. Such algorithms assume that orbital changes are dominated by close encounters, and that successive encounters are uncorrelated. We have compared the results of an Öpik code (H. J. Melosh and W. B. Tonks, Mete-oritics28, 398 (1993)) and a fast integrator (H. F. Levison and M. J. Duncan, Icarus108, 18 (1994)) to investigate the regimes of validity of the Öpik–Arnold approach. We investigate the transfer of ecliptic comets from Neptune-crossing orbits to observable Jupiter-family comets, the dynamics of Halley-type comets, and the transport of meteorites among the terrestrial planets. In all cases, the Öpik code overestimates the median lifetime of the small bodies, although both codes show a rapid initial loss of objects followed by a slow decay. For martian impact ejecta, some of which find their way to Earth as the SNC meteorites, the Öpik code substantially overestimates lifetimes because of its neglect of secular resonances, which rapidly pump eccentricities (B. J. Gladman et al., Science 271, 1387 (1996)).
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ABSTRACT: The Russian Phobos-Grunt spacecraft originally planned to return a 200-gram sample of surface material from Phobos to Earth. Although it was anticipated that this material would mainly be from the body of Phobos, there is a possibili-ty that the sample may also contain material ejected from the surface of Mars by large impacts. An analysis of this possibility is performed using the best current knowledge of the different aspects of impact cratering on the surface of Mars and of the production of high-speed ejecta that might reach Phobos or Deimos.22nd AAS/AIAA Space Flight Mechanics Meeting, Charleston, South Carolina; 02/2012
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ABSTRACT: Abstract— We have examined the fate of impact ejecta liberated from the surface of Mercury due to impacts by comets or asteroids, in order to study 1) meteorite transfer to Earth, and 2) reaccumulation of an expelled mantle in giant-impact scenarios seeking to explain Mercury's large core. In the context of meteorite transfer during the last 30 Myr, we note that Mercury's impact ejecta leave the planet's surface much faster (on average) than other planets in the solar system because it is the only planet where impact speeds routinely range from 5 to 20 times the planet's escape speed; this causes impact ejecta to leave its surface moving many times faster than needed to escape its gravitational pull. Thus, a large fraction of Mercurian ejecta may reach heliocentric orbit with speeds sufficiently high for Earth-crossing orbits to exist immediately after impact, resulting in larger fractions of the ejecta reaching Earth as meteorites. We calculate the delivery rate to Earth on a time scale of 30 Myr (typical of stony meteorites from the asteroid belt) and show that several percent of the high-speed ejecta reach Earth (a factor of 2–3 less than typical launches from Mars); this is one to two orders of magnitude more efficient than previous estimates. Similar quantities of material reach Venus.These calculations also yield measurements of the re-accretion time scale of material ejected from Mercury in a putative giant impact (assuming gravity is dominant). For Mercurian ejecta escaping the gravitational reach of the planet with excess speeds equal to Mercury's escape speed, about one third of ejecta reaccretes in as little as 2 Myr. Thus collisional stripping of a silicate proto-Mercurian mantle can only work effectively if the liberated mantle material remains in small enough particles that radiation forces can drag them into the Sun on time scale of a few million years, or Mercury would simply re-accrete the material.Meteoritics & Planetary Science. 01/2009; 44(2):285 - 291.
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ABSTRACT: Abstract— The recent discovery of the importance of Sun-grazing phenomena dramatically changed our understanding of the dynamics of objects emerging from the asteroid belt via resonant phenomena. The typical lifetimes of such objects are now expected to be <10 Ma, thus demanding a reassessment of our general picture of the meteorite delivery process. By analysing direct numerical integrations of ∼2000 test particles beginning in the v6, 3:1, and 5:2 resonances in the main belt, we have reexamined the orbital and temporal distribution of meteoroids that journey to Earth. Comparing the results with fireball data, we find that the orbital distribution of Earth-impacting chondrites is consistent with a steady-state injection of meteoroids into the 3:1 and v6, resonances. Because this is the most complete and unbiased data set concerning Earth-impacting meteoroids, the agreement leads us to believe that our model is accurate. The simulations predict a P.M. fall ratio for chondrites ∼14% lower than the observed value of ∼68%, which argues for a moderate bias being present in this statistic. Most interestingly, the typical meteorite transfer times predicted by our models are several factors lower than the typical chondrite exposure ages, which implies that these meteorites acquired most of their exposure in the main belt before entering the resonances. We discuss some processes that would allow such preexposure. The case of achondrites and iron meteorites is also briefly discussed.Meteoritics & Planetary Science. 02/2010; 33(5):999 - 1016.