Meteoritics & Planetary Science 40, Nr 8, 1241–1256 (2005) Abstract available online at http://meteoritics.org Decoherence time scales for “meteoroid streams” Adam PAULS and Brett GLADMAN* University of British Columbia, Department of Physics and Astronomy, 6244 Agricultural Road, Vancouver, British Columbia V6T 1Z1, Canada *Corresponding author. E-mail: [email protected] (Received 15 May 2004; revision accepted 19 August 2005) Abstract–We explore the orbital dynamics of Earth-crossing objects with the intent to understand the time scales under which an “orbital stream” of material could produce time-correlated meteorite falls. These “meteoroid streams” have been suggested to be associated with three well-known meteorite- dropping fireballs (Innisfree, Peekskill, and P¯Ìbram). We have performed two different analyses of the statistical significance of the “orbital similarity,” in particular calculating how often orbits of the same level of similarity would come from a random sample. Secondly, we have performed extremely detailed numerical integrations related to these three cases, and we find that if they were streams of objects in similar orbits, then they would become “decoherent” (in the sense that the day-of-fall of meteorites of these streams become almost random) on time scales of 104–105 yr. Thus, an extremely recent breakup would be required, much more recent that the cosmic ray exposure ages of the recovered falls in each case. We conclude that orbital destruction is too efficient to allow the existence of long-lived meteoroid streams and that the statistical evidence for such streams is insufficient; random fall patterns show comparable levels of clustering. INTRODUCTION from the Prairie and the MORP networks to filter out fireballs whose pre-atmospheric objects were likely cometary. Meteor streams are well-accepted and well-understood Morbidelli and Gladman (1998) showed that the orbital phenomena, in which an object of cometary composition distribution of the non-cometary fireballs (presumably mostly loses material during each orbit and populates a region of chondritic) was perfectly consistent with sources in the main orbital space near that of the parent comet with small particles asteroid belt, as is commonly accepted. that have escaped from the comet at low speeds. If this orbit is The fireball orbit database has been analyzed by several currently intersecting the orbit of the Earth, we see an annual workers to search for the possibility of “meteoroid streams,” meteor shower near the day of the nodal intersection. Due to that is, meteorite-dropping fireballs whose pre-atmospheric the recent and continual resupply caused by the repeated orbits would indicate that there is a stream of meteoroids in expulsion of dust at each perihelion passage, many very similar Earth-crossing orbits. We summarize below the meteoroids are placed on very similar orbits. Because of their three main cases, which we will later use as case studies for recent escape, the tiny meteoroids generate a meteor shower our detailed numerical integrations. The orbits are listed in with pre-atmospheric orbits clustered near that of the parent Table 1. comet (e.g., Messenger 2002). The situation is much more unclear regarding meteorite- P¯Ìbram dropping fireballs, that is fireballs for which the pre- atmospheric object is sufficiently large and strong to survive One of the largest fireballs with a well-determined orbit atmospheric entry and deliver intact fragments to the ground. is that of P¯Ìbram, an H5 ordinary chondrite which was Some fireballs have been observed by photographic camera observed by the European Camera Network on April 7, 1959 networks (Halliday et al. 1978; McCrosky et al. 1979; (Ceplecha 1977). On April 6, 2002, the Neuschwanstein Ceplecha 1977) or by ground-based observers (e.g., Brown chondrite (Spurn˝ et al. 2003) was also observed by the same et al. 1994) with sufficient coverage to compute a pre- camera network and calculated to have a very similar orbit to atmospheric orbit for the incoming object, some fraction of that of P¯Ìbram. They estimate that, based on 200 “meteor which will be non-cometary. Wetherill and ReVelle (1981) candidate” orbits taken from the MORP data, the probability and Halliday et al. (1996) analyzed photometric fireball data of getting an orbital match as close as that observed was 1 in 1241 © The Meteoritical Society, 2005. Printed in USA. 1242 A. Pauls and B. Gladman Table 1. Pre-atmospheric orbits of three fireballs of interest along with companion objects. Data are taken from Spurn˝ et al. (2003), Halliday (1987) and Brown et al. (1994). Here a is the semi-major axis; e is the eccentricity; i is the inclination relative to the ecliptic; Ω is the longitude of ascending node; ω is the argument of perihelion; and q is the perihelion distance. Companion objects (with italicized names) have very similar orbits to the corresponding fall in each case, as measured by the D' metric of the companion orbit relative to the original. The last digit in each measurement is uncertain. Object a(AU) eiΩωq(AU) D' P¯Ìbram 2.401 0.6711 10.482° 17.79147° 241.750° 0.7895 − Neuschwanstein 2.40 0.67 11.41° 16.82664° 241.20° − 0.009 Innisfree 1.872 0.473 12.27° 316.80° 177.97° 0.986 − Ridgedale 1.873 0.475 12.33° 316.01° 186.66° − 0.034 Peekskill 1.49 0.41 4.9° 17.030° 308° 0.88 − 100,000 and therefore the P¯Ìbram/Neuschwanstein similarity which had similar day-of-fall parameters and sets of labile was not chance, although reconciling the different trace elements, as determined by a multivariate statistical petrographic types (H5/EL6, respectively) and very different analysis, and proposed meteoroid streams as a mechanism to cosmic-ray exposure ages (CRE) (12/48 Myr, respectively) explain the pattern. Later, these authors (Lipschutz et al. was acknowledged to be problematic. 1997) noted that the well-observed Peekskill fall (Brown et al. 1994) was part of one of their identified streams. Thus, Innisfree although close orbital similarity with another fireball does not exist here, we will also study the orbital evolution of a The Canadian Meteorite Observation and Recovery “Peekskill” stream. Project (MORP) observed a fireball on February 5, 1977 Our approach to the problem will be to first attempt to (Halliday et al. 1978), and subsequently recovered the calculate the statistical significance of the level of Innisfree LL chondrite fragment. Three years later, the same “similarity” that has been observed in the growing worldwide network observed a second fireball (Halliday 1987) which database of fireball orbits. We will do this by examining the should have resulted in a fall near Ridgedale, Saskatchewan number of cases of similarity that one would find from a (and is thus known as Ridgedale even though no material was totally random distribution of Earth-impacting objects. We recovered). The calculated pre-atmospheric orbits of these will then proceed to perform numerical integrations of the two objects were quite similar, leading Halliday (1987) to three stream candidates discussed above to measure the time postulate the existence of a meteoroid stream. They noted that scale over which such material could retain a “stream-like” the 27 Myr CRE age was problematic in terms of the size characteristic. expected for the parent object. That is, a parent object larger than 140 m in diameter would have to be fragmented in order STATISTICAL SIGNIFICANCE to get two fireballs, and yet the CRE data shows that Innisfree OF ORBITAL SIMILARITY itself was either in a <4 m body when it hit the atmosphere or spent most of its CRE exposure on the surface of a 140-m The previous section listed three main arguments for the object (a problem we shall address in our conclusions). Those existence of streams of meteoroids: i) the existence of pairs authors did note that the light production and fragmentation fireballs with very similar pre-atmospheric orbits; ii) the pattern of Ridgedale was quite different from that of existence of “clusters” of fireballs with mutually similar Innisfree; they ascribed this to Innisfree having more orbits; and iii) the existence of meteorite falls with a correlated fractures before entering the Earth’s atmosphere. In a final day-of-fall and chemical trace elements. In this section, we paper, Halliday et al. (1990) examined the MORP and Prairie wish to examine the statistical strength of these claims. network (McCrosky et al. 1979) databases and selected The first step in this task is to calculate the frequency of several fireball groups as potentially corresponding to streams occurrence of “coincidences” in a random distribution of of meteoroids in Earth-crossing orbits; Innisfree/Ridgedale fireballs. This will serve as a calibration to determine are part of the most significant grouping that they found, and confidence levels in a real sample and thus judge the statistical that grouping was also proposed to be linked to a stream of significance of such coincidences. In analogy with familiar asteroids by Drummond (1991). Gaussian statistics, in this paper, we will use confidence levels of 17%, 2.5%, and 0.15% as “low,” “moderate,” and “high” Peekskill levels of significance. These percentages are half of the usual 34% and 0.3% values for the 1σ, 2σ, and 3σ tails of a Gaussian The last meteoroid stream candidate we will explore is distribution because the case in which fewer coincidences one lacking direct observation of two similar pre-atmospheric occur in a real sample than in a random distribution is of no orbits. Rather, Dodd et al. (1993) identified a set of meteorites interest. One can judge the claims for orbital streams Decoherence time scales for “meteoroid streams” 1243 moderately significant if the apparently non-random behavior 1.