Are Hermes and 1988 XB Parent Bodies of HED Meteorites?

Comparisons of Dates of Meteorite Falls and NEA Orbit Close Approaches

Charles A. Wood, Ron Fevig, Alexander Willman, Space Studies, University of North Dakota, Grand Forks, ND 58202; Walter Worman, Physics, Moorhead State University, Moorhead, MN 56560

Presented at the XXVIth Annual Lunar and Planetary Science Conference, March 1995

Every meteorite is essentially geologic float. Meteorites are not collected in situ, but come to Earth over complex, million year paths that make identification of their individual parent bodies difficult. Believing that unnoticed information lurks in disparate data sets, we are comparing the dates of fall of meteorites and the dates of closest approaches of near Earth asteroids. Many coincidental matches must occur, but two preliminary findings are promising. The orbits of both 1988 XB and Hermes approach Earth at about twice the Earth-Moon distance, and when they do, howardite, eucrite and diogentie meteorites fall at ~3 times their average annual rates.

A major goal in planetary science is the linking of individual meteorites with their specific parent bodies. In general, this has not been successful. Telescopic spectral studies have demonstrated that there are hundreds to thousands of asteroids with reflectivities similar to various classes of meteorites, each of which has tens to hundreds of members. The most celebrated linkage of asteroid to meteorites was the discovery of McCord et al [1] that the unique spectrum of Vesta is very similar to that of eucrite meteorites. More recently, Binzel and Xu [2] discovered a series of 5-10 km size asteroids with the same spectral features as Vesta, and which, from their orbital elements, are likely to be ejecta from Vesta. Some of these fragments are near the asteroid belt 3:1 resonance, where they may be perturbed into orbits that ultimately cross that of the Earth. There is thus a series of mechanisms which transport collisional fragments from the original eucrite parent body, Vesta, into near-Earth asteroids (NEAs), which may somehow spawn chips, that can land on Earth as meteorites. As recounted above, McCord and Binzel and Xu have discovered parts of this puzzle, and Cruikshank et al [3], apparently found the eucrite-like NEAs that would be the immediate parents of eucrite and associated howardite and diogenite (HED) meteorites. The difficulty with this pleasing scenario is that orbits of these particular NEAs (3551, 3908 and 4055) do not pass close to Earth when HEDs fall.

We have taken a different approach to searching for possible linkages between asteroids and meteorites. We are undertaking a statistical comparison of the times of fall of meteorites and the times of closest approach of NEA orbits with Earth. The Catalogue of Meteorites [4] and updates provide meteorite fall times, and we have calculated the date of closest approach to Earth, not of the NEA itself, but of its orbit. We are explicitly assuming that NEAs are accompanied in their orbits by a swarm of their own fragments. In our view, an NEA is simply the most visible member of a stream of particles, one of which is a few kilometers in size, with the rest being only centimeters to meters. Furthermore, we assume, like meteor streams in some comet orbits, that the NEA fragments are dispersed around the entire NEA orbit.

There is a growing body of evidence to justify this model. In 1982, Wood [5] discovered that about half of the H chondrites travel in meteorite streams, and Olsson-Steel [6] concluded that all NEAs that pass nearer than 0.1 AU have associated meteors. Additionally, the NEA 3200 Phaethon is apparently the source for the Geminid meteor shower [7]. Thus, the general trend is that meteorites, meteors and NEAs may be related, but which meteorite comes from which NEA?

Considering that there are more than 900 meteorite falls, and that each of nearly 300 NEAs may make two close orbital approaches to Earth, there will be many apparent linkages that are coincidental. All meteorite falls are actual events, but we can not be sure that any NEA debris hits Earth as meteorites. To improve the chances of this occurring we consider only those close approaches where the NEA orbits pass <0.02 AU from the Earth, about eight times the Earth-Moon distance. In order to detect possible linkages we examined meteorite falls near times of close approach for NEAs whose orbits come close to Earth twice, both pre- and post-perihelion. Even over short periods of 3-4 weeks, most meteorites fall in proportion to their annual rates, but not all. The most intriguing results thus far are for NEAs 1988 XB and 1937 UB Hermes:

1988 XB

Dates of closest approaches: Jun 26 (0.020 AU) and Nov 26 (0.007 AU)

DateYearMeteoriteType
Jun 131819JonzacE
Jun 151821JuvinasE
Jun 171870IbbenburenD
Jun 171877JodzieH
Jun 231939Chervony KutE
Jun 271924BerebaE
Jun 271931TatahouineD
Jun 291843ManegaonD
Jun 301957IbitiraE
Jul 21939WashougalH
Jul 61924JohnstownD
Jul 141953MedanitosE
Jul 141845Le TeilleulH
Summer1950GarlandD
DateYearMeteoriteType
Nov 121906KirbyvilleE
Nov 241910LakangaonE
Nov 25?1933MalvernH
Nov 301850ShalkaD
Dec 51868FrankfortH
19 HEDs in 54 d = 0.35/d vs. 34 total H&Es in 365 d = 0.09/d

1937UB Hermes

Dates of closest approaches: Apr 25 (0.003 AU) and Oct 30 (0.006 AU)

DateYearMeteoriteType
Apr121913Moore CountyE
Apr171974Aioun-AtrousD
Apr191933BrientE?
Apr221942KapoetaH
Apr241875NagariaE
May31925ChavesH
DateYearMeteoriteType
Oct 241899PeramihoE
Oct 291905BholghatiH
8 HEDs in 27 d = 0.30/d vs. 41 HEDs in 365 d = 0.11/d

Of all the 300 or so NEAs, these two have the maximum numbers of HED meteorites that have fallen near the pre- and post- perihelion close approach times. As a warning, however, some (or all!) of the listed meteorites might not be associated with the asteroids.

These two NEAs may account for 66% of all HED meteorite falls. HED meteorites fall at ~ three times their annual rates when the Earth is near the orbits of these three NEAs. 1988 XB appears to be an especially prolific stone-dropper, dispersing all three types of HEDs. There is only one diogenite associated with Hermes, which may therfore be a piece of the HED parent body’s basaltic crust and regolith zone (with only a little admixture of the underlying diogenites). 1988 XB may be from deeper within the parent body. Spectral observations were not made during Hermes one period of observation in 1937; this would be an important NEA to recover and observe with modern techniques. Similarly, spectral observations appear not have been made for 1988 XB. This analysis predicts that these two objects should be V types, like Vesta. It appears that there may be more V type NEAs that the three discovered by Cruikshank et al. [3].

References:

  1. McCord, TB, JB Adams & TV Johnson (1980) Science 168, 1445.
  2. Binzel, RP & S Xu (1993) Science 260, 186.
  3. Cruikshank, DP et al (1991) Icarus 89, 1.
  4. Graham, AL et al. (1985) Catalogue of Meteorites.  Univ. Az Press, pp 460.
  5. Wood, CA (1982) Lunar Planet. Sci. Conf XIII, 873.
  6. Olsson-Steel, D (1988) Icarus 75, 64.
  7. Fox, K et al (1984) Mon. Not. R. Soc. 208, Short Communications, 11P.

Copyright © 1995, C. A. Wood, et al. All rights reserved.
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