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Orientation and Oriented Meteorites

Modified from Image Source: Rasslava / iStock / Getty Images Plus

A meteorite develops orientation when it experiences a period of stable flight through Earth’s atmosphere and is gradually further ablated into an increasingly more stable and aerodynamic shape (cone or shield). The resulting meteorite can show evidence of flow lines, roll-over lipping and regmaglypts. Though features of orientation can be seen on any meteorite type, iron meteorites (e.g Sikhote Alin) best preserve the faintest and most distinct features that the meteor experienced as it smashed through our atmosphere.

 

 

 

 

Study of fluid dynamics helps researchers uncover the mystery of oriented meteorites using aluminum cones falling in water. Image Modified from the Source: shanghai.nyu.edu

As meteors rarely develop into perfect stable cones and can still experience significant ablation during inversion, tumbling, and gliding trajectories, the characteristics associated with meteorite orientation can appear somewhat subjective for the uninitiated. However, there are clear objective identifying characteristics associated with oriented meteorites that make them prized by collectors. One or more of the features listed below can be found on all oriented meteorites:

  1. Conical or shield shape where the leading surface or apex experiences the highest temperature and pressure and is ablated into a rounded shape, and the trailing surface is more or less flat (can be slightly concave, convex or a combination of both).
  2. Flow lines indicating a consistent direction of travel and, emanating from the apex of meteorite and leading back towards trailing surface, or flowlines can also occur in certain areas below the leading surface. The most dramatic flowlines occur in a radial pattern as seen below on the Lafayette meteorite.
  3. Roll-over rim/lip develops when the ablating/melting material begins to flow over onto the back side of a meteor or from a protuberance on the leading surface. The material that has lipped over can also have flow lines and even appear frothy with tiny bubbles in the lipped region. Some meteorites can even experience double lipping indicating that they experienced stable flight in two directions.
  4. Regmaglypts or fluting/feathering (long regmaglypts) indicating large scale ablation in a consistent direction radiating from the apex. In rare cases, the regmaglypts can occur in a radial pattern as well.
  5. Spattering of small molten droplets (called travelers by some collectors) that adhered on the meteorite’s trailing surface or behind a feature like a raised surface or protuberance. This feature is typically seen on iron meteorites though large droplets have been seen on the backside of oriented stony meteorites. Evidence of this feature does not by itself mean the meteorite is oriented, though it can be associated with it.

A word of caution: Meteorites that don’t display flowlines (2), lipping (3), or regmaglypts (4) can be mislabeled as oriented. Just because a meteorite is shaped like a pyramid or sits on a flat edge does not mean it is oriented. Some meteorites are clearly oriented, but if there are doubts, ask an expert.

Beautiful radial flowlines displayed in the Lafayette meteorite (Nakhlite). Image Source & Credit: AMNH & Chip Clark
Stunning fluted regmaglypts in the Chrysanthemum meteorite. Photo Credit: Emaan Baqai/UCLA Daily Bruin senior staff
Sikhote Alin with complete rollover lip/rim. Note lip that extends far into back surface on left and sputtering of material top right. SkyFall Collection.
Karakol fell May 9, 1840 in Kazakhstan and is an excellent example of a cone shaped oriented meteorite. Measured q = 50o. Image Source: www.meteorites.ru

 

 

 

 

 

 

 

 

Note that rotational spin plays an important role in the shaping and stability of a meteor but is not discussed in detail here. Also, the plasticity of tektites during flight leads to amazing orientation effects as seen in Australite flanged buttons.