Surface Characteristics of Iron Meteorites
Even though iron meteorites are more obvious to the untrained eye, they are actually much rarer than stones. Statistically, seven times more stone meteorites are seen to fall to Earth than irons. The following surface characteristics make irons much easier to spot in the field. A number of these features are also evident in stone and stony-iron meteorites, though they tend to be more pronounced in irons.
Thumbprints, known scientifically as regmaglypts, are small, rounded indentations found on the surface of many irons. Caused by melting during the meteorite's flight to Earth, they are not present in terrestrial rocks, though similar features can sometimes be seen on common stones in the desert, and are the result of wind-blown sand gently eroding exposed surfaces.
As a meteorite burns on its descent through the atmosphere, some of its molten surface forms droplets that move in tiny, delicate rivulets called flow lines. These fine patterns may be as thin as the strands of a spiderweb and might only be clearly visible through a loupe or magnifying glass.
Meteorites with a high degree of orientation also often acquire rollover lips when a small amount of molten material literally rolls over onto the meteorite's trailing edge. Rollover lips manifest themselves as delicate rounded rims on the reverse faces of oriented meteorites. If you've ever watched molten candle wax flow over the side of a saucer, you'll get the idea.
Fusion crust — a burned, dark, paper-thin rind on the outside of meteorites — is fairly common on stones, but rarely seen on irons. Only a freshly fallen iron will exhibit fusion crust, as the layer will decay quickly in Earth's moist, oxygen-rich atmosphere. The few iron meteorites that have been recovered shortly after a witnessed fall exhibit an attractive bluish-black gunmetal color — a testament to the extremely high temperatures they endured during their brief, fiery flights.
Occasionally a natural hole will form in an iron meteorite, when a portion of its surface melts completely through during flight. Perhaps 1 in 1,000 meteorites will exhibit a natural hole, and they are very highly prized by collectors for their aesthetic beauty.
The features described here are, for the most part, unique to meteorites, and just part of what makes these visitors from outer space such exciting and unusual collectibles. Keep your eyes open! Thousands of meteorites land on Earth every year, and there are still many out there waiting to be found.
For more information, photos, expedition reports, and a comprehensive guide to meteorite identification, please visit our Aerolite Meteorites website.