FROTHING AND VESICATION
Meteoroids with stable flights develop a high vacuum at their trailing side. The vacuum favors intensive degassing of volatile elements which, in turn, produces vesicles and sometimes larger bubbles or splash craters in the outer fusion crust. The longer the meteoroid maintains a stable flight, the more intense the degassing process and the resulting frothing formation.
Though most common on the trailing edge of meteorites with stable flight attitudes, the forming of bubbles and larger vesicles is not limited to those. The violent release, for example, of S from troilite, of H2O from serpentinite (in carbonaceous chondrites) or the release of CO2 during the combustion of graphite can lead to bubble formation on areas where one would expect it less; e.g., on the breast side or flanks of a specimen.
While continuing its fall toward the lower regions of our atmosphere, the meteoroid encounters low temperatures, often well below freezing point. This leads to a further contraction of the just-cooled fusion crust. Frost forms on the meteoroid; under special conditions it may even develop a layer of ice. The thermal stresses induced by low temperetaures often lead to tiny weblike cracks in the fusion rind. These delicate structures resemble the crazing on fired ware and are called contraction cracks (see last image on this page).
The color of fusion crust on stone meteorites is generally black. However, the quality ranges from a glassy jet black on basaltic achondrites (with vesicles as on crusts of lunar mare basalts) to a dull greyish black with shades of rich or lighter brown on ordinary chondrites.
Frothy, transparent, olive-green crusts, as on anorthositic lunar highland meteorites, or bright whitish and ash-colored fusion crusts as, for example, on aubrites, are rare exceptions. The thin fusion crusts on irons show a blueish black often changing in tempered colors. In general, the color of the fusion rind is mainly a function of the iron content, the mineralogic composition of the meteorite and the range of melting temperatures and subsequent viscosity of the melt on the surface of the meteorite (Schneider, et al. 2000).
Often, freshly-fallen chondrites display rather black fusion crust on one side and a more brown color of the crust on the opposite side. This is not a weathering effect. The color differences are due to variations in the magnetite contents in the crusts of opposite sides. These, in turn, are controlled by the supply of atmospheric oxygen and the temperature of the oxidation process. A high vacuum on the trailing side, for example, reduces or blocks the supply of atmospheric oxygen to the reaction.