METEORITE OR METEORWRONG?
vesicles in meteorites - vesicular fusion crusts
Most stony meteorites have vesicular fusion
crusts (for example, see Cynthiana). As the surface
melts when the meteorite passes through the atmosphere, gases in the meteorite is
are released. Some of that gas becomes trapped in the glassy melt when the melt
Many lunar meteorites are regolith breccias, and many of these have
fusion crusts that are thick and highly vesicular. The best examples are QUE 03069 and PCA 02007, although the effect can be also
seen in ALHA 81005 and Calcalong Creek. At one time, all of the
material of a regolith breccia was fine grained "soil" on the surface of the
Moon. Soil grains exposed at the very surface of the Moon absorbed ions emitted
by the sun as solar wind. Most of the ions were of gaseous elements like
hydrogen, helium, and nitrogen. Impacts of small meteoroids on the Moon mixed and
stirred the upper part of the regolith. In a location where there has not been a
recent large impact, nearly all the grains in the upper few meters of the
regolith will contain solar-wind implanted gases because over millions of years
all grains spend some time at the surface. On Earth, the solar wind is absorbed
by the atmosphere, so there are no Earth rocks with solar-wind implanted ions.
Some meteorites from the asteroid belt have solar-wind gases, but none have the
high levels found in lunar meteorites because the Moon is closer to the sun. When
the exterior of the meteoroid is heated by the friction of the atmosphere, it
melts and the gases are released, forming gas bubbles that get trapped in the
glass when the glass cools.
Backscattered-electron image of lunar
meteorite PCA 02007, a regolith breccia. At the top and right is the glassy,
vesicular fusion crust that occurs on the outside of the meteorite in
the vesicles are black in the image. At the bottom is the brecciated
interior of the meteorite. It was never molten, so there are no vesicles.
Longest dimension: 1.3 cm. The vesicles are all smaller than 1 mm in
diameter. Image courtesy of Ryan Zeigler.
This is a photo of tiny
lunar meteorite QUE 94281.
It also has a highly vesicular fusion crust because it is a regolith
Many people have contacted me
saying, "My rock looks just like QUE 94281." Above is a photo that
one such person sent me. Superficially, the rocks do resemble QUE
94281, but not in detail. QUE 94281 is a fragment broken from a
larger stone, so it has some rough edges, like the rocks above.
The fusion crust on QUE 94281 coats only part of the stone.
As in PCA 02007 above,
the interior of QUE94281 does not contain vesicles (although
that's hard to see in this photo) because it was never molten. The
rocks above are not regolith breccias, but terrestrial basalts.
Notice in the upper right stone that there are fewer vesicles
in the chilled top than in the interior - the opposite of what is
seen in meteorites.
A piece of vesicular basalt from New Mexico showing the smooth top
that was exposed to the atmosphere as the molten lava cooled.
If your rock has a vesicles on the inside, it's probably not a meteorite.
Such rocks are very common on Earth but are exceedingly rare among
vesicles and amygdules
vesicles in meteorites