Bilanga, stone, achondrite.
Fusion-crusted, oriented end piece.
Achondrite meteorites are very similar in
appearance to terrestrial igneous rocks. As
such they are very difficult to find unless the fall has been witnessed. There
are over a dozen different sub-classifications of achondrites, but many of them
have only one or two specimens associated with them. Only the more numerous
types have been included in this discussion.
achondrites are igneous in nature they are believed to have formed on
differentiated bodies in the solar system. Differentiated bodies are large
enough to have been completely molten at one time allowing heavier elements to
sink towards the center of the layered mass. This results in a body, like the
Earth, where there are chemically distinct core, mantle and crust areas. This
complete melting also removes all evidence of chondrules, hence achondrite, meaning
Johnstown, Colorado, USA,
HED group of achondrites comprises the related types
Howardites, Eucrites and Diogenites. Very careful analysis of the spectra of
these meteorites and comparison of them with the spectra of asteroids using the
Hubble Space Telescope provides compelling evidence that this group of
meteorites comes from asteroid 4 Vesta.
volcanic pyroxene and feldspar and correspond well spectrally to the
lava flows on the crust of 4 Vesta. Diogenites are
plutonic pyroxene in nature and match areas
of 4 Vesta that appear to be large craters exposing the interior mantle of the
asteroid. Howardites are a breccia of
Eucrites and Diogenites implying that fragments of these materials have been
fused together by subsequent impacts.
access area of the Hubble Space Telescope web site contains low resolution
images of the
surface of asteroid 4 Vesta including a false color map showing
areas that correspond to Eucrite and Diogenite material.
The SNC group of achondrites contains the
classifications Shergottite, Nakhlite, Chassignite and the single specimen
(ALH84001) Orthopyroxenite. The three main groups have similar chemical and
isotopic characteristics, but are unusual in
that they are relatively young for meteorites. Testing has shown these
meteorites to be only 1.3 billion years old compared to the 4.5 billion years
for typical meteorites. The age of ALH84001 is closer to 4.5 billion years. For
the SNC group to be this young they must come from a large body that was still
hot and volcanically active at that time.
Zagami, Nigeria. An SNC
The only bodies
in our solar system that are large enough to have been volcanically active 1.3
billion years ago are planetary in size. Scientists have again turned to
spectral analysis and have focused on the planet Mars as the SNC parent body.
Specifically, the area around Olympus Mons known as the Tharsis Bulge is a
possible candidate. Further confirmation comes from comparing gas ratios
contained in the SNC meteorites with atmospheric analyses from the Mars Viking
landers. Recent studies show an exact match.
On August 7,
1996 NASA issued a press release
announcing the stunning finding that evidence of 3.6 billion year old
microscopic fossils had been found in one of the known martian meteorites.
specimen, ALH84001, was the first meteorite found during the 1984 research
season on the Allan Hills ice field in Antarctica and is the oldest of the
SNCs, dating to about 4.5 billion years. A piece of ALH84001 is now on display
at the Smithsonian Institution in Washington, D.C.
Needless to say,
the claim of ancient fossils in meteorites from Mars has sparked much
controversy in the scientific community. The November 1996 issue of
Meteorite! Magazine provides a good overview of the
arguments for and against
ancient fossils on Mars.
SNC, is an extremely rare meteorite in an extremely rare
Nakhla, Egypt. Of
are achondrites that are very similar to
enstatite chondrites and are often called enstatite
achondrites. While there are some chemical differences between aubrites and E
chondrites, aubrites show a similar high content of the mineral enstatite and
are almost iron free. Aubrites also formed in a low oxygen area and are
believed to have formed by the melting and differentiation of E
Micro-structures in the
Aubrite, Pena Blanca Springs, Texas USA.
Ureilites are a very rare class of meteorite with
a relatively high carbon content of 2%. Most of the carbon is in the form of
graphite found in veins within an
olivine matrix, but some of the carbon takes the form of
microscopic diamonds. Because of the high carbon content and other trace
elements, it is thought that ureilites could have formed from or in conjunction
with carbonaceous chondrites.
Micro-structures in Kenna, a