New England Meteoritical Services

Achondrites

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.

Because 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 without chondrules.

Johnstown, Colorado, USA, Achondrite, diogenite.

The 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.

Eucrites are 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.

Millbillillie, Australia, Achondrite, eucrite.

The public 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 (Martian-origin) Achondrite.

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.

Life on Mars?

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.

The particular 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.

Nakhla, another SNC, is an extremely rare meteorite in an extremely rare classification.

Nakhla, Egypt. Of Martian-origin

Aubrites 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 chondrites.

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 Ureilite.


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