|Photograph of a specimen of Allan Hills (ALH) 84001, a 1.9 kilogram (4.2 pound) meteorite found in Antarctica. The little cube in the picture is 1 cm across.|
Like most meteorites, it was partly covered with smooth, dark, glassy material, called the fusion crust, which formed when the rock blazed through the Earth's atmosphere.
It was found in 1984 during the annual meteorite search in Antarctica. According to geologist Roberta Score, former laboratory manager in the meteorite curatorial facility and the explorer who actually found the meteorite, the rock looked greenish inside as it lay on the Antarctic ice. In the laboratory, however, it looks gray. (NASA photo.)
|View of a thin slice of ALH 84001 in a microscope reveals large crystals (up to 6 mm long) of orthopyroxene (a silicate mineral containing iron and magnesium) and a small grain near the top of the photo of plagioclase feldspar (sodium-calcium alumino-silicate), rendered glassy by shock waves.|
Orthopyroxene makes up about 95% of the rock, and the large size of the crystals suggests that the rock crystallized in a slowly-cooling magma body inside the Martian crust.
The crystals contain numerous cracks and are separated by crushed zones of much smaller crystals. These zones probably formed when high-pressure shock waves, generated by an impact, crushed portions of the large crystals. Crushed zones and other cracks in the rock contain the carbonate globules that have the features ascribed to biological processes. (Photo courtesy of David Mittlefehldt, Lockheed Engineering and Science Company.)
The meteorite is decorated with globules of carbonate minerals that seem to occur along cracks in the rock. These globules have a somewhat orange color and are small, only 0.1 millimeter across.
As discussed in The Evidence and the Debate, a big discussion centers on the origin of the globules, especially whether they formed from very hot fluids (more than 650 degrees Celsius) or cooler ones (between 0 and 80 degrees Celsius) . Life would not have survived high temperatures. (NASA photo.)
|When viewed in an electron microscope, it is obvious that the carbonate globules are complicated. This photograph is a colorized image of the intensity of electrons bounced back from a polished surface of a sample of ALH 84001. The colors represent different minerals. Green is orthopyroxene (the silicate with iron and magnesium), blue is glassy plagioclase feldspar, and the various shades of red and orange are carbonate minerals with a range in chemical composition. (Photo courtesy of Ralph Harvey, Case Western Reserve University.)|
|Studies of ALH 84001 have revealed the basic outline of the rock's history. It formed about 4.5 billion years ago in a relatively large magma body inside the crust of Mars. Its high abundance of one mineral (orthopyroxene) indicates that this mineral must have accumulated in the magma, probably near the bottom of the magma body, eventually forming the original igneous rock with large crystals of orthopyroxene. (Graphics by Brooks Bays, PSR Discoveries graphic artist.)|
|An impact blasted ALH 84001 4.0 billion years ago, ripping it from its deep location and probably placing it nearer to the surface in a pile of debris. The shock waves deformed the pyroxene crystals and converted the feldspar to glass. This event also heated the rock, allowing Ar gas to escape and resetting the potassium-argon clock, which allows scientists to determine the age of the impact.|
On the basis of the elemental compositions of the carbonate minerals, Ralph Harvey (Case Western Reserve University) and Harry Y. McSween (University of Tennessee) have proposed that the rock was 650-700 degrees Celsius after the impact and hot fluids rich in carbon dioxide circulated through the crater, depositing the carbonate globules along cracks. (Graphics by Brooks Bays, PSR Discoveries graphic artist.)
|In contrast to Harvey and McSween, most investigators, such as Allan Treiman of the Lunar and Planetary Institute and others at the Johnson Space Center in Houston and the Open University in England, believe that mineral compositions and the abundances of the isotopes of carbon and oxygen in the globules imply that the carbonates were deposited by relatively cool (no more than 80 degrees Celsius) flowing water enriched in carbon dioxide, after the rock had been deformed by impact.|
Determining the age of the carbonate globules is extremely difficult. Estimates range from 1.4 to 3.6 billion years. The age is not known accurately enough to link the formation of the carbonates to the 4.0 impact event, to the relatively wet era on Mars between 3 and 4 billion years ago, or to any time before it was blasted off Mars and sent our way. (Graphics by Brooks Bays, PSR Discoveries graphic artist.)
|Scientists in Switzerland, Japan, and the U.S. (Arizona, and California) have measured the time ALH 84001 was exposed to cosmic rays in space. This actually dates the time the meteoroid containing the rock was smaller than a few meters across; the interiors of larger objects are shielded from radiation. This time is between 16 and 17 million years ago, and may indicate when it was lifted off Mars by an impact as depicted in this artist's rendition. It could have been liberated earlier, however, as a large object, and the 16 to 17 million years simply dates a recent breakup of the object as it wandered in space. (Graphics by Brooks Bays, PSR Discoveries graphic artist.)|
|It is easy to determine how long a meteorite has been on Earth if it was seen to fall. Fortunately, we can also determine the residence time of other meteorites by determining the extent to which radioactive isotopes (produced by cosmic rays) have decayed. Useful isotopes for this purpose are carbon-14 and aluminum-26.|
Measurements done on ALH 84001 by scientists in Arizona show that the meteorite fell about 13,000 years ago. It was eventually spotted in 1984 by Roberta Score, and identified as a Martian meteorite in 1994 by one of Roberta's colleagues, Dave Mittlefehldt.
Now ALH 84001 is the focus of intense scientific scrutiny because of the possibility that the carbonate globules were formed in part by biological activity of ancient Martian life forms.
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