ALH84001 Origin
Meteorite From the Ancient Crust of Mars by G. Jeffrey Taylor
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.)