Chronology of the Creation of ALH84001
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.