Carbonate Globules Formed on Mars
by G. Jeffrey Taylor
There is little doubt that a group of meteorites come from
Mars. The evidence is described clearly in a report on the web site of the meteorite
curators at the Johnson Space Center (see link on our title page). The ratios of the
isotopes of oxygen indicate that ALH 84001 is also a member of that group, though it is
much older than the others (over 4 billion years vs 1.3 billion years or less). The
meteorite also contains trapped gases like those in the Martian atmosphere. It seems
highly likely that ALH 84001 comes from Mars, and there is not much debate about this
point. This is an artist's rendition of the impact that liberated ALH 84001 from the
martian surface. (Graphic by Brooks Bays, PSR Discoveries graphic artist.)
There is also little dispute
that the carbonate nodules formed before the meteorite arrived on Earth 13,000 years ago.
As seen in this photograph, which has been colorize to highlight compositional
distinctions, the prominent chemical zoning pattern is offset in some carbonate globules,
undoubtedly because of an impact event that took place before arrival on Earth. An arrow
points to the offset, shown prominently by the white bands. The formation age of the
carbonates also indicates a pre-terrestrial origin for them. Although the age of the
globules is highly uncertain, there is no question that they formed at least a billion
years ago, long before the meteorite landed in Antarctica. (Image based on
photograph by David Mittlefehldt.)
Carbonate Globules Formed from Liquid Water
by G. Jeffrey Taylor
Survival of life like that on Earth requires hospitable conditions,
most notably water. Data on the isotopic compositions of oxygen in the carbonate globules
indicate that the carbonates formed between 0 and 80 degrees Celsius, appropriate for life
to flourish. (NASA photo)
Alternative View of Temperature
There is a drastic difference of opinion about the
temperature at which the carbonate globules formed. This debate is not surprising in light
of how complicated the globules are. In this colorized image made with an electron
microprobe green is orthopyroxene (a silicate with iron and magnesium), blue is glassy
plagioclase feldspar, and the various shades of red and orange are carbonate minerals that
vary widely in chemical composition. On the basis of the elemental abundances in the
carbonate minerals, Ralph Harvey (Case Western Reserve University) argues that the
temperature could have been as high as 700 degrees Celsius, far too hot for any form of
life to survive. He argues that the high temperatures resulted from a meteorite impact. A
counter argument is that the system was not in chemical equilibrium, which Harvey assumed
when making his temperature estimate. Bacteria are capable of producing products quite
different than those made by purely inorganic chemical processes. On the other hand,
formation of the carbonate minerals from water ought to have produced some water-bearing
minerals, but these have not been detected.
