cited:It was known as early as 1872 (Tschermak) that the
Shergotty meteorite was a basalt that formed under
relatively oxidizing conditions, but it wasn’t until 1979
(Walker et al., Nyquist et al.) that it became apparent
to meteoriticists that the relatively young SNC*
meteorites may have come from the planet Mars. This
hypothesis was explained in detail by Wood and
Ashwal in 1981, but it wasn’t until Bogard and Johnson
(1983) found gas trapped in glass in shergottite
EETA79001 identical in composition to Martian
atmosphere (as measured by Viking experiments), that
it became widely accepted that SNC meteorites came
from the planet Mars. Also in 1983, Clayton and
Mayeda showed that SNC meteorites formed their own
subgroup on an oxygen isotope diagram with their own
fractionation line separate from that of the Earth or
HED parent bodies. In 1984, Becker and Pepin found
that nitrogen isotopes and N/Ar ratios were also typical
of Viking results, clinching the argument (figure I-1).
In 1995, Marti et al. found similar results in glass in a
second SNC meteorite (Zagami).
Briefly, Martian meteorites are igneous rocks (basalts
and cumulates) that have been shocked to various
degree. However, they are not badly brecciated. Table
I-1 gives the modal mineralogy. All, accept one
(ALH84001), have young igneous ages. They were
apparently blasted off Mars over the last 16 million
years, judging from their cosmic-ray exposure ages.
Several were seen to fall and were collected, others
were recovered from the ice in Antarctica.