QUESTION:
What are the implications of the unexpected amounts of silicates in the rock
analysis for the designation of certain meteorites found on earth as Martian
meteorites?  Is it still so certain if Mars is not as different from earth
as we thought?

ANSWER from Bruce Jakosky on August 7, 1997:

The designation of the Martian meteorites as having come from Mars is not
based on a similarity or difference in their mineral or elemental
compositions.  Rather, it is based on other differences that turn out to be
quite convincing.  The argument for a martian origin is as follows:

1) The relative abundances of the different isotopes of oxygen are different
from Earth rocks.  (The isotopes differ by having an extra one or two
neutrons in the nucleus.) The variation of the isotope abundances seen in
Earth (and Moon) rocks falls along a particular trend.  The Mars meteorites
fall along a different trend.  There is no way that one could change the
ratios to create this difference.  Differences such as this are seen,
however, in meteorites that formed in different places in our solar system. 
The conclusion is that these rocks cannot have come from either the Earth or
the Moon.

2) Eleven of the twelve meteorites solidified very recently from molten
rock, within the last 1.3 billion years.  This requires that they must have
come from a planet that is large enough that it would retain heat and be
geologically active more than 3 billion years after their formation.  This
really means they must have come from one of the larger terrestrial planets.
With Earth ruled out, this leaves only Mars and Venus.

3) The convincing tie to Mars comes from the composition of gas that is
trapped within a glassy component of two of the twelve meteorites.  The
glass was created, and the gas implanted into it, at the time that a
meteorite impacted the surface and ejected them to space.  The gas is
identical in composition to the martian atmosphere, and distinctly different
in composition from any other source of gas in the solar system.

Thus, at least 2 of the 12 rocks are pretty convincingly of martian origin.
The other 10 are tied to Mars by the geochemical similarities with the first
2.  In particular, the oxygen isotopes all group together rather than
separately.  All of the scientists who have thought about these questions
have concluded that it is most likely (but not certain) that these rocks
came from Mars.  If they didn't come from Mars, they would have to have come
from a very Mars-like planet, but there is no other such planet in our solar
system.