QUESTION:
I am fascinated by the findings of David McKay and associates on the
meteorite from Mars. How does the fact that the meteorite is 15 million
years old affect theories of life on Mars? How certain are scientists on
this evidence of life? Do you think that this finding is just one species
of life that did or does live on Mars, or do you think there are other
living things present?

ANSWER from Bruce Jakosky on March 6, 2000:
There has been a lot of controversy regarding the interpretation of data
from the martian meteorites. Let me try to give a relatively unbiased view
of the analysis and interpretations:

1) Essentially everybody accepts that these rocks are from Mars, based on
their ages, oxygen isotope ratios, and trapped gas evidence. If they are
not from Mars, then there is no other place in our solar system from which
they could have come.

2) The rock that McKay et al. have focussed most of their attention on
crystallized about 4 billion years ago. This means that it has been around
for a long time and could retain some evidence of conditions on Mars at
various times during Mars' entire geological history. The 15 million year
age refers to when it was knocked off of Mars by an asteroid impact. It's
been sitting on Antarctic ice on the Earth for some 13000 years.

3) The rock contains carbonate mineral deposits within it, which have to
have formed when water or other fluids flowed through the rock at some time
in its history. It is the presence of water at some time in the past that
makes this a good rock in which to look for evidence of life.

4) There is no agreement as to whether the chemical, mineralogical, and
morphological features within the rock are evidence that life existed in the
past. Many of the features seen in the rock could have been formed by the
presence of life. However, they also might have been formed by
non-biological, geochemical processes. Ultimately, this means that we
cannot tell from this rock whether life was present.

5) There is a growing concensus that the only way to find out about martian
life is to go to places on Mars where life would have had the best chance of
existing at some time in its history, to collect rocks that have the best
chance of retaining some geochemical memory of that life, and then bring
them back to Earth to examine. It is much better to have a rock for which
we know the history and context than to have a random rock (as the martian
meteorites certainy are).

6) Even with rocks that we understand, however, it may be extremely
difficult to determine whether life ever existed. Certainly, our experience
in the last few years with the martian meteorites underscores the difficulty
of interpreting evidence for life that, if it exists, may be very different
from terrestrial life. My own personal sense is that either life will be so
obvious that everybody will immediately accept the evidence for it or that
the evidence will be ambiguous and we will not be able to understand it or
interpret it easily.

7) Finally, nothing that we have found changes our view that Mars appears to
meet all of the environmental conditions required for an origin and the
continued evolution of life. Searching for life on Mars becomes, in some
sense, a test of the chain of hypotheses that we have developed concerning
the origin of life on Earth. If life arose as we think it did on Earth, we
should find evidence for life on Mars (and Europa); if we don't find it
there, we have to question our understanding of life on Earth. Similarly,
finding or not finding life on Mars will tell us lots about the general
abundance of life in the universe.