An Exopaleontological
Exercise
Long ago, Mars might have been
hospitable enough for microorganisms to develop.
The climate of early Mars was more like the
Earth's, with abundant water and a denser, warmer
atmosphere. Photos from the Mariner and Viking
orbiters show channel systems in the older
regions of Mars, similar to those produced by
rivers on the Earth. These channels often
surround basins where water could have formed
lakes. At
the same time when liquid water apparently was
present on Mars, life arose on Earth. Terrestrial
life developed between 4.1 and 3.5 billion years
ago. Before then, the crust was molten. The
oldest life forms--which resemble modern-day
cyanobacteria, simple photosynthetic microbes
that form layered sedimentary structures called stromatolites--occur
in rocks 3.5 billion years old. If life developed
on Earth during this period, it might have
developed on Mars, too.
If it did, it
probably left fossils. This prospect has spawned
a new field of science called
"exopaleontology." Exopaleontologists
are working on a strategy for fossil hunting.
The fossil record
in the oldest rocks on Earth, formed during the
time in history called the Precambrian, consists
of remnants of microbes and the chemicals they
left behind. The preservation of microorganisms
did not occur everywhere: It required certain
geologic conditions and environments. Precambrian
fossils consist mostly of stromatolites,
produced by stratified microbial communities
called "mats." Inside stromatolites,
paleontologists sometimes find preserved cells of
microorganisms and the organic compounds they
produced. But this happens only where organisms
were quickly entombed by minerals that
precipitated in their environment while they were
still alive.
This rapid
entombment is most common in water rich
environments, especially nutrient rich
springs that bubble to the surface and deposit
their dissolved minerals. Thermal springs,
such as the geysers of Yellowstone, and cooler
springs, such as those that form the tufa towers
at Mono Lake, are good examples. These also tend
to be environments that are particularly rich
with microbial life. ...
IFor long-term
preservation, over billions of years, the
minerals that entomb organisms must be chemically
stable and impermeable to water. only in this way
can they seal out the environment and shelter the
organic materials from decay. If
Exopaleontologists want to find 3.5 billion year
old fossils on Mars, they must target not only
the most common minerals, but also the most
stable: those that resist the ravages of
weathering, and retain their biological
information after long-term burial.
The most
widespread such minerals are silica, calcium
carbonate, and fine grained clays. ...
Planetary
geologists do not know whether Mars has
widespread deposits of silica, calcium carbonate,
and clays. The Mars Global Surveyor mission will
look for them, but until then we can make some
educated guesses. Carbonates are
an interesting case study. ... the planet
accumulated carbonate deposits and salts like
halite. Similarly, other rocks that are rare in
ancient areas on Earth, such as evaporates, may
be quite common on Mars. These rocks were
probably never buried or disturbed by the
tectonics that constantly reshape the Earth's
surface. The rocks in these ancient southern
highlands of Mars have likely survived with
little change.
If spacecraft can
find aqueous mineral deposits in those ancient
highlands, they will have found the best place to
hunt for fossils. Spacecraft orbiters have
instruments to look for key minerals like silica
and carbonate, using high resolution photography
(to identify geologic textures) and infrared and
gammaray spectroscopy (to identify the minerals).
After we know where to look, robots can collect
and analyze samples.
from Fossil Hunting on Mars
-- J.D.Farmer
, SETI Institute, NASA Ames Research Center
The Pathfinder
mission is using a special filter to look for
certain distinct patterns in the rocks that
indicate that fossilized silicon microbes are
there. The patterns will look like swirls or
spikes as the microbes push up towards the
sun. The do this so they can
"photosynthesize" or make their own
food. This type of fossil is called a
stromatolite.
from How will researchers test
for fossils on Mars?
-- Webquest
site
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