Motivation for Thin Section Analysis
When we look for fossils on Mars, we want to try to
emulate the process used on Earth to look for fossils as much as possible.
The process on Earth begins with a general geological survey to
identify potential fossil-bearing rocks—our astronauts will do this, and
the details are described in the section of this report on the geological
survey. The next step on
Earth is a visual analysis of the rock to look for cellular fossils, which
are omnipresent in many kinds of Earth rocks.
If a rock on Earth formed under conditions favorable to life, and
has not been too heavily altered by the ravages of time, it is bound to
contain fossils. Due to this
constant presence, a number of techniques have been developed to analyze
these fossils in various levels of detail.
The problem, however, is identifying which of the following options
is best suited to use on another planet, where the very presence of
fossils is questionable.
Thin Section Inspection: One of the
most common techniques used to identify fossils on Earth is the
preparation and inspection of a thin section.
A thin section is essentially a very thin slice of rock mounted on
a microscope slide. The slice
is so thin as to be translucent, and highly polished.
It can be examined under a microscope to look for morphological
fossils—i.e. the shapes of the remains of long-dead organisms.
By adding a polarizing filter to the microscope, a thin section can
also be used to characterize the types of minerals present in a rock,
information useful in identifying whether the rock formed under conditions
favorable to life. This
method, however, usually requires a long and tedious series of polishing
Fossil Extraction: Another way to look at microscopic fossils is
to dissolve the surrounding rock away, leaving only the fossils. The techniques to do this are well established, and allow
microscope examination of fossils in somewhat greater detail than in thin
section. However, it has a
number of drawbacks. It
requires the use of several extremely dangerous chemicals, including
highly corrosive hydrofluoric and nitric acids.
It also requires knowledge of the composition of the rock, in order
to know which chemicals to use to dissolve the rock.
This knowledge is often obtained from a thin section.
Finally, it requires the use of large amounts of water, which will
be a precious resource on Mars. (Brasier, 1980)
Microscope Inspection: An electron microscope can examine fossils in
much greater detail than any method using a light microscope.
Traditionally, however, electron microscopes have required involved
sample preparation, which can introduce artifacts that look like fossils.
While new types of electron microscopes are now available that
require no sample preparation, other problems remain.
Electron microscopes are heavy and power-hungry.
They use very sensitive detectors, which could easily be corrupted
by radiation in space. Finally,
they examine specimens in such great detail that their field of view is
very restricted, making it difficult to find fossils to examine.
From the above options, it is clear that the analysis of a thin section is the best way to visually look for fossils. Automating the process can eliminate its tedium. Machines exist which can prepare thin sections with no human intervention, and one will be used on the mission to prepare thin sections.
Principles of operation
Thin sections will be prepared using an automatic
thin-section-preparing device manufactured by Microtec Engineering, Inc.
The particular model we will use can go from a rough chunk of rock
to a finished slide with no human intervention, all in relatively little
mass and power (Microtec, 2000). Since it is a mechanical device, however, we will bring
enough spare parts to replace everything that could possibly break or wear
out in the machine.
The thin sections produced will all be analyzed under a binocular polarizing microscope to determine whether they meet the criteria for fossil life, presented earlier in this report.. Fossils should appear as brownish structures in the thin section due to their high carbon content (Schopf, 1999). The polarizing filter will be used on each section to identify the types of minerals present in the sample. Different minerals behave differently under polarized light, and so can be identified from a chart after observing their behavior. For fossil-bearing rocks, the mineral composition can indicate some of the rock’s formation conditions, and thus determine whether the sample meets another condition for fossil life.
This experiment shall be conducted on any sample of
rock that has been returned from the field.
If field analysis, including naked eye inspection and alpha proton
x-ray spectrometer results, suggests that the rock will not contain
fossils, analysis can be restricted to the polarizing filter to determine
its mineral composition for geological interest.
Time for experiment
of Life: the Discovery of Earth’s Earliest Fossils. Schopf, J. William. 1999.
Princeton, NJ: Princeton U. Press.
Brasier, M. D. 1980. London:
Chapman & Hall.
Microtec Engineering corporate web site: http://www.microtec.gj.net/.
Copyright © 2000 Massachusetts Institute of Technology
Comments and questions to firstname.lastname@example.org Last updated: 10 December, 2000