Introduction
  Definition of Life
  Motivation

Preliminary Steps
  Geological Survey
  Sample Collection

Present Life
  Spectroscopic Analysis
  Organic Analysis
  Biology Experiments

Past Life
  Thin Section
  Isotope Analysis
 

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.

Options

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 steps.

Chemical 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)

Electron 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.

Choice

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.

Equipment

Protocol

• Examine the rock specimen with a hand lens to determine if it contains any fine layers.  Such layering could suggest that the rock was built by microorganisms, in which case fossils would be located between the layers.  Enter into the computer the specimen number, slide number, and a brief description of the sample.  If the sample is layered, allocate two slides to it, and indicate that one is parallel to the layers and one is perpendicular.
• If the rock lacks layers, simply use the rock saw on the thin sectioner to slice off a fragment approximately two inches square.  If it has layers, prepare one piece cut parallel to the layers and one perpendicular, taking care that the pieces remain matched with the appropriate slides.
• Insert the rock piece and slide into the thin sectioner.  It should be ready at a thickness of about 30 microns, depending on the type of rock.  Retrieve the prepared slide from the device.
• Examine the slide at low magnification under a binocular microscope, so that the entire slide is visible.  Focus the microscope appropriately, and replace the eyepieces with the digital camera.  Take one picture of the section as a whole, and then replace the eyepieces.
• Examine the slide at 500x magnification.  Be sure to cover the entire area of the slide, and note any potentially fossil-like structures.  Upon encountering any such structure, take a digital picture.
• Upon completion of examination, note the presence of any of the following features:  filaments or clusters, large numbers of similar structures, variations between structures similar to that produced by genetic variation, dividing cells, brownish color
• Examine the slide under high magnification on the polarizing microscope.  Determine the mineral phases present, and take at least one picture of a representative sample of grain distributions, sizes, shapes, and types.
• If the structures in the rock meet all five of the above criteria, and/or are in a type of rock that formed under conditions favorable to life (sandstone, shale, mudstone, fine-grained silica, water-soluble minerals), mark the slide as high priority and file it away.  Otherwise, mark the slide as low priority and file it away.  In addition, return to sites that yield high priority samples to obtain new samples for confirmation.
• Regularly filter the water in the thin sectioner, and replace it if it gets too dirty or too low.

Use of this experiment

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
1 hour
 

References

Cradle of Life: the Discovery of Earth’s Earliest Fossils.  Schopf, J. William.  1999.  Princeton, NJ: Princeton U. Press.

Microfossils.  Brasier, M. D.  1980.  London: Chapman & Hall.

Microtec Engineering corporate web site: http://www.microtec.gj.net/.