Experimental Design

Experimental Design

Past Life

At present, the general scientific consensus is that the most likely kind of life to be found on Mars will be fossilized life. Thus, it is essential to the success of our mission that we have a solid set of experiments designed to detect fossils. Unfortunately, looking for fossils is not a simple task, requiring detailed microscopic examination and the collection of a multitude of samples, along with healthy doses of luck and intuition. Fortunately, despite these obstacles, it is possible to conclude with some certainty that a particular rock or structure of interest shows evidence of fossil life.

In fact, a level of great confidence can be reached with only two experiments on Mars, along with supplemental experimentation on Earth of returned samples. Such a feat can be accomplished due to the nature of the generally accepted definition of a fossil life-form. According to William Schopf, a leading expert on the identification of ancient microfossils, a rock structure must satisfy seven conditions in order to be accepted as a fossil.

While these conditions may eliminate some heretofore unheard of forms of life, such as life based on alternative chemistries or non-chemical life, they are sufficient to conclusively identify a structure as a fossil. By attempting to accomodate unknown types of life, however, we would have to loosen these conditions, and potentially declare abiotic structures to be fossils. Since it is far better to be cautious and perhaps miss some unknown life-form than to announce the discovery of a fossil which turns out to be a microscope artifact or similar abiotic relict, we will abide by these seven conditions.

But how can two experiments establish all of these conditions? By being relatively general, of course. In fact, the first five of these conditions are determined by one experiment, the sixth is determined when the sample is collected in the field, and the final condition is established by a second experiment. The first experiment is the preparation and examination of a petrographical thin section of the sample rock, and the second is the determination of the carbon and sulfur isotope ratios in four different classes of materials in the sample rock.

A petrographic thin section is an amazingly useful tool in geologic and fossil analysis. Essentially, it is a slice of rock so thin that it is transparent, which can then be analyzed under a light microscope to characterize its internal structure. Ordinarily, the preparation of a thin section is a laborious task, requiring hours of cutting, grinding, and polishing, using large amounts of water and many different grits of abrasive. Such a process is wholly unsuited to a Martian laboratory. At least one company, however, produces a device capable of doing all of this hard work automatically, using very little power and very little weight. The model pictured here can take a sample from an irregular chunk of rock all the way to a precisely polished thin section in a relatively short period of time. Additional details can be found at Microtec Engineering's web site, but suffice it to say that this device is very impressive and eminently capable of being used in space and on Mars.

Once a thin section of a sample has been prepared, it can be visually inspected for signs of fossil life. All of the first five criteria for the identification of a rock feature as a fossil are satisfied by a close inspection under a binocular microscope. Visual inspection can determine whether a feature is organic by color, the complexity of cellular structure, the presence of cell division, the existence of a multitude of similar objects, and the nature of variations among those objects. Samples exhibiting all of these characteristics will be returned to Earth for further analysis and confirmation by independent experts.

The other necessary experiment is an isotope ratio determination. In particular, past biological activity leaves signatures in four distinct rock components: leftover organic material, carbonate minerals, sulfide minerals, and sulfate minerals. Most forms of autotrophic metabolism tend to increase the relative abundance of carbon-13 in carbonate minerals and decrease the abundance of carbon-13 in the remaining organic material. Other forms of metabolism, which rely on sulfur as an energy source, increase the abundance of sulfur-35 in sulfate minerals and decrease its abundance in sulfide minerals. These characteristics lead to an interesting experimental challenge: how does one separate these different minerals to observe these different effects?

The answer is provided by the chemical properties of the minerals involved. Both organic carbon and sulfide minerals react with oxygen at elevated temperatures to form gaseous carbon and sulfur oxides, which can have their isotope ratios determined by mass spectroscopy. Carbonates and sulfates are stable under these conditions, however, and remain in the rock. Once no more CO₂ and SO₂ is generated by the rock in oxygen, the rock sample can be removed and ionized directly in the mass spectrometer to determine the isotope ratios in the carbonates and sulfates. Samples bearing unusual isotope ratios can be returned to Earth for more detailed analysis. While they may not have visible fossils, it is common for the fossils in a rock to be destroyed over time, leaving only carbon behind as evidence of life. Thus, a fossil-free rock with characteristic isotope ratios can also suggest possible fossil life, but can never confirm it. If the isotope ratios can be confirmed by labs on earth, then such a sample can be taken as tentative evidence for past life.

Finally, much testing can done directly in the field. When a sample is taken, it should be analyzed by an APX spectrometer, which can determine its elemental composition with high accuracy. If it contains no carbon, it is of little use to look for life in the sample, and so it can be discarded. Also, on the initial collection, the astrogeologist should be able to determine the rock's formation conditions from its mineral types, identifiable on sight. More detailed mineralogical analysis can be done from the thin section, and final evidence of formation conditions can be found on Earth using an ion microprobe to measure the rock's age and the temperature at which it formed.

Major Experimental Divisions to search for past life

Petrographic Thin Section

Isotope Analysis