Motivation for Isotope Analysis
When we look for fossils on Mars, we do not
necessarily expect to find recent fossils, or fossils in unaltered rocks.
Instead, evidence of fossils may be present in heavily
metamorphosed and weathered impact crater ejecta, or in rock strata
strongly heated and altered by nearby magma flows. Thus, we need a way to find signs of fossil life in rocks
that have been poorly preserved.
The solution to this problem is to go beyond the
traditional methods of fossil hunting, to go beyond microscopes and
visible fossils and instead look for chemical evidence of life.
ratio analysis: Chemical
elements exist in multiple forms with different masses, called isotopes.
This includes the elements of life, such as carbon, nitrogen,
oxygen, and sulfur. Different
isotopes are denoted by mass number, such as 12C and 13C.
While isotopes of the same element have the same chemical
properties, their masses can affect how they are used in chemical
reactions. In particular, the
metabolic reactions of life which process the above elements
preferentially use lighter isotopes.
In the cases of carbon and sulfur, the relative amounts of the
various isotopes are preserved in fossils and surrounding abiotic rocks.
These relative amounts of isotopes can be measured in the form of
isotope ratios, and the ratios of interest are 12C/13C
and 32S/35S. Fossils
will have higher values of 12C/13C compared to the
surrounding non-biological rocks. Sulfide minerals, created in non-volcanic settings by
microorganisms, will have higher values of 32S/35S
than sulfate minerals, which formed the raw material used by microbes to
form sulfides. These
differences remain even after rocks have been so heavily altered so that
no recognizable fossils remain, and only raw organic material remains of
the fossils. These isotope ratios can be measured by mass spectroscopy for
various materials in rocks to infer the presence or absence of life.
In addition, different kinds of metabolic reactions produce
different 12C/13C and 32S/35S
ratios, so these isotope ratios can indicate the types of metabolic
reactions used by past life. The
main problem with this method is the need to measure the ratios very
accurately, and to account for any non-biological reactions that can
produce similar isotope ratios. Fortunately,
these reactions are known, and so their influence can be inferred from the
type of rock being sampled.
Molecular fossil analysis: Even when fossils have been rendered completely
unrecognizable due to heavy alteration of their host rocks, and those
rocks have been subjected to high heat and pressure for millions of years,
some complex organic material will remain.
The molecules present in this leftover tar-like material are called
molecular fossils, and can indicate the source of the organic material. Certain classes of molecules are produced by the
decomposition of different kinds of living things, so the specific
molecules present can indicate whether the organic material came from
fossils or other processes, and even what kind of fossil was originally
present. The molecules present can be determined by simple mass
spectroscopy. The primary
drawback to this method is that the kinds of complex organic molecules
generated by non-biological processes are not known to a great level of
detail, and so results that seem to infer life could actually be caused by
Since the isotope ratios produced by biological and
non-biological processes are known in greater detail than the molecular
fossils generated by such processes, we will perform isotope ratio
analysis to look for chemical evidence of past life.
Principles of operation
According to William Schopf (1999), a leading expert in the identification of ancient fossils, metabolism leaves isotopic signatures in four different materials: organic carbon (kerogen), carbonates, sulfides, and sulfates. Metabolic enzymes preferentially use lighter isotopes of carbon and sulfur, leaving heavier isotopes in the non-biological environment. The metabolic products are preserved as sulfides and kerogen, while the leftovers are preserved in carbonates and sulfates. The isotopic situation is simple in carbonates and kerogen: the kerogen should have a lower value of 12C/13C, while the carbonates should have a higher value. The chemistry of sulfur is more complex, since its minerals can be either feedstock for metabolism or metabolic waste, depending on the process. Schopf (1999) described the results of research on with sulfate-reducing bacteria, which would lead to higher 32S/35S in sulfate minerals than sulfides; however, sulfide-oxidizing bacteria are also possible, and would reverse the sulfide vs. sulfate trend. Regardless, however, the two phases should show different 32S/35S ratios. The determination of these ratios will be done in the laboratory module. Separating the kerogen, carbonates, sulfides, and sulfates will be accomplished by combusting the kerogen to CO2 and the sulfides to SO2 at relatively low temperatures, until no kerogen or sulfides remain. The gases produced will be analyzed by the GC/MS to determine their 12C/13C and 32S/35S ratios. Then, part of the remaining sample, which now only contains carbon and sulfur in sulfate and carbonate minerals, is directly introduced to and ionized in the mass spectrometer, yielding 12C/13C and 32S/35S isotope ratios for the highly stable carbonates and sulfates.
As discussed under principles of operation, the influences of life can be seen in the differences between the 12C/13C and 32S/35S ratios in kerogen, sulfides, carbonates, and sulfates. Living material and its waste products tend to concentrate lighter isotopes, leaving relatively more heavy isotopes in the environment. In fossil systems, the kerogen is the remnant of the living material, and the sulfides are its waste products. Thus, any differences between the 12C/13C and 32S/35S ratios in these materials and the ratios in the non-biological carbonates and sulfates could signify the possible presence of life. If Martian life uses the same metabolic reactions as Earth life, then these two isotope ratios should be higher in the kerogen and sulfides. However, other reactions are possible, so other differences in isotope ratios could also signify life. The final analysis will depend on other factors, including detailed knowledge of the composition and formation conditions of the rock, which will be determined on Earth.
Use of this experiment
Field analysis of samples should indicate the
presence of carbon and/or sulfur. If
either is present, the isotopic measurements are performed.
Time for experiment
Cradle of Life: the Discovery of Earth’s Earliest Fossils. Schopf, J. William. 1999. Princeton, NJ: Princeton U. Press.
Copyright © 2000 Massachusetts Institute of Technology
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