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A
survey of the total organic and inorganic carbon (TIC/TOC) inventory
is the highest level assessment of both biological history and
habitability of a site. Moreover, it is a measurement of interest
not only on Mars, but on any planetary body. By
addressing TOC, we perform a high-level assessment of habitability
and biological potential that does not depend on any specific
molecular model. The ratio of organic to inorganic carbon is a
model independent indicator of the extent to which chemical energy
is available to biological entities. The 13C/12C
ratio is a similarly generic indicator of biotic vs. abiotic material.
13C is depleted in biological
systems, enhanced in carbonates. From a paleontological point
of view, this measurement helps us understand the evolution of
Martian organics. Do organics graphitize as on Earth, oxidize
to CO2, or do they form stable,
possibly detectable intermediates?
Purely
from an analytical stand-point, TIC/TOC is best measured by oxidizing
the organics either chemically or electrochemically. In the chemical
approach, inorganic carbon is converted to CO2
by an acid solution, while organic carbon is converted in an oxidizing
solution. In the electrochemical technique the organic carbon
is oxidized at a boron-doped diamond electrode. Assuming future
Mars lamders will offer sample collection and grinding, the challenges
to be addressed are the oxidation mechanism itself and the CO2
detection against the background of a CO2-rich
atmosphere.
We
are currently working to develop a Total Inorganic Carbon / Total
Organic Carbon (TIC/TOC) analyzer capable of:
(1) determining quantitatively the relative proportions of total
inorganic carbon (TIC) and total organic carbon (TOC) in planetary
surface and near-surface samples.
(2) determining stable carbon isotope ratios for the TIC/TOC
fractions.

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