Isotopic
Characterization of Martian Methane by CRDS - 7th International
Mars Conference
Abstract
#3233
John D. Kessler1 (jdkessle@princeton.edu), T.C. Onstott1, K.K.
Lehmann2
Whiticar (1999)
Introduction:
Fluxes, Sources, and Sinks
Design
and Results
1Department of Geosciences Princeton University 2Department of Chemistry University of Virginia
Fig. 1.
Fig. 2.
Measurements of the stable isotopes of CH4 (δ13C-CH4 and δ2H-CH4) potentially
could resolve the outstanding question of what are the sources and sinks of
methane in the Martian atmosphere, including the hypothesis that it is
biologically generated or consumed IF:
1) Both C and H isotope ratios are measured
2) The precisions of the measurements are high
(0.3 for δ13C-CH4 and 5 for
δ2H-CH4). (Fig. 1)
In Mars Analog sites,
stable isotope data has determined CH4 sources. Figure
2 shows experimental data from the
South African and Canadian Precambrian shields compared to the conventional
fields for microbial and thermogenic CH4 after Schoell
(1988). Samples fall along linear
trends (dashed arrows) from the most 13C-enriched abiogenic end-members to more 13C-depleted and 2H-enriched biogenic end-members. Analytical errors are ± 0.3 on δ13C values and ±5 on δ2H values and are smaller than the plotted symbols. Adapted from Ward et al.,
(2004). The black arrows represent the
fractionation of CH4 observed for mesophilic methanotrophic
cultures grown in the lab.
What
is it? Laser light is injected into a cavity and
reflects between two high reflectivity (R = 99.9976%) mirrors. After the laser is turned off, the
exponential decay in laser intensity vs. time (or path length if you multiply
by the speed of light) is detected as it is transmitted through one of the
mirrors. The exponential decay rate is
related to the concentration of a constituent.
High sensitivity is
achieved due to effective path
lengths of 10+ km and because
it measures exponential decays eliminating noise caused by fluctuations in the initial laser intensity. Our current standard deviation for 49 measurements of τ is 0.07% giving a standard error of 0.07% /√49 = 0.01%. This
error allows us to measure δ13C-CH4 and δ2H-CH4 to within 0.3 and 5,
respectively.
Methane Isotope CRDS at Princeton University
To greatly simplify instrumentation, we have used only ONE laser to measure the absorption peaks for 12CH4, 13CH4, 12CDH3 and CO2. Coarse adjustment of the wavelength of the Distributed Feedback Laser was conducted by varying the temperature of the thermoelectric cooler from 0șC to 70șC to change the wavelength
by 7nm. The center wavelength is 1653nm.
REFERENCES:
Schoell,
M., Multiple Origins of Methane in the Earth, Chemical Geology, 71:1-10, 1988.
Ward,
J. A., et al., Microbial Hydrocarbon Gases in the Witwatersrand Basin, South Africa: Implications for the
Deep Biosphere, Geochim. Cosmochim. Acta, 68:3239-3250, 2004.
Whiticar,
M.J., Carbon and hydrogen isotope systematics of bacterial formation and oxidation of methane, Chemical Geology, 161, 291-314, 1999.
