2000 Annual Science Report
NASA Jet Propulsion Laboratory Reporting | JUL 1999 – JUN 2000
Studies of Biogenic Greenhouse Gases on the Early Earth
Research by the Blake group as part of the NABI effort led by P.I. K. Nealson centers on isotopic studies of biogenic greenhouse gases as evidence for life on the large scale appropriate for remote sensing approaches and on global biogeochemical investigations. Research over the past year has dealt primarily with topics associated with the early and present biogeochemical evolution of the Earth, but our efforts over the coming year(s) will focus strongly on developing new tools for the exploration of Mars and other extraterrestrial environments.
In collaboration with the Nealson and Yung groups, our recent research has involved the laboratory and observational investigation of the biogeochemistry of greenhouse gases relevant to the early Earth. Particular emphasis is placed on the quantitative examination of biologically specific tracers such as stable isotope fractionation in these gases, and on whether or not these signatures can be reliably detected in extraterrestrial solar system bodies and exo-planetary systems Our initial efforts have dealt with nitrous oxide, or NNO, and we have now characterized in detail the fractionation induced by the photolysis of NNO first predicted by Prof. Yung using a combination of non-linear spectroscopic light sources and mass spectrometry or FTIR spectroscopy (Rahn et al. 1998, Zhang et al. 2000, Turatti et al. 2000). We are now extending this work to include biochemical fractionation with Dr. Lisa Stein and are working with the Yung group to model the global biogeochemistry of nitrous oxide. Part of this effort is now funded by the NSF Atmospheric Chemistry office through a grant to Prof. Yung, and the early results made possible by NABI support were critical to this award.
Neither approach used in the lab to date is compatible with the stringent space and weight requirements of in situ planetary exploration strategies. We are therefore developing new technologies that should enable the in situ measurements of abundances and stable isotope ratios in important radiatively and biogenically active gases such as carbon dioxide, carbpm monoxide, water, methane, nitrous oxide, and hydrogen sulfide to very high precision (0.1 per mil or better for the isotopic ratios, for example). Such measurements, impossible at present, could provide pivotal new constraints on the global (bio)geochemical budgets of these critical species, and could also be used to examine the dynamics of atmospheric transport on Mars, Titan, and other solar system bodies. We believe the combination of solid state light sources with imaging of the IR laser induced fluorescence (IR-LIF) via newly available detector arrays will make such in situ measurements possible for the first time. Even under ambient terrestial conditions, the LIF yield from vibrational excitation of species such as water and carbon dioxide should produce emission measures well in excess of ten billion photons/sec from samples volumes of order 1 c.c. These count rates can, in principle, yield detection limits into the sub-ppt range that are required for the in situ isotopic study of atmospheric trace gases.
While promising, such technologies are relatively immature, but developing rapidly, and there are a great many uncertainties regarding their applicability to in situ IR-LIF planetary studies. The support from NABI was used to conduct pivotal modeling tests of this approach, which has just resulted in a successful proposal to the PIDDP program (funding start 5/00). PIDDP now provides the larger funding base to actually fabricate these new tools through a three-year program to combine microchip near-IR lasers with low background detection axes and state-of-the-art HgCdTe detectors developed for astronomical spectroscopy to investigate the sensitivity of IR-LIF under realistic planetary conditions, to optimize the optical pumping and filtering schemes for important species, and to apply the spectrometer to the non-destructive measurement of stable isotopes in a variety of test samples. These studies form the necessary precursors to the development of compact, lightweight stable isotope/trace gas sensors for future planetary missions. Within the NABI consortium, we will continue to work with the Nealson and Yung groups on modeling of the potential biological activity on Mars and the early Earth, and on the isotopic signatures of various model biospheres. This research is groundbreaking, and provides the necessary context for IR-LIF to be used as a robust diagnostic of extant or extinct (through its sampling of gases trapped in Martian soils and ice caps) biota on and underneath planetary surfaces.