nai

Project Progress

Overview: The Search for Biomarker Gases

Our team is engaged in a search for local sources of methane, water, and other biomarker gases on Mars, and for associated chemical species, using astronomical remote sensing techniques. We acquire simultaneous maps of atmospheric gases in the key infrared spectral region (wavelengths 3.0 – 3.8 μm) where aliphatic and aromatic organic gases have strong vibrational bands. Gases such as H₂O, HDO, and CO₂ also have strong spectral signatures in this region, and are readily measured (simultaneously) in the spatial footprints searched for biomarkers (e.g., Figure 1). Their signatures assist in identifying possible correlations of biomarkers and water, and in quantifying mixing ratios for trace gases by easing correction of topographic and temperature effects (using CO₂). The D/H ratio (especially) offers potential for identifying the locale of active vents and for discriminating near-surface from sub-permafrost release zones. These are necessary first steps towards distinguishing the depth of potential biomarker release, and they help to test geochemical vs. biotic generation mechanisms.

{{ 1 }}

During this (and the prior) reporting period, we introduced (and tested) several key improvements to our software for pipeline processing of astronomical spectra. Specifically, we developed new techniques to account for subtle instrumental effects, thereby achieving an order of magnitude increase in sensitivity. These new tools include improved correction for anamorphic optics (with milli-pixel precision), removal of internal scattered-light, correction for variable resolving power (along and across the slit), removal of spectral fringing (using Lomb periodogram analysis), and so forth.

We also introduced the highly advanced GENLN2 atmospheric model into our pipeline data processing, for synthesizing and removing telluric signatures from the measured Mars spectra. For example, the (terrestrial) atmospheric model now accounts for pressure shifts in individual spectral lines. The molecular database accessed by the model was also updated to include the latest experimental and theoretical results, e.g., parameters for many spectral lines of H₂O and C₂H₆ were recently corrected and extended.

We re-analyzed our entire spectral database, consisting of searches spanning 3 Mars years and acquired with three instrument-telescope combinations. We identified a previously unknown band of isotopic CO₂ that overlaps the CH₄ P-branch region near 3.35 μm (v₃ vibrational fundamental band), and we evaluated its potential for introducing spectral confusion into biomarker searches. This CO₂ band does not extend to the CH₄ R-branch search region where most of our data are acquired.

Scientific and Technical Details:

Because of its potential biological significance methane on Mars has been sought for nearly 40 years. The earliest investigations ^{2, 3, -4} averaged over much of the dayside hemisphere, and the local abundance could be significantly larger if methane is released mainly at a few active sites. The large spatial variation found in sub-surface hydrogen at mid-latitudes by Mars Odyssey suggests that methane searches should feature comparable spatial resolution. Recent searches for local release have featured spatial-spectral mapping from Earth (IRTF, Gemini-South, Keck) and from Mars orbit (MEX-PFS).

Methane detection is difficult, even with long slit mapping spectrometers at ground-based observatories, because of four major limitations: high atmospheric opacity, limited sensitivity, low spatial resolution, and sparse temporal coverage. Earth’s atmosphere is opaque to the strongest methane lines (Q-branch) even atop Mauna Kea (14,000 feet altitude), and weaker lines (e.g., P2 and R0) are severely extinguished even at the largest geocentric Doppler shifts. We search at times of high geocentric Doppler-shift to minimize terrestrial extinction (Figure 2).
The sparse temporal coverage (observing runs of a few days) permits limited mapping in longitude. By combining results from many runs, we achieved fairly complete coverage of the Mars globe – albeit not at every season (Fig. 2).

{{ 2 }}

The most compelling results from these searches are: 1) evidence supporting the detection of methane, 2) evidence for spatial variability that implies release from discrete vents and requires a CH4 lifetime of weeks rather than the 300-plus years implied by photochemistry, and 3) evidence for seasonal variations in its release.

The measurement approach:

Conducting searches for biomarker gases is particularly convenient at infrared wavelengths owing specifically to rovibrational C-H stretching modes in hydrocarbons, which fall in the range ~ 3.2 – 3.6 μm. More generally, in the 2-5 μm spectral region, many molecules of possible biological and geothermal origin have strong signatures, for example H₂O, HDO, CO₂ (626, 627, 628), CH₄, C₂H₆, H₂CO, CH₃OH, C₂H₂, C₂H₄, SO₂, OCS, N₂O, NH₃, HCN and CH₃Cl. Comparison with CO₂ eliminates many sources of systematic error that would otherwise limit extraction of accurate mixing ratios for trace species.

Observations are done using use high-resolution spectrometers (λ/δλ⇑> 40,000) at high altitude observatories, minimizing the effect of telluric extinction. Our current dataset of biomarker gases extends over a period of 10 years (see Fig. 2), with data from three instruments: CSHELL (Cryogenic Echelle Spectrograph) at NASA-IRTF (Infrared Telescope Facility), NIRSPEC (Near Infrared Spectrograph) at Keck-2, and PHOENIX at Gemini South. Most of the observations were performed using CSHELL, which provides high-resolving power (λ/δλ⇑ = 40,000) but limited spectral grasp. On the other hand, the new generation cross-dispersed instrument at Keck-2, NIRSPEC, achieves similar resolving power and has a spectral grasp almost 40 times higher than CSHELL. This allows the simultaneous detection of multiple species (Figure 1), thereby permitting the precise extraction of atmospheric abundance and isotopic ratios. Test frames on Mars have been obtained using NIRSPEC since 2003, however, the first sensitive campaign was in January 2006 (2 half-nights), when we achieved extremely high sensitivity for methane (1σ ⇑= 1.5 ppb) and other biomarkers.

In our earlier investigations, the removal of telluric features was obtained by differencing spectra taken at different latitudes. For well-mixed species, this approach reduces significantly the intensity of residual spectral. The extraction of absolute spectral signatures is, by contrast, extremely complex. It requires very precise frequency calibrations (to better than one-hundredth of a pixel), and the telluric transmittance spectrum must be synthesized at very high resolution (approaching 100 m/sec).

Examples of the obtained retrievals can be seen in Figures 3, 4, and 5. Figure 3 shows residuals for averages of 5 rows mapping different latitudes along the spectrometer slit – strong lines of CO₂ are seen in each spectrum. The high precision of the residuals (shown multiplied by 10) permits us to retrieve absolute abundances with high confidence. For validation purposes, we compare the surface pressure and temperature retrieved from these spectra with values obtained from a Global Circulation Model , for this season and these observing conditions. Similar agreement is obtained when comparing our retrieved water burdens with MGS-TES measurements of water vapor. Most important is that we sample CO₂ simultaneously, permitting extractions of the atmospheric column and temperature for each sampled footprint, essential in the retrieval of abundance ratios.

{{ 3 }}

Figures 4 and 5 show spectra of a new band of isotopic (628) CO₂ discovered by us during this study (Villanueva et al. 2007). This band does not appear in any spectral database nor is it found in the laboratory literature, nor had it been reported to be present on Mars until now. The band can be used to retrieve atmospheric parameters (pressure and temperature) and they agree with values obtained from another 628 band (pressure and temperature, Figures 3b and 3c) and from GCM models. The new band can interfere with biomarker searches in the P-branch region of CH4 region. Fortunately, it does not affect searches in the R-branch region, and we have found no other CO2 bands that overlap the CH4 R-branch region that we target.
Our absolute retrievals of surface pressure and gas temperature (at the surface and in the first scale height) agree well with GCM models (Figures 3b, 3c, and 4). Together, they validate the rigor of our observing, data reduction, and analytical approaches. In the coming reporting period, we will publish detailed results of our search for methane and other biomarker gases.

{{ 4 }}

{{ 5 }}

References

1. D. P. Edwards, NCAR Tech. Note NCAR/TN-367+STR (1992).
2. W. C. Maguire, Icarus 32: 85 (1977).
3. M. Lellouch et al., Planet Sp. Sci. 48(12-14): 1393 (2001).
4. V. A. Krasnopolsky, G. L. Bjoraker, M. J. Mumma, D. E. Jennings, J. Geophys. Res. 102 (E3): 6525 (1997).
5. M. A. Mischna, M. I. Richardson, R. J. Wilson, D. J. McCleese, J. Geophys. Res. 108 (E6), 5062 (2003).
6. G. L. Villanueva, Ph.D. Thesis, Freiburg University, ISBN 3-936586-34-9 (2004).

Invited Talks and Reviews

“The Current Status of Methane on Mars”, M. J. Mumma, Committee on the Origin and Evolution of Life, Natl. Res. Council (Keck Center, Washington, D.C.)

“The Current Status of Methane on Mars: Geochemistry or Biology?”, M. J. Mumma, Centro de Astrobiologia (Madrid, Spain)

“Methane on Mars: Geochemistry or Biology?”, M. J. Mumma, Dept. of Astronomy and Physics, The College of New Jersey (Trenton, N. J.)

“A Sensitive Search for Life Signatures in the Martian Atmosphere”, G. L. Villanueva et al., Bioastronomy Conference (San Juan, Puerto Rico)

“The Search for Life at Infrared Wavelengths”, G. L. Villanueva, Review talk at the Astrobiology Graduate Conference (AbGradCon), (San Juan, Puerto Rico).

“Is Mars Alive?”, P. Mahaffy, G. L. Villanueva, J. Eigenbrode, in the “About Goddard” Monthly Seminar, NASA ? Goddard Space Flight Center (Greenbelt, MD)

“Infrared Signatures of Biomarker Gases”, G. L. Villanueva et al., European Southern Observatory Workshop on Planetary Systems (Santiago, Chile)

“A Deep Search for Biomarkers on the Martian Atmosphere”, G. L. Villanueva et al., Dept. of Physics, The Catholic University of America (Washington, D.C.)

“Is Mars a Dead Planet?”, G. L. Villanueva et al., Solar System Exploration Division, NASA ? Goddard Space Flight Center (Greenbelt, MD)