Haris Risis leads this effort to develop active laser instrumentation that is capable of sensitive remote measurements of atmospheric trace gases from planetary orbit. Such laser-based instrumentation can provide unprecedented information on planetary atmospheric composition, chemistry and evolution. For example, remote measurements of methane (CH4) and other biogenic molecules (such as ethane and formaldehyde) on Mars could provide seminal information in the search for possible existence of life on Mars. Measurement of CH4 at very low concentrations (<1 ppb) from orbit will dramatically improve the spatial resolution and sensitivity of the search for CH4 vents, which may indicate subsurface biogenic activity.

On Earth, several trace gases such as CH 4 and CO2 are strong greenhouse gases. Earth's atmospheric CO2 concentration is increasing rapidly, yet the locations and dynamics of the CO2 sinks are poorly understood. Measurement of greenhouse gas concentrations from orbit will help determine and monitor their global distributions, as well as the dynamics of their sources and sinks; these, in turn, will help us understand the changing processes in the Earth's atmosphere.

Our trace gas measurements make use of Differential Absorption Lidar (DIAL), which measures the absorption of the laser pulse by a molecule when tuned to a wavelength, λ1, which is coincident with an absorption line, relative to a laser pulse tuned to a wavelength, λ2, which is slightly off the absorption line. The wavelengths are alternated at kilohertz rates, minimizing any potential changes from the surface reflectivity and background. Our proposed DIAL spectrometer will operate in the near-IR region (3-4 μm) in a nadir viewing geometry, using the strong laser echoes from the surface.

This wavelength region is unique for these applications. Current laser and detector technologies have limited space lidar measurements to the short IR (~1 μm) and visible wavelengths. However, most molecules of biologic interest have relatively weak absorptions in those regions but much stronger absorptions at 3-4 μm. Therefore, lidar measurements at 3-4 μm will have a significant detection sensitivity advantage for measuring trace gases. Likewise, most molecules of biogenic interest?such as methane, ethane, water, formaldehyde?have strong absorption spectra in the 3-4 μm range.

The instrument's operating parameter goals are: 1) sensitivity of <1 ppb in a 1 km path length; this is possible because of the high spectral purity of the laser transmitter and sensitive detector; 2) spatial resolution of ~0.1 x 5 km for high CH4 concentrations; and 3) global coverage and continuous measurements in sunlight, darkness and at all latitudes throughout the Martian year.

Although this work targets primarily CH4, there is growing interest in the development of lidar that uses the much stronger gas absorption lines in the longer wavelength region to measure other trace gas concentrations from space. The IR laser sounder approach can be used to measure concentrations of many trace gases for many planetary science applications.

The trace gas detection effort has made significant progress over the last year. The group has successfully demonstrated CH4 and H2O detection in a gas cell (see figure). We have also demonstrated the tunability of the transmitter. Tuning can be achieved over a wide range of wavelengths (3.1-3.6 μm) by choosing the appropriate temperature and period of the crystal (periodically poled lithium niobate) in the optical parametric oscillator (OPO).