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| Titel |
Using Infrared Laser Heterodyne Radiometry to Search for Methane in the Atmosphere of Mars |
| VerfasserIn |
Richard Passmore, Neil Bowles, Damien Weidmann, Kevin Smith |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250043859
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| Zusammenfassung |
| Introduction
Methane has been detected in the atmosphere of Mars by several research teams in the
last few years. Ground-based observations [1][2] and space-based instruments (e.g. the
Planetary Fourier Transform spectrometer on Mars Express [3]) have reported low levels of
methane gas (approximately 10 ppb) in the Martian atmosphere. Methane detection is
important as its presence could imply a biological origin, and Martian methane sources are
still unknown. However, current methane concentration measurements are at instruments’
lower limits of detection.
The viability of remote sensing using infrared laser heterodyne radiometry (LHR) to
detect methane in the Martian atmosphere is investigated. The LHR technique allows
high spectral resolution (greater than 0.001 cm-1) measurements over a narrow
spectral range (~10 cm-1) when a distributed feedback quantum cascade laser
(QCL) is used as local oscillator. The advantages of such an instrument, including its
compact lightweight design, over current remote sensing spectral instruments are
reviewed.
The Laser Heterodyne Radiometer
Laser heterodyne radiometers have been used extensively, and with much success, for
atmospheric studies such as work on stratospheric ozone [4], mainly because the ultrahigh
spectral resolution of the instrument allows fully resolved narrow molecular absorption
line-shapes, which contain information on vertical concentration profiles. It has been
shown that a carefully selected specific high resolution micro-window provides as
much vertical profile information as a medium resolution radiometer covering a
broad spectral range [5]. In addition to the high spectral resolution, the LHR is also
extremely compact and robust and so has a significant advantage when targeting specific
trace species over larger instruments such as high-resolution Fourier Transform
spectrometers.
Quantum Cascade Laser as Local Oscillator
At the heart of the current generation infrared LHR is the use of a Quantum Cascade
Laser (QCL) as the local oscillator. QCLs are an ideal local oscillator for this instrument as
they emit in the mid infrared region where molecular “fingerprints” lie, they provide the
necessary optical power and have spectral purity in the kHz to MHz range [5]. They have the
advantage of continuous frequency tuning over a specific spectral window that can be
precisely tailored to specifications. They also have the advantage of being compact,
robust and reliable devices which make them ideal candidates for flight and satellite
deployment.
Instrument Development
Although heterodyne spectroscopy is not a new idea, recent advancements in local
oscillator technology offer the possibility of significant instrument miniaturisation relevant to
space deployment. We present our current work on the LHR which involves adapting an
existing 10 μm laser breadboard design to operate at 7.7 μm in order to target the ν4
fundamental band of methane. The optical and mechanical designs of the instrument, as well
as an evaluation of the LHR’s flight potential, are discussed.
References
[1] Krasnopolsky et al. (2004) Icarus, 172, 537-547. [2] Mumma et al. (2009) Science,
323, 1041. [3] Formisano et al. (2004) Science, 306, 1758-1761. [4] Weidmann et al. (2007c)
Applied Optics, 46, 7162-7171. [5] Weidmann et al. (2007b) Review of Scientific Instruments,
78, 73107. |
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