A study on planetary atmosphere by the laser heterodyne spectroscopy

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Laser heterodyne spectroscopy is the most powerful technique for remote sensing of planetary atmosphere because of its high spectral resolution, high spatial resolution, and high signal-to-noise ratio. In this study, a new tunable diode laser heterodyne spectrometer with a wide spectral range has been developed for planetary atmosphere observation.First, a small-sized laser heterodyne spectrometer with four tunable laser diodes was developed to covert the infrared emission or absorption spectra from planetary atmosphere into the radio wave frequency range. An acousto-optic radio spectrometer was also developed to obtain the spectra in the converted radio frequency range.Next, an optics for interface with the infrared telescope of the Institute of Space and Astronautical Science was made to observe the Venusian atmosphere for testing the instrument. However, observation scheduled in November 1997 was not carried out because of the trouble of the infrared telescope. Then the test observation was carried out with the infrared simulator of National Astronomical Observatory in April 1998. However, the infrared spectra of the Venusian atmosphere were not obtained because of bad weather.Finally, laboratory test and observations of the Earth's atmosphere using the sun or the moon as a light source were performed to check the performance of the developed instrument. These tests revealed that the signal-to-noise ratio of the spectrometer is not enough, while the moon observation demonstrated that weak infrared spectra can be measured with this instrument. The instrument is now under improvement and test observation of the planetary atmosphere will be performed again.