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Laboratory for Atmospheric and Space Physics

Discovery of the Ultraviolet Airglow of Mars

Each spacecraft made two observations of the limb of Mars during the fly-bys, the first by looking ahead and the second by looking perpendicular to the flight direction.  The observations began when the image of the limb was about 200 km above the surface.  The motion of the spacecraft carried the image downward through the atmosphere.  During the 3 second scan time of the spectrometer; the image moved downward 21 km.  Starting at 200 km, airglow emissions appeared in the spectral channel 1900 to 4000 A (190 to 400 um).  The strongest spectral features were from carbon monoxide and ionized carbon dioxide.  In addition, a spectral emission from atomic oxygen appeared.  All of these emissions were the result of the action of solar ultraviolet radiation on carbon dioxide.  These observations showed that carbon dioxide was the major constituent of the atmosphere.  The measurement of the intensity of the airglow emissions as a function of altitude gave the scale height that was used to determine that the temperature of the upper atmosphere was 350 K.


One of the first spectra obtained by Mariner 6 is shown below.  The altitude of the limb image was 167 km and the sun was directly overhead (Solar Zenith Angle (SZA)).  The spectra scan was from 1900 A to 4000 A.  The intensity of this uncalibrated spectrum is given in data numbers.

All of the high quality spectra were calibrated and averaged for the publication of these first observations of the Mars airglow (Barth, et al., 1971).  This composite spectrum, with the identification of the spectral features, is shown below.

The limb spectra obtained during the fly-bys of Mariners 6 and 7 are available.  There are eight spectra for each spacecraft. They are in two files labeled: Mariner6_N.dat and Mariner7_N.dat.  They are in binary format and may be read using IDL.  A procedure is available that will read these data files.  It is named:  The command line is plot_M6_7_n,m,n.  The m designates the spacecraft: 6 for Mariner 6 and 7 for Mariner 7.  The n designates the number of the spectrum in each file; 1-4 for the first limb crossing and 5-8 for the second.  For example, to plot the spectra shown above, the command line is: plot_M6_7_n,6,6.   Record n=0 is the wavelength.