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Laboratory for Atmospheric and Space Physics

First Observations of Atomic Hydrogen

Atomic hydrogen escaping from the Mars upper atmosphere was observed in the short wavelength channel of the Mariner spectrometer starting at a distance of 24,000 km from the center of Mars continuing until the tangent limb reached the surface.  The short wavelength channel scanned from 1100 A to 2100 A in the second order.  A spectrum obtained when the tangent limb was 190 km above the surface is shown below.  This spectrum is the average of four limb crossings, two each from Mariner 6 and Mariner 7.  The second order wavelength scale shows the Lyman alpha emission at 1216 A.  Lyman alpha also appears in the third order at an apparent wavelength of 1824 A on the wavelength scale.  The atomic oxygen line at 1304 A is also present.

The intensity of the Lyman alpha emission as a function of planetocentric distance is shown in the figure below which is taken from the published paper that first reported the results of the Mariner 6 and 7 fly-bys [Barth et al., 1971].  These data were analyzed using radiative transport theory and a model of planetary exospheres [Anderson and Hord, 1971].  The results indicated that the temperature at the base of the exosphere (250 km) was 350° K and the atomic hydrogen density was 3 x 10^4 atoms / cm^3.

The short wavelength limb spectra obtained during the fly-bys of Mariners 6 and 7 are also available for downloading.  There are eight spectra for each spacecraft, for each channel. They are in two files labeled  : Mariner6_G.dat and Mariner7_G.dat which are the G channel data.  There are two more  N channel data files:  Mariner6_N.dat and Mariner7_N.dat   They are in binary format and may be read using IDL.  A set of  procedures are available that will read these data files.  They are named: (G channel), (N channel).  The command line is plot_M6_7_g,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.  Record n=0 is wavelength.