subsequent contraction of the thermosphere back to original levels weeks later. Comprehensive thermospheric general circulation models are now in existence (see Figure 10.1), and they are quite successful in accounting for the overall MGS temperature observations.¹⁰ However, the dynamical and radiative processes that drive the Mars lower and upper atmospheres on the short time scales corresponding to short-lived dust storms are yet to be explained, and of course no direct information on the winds is available.

The only in situ measurements of the thermal plasma composition, density, and temperature in the ionosphere of Mars were obtained by the retarding potential analyzers carried aboard the two Viking landers¹¹ and by the mass spectrometers mentioned above. The retarding potential analyzers provided two vertical profiles of the densities of the three most abundant ions (O₂⁺, O⁺, and CO₂⁺) in the altitude region of about 120 to 300 km. The observations confirmed theoretical suggestions that the most abundant ion is O₂⁺; at first this seemed a surprising result because of the practically total absence of neutral molecular oxygen in the upper atmosphere of Mars. This finding demonstrates the importance of ion chemistry in ionospheres. The retarding potential analyzers also provided information on the ion and electron temperatures, but in a limited altitude range and along only two profiles.¹² These temperatures were found to be a few thousand degrees, which cannot be explained by extreme ultraviolet

FIGURE 10.1 Results of model calculations of upper-atmosphere temperatures and winds at Mars, in a plot of latitude versus local solar time (LST), for solar maximum and northern summer conditions at ~200-km altitude. The isotherms shown are in 10-K intervals; superimposed arrows represent the magnitude and direction of the neutral winds. The winds diverge from midafternoon and converge after dusk or before dawn. Neutral temperatures reach 321 K (dayside, LST = 16) and decline to 111 K (south polar night). Maximum winds reach 326 m/s across the terminators and near the poles. SOURCE: S.W. Bougher, S. Engel, R.G. Roble, and B. Foster, “Comparative Terrestrial Planet Thermospheres: 3. Solar Cycle Variation of Global Structure and Winds at Solstices,” Journal of Geophysical Research 105:17669-17692, 2000. Copyright 2000 by the American Geophysical Union. Reproduced by permission of AGU.