A comparative study of Venus and Mars: upper atmospheres, ionospheres and solar wind interactions

Both Venus and Mars have extended atmospheres with CO2 dominating in the lower thermosphere and O at higher altitudes. Although O is depleted at Mars in the lower thermosphere, the low gravity can make O to be larger at Mars than at Venus for altitudes above about 250 Km. Hot neutrals exist both at Venus and Mars, but the densities are large enough at Venus alone to play a significant role in the solar wind interaction. The venus atmosphere is found to be very insensitive to solar EUV variability, with the exospheric temperature changing only by about 60 K over a solar cycle. In contrast, the exospheric temperatures at Mars, deduced mainly from plasma scale heights, indicate a variation from 150 K to 400 K over a solar cycle. However, no direct measurements of the neutral atmosphere over a solar cycle exist to make a definitive statement. On Venus the major ion is O+ in the upper ionosphere, while O2+ dominates in most part of the Mars ionosphere. During solar minimum, when the ionosphere is weak, the solar wind compresses the Venus ionosphere to its limiting altitude in the photodynamical regime, forming a thick ionopause. This thick ionopause is also seen at times of high Psw during solar maximum. Mars ionosphere is weak and its ionopause is also observed to be thick, similar to the one observed at Venus. This provides evidence for direct interaction of solar wind with the ionosphere of Mars suggesting that Mars does not have an intrinsic magnetic field of any consequence. However, ionospheric outflows from Mars, similar to those on Earth have been observed on Phobos-2 which have been interpreted as an evidence for Mars intrinsic magnetic field. (author). 186 refs., 17 figs