Venus Unveiled

Venus has an Earthlike mass and surface gravity, the latter being 8.9 meters per second per second, only slightly less than Earth’s. Venus is in a very nearly circular orbit about the Sun with orbital period (year) equal to 224.65 Earth days. Venus rotates much more slowly than Earth, however, and this has many consequences for the atmospheric dynamics, since it greatly reduces the Coriolis accelerations that do so much to organize Earth’s large scale atmospheric circulations. In fact, the rotation of the planet is retrograde — i.e. opposite in direction to the rotation of the orbit. The siderial day on Venus — the period with which the star patterns would repeat, if you could see the stars from the surface — is 243 Earth days, but since this is in the retrograde direction, it adds to the angular velocity of the planet relative to the Sun. Thus, the rate of rotation relative to the Sun is 1/224.65 + 1/243 rotations per Earth day, leading to a solar day of 1/(1/224.65 + 1/243), or 116.7 Earth days. This is the time between sunrises, as would be seen from the planet’s surface. Thus, Venus’ solar day is roughly half of its year, as illustrated in the sketch below (credit K. Fuller).

You might think that the long day would result in the dayside heating to extreme temperatures while the darkness-plunged nightside plummeted to relatively frigid values. In fact, because of the dense 92 bar atmosphere, it takes a very long time for most of the atmosphere to heat up or cool down, and there is little day/night variation over most of the depth of the atmosphere. Higher up, however, there is a diurnal and seasonal cycle, as illustrated by the black vs. green lines in the accompanying sketch — Venus Express in fact found indications that the diurnal cycle extended deeper into the atmosphere than this traditional sketch suggests, with significant temperature variations penetrating to 45 km. altitude. The atmosphere of Venus is nearly pure carbon dioxide, with a few percent of nitrogen thrown in. It also contains traces of water vapor, which though tiny, contribute significantly to the greenhouse effect of the atmosphere.Most of the greenhouse effect comes from the carbon dioxide, however, which by itself is sufficient to raise the surface temperature most of the way toward its observed value of around 470C. A key feature of the atmosphere of Venus is the sulfuric acid cloud deck. These clouds account for the high reflectivity of Venus, but because they also reflect infrared back to the surface (unlike water clouds, which absorb and emit), they have a warming effect as well, and constitute the second most important factor in the greenhouse effect of Venus after carbon dioxide. Radiation model calculations demonstrate that the clouds have a pronounced net cooling effect on the planet, when both factors are taken into account. The cloud deck comes from combinations of sulfur dioxide with water, but the nature of the sulfur cycle allowing the cloud deck to be maintained is currently a matter of considerable uncertainty.

The VEX instruments

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