The atmosphere of Venus, like that of any other planet, is kept in place by gravity. The extremely high temperatures at the surface of Venus cause its atmosphere to expand somewhat, but the atmosphere is already gaseous (about 97% carbon dioxide, with almost no water vapor) and cannot evaporate. Nevertheless, there is a steady slow leakage of the atmosphere to space as atoms (usually in the form of charged ions) are accelerated to escape velocity. The solar wind contributes to this effect, and the total rate of loss from Venus's atmosphere is estimated at about 10^25 ions per second.

To put this number in perspective, just over 6 x 10^22 (Avogadro's number) CO2 molecules, with a total of 1.2 x 10^23 atoms which can be ionized, weigh a total of about 14 grams. If the 10^25 lost ions per second came entirely from the dissociation of CO2, the total weight would be a bit more than 1 kilogram. This corresponds to a mass loss rate of about 3.5 x 10^6 kilograms per year. The total mass of Venus's atmosphere is about 4.8 x 10^20 kilograms, so at the current rate of loss it would take about 1.4 x 10^12 years for Venus to lose 1% of its atmosphere.
Source(s):
http://www.aip.org/history/climate/Venus.htm
http://www.lpi.usra.edu/meetings/lpsc2009/pdf/1408.pdf
http://en.wikipedia.org/wiki/Atmosphere_of_Venus

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