Expert: Courtney Seligman - 5/13/2011

Question
Dear Courtney,

I've been trying to get information about Venus' gross atmospheric loss rate.  Specifically, how much of its atmosphere is lost to space each year from all factors (solar wind, etc.) assuming no atmospheric replenishment (volcanic out-gassing, etc.).

I'm assuming this rate must be greater than that of Earth because of its closer proximity to the Sun, as the two planets have similar gravitational abilities to hold an atmosphere.

Although not a scientist, I'm working on a research project to determine how much volcanism would be required to maintain the current atmospheric pressure on Venus over time.

Can you tell me what this value is, or direct me to a site where I might find it?  Also, I would appreciate your thoughts on this topic.

Thank you so much,
Bob

Answer
The rate of loss of low mass atoms, such as hydrogen, is accelerated by the effects of the solar wind. The best estimate I can find of the increase involved is about 30% of the (normal) thermal loss rate, which depletes hydrogen compounds in the upper atmosphere (and by the expansion of such compounds from the lower atmosphere, eventually all of the hydrogen compounds throughout the atmosphere) over time scales which are short compared to geological time. But for heavier atoms, such as nitrogen and oxygen, neither solar wind nor thermal effects are believed to have any significance.

In other words, hydrogen and hydrogen compounds such as water vapor should be lost in periods of a few hundreds of thousands or millions of years. Heavier gases and their compounds should not be lost, despite Venus' closer proximity to the Sun, and its lack of a magnetic field.

As far as how much volcanism would be required to replace the lost gases, obviously none, for anything other than hydrogen compounds. For those, active volcanism would increase the water vapor and other hydrogen compounds in the lower atmosphere, and over a period of time, decrease the acidity of the clouds, which sequester most of the water vapor in the atmosphere. That would allow an increased outward flow of hydrogen gases and compounds into the upper atmosphere, which at equilibrium would be equal to the rate of release at the surface. Right now, both values are close to zero. If the planet became very volcanic again, the release of gases into the atmosphere would increase, gradually increasing the amount of water in the cloudes, and eventually resulting in an increased rate of loss in the outer atmosphere. Any amount of increased volcanism would increase the water and water vapor concentration, but only on a temporary basis. If massive volcanic activity took place, there might be a noticeable change in the amount of water vapor over long (compared to human lifetimes) periods of times, but it would persist for only as long as the volcanic activity, and things would return to "normal" within geologically short periods, once the volcanic activity ceased.

So there really is no specific answer to your question. None of the heavier gases are being or would be lost, regardless of the rate of volcanism, and although the current "balance" of hydrogen compounds would increase if volcanism increased, it would not prevent the continual loss of hydrogen, and the balance would return to minimal values once the volcanism ceased (or at least, within a geologically short time afterward).