|Is any water vapor escaping the earth's surface
into space over time and is there an estimated
|Question Date: 2001-10-15|
The answer to your question is yes but it is
small. A small amount of H2O breaks
down when attacked by UV radiation... some of the
oxygen recombines as ozone which is
gravitationally bound to earth. However diatomic
hydrogen does in fact achieve escape velocity and
leaves the earth. So while strictly speaking water
does not escape, hydrogen does. A place to read
about this is in the book moons and planets by W.
Hartmann or any other decent planetary science
Some astronomy books have decent
sections on planets; others are too deep sky
oriented and don't discuss planetary atmospheres
Yes, water does escape into space -- there are
several mechanisms, but the most common one is
breakup of water via ultra violet radiation, which
frees a hydrogen atom. This atom is not
gravitationally bound to the earth as hydrogen is
so light that typical thermal energies can exceed
escape velocity relatively easily. As for how
much, that is a really good question, and one I
don't know off hand.
However, the people who
do climate modeling need to be able to model
various gas concentrations and water vapor is very
important, so they must have some sort of
estimates. -- A good source would be writing (or
email) to the U.S. Geological Survey -- their Web
click here --
also look at
click here, too
Sorry I can't
estimate the loss rate for you -- but I am sure
these people can do it.
Water vapor, e.g., the atmosphere, is "held
against" the earth by the earth's gravity. There
may be a trace amount of the atmosphere escaping
over time, but it would not be a significant
amount. In order for a significant amount of
water vapor to escape, it would need to achieve
escape velocity (the same as a rocket) to leave
the earth's atmosphere.
My Resident Science Expert tells me that the
loss should be very small, because of gravity.
Hydrogen and helium are escaping because they are
light and can reach escape velocity - molecules
have some amount of kT worth of energy (maybe 3/2
kT at room temperature), which gives them a
kinetic energy of 1/2 m v2.
Since m, the
mass, is so small for hydrogen and helium, the
velocity is large. But for things like nitrogen,
oxygen, and water, m is larger and v is smaller
and so very few molecules will reach escape
velocity in the upper atmosphere.
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