via Discovery News.
The Earth is losing atmosphere at a rate of 5 X 10^25 (50,000,000,000,000,000,000,000,000) molecules per second. Quite a large number, though not large enough to put us in any immediate danger. Christopher Russell, a professor of geophysics and space physics at the University of California, Los Angeles measured the rate and discussed it with colleagues present at the American Geophysical Union conference – the largest international comparative planetology conference in the calender. Their discussions led to the realisation that the loss rate was greater than either Mars or Venus. These two planets have very little magnetic shielding from the solar wind, which can scour off material from atmospheres. So what is going on?
Well, in my uninformed view, this may just be the economies of scale going on. The Earth has a very large and thick atmosphere, so losing a tiny fraction of it leads to quite large numbers, larger than the comparitively vacuum like Venus and Mars. This can affect atmospheric loss in two ways – partial pressure and availability for scouring.
In terms of partial pressure, to put it simply, if a room is half-filled with oxygen and half with nitrogen, they will independently shuffle about until the entire room will have an equal oxygen-nitrogen mix. This is because as well as overall pressure, individual gas constituents like to have their contribution or partial pressure to be equalised. In the case of an atmosphere, the gases are pushing outward towards their abundances in space. They don’t all push at the same rate as gravity pulls back and reduces the incentive to go, so as gases get closer to the surface they can have a larger stable reduction in pressure with altitude. In the case of Mars and Venus, they have lost the majority of their main constituents and are left with a more stable fall off in overall pressure. Additionally, the lighter elements (able to escape gravity) have run off faster than the heavier elements, so what is left clings harder to the planets.
In terms of scouring area, the Earth is a lot bigger than Mars and a bit bigger than Venus. The atmosphere extends beyond its surface, therefore presenting a bigger disc to the solar wind than that of Venus or Mars (even if all the atmospheres were of identical sizes). Earth’s atmosphere is thicker at the surface and so remains thick and relatively easy to strike far farther out than those of Mars or Venus do. This makes the atmosphere ripe for scouring and all the magnetic shielding in the world (as it happens) won’t completely negate that. In addition, the present warm period created by global warming as well as the localised heating of the aurora (created by the magnetic shielding) can up the loss rate.
As was once said about the Sun’s corona when the idea of the solar wind (the hot gas escaping radially from the Sun) was first conceived through thermodynamic considerations “The Solar Corona won’t be contained” – and neither will the atmosphere, until it starts to run out, then it will fall behind Venus and Mars in terms of loss rate.