How fast are we losing our atmosphere?

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.

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2 responses to “How fast are we losing our atmosphere?

  1. Sorry to put a dent in your article, but Venus has an atmosphere that is 93 times thicker then the Earths.
    Two factors that could be contributing to our rate versus Venus though is that most all of the lighter elements that are on Venus have already been stripped away like hydrogen and helium. It’s also believe that the earth is in the process of flipping it’s magnetic poles, during which time the magnetic field is rapidly decreasing in strength until it flips, and then it will build itself back up to it’s full strength.

    • That’s the trouble with quick rants, they tend to be quick and ranty. What I should’ve made clear was that although Venus has a strong surface atmosphere due to volcanic replenishment, the upper atmosphere is depleted with respect to that of the Earth at similar altitudes, this is because of the removal of the elements with larger scale heights (the lighter elements you mentioned, which, fortunately, I did remember to put into the post). However, the strength of the Earth’s magnetic field is now believed (through magnetic records stretching back about a thousand years and higher precision sea floor records going further) to have not yet lost a lot of its strength, although it has moved about a lot and is likely to accelerate loss soon. It should also be noted that the Earth’s exosphere, the point between the upper atmosphere and interplanetary space, extends around fifty times further out than the sunward part of the magnetosphere and around twice the measured distance of the magnetotail, though the change in partial pressure doesn’t have as strong an impact as if the solar wind was scouring further down. Any of this low density material further exposed in the region of the relatively wildly fluctuating magnetopause altitude would have a minimal though increasing impact on downwind measurements at this time. Or to put it more simply, the bulk of the proportion of the material in the atmosphere that is there because the magnetic field protected it from being stripped away is held further down than the balance of solar wind-terrestrial magnetic fields has reached.

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