This week's #AstroFriday question was submitted by Chuck Dugan of Arizona Project WET.
Chuck: During many of my lessons to my Pinal County fourth grades, I teach that (generally) Earth is a CLOSED system as far as water is concerned. We do not 'leak water' into space, and there are no 'water trucks' delivering new water to our planet! That, in general, the amount of TOTAL WATER we have on the planet is the SAME AMOUNT we've always had; the water we drink today is the same water that dinosaurs were doing what dinosaurs do in their water tens of millions of years ago. Drought is a much more complicated issue related to PERCENTAGE of FRESH and ACCESSIBLE water available to an expanding population in changing weather and planetary conditions. Yet, we hear of planets like Mars, where there WAS water and an atmosphere millions or even billions of years ago. On Mars, much of the atmosphere has been lost, and the water has largely 'gone to ground', or been lost to space over millennia. My question then is...Is there a process by which Earth is actually LOSING WATER to space? If so, can it be measured and tracked? Given our planet's mass, position in the solar system, and with its protective atmosphere, I'm not aware that we are losing water to outer space, but I'd like to confirm this hypothesis.
Astronaut Ricky Arnold: You are correct. Earth is not losing water at a meaningful rate because Earth, unlike Mars, has a cold trap in the stratosphere that freezes out most water and keeps it closer to the Earth's surface. Additionally, higher up in the atmosphere there is an ozone layer that blocks ultraviolet radiation that would otherwise split up water molecules into hydrogen and oxygen, allowing the very light hydrogen to escape. Finally, the Earth's higher escape velocity (due to greater mass and thus gravity) helps it hold on to all of its atmospheric constituents better than Mars.
Q&A in Pictures