Hydrogen restoration

kw: interstellar medium, analysis

Oh, wow, I just read an interesting speculation. It seems we could be in danger of losing our air. The Solar System (SS) is presently in a "local bubble" of extra-thin interstellar gas, but we are moving through it and some time in the future we'll exit into thicker gas. Most of the interstellar medium (ISM) is pretty thin, though it is as much as 100 times as dense as our current surroundings. But there are big clouds of hydrogen out there, "molecular clouds" (MC) that are as much as ten million times as dense as the gas through which the SS is moving. If we pass through a MC, it is thought that lots of hydrogen will be added to the Earth's atmosphere, combine with oxygen to make water, and it could get harder to breathe.

Really? This is worth a bit of examination. Some real numbers:

• More than half of the ISM has a density of 0.2-0.5 atoms or molecules per cubic centimeter (cc). Another 10-20% is 10-20 times as dense as that. Most of those molecules are neutral hydrogen, H₂. From this point we'll just call everything "particles".
• The gas in the local bubble has roughly 0.1 particle/cc (~100,000/cu m).
• The density within MC's ranges from 1,000 to 1 million particles/cc.

What is the density of normal air at sea level? It is a mix of gases, but the standard molar volume at one atmosphere of pressure and at 0°C (32°F) is 22.4 liters and contains 6.022x10²³ particles, which comes to 2.7x10¹⁹/cc. That is between 27 trillion and 27 quadrillion times as dense as a MC.
But, of course, were the SS to move through an MC, the Earth's average velocity relative to the MC would be at least 30 km/s, our orbital velocity. How much gas would we sweep up at that velocity? The Earth's cross section (intercept area) is about 1.3x10¹⁸ cm². The velocity in cm/sec is 3 million, so the volume of MC swept up per second would be 3.8x10²⁴ cc. The number of hydrogen molecules entering the atmosphere would range from 3.8x10²⁷ to 3.8x10³⁰. The mass of hydrogen gathered would then be between 13 and 13,000 kilograms per second.

That sound like a lot. What is its proportion to the whole? The mass of the atmosphere is about 5.3x10¹⁸ kg. Twenty percent is oxygen (just over 10¹⁸ kg). One kilogram of hydrogen combines with 8 kg of oxygen to form 9 kg of water, so the larger figure from the densest MC would remove 104,000 kg of oxygen from the atmosphere each second.There are about 31.56 million seconds in a year, so the oxygen consumed per year would be 3.3 trillion kilograms. Again, it sounds like a lot, but it is 1/320,000th of the available oxygen. Thus we can conclude that, if the SS catches up to a thick MC (or the converse), one percent of our oxygen will be consumed in 3,200 years.

Let that settle in: 1% in 32 centuries. In terms of a human lifetime,it is nothing, but in more "geological" terms, it is significant. The earth loses 3 kg/sec of hydrogen to space, because water in the upper atmosphere is broken apart by UV light, and the hydrogen escapes. Over millions of years, this will cause the oceans to be lost. Even a thin MC would counteract this effect, and delay significantly the eventual loss of our oceans. That is not a bad thing!

When is this likely to happen? The closest MC, in the direction of the constellation Taurus, is 400 light years away. But we are moving "sideways", parallel to it. The nearest MC that we are getting closer to, as well as I have been able to determine, is a bit more than 1,000 light years away. Because the SS and the MC are in similar orbits around the center of the Milky Way, the approach speed is relatively low, at most 10 km/s. At that velocity, it takes 30,000 years to go a light year. It will take 30 million years or so to catch up to that MC. I guess we won't have to worry about the consequences of that for a while!