Geo-Corona

Space between the planets looks crystal clear to the unscientific eye. But there is indirect evidence that a very thin gas pervades at least some parts of it. Scientists have argued for years about this tenuous stuff: one theory holds that interplanetary space is filled with "resident" gas that has nothing to do with the planets; another claims that the outer fringe of the sun's glowing corona sometimes reaches out as far as the earth's orbit. The issue remained in doubt for the simple reason that no one had actually sampled interplanetary space, but in Britain's New Scientist Professor Josif Shklovsky of Moscow's State Astronomical Institute tells how Soviet space probes have measured the gas directly.

The Russian space scientists reasoned that any gas outside the earth's atmosphere would be ionized--broken into electrically charged particles--by the sun's radiation. So they furnished their space probes with ion traps: simple instruments that give electrical signals whenever a charged particle hits them. The Soviet moon probe launched on Sept. 12, 1959 carried four traps adjusted to respond to ions of different energies, and it telemetered 12,000 measurements back to the earth.

After Professor Shklovsky and his team of astrophysicists analyzed the data, they concluded that the earth has a "geo-corona" of very thin ionized gas that extends out about 14,000 miles. Beyond 15,000 miles the Russians found no measurable ions, and Shklovsky believes that true interplanetary space has little or no resident gas. One possibility is that the streams of high-energy particles that shoot out of the sun (and probably cause the earth's Van Allen radiation belt) sweep the solar system clean of any gas that leaks into it.

The gas in the earth's corona, Astrophysicist Shklovsky reasons, is mostly hydrogen which came originally from the earth's oceans. Water vapor works its way up from the lower atmosphere. When it reaches about 60 miles, its molecules are broken into oxygen and hydrogen by solar radiation. The hydrogen, being lighter, tends to rise, and above about 1,000 miles it becomes the main constituent of the atmosphere. Some of its molecules get hot enough and move fast enough to reach escape velocity and leave the earth entirely. Moscow's Professor Shklovsky believes that enough hydrogen has escaped in this way to lower the level of the earth's oceans by several yards during the long span of geological time.

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