Three-quarters of Earth’s atmosphere sits within the bottom 6.8 miles (11 km). At the space station’s altitude of 250 miles (400 km), there’s next-to-no air. But the faint breath of Earth continues into space as the geocorona or exosphere, the outermost layer of the atmosphere. Older estimates put its size at around 1.5 Earth radii or 12,000 miles out in space. But a recent discovery based on observations by the Solar and Heliospheric Observatory (SOHO) shows that it goes much, much further — all the way to the moon and beyond!
Scientists weren’t aware of how big it was until they took a look back at older observations made by the orbiting solar observatory. One of the spacecraft’s instruments picked up the signature of hydrogen up to 391,000 miles (630,000 km) above Earth’s surface or 50 times the diameter of the the planet. The geocorona is the extreme extent of Earth’s atmosphere and composed of hydrogen atoms. They’re very sparse with about 70 atoms per cubic centimeter (about the size of a fresh pencil eraser) at 37,000 miles (60,000 km) above Earth’s surface, and only about 0.2 atoms at the moon’s distance. On Earth, we’d call either place a vacuum.
Apollo 16 astronauts used the first telescope ever taken to the moon to capture the first photograph of the geocorona using a camera sensitive to ultraviolet (UV) light. Their images show a modest glow around the Earth. Little did they know at the time that they were actually within in the geocorona as they stood on the moon.
You can only see the geocorona from space because it shines in extreme ultraviolet light which is both absorbed by Earth’s atmosphere and invisible to the eye. With no atmosphere, the moon was perfect for UV photography. Besides the geocorona the astronauts took photos of stars, nebulae and galaxies with the instrument from April 21-23, 1972.
The new study also revealed that sunlight compresses hydrogen atoms in the geocorona on Earth’s dayside, and also produces a region of enhanced density on the night side. The source of the hydrogen is none other than the “H” in good old H2O and comes primarily from the oceans. Water molecules released into the atmosphere are broken apart by the sun’s ultraviolet light into hydrogen and oxygen. Hydrogen is the lightest of the elements and escapes into space to create the corona. Hydrogen coronas also surround Mars and Venus, but they’re nowhere near as extensive as the Earth’s because those planets have far less water.
What I find truly fascinating about coronas is that they can be used to indicate the presence of water on planets around other stars. A large hydrogen corona could point to a massive ocean, making it a good place to look for potential signs of life. Hydrogen coronas seem like such nothing things, yet they could make one of the best tools ever for discovering possible habitable planets.