Earth's Atmosphere Extends Far Beyond the Moon, Scientists Discover

Scientists have just discovered Earth's atmosphere extends well beyond the moon, reaching out almost 400,000 miles into space. Using data from over 20 years ago, a team of researchers measured just how far the outermost part of the atmosphere goes—a region of space known as the geocorona.

Earth's atmosphere is made up of layers of gas surrounding the planet and held in place by gravity. The different layers are known as the troposphere, the stratosphere, the mesosphere, the thermosphere and the exosphere. The geocorona—where the atmosphere merges with outer space—is a cloud of hydrogen atoms.

In a study published in the Journal of Geophysical Research: Space Physics, researchers led by Igor Baliukin, of Russia's Space Research Institute, found the geocorona extends twice the distance of the lunar orbit. "The Moon flies through Earth's atmosphere," he said in a statement. "We were not aware of it until we dusted off observations made over two decades ago by the SOHO spacecraft."

The hydrogen cloud was mapped in the 1990s by NASA and the European Space Agency using the Solar and Heliospheric Observatory (SOHO) instrument. The initial aim of SOHO was to study the Sun's deep core, outer corona and solar wind. It orbits Earth at a distance of almost a million miles—that's around three times the distance between us and the Moon.

Earth atmosphere
Image showing Earth's geocorona and the SOHO instrument. ESA

Baliukin and colleagues decided to retrieve old data from SOHO to find out if it could provide a more detailed view of Earth's atmosphere.

Hydrogen atoms and the Sun interact in such a way that it can only be observed from space—the light emitted is absorbed by Earth's atmosphere, so cannot be observed terrestrially. Because of its distant orbit, SOHO was able to measure the light being emitted in the geocorona to find out where the hydrogen cloud ended. Findings showed the geocorona extends over 390,000 miles.

Geocorona from Moon
The geocorona seen from the Moon. NASA

The discovery means that the Apollo astronauts who visited the Moon in 1972, to place the first telescope on the lunar surface, captured an image of the geocorona without knowing they were still inside Earth's atmosphere. "At that time, the astronauts on the lunar surface did not know that they were actually embedded in the outskirts of the geocorona," study co-author Jean-Loup Bertaux explained.

The researchers say the findings will be important for understanding the atmospheres of other planets—especially ones beyond the solar system where the presence of water is promising.