The Earth’s atmosphere actually reaches to the Moon — and then some!

The Earth and its hydrogen envelope, or geocorona, as seen from the Moon. This ultraviolet picture was taken in 1972 with a camera operated by Apollo 16 astronauts on the Moon.
Image credits NASA via ESA.

New research says that Earth’s gaseous cover reaches up to 630,000 kilometers away, over 50 times the diameter of the planet. The findings were drawn from observations by the ESA/NASA Solar and Heliospheric Observatory, SOHO.

More is actually more

“The moon flies through Earth’s atmosphere,” says Igor Baliukin of Russia’s Space Research Institute, lead author of the paper presenting the results. “We were not aware of it until we dusted off observations made over two decades ago by the SOHO spacecraft.”

The interface between Earth’s atmosphere and outer space, the place where the two kind-of mix together, is called the geocorona. Starting from data recorded by the SWAN instrument on the SOHO, the team calculated precisely how far away the outskirts of the geocorona are. It’s twice as far as the Moon, they report.

Illustration of the geocorona (not to scale).
Image credits SOHO / ESA.

Apollo 16 astronauts — some of the last people to ever land on the moon — took a telescope up to our planet’s natural satellite which they used to capture an image of Earth’s geocorona, glowing brightly in ultraviolet light. This UV light was produced by the interaction between sunlight and hydrogen atoms. The atoms emit (but also absorb) a very particular wavelength of UV light known as Lyman-alpha when interacting with sunlight. We can’t really see it down here, as Lyman-alpha light is quickly absorbed and dissipated in the atmosphere — but you can see it from outer space.

“At that time, the astronauts on the lunar surface did not know that they were actually embedded in the outskirts of the geocorona,” says Jean-Loup.

SOHO currently orbits much further away from Earth than those astronauts were located — some 1.5 million kilometers away from our planet, towards the sun. This distance offered a good vantage point to observe the geocorona. Furthermore, the SWAN instrument aboard SOHO comes equipped with a hydrogen absorption cell, which the team used to selectively measure Lyman-alpha light from the geocorona and filter out that of hydrogen atoms further away in interplanetary space.

They report that Earth’s geocorona gets smooshed down on its dayside (the one facing the sun), which in turn produces an area of higher hydrogen density on its night side. “Dense” here is a relative term — this isn’t the atmosphere we known and love down at the surface level. Hydrogen densities peak at about 70 atoms per cubic centimeters at roughly 60,000 kilometers above the surface, and dip as low as 0.2 atoms per cubic centimeters around the moon.

“On Earth we would call it vacuum, so this extra source of hydrogen is not significant enough to facilitate space exploration,” says Igor.

“There is also ultraviolet radiation associated to the geocorona, as the hydrogen atoms scatter sunlight in all directions, but the impact on astronauts in lunar orbit would be negligible compared to the main source of radiation – the sun,” says Jean-Loup Bertaux.

SOHO’s SWAN instrument imaged Earth and its extended atmosphere on three occasions between 1996 and 1998. The team decided to retrieve this data from the archives for further analysis — and that’s how this discovery was born.

“Data archived many years ago can often be exploited for new science,” says Bernhard Fleck, ESA SOHO project scientist. “This discovery highlights the value of data collected over 20 years ago and the exceptional performance of SOHO.”

The paper “SWAN/SOHO Lyman‐α mapping: the Hydrogen Geocorona Extends Well Beyond The Moon” has been published in the Journal of Geophysical Research: Space Physics.