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Earth’s Atmosphere is Bigger Than We Thought, It Actually Engulfs The Moon

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The boundary between Earth’s atmosphere and space is the Kármán line, the point at 100 kilometres (62 miles) altitude where aeronautics end and astronautics take over.


But Earth’s atmosphere is way more complicated than that. There’s even some debate about where the Kármán line should be. Now a team of astronomers has discovered that it’s bigger than we even thought – extending all the way out to the Moon, and as far again.

The region: Geocorona

The region is called the geocorona, part of an atmospheric layer called the exosphere. It’s a tenuous cloud of neutral hydrogen that glows in far-ultraviolet light.

It’s been hard to measure because it’s so thin, ; previously, its upper limit was thought to be around 200,000 kilometres (124,000 miles) from Earth, because that’s the point at which solar radiation pressure would override Earth’s gravity.


According to data from the Solar and Heliospheric Observatory (SOHO) co-owned by the European Space Agency (ESA) and NASA, that limit doesn’t even come close. The geocorona, scientists have found, extends out to as much as 630,000 kilometres (391,000 miles). Which means it hugs our beloved moon.

Geocorona taken from the Moon by Apollo 16 astronauts. Credit: NASA

In fact, at an average distance of 384,400 kilometres (238,855 miles), it’s almost smack-bang in the middle of it.

An observation over two decades ago

SOHO made these observations over two decades ago, between 1996 and 1998. The data had just been sitting in an archive, waiting for someone to get around to analysing it.

 Illustration of the geocorona
Illustration of the geocorona (not to scale). Credit: ESA

The readings had been taken specifically to map the geocorona using the observatory’s SWAN instrument, a sensitive piece of equipment designed to measure far-ultraviolet emissions from hydrogen atoms, called Lyman-alpha photons.

Why are we unaware of?

Why can’t we see these from Earth? Because they’re absorbed by the inner layers of the atmosphere – so we need instruments out there in space to look for them. Apollo 16 astronauts, for instance, were able to take a photograph of the geocorona in 1972 – not even knowing they were still inside it.

On Earth’s day-side, the hydrogen atoms are compressed by sunlight, resulting in a density of 70 atoms per cubic centimetre, thinning out to 0.2 atoms at lunar orbit (that’s not very dense at all – it’s still effectively a vacuum).

On the night side, the hydrogen density is higher due to solar radiation pressure – it kind of ends up looking a comet tail.

While the hydrogen atoms do scatter ultraviolet radiation, the quantity is negligible, especially compared to the vast amounts being blasted out by the Sun, making space a hazardous radiation environment for astronauts.

What the discovery does mean?

What the discovery does mean is that any space telescopes within the geocorona will likely need to adjust their Lyman-alpha baselines for deep-space observations.

You know what’s even more interesting? That means no human being has ever actually been outside of Earth’s atmosphere. Guess we’ve got some work to do.

Originally written by MICHELLE STARR for ScienceAlert 
Source / Journal Journal of Geophysical Research: Space Physics
Via / Provided by: ScienceAlert

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