We Just Discovered The Moon Moves Through Earth's Atmosphere

[TexasTowelie]

Where Earth’s atmosphere merges into outer space, there lingers a cloud of hydrogen atoms called the geocorona that extends beyond the moon. Its full extent was recently revealed in old data taken by the SOHO spacecraft. (Illustration only — not to scale) ESA


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.

Read more: http://astrobob.areavoices.com/2019/03/01/we-just-discovered-the-moon-passes-through-earths-atmosphere/


The Earth and its hydrogen envelope, or geocorona, as seen from the moon. This ultraviolet picture was taken in 1972 by John Young, Apollo 16 commander. NASA

[Igel]

1. Taught my kids about the exosphere last fall.

It was tough going, because usually the atmosphere is defined as ending with the thermosphere if you're focused on the Earth. It's well defined, it's clearly Earth-bound, and it's clearly part of what you need to cover for Earth science. For atmospheric science, not so clearly, but it's there anyway.

But for planetary science there's clearly no good boundary for the atmosphere, so the exosphere gets included.

It's a question of definition: What do you need the word "atmosphere" to include?

Moving the boundary of the exosphere is like saying, "Oh, noes, my water is contaminated with arsenic!" At one point, if the contamination was less than 1 part per 1000, it was clean, but suddenly we can detect one part per 1,000,000 and it's a huge problem. But "my water is clean!" was met with objections at sensitivies of one part per 1,000,000,000, and suddenly that clean water was instant death. Then formerly clean water was lethal at one part per trilliion. "There's an atom of arsenic in my swimming pool, it's a Superfund site!!!" is on the horizon.

It's the same for the "surface" of the Sun. Or placing the (ever arbitrary) edge of the Solar System in the Kuiper Belt. Or the limits of the US at some arbitrary distance by law or based on the continental shelf. Or fudging what "navigable waters" are--are they where you can float a supertanker, a 20-foot boat, a canoe, or a duck? And would you have to engage in significant portage to get that vessel (or duck) from where it's floating to where it could actually get to a sea (except maybe the Salton or Dead or Aral)?