Note (12/2015): Hi there! I'm taking some time off here to focus on other projects for a bit. As of October 2016, those other projects include a science book series for kids titled Things That Make You Go Yuck! -- available at Barnes and Noble, Amazon and (hopefully) a bookstore near you!

Co-author Jenn Dlugos and I are also doing some extremely ridiculous things over at Drinkstorm Studios, including our award-winning webseries, Magicland.

There are also a full 100 posts right here in the archives, and feel free to drop me a line at with comments, suggestions or wacky cold fusion ideas. Cheers!

· Categories: Astronomy, Physics
What I’ve Learned:

Exosphere: It's where the innie space becomes an outie.
“Exosphere: It’s where the innie space becomes an outie.”

The earth’s atmosphere is sort of like a family. Everybody stays pretty close… but some stay closer than others. So the troposphere, the nearest layer, is like that aunt who comes over every weekend you can’t get rid of. The mesosphere, you maybe see at holidays, and it sends you stupid-looking sweaters for Christmas.

Or pictures of itself in stupid-looking sweaters. The mesosphere’s side of the family was always a little weird.

But the exosphere? No. The exosphere is the black sheep of the atmospheric family — and it likes it that way. You might get the exosphere to RSVP “NO!” to a family reunion, but that’s about it.

In planetary (as opposed to familial) terms, the exosphere is the outermost layer of influence, where individual molecules are still bound by gravity, but there’s nothing you could call an “atmosphere” to be found. Around Earth, the exosphere contains hydrogen, with a little bit of helium, carbon dioxide and oxygen flitting around. But not in a crowded way. It’s less “Times Square at rush hour”, and more “fans at a Miami Marlins baseball game”.

The spot where the exosphere begins is called the thermopause. That might sound like a fancy name for “hot flashes” — and if you happen to be a planet, that’s not too far from the truth. The thermopause marks the boundary of the Earth’s energy system, and the exosphere doesn’t have enough molecular oomph to be part of that.

Of course, the planet has good and bad days, just like the rest of us. So on a high-energy day — maybe it’s summer, or the sun is flaring, or Earth got its ass out of bed early for yoga — the exosphere might start six hundred miles or more above the surface. On days with less energy — Sundays, I’m assuming, and the day after Thanksgiving — the exosphere might start half that high, around three hundred miles up. Some days it’s just harder to push yourself up against outer space, you know?

Where the exosphere ends depends on how you feel about outer space. Or at least how you feel about defining it. If you prefer your outer space to start where the radiation from earth ends — light, heat, the glow from Ryan Seacrest’s front teeth, all of that — then your exosphere ends six thousand miles above the earth, give or take a few hundred miles.

If you want to be all technical about it, and consider where the gravitational pull of the earth on atoms of hydrogen gives way to radiation pressure from the sun, letting those atoms escape out into the ether, that’s a different story. That happens a wee bit further out — say, a hundred and twenty thousand miles up the chute. Or halfway to the moon, if we’re getting other celestial objects involved now.

Speaking of which, other globs of space rock — including our moon — have exospheres, though at lot of them don’t have much of anything else. So an exosphere is sort of the bare minimum possible, in lieu of something more substantial.

Kind of like Ryan Seacrest, apart from his teeth — or the stands at a Marlins game. Neat.

Actual Science:
Universe TodayExosphere
University Cooperation for Atmospheric ResearchExosphere – overview
Science DailyHow the moon gets its exosphere
CBS NewsNASA moon mission targets lunar dust, “exosphere”
University College LondonDione’s thin oxygen exosphere

Image sources: Surfline (atmospheric layers), (Christmas sweater), Buzzfeed (Marlins fans [both of them]), The Richest (Tom Cruise’s shiny, angry, shiny teeth)

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