Mercury A to Z: Exosphere

Hello, friends!  Welcome back to this year’s A to Z Challenge.  My theme for this year’s challenge is the planet Mercury, and in today’s post E is for:


When I was preparing for this A to Z series on Mercury, a friend and I were joking that I should do “atmosphere” for the letter A.  The body of the post would simply say: “There isn’t one.”  And that would be the end of it.  But that wouldn’t be 100% true, and it wouldn’t be fair to poor, little Mercury.  Mercury does, in fact, have an atmosphere.  An extremely thin atmosphere, so thin it’s almost nonexistent.  But it is not entirely nonexistent.

Scientists usually refer to Mercury’s atmosphere as an “exosphere” to help distinguish it the thicker, heavier air layer that the word atmosphere traditionally implies.  Mercury’s exosphere is made of a little hydrogen, a little helium… there’s a little oxygen and a little sodium… a little potassium… a little calcium… there’s a little of a lot of different things, which adds up to not very much.

The hydrogen and helium presumably come from the Sun.  As the solar wind washes over the planet, hydrogen and helium atoms get tangled up in Mercury’s magnetic field and end up being incorporated (temporarily) into Mercury’s exosphere.  Some of the helium may also come from the radioactive decay of elements like uranium in Mercury’s crust.  As for the oxygen, sodium, and everything else, that stuff probably outgasses from the planet’s interior.  When Mercury formed, certain gases were trapped inside, and those gases have been very slowly leaking out of the planet ever since.  This outgassing process may help explain why Mercury appears to be shrinking (but we’ll talk about that in a future post).

But any gas you might find in Mercury’s exosphere is only there temporarily.  Mercury’s low gravity, plus the intense heat of the Sun, plus the constant pressure of the solar wind “blowing” on the planet, mean that Mercury’s exosphere is constantly blowing off into space.  Just as quickly as Mercury can gain a few atoms worth of atmosphere, he’ll lose them again.  In fact, as you can see in the totally legit Hubble image below, Mercury has a very faint comet-like tail of atmospheric gases, billowing off into space.

Cartoon image of Mercury, singing "You Take My Breath Away" to the Sun, while Mercury's atmospheric gasses blow off into space as a comet-like tail.

Just kidding.  That’s not really a Hubble image.  The Hubble Space Telescope has never observed Mercury.  Due to Mercury’s proximity to the Sun, trying to image Mercury would run the risk of burning out Hubble’s optics.  Some other space telescope must have taken that picture.


Today, I want to recommend a book simply titled Mercury, by William Sheehan.  It’s part of a series of books on the Solar System called Kosmos.  I’ve read a few of these Kosmos books now, and they are all wonderful.  Finding a book about one specific planet can be difficult (unless that planet is Mars), so if there’s a specific planet you want to learn more about, I highly recommend checking out the Kosmos series.

Also, if you want to see a for real picture (a for real for real picture) of Mercury’s comet-like tail, click here.

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