Sunday, 19 February 2012

Ions are the 99%

Remember what you were taught in chemistry at school? It was very exact. Water is solid at 0°C, gas at 100°C, and liquid in between. Metals are solids; oxygen and nitrogen are gases. Gases like oxygen, nitrogen, hydrogen and chlorine are invariably molecules containing two atoms of each element. Atoms like their outer "shells" to be "full" and join up with other atoms to reach this stability. Oxygen is electronegative: it likes to grab electrons. You never hear of an oxygen giving up its electrons to some other type of atom.

But there is nothing universal about this.

These are what, by A level at any rate, we call "standard conditions" - 25°C, Earth's atmospheric pressure, Earth's gravity, and with all the protections of Earth's atmosphere from the violent radiation from space.

Of course, we learn that it's not like that everywhere. The pressure at the bottom of the sea, for instance, is intense - "if you went there, you'd end up the size of a chip," my Chemistry teacher told us when we were about 13. And up at the top of the atmosphere, in the ozone layer, you hear about photodissociation and ultraviolet light (the type that is dangerous if you're out in it too long) snapping ordinary two-atom oxygen molecules in half, leading to each single oxygen atom joining up with a normal two-atom pair to make a three-atom molecule of ozone.

But all this is just on our little planet Earth: a tiny, tiny place in our great Universe.

(The Pale Blue Dot.)

What about on other planets? Well, we know the gas giants, such as Jupiter, are made largely of hydrogen. Jupiter is almost 318 times more massive than Earth, though less dense; and its gravity is gigantic. It is therefore hypothesised that hydrogen in its core is likely to be solid, behaving like a metal. (If you look on the Periodic Table of the elements you'll see that hydrogen, H, actually rests right above lithium, sodium, potassium etc. - due to having one electron in the outermost "shell"; and arrangements in common like this create characteristics in common.)

(Jupiter from NASA/JPL/Cassini's Photojournal. The Great Red Spot is on the right. On the left is a black circle - it's the moon Europa's shadow. It's worth zooming in!)

But Earth and Jupiter are planets. That means they're compact, cold and - in our cases; not in the case of Mercury, for example - there is a protective atmosphere. Off a planet, this stabilising gentleness is gone.

In space, you often get one atom - or fewer - knocking about in every cubic centimetre (it varies, of course, for example whether you're near a star or in a nebula or inside or outside a galaxy, etc. etc. In the meantime, you might enjoy this little bulletin of interstellar medium facts, from a lecture in Ohio). At sea level, the "standard conditions" on Earth, you get 100,00,000,000,000,000,000 or so. Marcus Chown likes to remind us that atoms are so numerous that every breath that you take will contain an atom breathed in by Marilyn Monroe!

This of course makes it pretty easy for molecules to find and bond with each other. In space, to be able to do this is very rare.

Apart from on nice cool compact places like planets, the only places you're likely to find actual molecules are inside nebulae. It wasn't until last August that it was announced that molecular oxygen - the type of oxygen we breathe in - was discovered in space. Molecular hydrogen of course is better known, and carbon monoxide - the same type of carbon monoxide that is poisonous - is a good "tracer". That means that it's easy to find by its spectrum, so astronomers look for it as an indication of what else is going on around the place.

There is, according to the APOD I nicked it from anyway, molecular gas here. It's been able to form molecules because - although even though those dark "pillars", similar to the marvellous "Pillars of Creation", are devastatingly empty and thin compared to what we know - the environment is dense enough to block out a lot of light. ("Light" is a loose term for what stars give out. You've probably heard of ultra-violet radiation damaging your skin. That's the same type of thing as light, but it's a shorter wavelength we can see. Shorter still are X-rays. Hot stars and energetic environments give those out too. Longer include microwaves, infra-red, etc.) This does two things. Firstly, it allows the gas to cool and condense. Secondly, a lot of light in space (electromagnetic radiation) is "ionizing": it knocks the electrons' atoms right off!

99% of atoms in space are ions. Lone electrons, or (as "ion" usually means) a charged nucleus - a proton if it's hydrogen, or a ball of protons and neutrons if it's anything else. Some of these nuclei may retain some of their electrons. This completely changes their properties - they become much more affected by electric and magnetic fields, for example.

Stars are almost all ions - unless they're incredibly cool stars. So is most of the interstellar medium. All that radiation flying around is no match for poor lonely atoms. They might find an electron and combine with it, but chances are it'll be knocked off again before too long.

And this is the norm. The orderly, neutral molecules that make up the Earth we know behave as only 1% of the matter in the Universe behaves. The upper atmosphere is full of ions that bear the brunt of the stronger radiation from the Sun. By having their electrons knocked off, they absorb the energy and let the rest of the planet go relatively unmolested!

This ionization is what we noticed going on when we discovered the "Pea" galaxies: that oxygen, that really electronegative atom that loved electrons, was present and getting two electrons knocked off. (Of course there was a great deal more hydrogen, but oxygen shows up better in the spectrum.) This happens pretty frequently in space, of course, but things were really firing up in those peas!

One of the units I'm studying this semester is called "Astrophysical Plasmas". You'll have heard that matter is a solid, a liquid or a gas. If they taught you much science, you'll have heard of the fourth state: plasma. Plasma, as you've probably guessed by now, is the state of matter when some or all of the atoms' electrons have been torn off, whether by radiation or electricity or intense heat. It behaves like a gas - even in the centres of stars where it is millions of times denser than any environment you get on earth. Most of the Sun is a plasma, as is the solar wind that triggers the Aurora.

You'll have seen gorgeous pictures of the Aurora from the ground, for example this lovely picture from Alaska Photographics - and also this footage of the Aurora in real-time from the Bad Astronomer is breathtaking to watch.

Looking at it from space, you can see that it's going on very high up in the atmosphere . . .

. . . and, in fact, that the Earth's magnetic field direct the charged particles from the solar wind to form rings around the poles (in this case, the South pole - the Aurora Australis).

The Aurora is caused when charged particles strike oxygen and nitrogen in the magnetosphere of the Earth's upper atmosphere. The molecules don't zoom around in those dancing curtains. It's different areas being struck at different times - like the light from a torch moves around when you point the torch in different directions.

What happens is ionization, or excitation of an electron - the same mechanism, but without enough energy to actually kick the electron free! Two charged particles from the Sun strike, say, two nitrogen atoms. One loses its electron altogether - and becomes an ion, one of the "99%". The other's electron gets "excited", into a higher energy state, but doesn't actually lose the electron. Later, the nitrogen ion finds an electron (maybe the one it had before, maybe another) to recombine with. This releases energy in the form of blue light. The other nitrogen atom's electron also falls back into a lower-energy state, releasing red light. (This process is described here.)

The oddness of ions doesn't stop there. Our lecturer gave us the example of a gyroscope: that when you push it forwards, it will move left or right; and charged particles can behave in this counterintuitive way, too. (We write about parallel and perpendicular vectors quite a lot in Astrophysical Plasmas - and, if you don't mind, I'm not going to go into that here, for I may well make a fool of myself.)

I mentioned earlier that the properties change and that magnetic fields have an effect on them. Ions in a magnetic field will gyrate as if they are sliding along a spring: round and round (in opposite ways depending on their charge!), and along, sometimes at right angles to forces acting on them. And sometimes they will reach a point in the magnetic field where they are "mirrored" - it is as if they hit a brick wall and are bounced straight back in the other direction.

And this is what helps create the van Allen belts around our little blue planet.

The van Allen belts, although - like the Earth's upper atmosphere - help protect us from solar radiation, are dangerous areas spacecraft have to watch out for. They are lobes of ions from the solar wind and our own atmosphere that are held in place by the Earth's magnetic fields. Some ions travel along in a banana-shaped object from the North Pole to the South, and vice versa - because when they stray too near the pole, the Earth's magnetic field lines become closer and closer together, and eventually this causes the "mirroring" of the particle - and it will zoom back off in the banana-shaped orbit. It's a bit like a skateboarder on one of those amazing curved platforms in the park, who doesn't go quite fast enough to get to the top and rushes back down again. He speeds up as he reaches the bottom and zooms his way back up the other end - but slows again towards the top. It's a constant motion, like a pendulum; potential and kinetic energy swap places again and again as the particle goes back and forth.

Charges make particles do very strange things.


HenstridgeSJ said...

Very interesting. Thank you!

Rebecca said...

Interesting post :) especially for a noob like myself!

midali said...
This comment has been removed by the author.
Alice said...

Thanks everyone! I wasn't very satisfied with this post - it didn't hang together well - and I guess I wasn't really separating out the "you need to know this to get this" bits from the "this is what happens" bits, so it seemed a mess. Perhaps this is because I wrote it over two separate nights.

I'm still trying to work out what on earth has happened to the font, which seems to keep changing - and becoming more varied and horrible and cramped - all by itself!!

midali said...

Great article Alice, thanks!