The tides of Venus

The tides of Venus

Impossible chemistry: making the unreactive, react

In my line of business, diamonds come under the heading “consumables”. Even the hardest thing known to mankind is sometimes no match for high-pressure science. Luckily since being in Australia (and since I’ve needed to get my own funding for diamonds) things have been pretty sedate - I’ve not so much as chipped anything carbon-based.

Last week a friend of mine, Chrystèle, got a paper accepted presenting the reaction of xenon with ice. I was lucky enough to have been part of the experimental team. For that experiment, I can exclusively reveal, we really did explode a diamond. I remember hearing it, through a 10cm-thick lead door, explode with a small “pop”. What was left was literally dust. But then again, this poor gem had been part of a cell creating conditions of pressure 800,000 times atmospheric pressure and 1500K. It had been squeezed and had massive laser power dumped through it for a total of about two days. It was just as well we already had the result at the point it decided to give up. That’s how hard it is to react xenon with water.

Xenon is an incredible element, but there’s one thing it doesn’t like doing all that much: reacting with other elements. Cast your mind back to high school chemistry and you know why this is. Like the other noble gases, argon and neon to name a couple, they have full outer shells of electrons, no room to accept any others and none to give away for the formation of bonds with other elements. In fact the first compound with a noble gas wasn’t discovered until 1962.

The Noble gases, the ‘Downton’ on the periodic table.

It’s this full shell that makes the noble gases pretty unreactive; they like to sit on their own being, well, noble. This unreactivity makes them pretty useful for creating neutral environments to play with other materials. Lots of gloveboxes will be filled with argon, and lightbulbs will often be filled with neon and other noble gas mixtures.

But what hasn’t really been explored is what happens if we literally force theses unreactive elements together with other with brute force pressure. It’s like taking the most placid people you know and keep putting them in a lift. Ten people in they may start to grumble, but I bet if you put 50 of them in there a few cross words would be said …

A crowded lift can be like a high-pressure experiment Flickr - ..::K2::..

What about putting 50,000 of them in there? That’s what we were exploring with xenon: if we pressurised it with water, would it react? Would this answer a mystery of the solar system?

Xenon, like most of the other noble gases, is found naturally in our atmosphere. But, in a long-time puzzle, there just isn’t enough of it compared with what’s expected. That’s one of the questions Chrystèle has been researching for a while now: where has all the xenon in Earth’s atmosphere gone?

Having done grand work in teasing out what has happen to the Earth’s xenon, by discovering its reaction with quartz, she’s now turned her attention a little more further from home. All of the gas giants could have the same problem: not enough of the noble gases in their atmospheres. The thought is if we know this is occurring, and how, we would understand a lot more about how the gas giants were built and evolved.

So could the noble gases have been gobbled by these planets’ interiors? If so, there has to be a stable way that they bind to the planetary materials. If xenon got sucked into the interior and didn’t react it would soon bob back up to the planetary surface and we really should see it there.

Who ate all the Noble gases? Uranus pictured by the Voyager 2 spacecraft. NASA

Two of the gas giants, Uranus and Neptune, are known as icy gas giants as they are made of water (mixed with ammonia and methane). Hence, water was chosen for the experiment. Chrystèle (with me watching in awe) carefully loaded xenon and water into a diamond anvil cell to create the massive pressures. Using lasers to heat the sample within the diamonds, we watched the sample with x-ray diffraction which gave us a clue to what was happening to the arrangement of atoms sitting in there.

Sitting there, at pressures and temperatures that you would find in the centre of the gas giant planets, we did notice a reaction. A whole new material was created, one that the team later determined to be made up of xenon, oxygen and hydrogen. A reaction product between xenon and water, success!

Even the noblest of elements, given the right provocation, will deign to react with the others. So we now have a way that xenon can be stored in the centre of Uranus and Neptune, can the other noble gases do this too? There is a lot more noble gas chemistry to come!

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