We are all taught at school that the three states of matter are solids, liquids and gases. It makes sense, and we don’t tend to question it. After all, we walk on the solid earth, drink liquid water and breathe the gaseous air. Without any one of these three states of matter, our lives would be impossible. So perhaps it’s worth thinking about states of matter more carefully. We’ll start by considering the composition of the Earth.
What is Earth made of?
The Earth is basically a solid ball of rock, right? Well, sort of – it’s certainly more solid than Jupiter, but it has its own liquids and gases too.
How much of the Earth is liquid? It is a commonly mentioned fact that water covers 71 % of the Earth. However, that is measure of the area of the surface, not the volume or mass of the 3D planet. The oceans may be impressively deep – with an average depth of about 4 km – but that pales into insignificance compared with the 6 400 km to get to the centre of the Earth. Phys.org have a great picture showing the size of the oceans, rivers, lakes and groundwater if balled up into a sphere. It looks like a lot of water, but it’s actually only 0.12 % of the volume of the Earth.
But water isn’t the only liquid on Earth. It’s not even the most common liquid on Earth! (Maybe we should say ‘in’ Earth, rather than ‘on’ Earth.) The Earth has a layered structure, with the crust sitting on top of the mantle, which itself surrounds the core.
The mantle and core are both classified as having an outer and inner region. The outer core, shown in orange in the diagram above, is in the liquid state, and accounts for the Earth’s magnetic field. The crust is so thin, on this scale, that the blue line surrounding Earth on the diagram has been drawn too thickly to represent it properly. Given that the oceans are a small part of the crust, you can see that the oceans are much less relevant than the outer core in answering the question “How much of the Earth is liquid?” The outer core occupies about 15 % of the Earth’s volume. Because it is much more dense than the surrounding mantle, that volume is about 30 % of the Earth’s mass. In contrast, the mass of water is about 0.02 % of the Earth’s mass!
So how much gas is on Earth? Well, all we have to do is state the volume of the atmosphere and we can make similar comparisons? True, except there is no well-defined edge to the atmosphere. It’s obvious where the solid bit of Earth starts and stops, less obvious where the atmosphere ends. If we take the edge of the atmosphere to be the ‘Karman line’, at an altitude of 100 km, then the atmosphere is approximately 4.5 % of the Earth’s volume. The average density of the Earth’s atmosphere is so small, however, that this reasonably impressive percentage by volume is only 0.00009 % (or 0.9 parts per million) by mass.
So, to conclude, here are the ‘states-of-matter-compositions’ of Earth, by volume and by mass, showing solids (purple), liquids (blue) and gases (yellow).
The mass of the gases on Earth is so small that any ‘yellow’ is obscured by the black edges…
What is the solar system made of?
Let’s show the composition of the solar system, using the same colour scheme.
Your eyes are not deceiving you. In relative terms, there is basically no solid, no liquid and no gas in the solar system. The solar system is the sun with a few (quite a lot) small things orbiting it. In case you think we are not giving Earth due credit, not to mention Jupiter and Saturn, consider this: the sun is 99.85 % of the mass of the solar system. But surely the sun is a ball of burning gas, so that the ‘bar’ above should be yellow? No. The sun is almost entirely a plasma (we tricked you by not explaining that the colour scheme includes ‘green = plasma’). A plasma is the state of matter obtained when the particles in a gas have enough energy to become ionised, that is the electrons can escape from their atoms/molecules. The diagram below represents a hydrogen gas and the same particles when fully ionised, i.e. in plasma form.
Two ways to make a plasma are to heat a gas, or apply a sufficiently large voltage to it, but in either case the effect is to provide energy for ionisation. Plasmas really are another state of matter, not just ‘hot gases’. If you want to think of them like that, then we can counter your description by saying that liquids are just hot solids and gases are just hot liquids! Plasmas behave differently from gases – for example, they conduct electricity and respond to magnetic fields. One interesting point is that at the core of the sun, where the nuclear reactions are taking place that heat our planet, conditions are so extreme that the density is greater than that of solid lead and yet it is still a plasma!
The sun is almost entirely plasma, and therefore so is the solar system. No solids, liquids or gases anywhere, relatively speaking.
What’s the universe made of?
By extension of the previous argument, since our solar system is pretty typical, and since the interstellar medium is largely plasma, the visible universe is 99.999 % plasma.
But the universe is not all visible. Using the same colour scheme as before, here is the universe. You will have got the hang of this by now and won’t be expecting any purple, blue, yellow or green…
Well, there’s some green…
Galaxies rotate, and the rate of their rotation is a factor of their mass. If we add up all the mass from the stars and other stuff we can detect, we don’t find enough mass to account for the rate of rotation of the galaxies. Therefore, either:
- Our understanding of how and why galaxies rotate is wrong, or
- There is extra mass out there that for some reason we can’t detect
Scientists prefer the second of these explanations, and call the extra stuff ‘dark matter’.
In addition, there seems to be a need to invent another type of invisible stuff, called ‘dark energy’. It has been known since the work of Hubble in the 1920s that the universe as a whole is expanding. Until 1998, it was assumed that the expansion is slowing down, due to the mutual gravitational attraction of all the mass in the universe. However, in 1998 it was found that the expansion of the universe is actually accelerating. ‘Dark energy’ is the name for the invented phenomenon required to make this accelerated expansion happen. No-one knows what either dark matter or dark energy is, and there’s a lot of them (see chart above). They are postulated to make sense of other phenomena. It’s either that, or throw out all the physics describing gravity and the motion of objects, and start again. And that physics has done pretty well in predicting lots of stuff that has come true, so there is a reluctance to throw it out. Still, it’s weird to look at the chart of the universe above and think that the green bit is the only bit we can detect…
We’ve covered a lot of ground, from the starting point of elementary school science! So here’s a summary diagram.
Maybe now you won’t go blindly accepting that everything is made of solids, liquids and gases. And in case you weren’t convinced, here’s a list of other states of matter that we haven’t even mentioned…