The Dalek Invasion of Earth

They dare to tamper with the forces of creation?

Diagram of Earth's interior

Interior of the Earth (courtesy of

The inner core of the Earth is a ball of solid iron about 2400 km across – 2/3 the size of the Moon. Its temperature is 5400 °C – about the same as the surface of the Sun. And it’s freezing.

When the Earth first formed, its interior was molten all the way through. Gradually it cooled, and the centre solidified. Although still incredibly hot by human standards – iron melts at about 1500 °C in the open air – the high pressure caused by the weight of all the Earth pressing down upon the core raises its melting point so that it freezes solid even at these sunlike temperatures. The interior is still cooling, and the solid inner core is slowly growing as the lowest layers of molten iron in the outer core freeze onto it.

Now freezing releases heat. If that seems an odd statement, think of it this way. You have to put in heat to melt a solid, using energy to break the molecular bonds. So if you reverse the process, as a liquid freezes that same heat must be given up.

That’s important in the Earth’s core. The heat released by freezing at the boundary between the inner and outer cores drives convection currents that make the liquid outer core roil and swirl restlessly.

Iron, of course, is a magnetic material, and all that circulation of liquid iron generates a powerful magnetic field. This field reaches to the surface of the Earth and far beyond into deep space. It allows seafarers to navigate with a magnetic compass, shields our planet from the full anger of the Sun, and channels the solar wind to the polar caps to create the shimmering curtains of the aurora.

As we move up through the outer core, the pressure drops and the temperature decreases. Once we get about 3500 km out from the centre, we hit another boundary. Above this depth, the composition changes. We are too high up for the heavy iron that sank towards the centre. Instead we have something more like ordinary rock, predominantly silicates, but under temperatures and pressures that are far from ordinary. This is not the flowing fluid of the outer core : neither is it the crystalline solid of the inner core or the crust. Instead it is an incredibly viscous, slowly flowing material called the mantle.

To get a handle on how this stuff behaves, we can look at the world’s longest-running scientific experiment – the pitch drop experiment.

Tar pitch is about the most viscous substance in human experience. To get an idea of what this means, go into the kitchen. Fill a glass with water, and stir it. Easy, isn’t it? That’s because water had low viscosity. Now try stirring a jar of honey. That’s a lot harder – the viscosity of honey is about ten thousand times that of water. If you have some peanut butter, give that a stir. It’s about 25 times more viscous than honey, and about the most viscous thing you’re likely to have lying around.

Tar pitch is a million times more viscous than peanut butter.

The original pitch drop experiment started at the University of Queensland in Brisbane in 1927, and is still going strong. It’s quite simple, really: a funnel is filled with tar pitch, and a drop slowly forms at the bottom of the funnel as the tar flows out, eventually dropping off, and then a new drop forms. They’ve recorded a drop every eight years or so. Just recently, a similar experiment at Trinity College Dublin was the first to record a drop falling on camera. (The Queensland experiment missed filming its most recent drop because the camera was offline.)

Mantle is ten trillion times more viscous than tar. Ten million trillion times more viscous than peanut butter.

So it’s incredibly stiff, but still more like tar than ordinary rock: it has no crystalline structure, and flows, however slowly, under pressure.

That lot makes up the bulk of planet Earth. On the outside edges there are some details: mantle that flows more readily thanks to the low pressure near the surface and, floating on top of it, some solid plates of cold, brittle rock on which various biological organisms live out their brief, meager lives.

So what the hell are we to make of the Dalek’s plan to remove the core of the Earth? First of all, if they want to do this by drilling a hole through the crust, Bedfordshire is a terrible place to do this. You have to drill through 30 km of crust there, as opposed to less than 10 km on the ocean floor. If underwater operations are too much of a drag, somewhere like southwest Ireland would still be a lot easier.

Wherever they drill, they are going to drop some device down into the core and suck out all the molten material. Presumably through some kind of magnetic funnel, but this is the Daleks so I wouldn’t put it past them to use a giant bendy straw. However they do it, what are the effects?

The first thing this would do is reduce the pressure on the solid inner core. This is still very hot, so once the pressure drops it will start to melt. As it does so, it will be sucked out along with the outer core, leaving the Earth entirely hollow.

(Top tip : never try to find scientific information on this subject by googling “hollow earth”. You will descend into a swirling vortex of maniacs and conspiracy theorists.)

Imagine the interior of the Earth at this point. It’s a great hollow cavern 7000 km across. There’s no gravitational force. This is because a particle inside a spherical shell experiences a force towards the nearest part of the shell, and an opposite force towards the furthest part. The lesser distance of the nearest part, and the greater volume of the furthest part, exactly cancel each other out, resulting in zero net force.

And it’s hot. The inner surface is at a temperature of around 3500 °C, hot enough to glow red. Not only is it hot, but it can’t cool down, except by losing heat slowly upwards through the remaining shell of the Earth. It’s radiating heat into the vacuum inside the Earth, but any given patch of this inner surface will not only lose heat through radiation, it will also absorb radiation emitted from the rest of the surface. These two processes exactly balance each other. Like gravity, radiation follows an inverse square law, and so the same mathematics that tells us the gravitational force inside the shell is zero also tells us that there is no net loss of heat.

It’s also melting. With the removal of the core, and hence the removal of its gravitational pull, the pressure on the mantle has dropped. That pressure was the only thing keeping it solid at these high temperatures, and with the loss of that pressure it will undergo decompressive melting. Indeed, this will happen as the core is being removed, so we can expect the mantle to liquefy and be sucked out as well in its turn.

Of course, as you are removing the mantle from the inside out, you will be taking the hottest material first and gradually working outwards into progressively cooler mantle. At some point, you will reach a level where the temperature is low enough that the mantle will not melt even though the pressure has been radically reduced. Actually calculating where that point is would be a substantial research project, and one for which it would be difficult to obtain funding. We can, however, put some upper bound on the answer. The temperature at which mantle rock melts on the surface of the Earth is about 1300 °C. That corresponds to a depth of about 200 km or so. Our remaining shell of the Earth isn’t going to get thinner than this, and may stay somewhat thicker.

So by the end of all this the Daleks have removed about 90% of the Earth’s volume, leaving behind a brittle solid crust sitting on top of a thin spherical shell of ductile rock. We have to hope that this lower layer is strong enough to hold together under its own self-gravity and the weight of the crust above it, otherwise this planet is going to implode like a cheap meringue.

Even if that doesn’t happen, the consequences for life on the surface will be catastrophic. All of this planetary-scale geoengineering will at the very least cause the crust to buckle and fracture as the mantle beneath is disrupted and removed. It will be like every earthquake, every volcano and every tsunami in history hitting all at once.

With the Earth’s mass reduced by more than 90%, the surface gravity will drop by the same fraction. The Earth will be no more able to hold on to an atmosphere than the Moon – indeed, even less so, as the Moon’s surface gravity is one sixth of Earth’s. Not only will there be nothing to breath, but there will be no protection from the Sun’s hard UV and X-rays. And as if that weren’t enough, the Earth’s magnetic field will have vanished along with the liquid core, leaving the planetary surface fully exposed to the solar wind.

The surface will freeze, of course. In the absence of an atmosphere, the equilibrium temperature for the Earth is about -18 °C. The greenhouse effect may have become a threat in recent decades, but it is still the only thing that keeps our planet habitable. Given that the Daleks want to zoom the hollow Earth around in space, however, we can assume that the temperature will drop even further. Gradually the remaining mantle layer will cool, becoming brittle, and sooner or later it will be meringue time – unless the Daleks have some cunning plan for preventing this.

Quite how this zooming about is supposed to be achieved is unclear. All we know is that the Daleks intend to place some kind of power system within the hollow Earth. It will need to be anchored somehow to the inner surface to prevent it drifting out of position, otherwise it would crash into the inside of the Earth whenever the planet moved. Beyond that, it’s hard to say.

What’s even harder to discern is why the Daleks are carrying out this apparently bonkers plan. There doesn’t seem to be any practical purpose that couldn’t be achieved a lot more easily and with a lot less risk simply by building a fleet of spaceships. Such as, for example, the spaceships they used to invade Earth with in the first place. The whole scheme just seems entirely redundant. Whatever purpose it might serve, it is not one that is apparent from the story or from any rational consideration.

I reckon they’re just doing it for a laugh.


9 thoughts on “The Dalek Invasion of Earth

  1. A lot of people have written on the fact that the Daleks plotting to hollow out the earth and fly it around like a space ship is utterly ludicrous — I’ve seen more than a few who go as far as to simply discard it and assert that the Daleks had some other, better plot, and the whole “Hollow out the earth and fly it around like a space ship” thing was a cover.

    What I find more interesting is that they’re not the only ones to do this. See also: Zanak; Mondas. For some reason, *everyone* seems to want a planet they can fly around.

    • The notion does crop up several times, but it’s differently motivated each time. Mondas has its roots in the whole Velikovsky “Worlds in Collision” thing – and it’s a while since I’ve watched Tenth Planet, but I don’t recall that Mondas’s travels through space were under any kind of control. Zanak is a classic example of Douglas Adams taking an intellectual joke and turning it into a plot – and within the story, Zanak is what allows the Pirate Captain to pursue his mad schemes.

      But in this story, there’s no indication of what the Dalek can do with a mobile Earth that they can’t do with their existing spaceships, and there’s no particular literary or mythical reference or intellectual conceit as far as I can see. It’s just straightforwardly mental.

      • @Iain: In ‘Attack of the Cybermen’, there’s this one sort of out-of-left-field line where Griffiths asks the Doctor how a planet can travel around off it’s orbit, and the Doctor says “Mondas had a propulsion unit. A tribute to Cyber-Engineering, though they should want to push a planet through space, I’ve no idea.”
        The popular theories among fan-scholars I’ve read are either that the Cybermen installed a propulsion unit in Mondas at some point after it had been knocked out of orbit in order to control their return, or that the ancient Mondasians had developed orbit-shifting technology to avoid some cosmological catastrophe (Usually assumed to be the same events that forced Homo Reptilia into hibernation, though that timetable does stretch disbelief), but cocked it up and sent themselves shooting off into space by accident. There is, though, an older notion that is alluded to in some of the novelizations, that the Mondasians deliberately took their planet on walkabout in the short-sighted notion that a life of cyberneticization and planetary piracy would be awesome (The novelization of The Tenth Planet says almost exactly this, except that it supposes the Cybermen originated not on Mondas, but on Telos, and that they’d found, conquered, and modified Mondas to be their flagship)

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