First look
The idea, in plain words
The deepest hole ever drilled is in the far north of Russia. Work on it began in 1970, and by 1989 it had reached 12,262 metres. It is 23 centimetres wide. The centre of the Earth is 6,371 kilometres down, so after nineteen years the drill had gone less than a five-hundredth of the way. Nobody has seen the inside of a planet. It has had to be worked out.
Earthquakes do the work. A big one shakes the whole planet, and the shaking travels right through it to instruments on the far side. There are two kinds of wave. One kind can pass through liquid and one cannot. On the far side of the Earth from an earthquake, the second kind never arrives. Something liquid is in the way.
Thousands of earthquakes, timed at stations all over the world, give the full picture. At the middle is a ball of solid iron and nickel 1,221 kilometres in radius, at about 5,400 degrees. Round it is 2,300 kilometres of the same metals, molten. Round that is 2,900 kilometres of hot rock, the mantle. The crust that carries every mountain and ocean is about 30 kilometres deep under the land. On this drawing it is thinner than the line.
The molten metal is always moving, and moving metal makes magnetism. So the whole Earth is a magnet, and its pull reaches far out into space. A thin, fast wind of particles blows from the Sun all the time. The Earth’s magnetism turns most of it aside, and the wind in turn pushes the magnetism into the shape of a teardrop, blunt towards the Sun with a long tail behind.
Jupiter has no iron sea, but it has a shield all the same. Less than a quarter of the way down to its centre, the hydrogen is squeezed so hard that it turns into a liquid that carries electricity as a metal does. That makes a magnet 16 to 54 times as strong as the Earth’s. The blue glow in this picture is particles steered down onto Jupiter’s north pole: lights like our northern lights, on a planet eleven times as wide.
Mars shows what happens without one. Its rocks still hold traces of a magnetism it had about 4 billion years ago, but today it has no shield round the whole planet. A spacecraft called MAVEN watched the wind from the Sun knocking gas off the top of its air, and found that most of the air Mars ever had has been lost to space. What is left is too thin for water to stay liquid on the ground.
Go deeper
That was the idea. The same subject is written again below for readers who want more, each on a page of its own.
Words used here
- Differentiation
- The sorting of a young, molten world into layers, with heavy metal sinking to the middle and lighter rock floating above it.
- Magnetic field
- The region round a magnet, or round moving electric charge, in which other magnets and charges feel a force.
- Magnetosphere
- The space round a planet ruled by its own magnetic field, which turns the solar wind aside.
- Solar wind
- A stream of charged particles that blows outward from the Sun all the time.
- Aurora
- Light in the sky near a planet's magnetic poles, made when charged particles strike the upper air.
- Terrestrial planet
- A planet made mostly of rock and metal with a solid surface: Mercury, Venus, the Earth and Mars.
- Gas giant
- A very large planet made mostly of hydrogen and helium, with no solid surface: Jupiter and Saturn.
- Atmosphere
- The layer of gas held round a planet or moon by its gravity.
Where this comes from
- Wahl and others, Comparing Jupiter interior structure models to Juno gravity measurements (2017). Hydrogen turns metallic at about 80 to 90 per cent of Jupiter's radius
- NASA, Earth facts
- USGS, The shadow zone
- NASA, Jupiter facts
- NASA, Mars facts
- NASA, InSight science highlights
- University of Colorado, MAVEN findings reveal how Mars' atmosphere was lost to space (2017)
- Wikipedia, Kola Superdeep Borehole
The words are written for this site. The pictures are drawn by the page from the figures in the lesson, and each one says underneath what it is based on.