Magnetic Fields
Every planet's magnetosphere is different. Saturn's is uniquely aligned, Uranus's is chaotic, Venus and Mars don't have one at all.
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Planets generate magnetic fields the same way an electromagnet does — by moving an electrically conducting fluid around. Inside Earth, that fluid is liquid iron in the outer core, sloshing as the planet rotates and as heat from the inner core convects it. The combined motion of a conducting fluid in a rotating frame is called a dynamo, and it spins up its own magnetic field for free. Without convection, or without rotation, or without a conductor, the field decays.
That's why Mars doesn't have a global magnetic field today. Four billion years ago it did — we can see frozen-in magnetisation in southern-hemisphere crustal rocks where the dynamo's signature got locked in. But Mars is small, so its interior cooled faster than Earth's, the iron core froze, convection stopped, and the dynamo died. Solar wind has been stripping Mars's atmosphere ever since — there's no magnetic shield deflecting it. Venus is the opposite case: hot inside, no shortage of conductor, but its 243-day rotation is so slow that the dynamo never spins up properly.
Then there's Saturn — a gas giant with a dynamo so cleanly aligned with its rotation axis that the magnetic tilt is essentially zero. Every other dynamo we know has at least some offset (Earth 10.5°, Jupiter 9.6°), but Saturn's is < 1°. Nobody's sure why. The leading theory is that Saturn has an unusual layer structure that mechanically smooths the field's symmetry, but the mechanism is debated. Saturn's near-perfect alignment is one of the unsolved puzzles of planetary physics.
Magnetic-axis tilt (angle between the dipole and the rotation axis) for each planet with a known dynamo: Mercury 0.7°, Earth 10.5°, Jupiter 9.6°, Saturn ~0°, Uranus 58.6°, Neptune 46.9°. Venus, Mars, and Pluto have no globally significant dipole today. Uranus and Neptune's wildly off-axis dipoles are also offset from the planet's centre — Uranus's by about 1/3 of the planet's radius. The conventional dynamo (iron core) can't easily produce that geometry; the favoured explanation is that Uranus and Neptune host their dynamos in an outer mantle layer of ionic water + ammonia + methane, not in a deep core.
Magnetospheres extend well beyond the planet itself. Earth's reaches about 65,000 km sunward (where it bows against the solar wind) and stretches into a long tail on the night side. Jupiter's magnetosphere is the largest physical structure inside the solar system that isn't the Sun's heliosphere: its magnetotail reaches Saturn's orbit, around 1.4 billion km long. The tail is shaped by the solar wind pushing the field downstream; the upstream nose gets compressed.
Particles trapped in the magnetic field create radiation belts — the Van Allen belts at Earth, the much more intense ones at Jupiter (one of the reasons missions like Juno fly polar orbits, avoiding the equatorial belt). Where field lines converge near the magnetic poles, charged particles spiral down and excite the atmosphere — that's the aurora. Every magnetised planet has aurorae; we've imaged them on Earth, Jupiter, Saturn, Uranus, and Neptune.