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6.30 Magnetic Fields of the Terrestrial Planets

The mysterious power of the Earth's magnetic field was utilized in navigation for hundreds of years before it was truly understood. It wasn't until 1957, when the first artificial satellite Sputnik was launched, that we really started to explore the magnetic environment surrounding the Earth. Earth's magnetic field extends about 10 times the Earth's radius from the planet in the direction of the Sun. It protects us from harmful solar wind particles, deflecting them around the planet into a tail extending about 25-30 Earth radii away from the Sun.

View of Miranda on the moon showing light and dark banded scarps near the boundary of the banded ovoid and a deep graben that bounds the ovoid in this region. Click here for original source URL.

Out of all the planets we've studied, only Venus has no substantial magnetic field and shows no sign of having one in the past. This could be due to the planet's extremely slow spin rate — it takes 243 Earth days for Venus to rotate once around its axis. Without fast rotation, it would be difficult to initiate electric currents in the core, even if it was molten. Another explanation could be that there isn't enough sulfur in the Venusian core to lower the melting point, so it remains solid, prohibiting dynamic motions in the core. The Earth's Moon also lacks a magnetic field, probably for the same reasons: its rotation is slow, and it lacks a hot, fluid core. 

The internal structure of the inner planets. Click here for original source URL.

Mars also has no magnetic field. However, the red planet probably had a substantial field in the past, perhaps when the core was still molten. Mars is much smaller than the Earth, so it cooled faster, and its core solidified much earlier in the solar system's history. Data from the Mars Global Surveyor show that patches of the surface appear to be permanently magnetized. This also happens on the Earth — in fact, the magnetized rocks on Earth's ocean floors record magnetic field pole reversals in Earth's history. The cause of these reversals, which may have also occurred on Mars, are still not understood.

Mercury poses yet another magnetic mystery. Although it spins very slowly, it has a significant, though small, magnetic field. Mercury's small size would make it difficult to insulate the core and keep it molten. According to its density, the core of Mercury is probably composed of pure iron. Without any sulfur mixed in to lower its melting point, a liquid core is even less likely. The origin of Mercury's magnetic field is not yet explained, but scientists hope the NASA mission Mercury Messenger will return new data on the issue.

Many questions about planetary magnetic fields remain. What causes Earth's field to reverse? Do the other planets experience similar reversals? What drives the magnetic field of Mercury, and what is the reason for Venus's lack of a field? Further study of the electromagnetic environments of the planets could lead to answers to these questions within our lifetimes.