Nature Feb. 121 restates a puzzle about Mercury known since the Mariner 10 encounters (1974–5):
…why, against expectations, does Mercury have a global magnetic field?
The planet’s diminutive size means that it should have cooled quickly after it formed. Any molten core would have become solid, or almost completely so. A magnetic-field-generating dynamo in an outer core (as is the case for Earth) should have long since seized up. But the news from Mariner 10 meant a rethink was needed, and since then attempts to account for the magnetic field have centred on how the core might have remained largely molten.
Tim Lincoln mentions a new theory by Aharonson et al. published in Earth and Planetary Science Letters (2004) that maybe the original field was frozen into an uneven crust. We may not be able to tell until the MESSENGER spacecraft, launching this May, sends back its first data in 2007.
Planetary magnetic fields are astounding in their diversity. Mercury has a small global field (but should not); Venus, our twin, has essentially none; the gas giants all have fields but those of Uranus and Neptune are extremely tilted and off center, while Saturn’s is almost perfectly aligned with its pole axis. According to theory, this cannot be. Earth, fortuitously, has a perfect field to shield its inhabitants from cosmic rays and solar flares, but its strength is dropping rapidly.
In this “golden age of planetary discovery,” we should soon be gathering more data to explain these mysterious phenomena. If trends continue, however, each solution will breed new problems. At this point, it does seem quite incredible that tiny Mercury should retain any global field at all, if it were really 4.5 billion years old. Note the surprise in Lincoln’s writing over this mystery: “against expectations…would have…should have…a rethink was needed.”