杏吧原创

Mercury has always had a warm, gooey heart. Now NASA鈥檚 Messenger spacecraft has revealed that the planet鈥檚 iron core has been generating a magnetic field for the past 3.8 billion years or so, potentially making it the most persistent among the solar system鈥檚 planets.

Before deliberately crashing into Mercury last week, Messenger had kept its distance for most of its four-year mission, taking a highly elliptical orbit that brought it no closer than about 200 kilometres from the scorched surface. In the months before its swan dive, it circled lower and lower, eventually swooping down to just 15 kilometres above the surface. That gave it a nice close-up view of the planet鈥檚 magnetic field.

Astronomers already knew that Mercury has a magnetic field about 1 per cent the strength of Earth鈥檚, and that the rotation of liquid iron in the core generates the field, just as happens inside Earth.

But when Messenger dipped below an altitude of about 100 kilometres, it saw an even weaker magnetic signal coming from the rocks on the surface.

Fridge magnets

鈥淚t鈥檚 like having a little patchwork of refrigerator magnets,鈥� says at the University of British Columbia in Vancouver, Canada. 鈥淲e鈥檇 flown over this area 20, 30 times before, and we hadn鈥檛 seen this. We would never have been able to see these signals if we hadn鈥檛 been able to fly very close to the planet.鈥�

The signal changed from non-existent at an altitude of 150 kilometres to strongest at 15 kilometres. That confirmed it was definitely coming from the crust, and was not the result of the core鈥檚 magnetic field interacting with charged particles from the sun, which would have produced a consistent signal at higher altitudes.

The magnetism was strongest in terrain estimated to be between 3.7 billion and 3.9 billion years old, based on how cratered the surface is 鈥� older rocks are more pockmarked. Rocks become magnetic by taking on their planet鈥檚 field while still molten, and this field becomes locked in as they cool and solidify. The presence of magnetism in terrain of this age therefore suggests that Mercury had a magnetic field as early as 3.9 billion years ago 鈥� almost the entirety of the planet鈥檚 4.5-billion-year history.

If that ancient field has persisted until today, it would make Mercury the planet with the longest-lasting magnetic field known. Earth鈥檚 earliest trace of magnetism dates back just 3.5 billion years.

But Messenger can鈥檛 tell us the entire history of Mercury鈥檚 magnetic field, notes Hao Cao of the California Institute of Technology in Pasadena. 鈥淲e actually don鈥檛 know whether Mercury always had a magnetic field in between 3.8 billion years ago and now,鈥� he says. 鈥淭hat鈥檚 a big puzzle. I could very well imagine that back then Mercury had a magnetic field, then for most of its history did not have one, and the field we see now turned on recently.鈥�

The next visitor to Mercury, a Japanese-European satellite called , could settle the issue if it comes up with better global maps of the crustal magnetic fields after arriving in 2024.

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