Mercury likely has a partly molten core, a new study indicates. This molten material may be generating the planet鈥檚 weak magnetic field, whose existence has been a puzzle since its discovery more than 30 years ago.
Earth鈥檚 magnetic field is generated by molten material in its core, but Mercury is so small that it was expected to have cooled and solidified long ago. But when NASA鈥檚 Mariner 10 spacecraft flew by the Sun-scorched planet in 1974 and 1975, it detected a magnetic field.
Even though the field is just 1% as strong as Earth鈥檚, its detection was taken by some scientists as an indication that Mercury had somehow avoided having its core solidify. Others argued that the core had already solidified but that back when it was still molten, the planet鈥檚 crust had become magnetised, leaving behind a 鈥渇ossil鈥� magnetic field.
Advertisement
Now, measurements of Mercury鈥檚 fluctuating spin rate are providing new evidence that Mercury really does have molten material inside it. Jean-Luc Margot of Cornell University in New York, US, led a team that made the spin measurements.
鈥楧isco ball鈥�
They bounced radar signals off of Mercury using the 305-metre Arecibo radio dish in Puerto Rico and the Goldstone 34-metre dish in California, US. Observations of the returning signal were made with these two dishes, as well as the 100-metre Robert C. Byrd Green Bank Telescope in West Virginia, US.
The signals reflect off of irregularities in Mercury鈥檚 surface, turning it into something of a 鈥榙isco ball,鈥� Margot says, with patterns of reflected radio waves sweeping across the solar system. The faster Mercury spins, the faster these patterns move, so watching them allowed the scientists to monitor its rotation rate.
Mercury spins once on its axis every 59 days, but the Sun鈥檚 uneven gravitational pull makes the planet鈥檚 spin rate fluctuate slightly over the course of its orbit. The size of these fluctuations depend on whether Mercury is solid or liquid inside 鈥� just as eggs spin differently depending on whether they are raw or cooked, Margot says.
The researchers measured the fluctuations to be about 1 part in 10,000, which is larger than the expected rate for a completely solid planet, suggesting Mercury is partially molten inside. They suspect that the inner part of the core is solid but that its outer regions are still molten.
Distant building blocks
That would be possible if its core, thought to be mostly iron, contains some sulphur as well. 鈥淚f you mix in a small amount of sulphur with iron, you can lower the melting temperature by several hundred kelvin,鈥� Margot told New 杏吧原创.
But if Mercury does contain some sulphur, its presence presents a puzzle of its own, since sulphur could not have condensed out of the early solar nebula so close to the Sun. This suggests that some of the building blocks, or planetesimals, that coalesced to form Mercury came from farther out, where sulphur could have condensed along with iron and other elements.
鈥淭hose planetesimals colliding with each other eventually bring all these elements together and that鈥檚 what you end up with to form the planet,鈥� Margot says.
Sean Solomon of the Carnegie Institution of Washington in Washington, DC, US, says this is a likely explanation for Mercury鈥檚 sulphur, if it really does contain a substantial amount. 鈥淚f there is sulphur in Mercury鈥檚 core, then as Margot has stated, it was likely contributed by planetesimals that formed farther from the Sun than Mercury鈥檚 current orbit,鈥� he told New 杏吧原创.
Other elements
But he says a variety of other elements besides sulphur could also keep Mercury鈥檚 core molten.
NASA鈥檚 Messenger spacecraft is on its way to Mercury, and is set to start orbiting the planet in 2011 following several flybys (see Messenger probe nudged towards Venus).
Messenger 鈥� short for Mercury, Surface, Space Environment, Geochemistry and Ranging 鈥� will help provide a more definitive understanding of the planet鈥檚 core. The radar data give a 95% confidence that the interior must be partially molten, but a better understanding of the distribution of mass within Mercury from Messenger measurements should sharply reduce the uncertainty.
鈥淎s soon as Messenger gets there [it] will make the interpretation of our results much more precise,鈥� Margot says.
The researchers have also discovered a longer term shift in Mercury鈥檚 spin rate, with a period of about 12 years. They will continue to monitor the planet鈥檚 spin rate in the hopes of determining the source of this shift, which might be the result of Jupiter鈥檚 gravitational pull. They also plan to study Venus to see if its spin fluctuates.
