Published online 29 September 2011 | Nature | doi:10.1038/news.2011.561

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Close-ups reveal a weirder Mercury

MESSENGER spacecraft results challenge theories about the planet's early history.

High-resolution images from MESSENGER reveal previously unknown landforms on Mercury's surface.JHU/APL

The first major release of results from NASA's MESSENGER spacecraft, which settled into orbit around Mercury last March, is forcing researchers to reconsider some of their most fundamental ideas about the nature and history of the Solar System's innermost planet.

The findings published today in Science include a previously unknown type of landform1 and evidence of volatile elements2 that most researchers assumed had been baked out of Mercury long ago, as well as five other reports describing the planet's landscape3, surface chemistry4 and magnetic field 5,6,7.

For MESSENGER researcher and planetary geologist David Blewett of the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, the take-home message from the findings is clear: "Mercury is weird; everything about it is weird. We don't know what kind of rocks it's made of, we don't know its colour and it's not depleted of volatiles like everyone thought."

It was Blewett and his team who, examining the craft's highest-resolution images to date, discovered depressions scattered along the floors, walls and central peaks of craters in Mercury's northern hemisphere. These irregularly shaped hollows, which are unlike any landforms previously known to researchers, range from tens of metres to a few kilometres across and look fresh enough not to have been altered by meteorite impacts during the long history of the planet1.

The hollows do not seem to have resulted from volcanic eruptions, says Blewett. However, they do look a little like the 'Swiss cheese' terrain seen at the south polar region of Mars. There, solar heating causes deposits of carbon dioxide ice to sublimate — or change from a solid directly into a gas — carrying bits of adjoining material away from the surface in the process.

By analogy, Blewett and his team propose that temperatures beneath Mercury's surface should be cool enough for some volatiles to remain stable. But debris striking the planet would deliver enough energy to trigger their release, hollowing out the surrounding terrain in the process, says Blewett.

"The hollows are indeed a puzzle, and I think that the leading explanation is sublimation," says David Rothery, a planetary scientist at the Open University in Milton Keynes, UK, who was not involved in the study.

The researchers estimate that, in the northern basin Raditladi, it would take 70,000–200,000 years to remove a centimetre of the surface by this process, suggesting that the hollows there formed over billions of years.

Volatile personality

But how could tiny, Sun-baked Mercury — the Solar System's smallest planet — retain a substantial supply of elements that would so easily exit the surface? Most models that seek to explain Mercury's immense iron core — which makes up a much larger fraction of the planet's volume than the core of any other terrestrial planet — require that the planet was exposed early on to searing heat.

Those models start off with a Mercury that was closer in size to Earth, with a much thicker crust and mantle than it has today. The models assume that either a large impact sheared off much of the rocky material soon after Mercury formed, or that the young Sun went through a hot phase and boiled off the planet's outer layers like a blowtorch.

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But the latest observations from MESSENGER suggest that neither scenario is correct. X-ray spectrometer results from the craft2 suggest that sulphur is at least ten times more abundant on Mercury's surface than it is in Earth's mantle.

Gamma-ray spectrometer results4, meanwhile, show that the ratio of potassium to thorium on Mercury's surface resembles that of the other terrestrial planets. Both results suggest the planet was not subject to high heat in the past and may have formed with the thin mantle seen on it today.

"What we're saying now is that all these exotic theories that have been proposed to explain Mercury's formation don't really pan out," says Patrick Peplowski of the Applied Physics Laboratory, who led the gamma-ray spectrometer study.

That still leaves the puzzle of how Mercury and its massive iron core formed. Some theorists have proposed that the mix of materials that coalesced to form Mercury — culled from the disk of gas and dust that circled the Sun — was rich in iron. But it's unclear why the other terrestrial planets in the Solar System wouldn't have ended up with a similar composition.

Such questions have far-reaching importance, says Blewett. Mercury is the closest analogue in our Solar System to rocky exoplanets orbiting close to their parent stars. "We can't say that we really understand how these planets form until we get Mercury figured out." 

  • References

    1. Blewett, D. T. et al. Science 333, 1856-1859 (2011). | Article | ChemPort |
    2. Nittler, L. R. et al. Science 333, 1847-1850 (2011). | Article | ChemPort |
    3. Head, J. W. et al. Science 333, 1853-1856 (2011). | Article | ChemPort |
    4. Peplowski, P. N. et al. Science 333, 1850-1852 (2011). | Article | ChemPort |
    5. Anderson, B. J. et al. Science 333, 1859-1862 (2011). | Article | ChemPort |
    6. Zurbuchen, T. H. et al. Science 333, 1862-1865 (2011). | Article | ChemPort |
    7. Ho, G. C. et al. Science 333, 1865-1868 (2011). | Article | ChemPort |
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