Published online 30 April 2009 | Nature | doi:10.1038/news.2009.423

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Mercury not so like the Moon

Messenger spacecraft reveals extensive volcanism on the planet Mercury.

MercuryMercury is less like the Moon than researchers thought.NASA

Until recently, the layers of rock forming Mercury's crust were thought to be a lot like those of the Moon: cold, dead and full of the bright minerals found in the Earth-facing lunar highlands.

But after two visits from the Messenger spacecraft, the planet is looking less like the Moon and more like a place that has been ripped by fault-grinding tremors and pierced by planet-surfacing bouts of volcanic eruption — things associated with the bigger boys of the inner solar system. Mercury, it seems, can punch above its weight.

"This is not a dead world," says Thomas Watters, a planetary scientist at the Smithsonian Institution in Washington DC, "but a planet that has been very active in spite of its size."

Messenger scientists are finding that episodes of volcanic and tectonic activity have not only been much more common than thought, but also much more recent. And chemical differences between Mercury and the Moon are suggesting that their crusts may have evolved in completely different ways.

Watters and other colleagues on the Messenger team report on the NASA spacecraft's observations in a suite of papers published today in the journal Science1, 2, 3, 4, 5. After a third flyby in September, Messenger will settle into a polar orbit around Mercury in 2011.

Srange brew

There is solid evidence from moon rocks, for example, that, on the Moon, there was once a primordial magma ocean. As the molten brew cooled, certain minerals such as feldspar crystallized first and rose to the top, forming an ancient flotation crust that can be seen today as the lunar highlands, protruding through the low-lying flows of ancient basalt that form the Moon's mare — its 'seas'.

Mercury craterMessenger has revealed volcanic features on the surface of Mercury.Science

To telescopes on Earth and even to Mariner 10, a spacecraft that passed by Mercury in 1974 and 1975, the surface of Mercury has seemed to reflect light as brightly as the Moon. Many have assumed from this that Mercury has some of the Moon's same feldspar-rich stuff on the surface and therefore might have had a magma ocean and flotation crust too.

Messenger is able to measure reflectance more accurately than earlier observers have and is finding that the surface is much duller than previously measured. Spectrometers are also hinting that the surface doesn't have much feldspar in it. That's probably because of the other surprising Messenger finding: the planet has probably been resurfaced by volcanic flows.

Under pressure

Mariner 10, which mapped about 45% of the surface, revealed some flow-like features, but there was debate as to whether they were volcanic in origin or if they were debris flows associated with asteroid impacts. Messenger, which has now seen 90% of the surface, has found volcanic features and smooth plains — probably recent eruptive flows — covering 40% of the surface.

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Even the rest of the planet's surface has the same look as the recent flows — just slightly older and more 'weathered'. If a flotation crust ever existed on the planet, it has been completely buried, says Brett Denevi, a postdoctoral researcher at Arizona State University in Tempe and first-named author of one of the Science papers.

Complicating the story, however, is the other trend emerging from Messenger's observations: criss-crossed by cracks and faults, the planet probably experienced tectonic crunching throughout its history, rather than just at its start. The conventional wisdom after the Mariner 10 mission was that Mercury shrank as it cooled early on, which led to contractional tectonic features such as thrust faults — breaks in the crust. The new evidence of volcanism makes this contraction theory seem less likely, because contraction squeezes things shut, closing off vents and pipes, and so making it difficult for volcanoes to work. "It may mean the contraction early on wasn't enough to preclude volcanism," says Denevi. 

  • References

    1. Glassmeier, K. H. Science 324, 597–598 (2009).
    2. Slavin, J. A. et al. Science 324, 606–610 (2009). | Article | ChemPort |
    3. McClintock, W. E. et al. Science 324, 610–613 (2009). | ChemPort |
    4. Denevi, B. W. et al. Science 324, 613–618 (2009). | ChemPort |
    5. Watters, T. R. et al. Science 324, 618–621 (2009). | ChemPort |
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