Scientists have argued about the origins of Mercury's smooth plains and the source of its magnetic field for over 30 years. Now, analyses of data from the January 2008 flyby of the planet by the MESSENGER spacecraft have shown that volcanoes were...

MERCURY: The MESSENGER spacecraft took this image of Mercury in January showing a side of the innermost planet that astronomers had never laid eyes on before.NASA/JPL

The first flyby of the planet Mercury in more than 30 years is resolving some long-standing puzzles about the closest planet to the sun. Among the findings: the planet's iron-rich core seems to be shrinking, causing its crust to buckle and crack.

These and other results, documented in 11 papers published today in Science, come from the first round of data from the MErcury Surface, Space ENvironment, GEochemistry and Ranging (MESSENGER) spacecraft, which buzzed Mercury earlier this year.

Mercury, the smallest of the eight planets, is unusual among its brethren in that it is mostly core. Based on its density, researchers know the core is mostly iron, and they estimate that it makes up at least 60 percent of the planet's mass and accounts for 75 percent of its diameter.

The last time a spacecraft studied the planet up close was from 1974 to 1975, when the Mariner 10 probe flew past and took images of some 45 percent of the diminutive world. MESSENGER is the first spacecraft designed to orbit Mercury.

To do so, it will make three flybys of the planet, adjusting its trajectory after each encounter. During the first flyby on January 14,...
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