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Mercury Flyby Reveals Active (but Shrinking) Core

Cracks in the planet's crust imply a contracting world
Mercury



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, MESSENGER flew within 125 miles (200 kilometers) to snap new shots and perform other tests.

Mariner 10 revealed that Mercury has a magnetic field around its equator—the only one of the solar system's inner planets besides Earth that has one. That left researchers wondering whether it is generated by the core or if a field frozen in the crust is its source.

MESSENGER found no signs of widespread crustal magnetism, indicating the core has a flowing—molten—outer layer similar to Earth's core to support the electric currents necessary to generate a magnetic field.

The exact thickness of that layer is unknown, but as it gradually cools, it should form solid chunks that sink like iron snow into the denser, solid inner core, also of unknown size, says geophysicist Sean Solomon of the Carnegie Institution of Washington's Department of Terrestrial Magnetism, the mission's principal investigator.

Because solid iron is denser than the liquid form, "as you grow the inner core, the whole volume of the core shrinks," he says, causing the planet itself to contract. "We are understanding how much shrinking there has been."

That understanding comes from images captured by MESSENGER that show elongated "scarps," or steep slopes riddling the surface. Researchers interpret these as places where one piece of crust has slid underneath another and forced it upward.

MESSENGER took photos of an additional 20 percent of the planet that Mariner missed as well as areas that it had covered, but with light coming in at a different angle. This allowed researchers to identify additional scarps missed by its predecessor. They estimate that Mercury has one third more cracks than they had counted.

Another 30-year debate has revolved around smooth patches of rock in the planet's craters, which could represent hardened lava or material ejected during asteroid impacts.

Researchers say the new images reveal geological details indicating that the patches were indeed volcanic in origin.

In another study, the craft used its Fast Imaging Plasma Spectrometer to identify energetic charged atoms and molecules (plasma) caught in Mercury's magnetic field.

The MESSENGER data reveal charged silicon, sodium, sulfur and water, which researchers conclude was dislodged from the planet's surface by the sun's own stream of charged particles, the solar wind.

Solomon says that studying Mercury provides a useful test case for ideas about how planets assemble and operate. He says he expects to learn even more when MESSENGER enters orbit March 18, 2011.

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