Mercury: The Incredible Shrinking Planet

In our quest to find planets around other stars, we often forget that we have alien worlds of our own, just a spacecraft-hop away. Look at the inner solar system: four terrestrial planets, formed by the same processes and at the same time, yet each is undeniably unique. And we don’t understand why.

That’s why NASA has headed back to Mercury.

Caloris basin by Messenger
Enhanced color image of the Caloris basin and surrounding area on Mercury's surface. The orange tinges mark places where past volcanism has affected the surface. Click on the image for a larger view.
Science / AAAS
The Messenger spacecraft (short for MErcury Surface, Space ENvironment, GEochemistry, and Ranging), launched in 2004, is designed to answer lingering questions and outright debates about the closest planet to the Sun. What’s with the weird-looking surface? Why does Mercury have a magnetic field? Is that water ice in those permanently-shadowed polar craters?

Answers are pouring in from the spacecraft’s first flyby on January 14th, including images of 20% of the surface that Mariner 10 (the only other craft to visit the planet) left largely unseen during its flybys in 1974 and 1975.

The Messenger results appear in 11 reports in the July 4th issue of Science. During the flyby, the first of three before it settles into orbit in 2011, the spacecraft confirmed that volcanism created many of the smooth plains seen across the planet. That's especially evident around the Caloris basin, a gigantic impact crater with a diameter over half that of the planet. High-resolution images also show eruptions from isolated volcanic vents.

One instrument, a spectrometer that records ultraviolet, visible, and near-infrared sunlight reflected off the planet’s surface, found significantly less iron in the surface than is present on the other terrestrial planets and the Moon. Ground-based astronomers had previously come to the same conclusion, though their observations weren't nearly as detailed. The dearth of iron is especially odd because Mercury’s iron core comprises 60% of its total mass — twice that of any other planet — and volcanic flows on Earth are usually iron rich.

Mercury shield volcano
An image taken by MESSENGER's MDIS of a dome feature scientists think may be a shield volcano. The "kidney-shaped" hole is thought to be a depression from an explosive volcanic eruption.
Science / AAAS
Messenger’s images also verify that the tall scarps and “wrinkle ridges” Mariner 10 saw extend across a significant portion of the planet’s surface (if not all of it). And the explanation for these features? Mercury shrank. A lot.

As Messenger principal investigator Sean Solomon (Carnegie Institution of Washington) explains in a Science podcast, the planet contracted between 3 and 4 billion years ago when its inner core cooled and solidified. The total shrinkage wasn't much in relative terms &mdash just 0.05% to 0.1% — but that was enough to create overlap faults across the surface similar to those made by crashing tectonic plates on Earth. The lost heat that caused the contraction may have been converted into the energy required to maintain the magnetic field generated within the core.

The Mercurian Magnet

Mercury’s magnetic field perhaps surprises astronomers more than anything else about the planet. The field appears to result from an active source, as opposed to being frozen into the surface. As Solomon explained in a press teleconference earlier today, the energy to drive the field may come from turbulence in the planet's outer core caused by iron as it solidifies and sinks.

The highly-dynamic field interacts with the solar wind along the magnetosphere’s boundary, a miniature version of the envelope that protects Earth from the solar wind and cosmic rays. Solar particles still punch their way through Mercury’s “flimsy” magnetosphere, though, often impacting the surface, explained FIPS project leader Thomas Zurbuchen (University of Michigan) in the teleconference. These particles can change the surface’s color and eject ionized material into the planet’s thin atmosphere or directly kick ions from the atmosphere into space. The atmosphere is so thin that its atoms are more likely to collide with Mercury’s surface than each other.

Drawing of the interconnections between Mercury's various parts — interior, surface, atmosphere, magnetosphere, and nearby space environment.
Science / Solomon et al.
Messeenger's Fast Imaging Plasma Spectrometer (FIPS) detected a slew of ions — including sodium, sulfur, calcium, and even water — in the atmosphere and magnetosphere. These ions surround the planet in a cloud and form a comet-like tail pointing away from the Sun.

“What is in some sense a Mercury plasma nebula is far richer in complexity and makeup than the Io plasma torus in the Jupiter system,” said Zurbuchen in a prepared statement.

Still, a lot of questions will have to wait until Messenger settles into orbit, after which it can study Mercury long term and confirm these preliminary results. The Sun will be more active in 2011, too, and mission scientists expect a spectacular shower of information as the faster and more turbulent solar wind interacts with Mercury’s magnetosphere, atmosphere, and surface.

“These are very exciting results for me,” said investigator William McClintock (University of Colorado, Boulder). “I can’t wait until orbital observations begin.”

For more information and images of the iron planet, visit the Messenger website.

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