Volcanoes on Mercury solve 30-year mystery

Planetary contractions
Scientists had hypothesized that Mercury underwent a significant contraction as its iron-rich core cooled, based on the results of the Mariner 10 mission. Images from that mission showed escarpments cut across much of the surface, indicating significant shortening of the planet's crust. The escarpments often deform other geological features.

"There are some craters that are just cut in half," Solomon said.

Messenger found more of these faults than Mariner 10 originally did, suggesting that the strain from the planet's contraction was at least one-third greater than scientists originally thought, one of the Science papers stated.

Essentially, as the hot, dense core cooled, some of the material would have solidified, sinking toward the center of the planet, forming an inner, solid core. When most solids cool, they also contract. In the case of Mercury, the planet's diameter was only decreased by about one-tenth of 1 percent, but in geologic terms, "it's a pretty big shrinkage," Solomon said.

When Messenger settles into orbit around Mercury in 2011 and gets a closer look at the escarpments, they may serve as "a record we can read" to determine when and how much contraction took place, and whether it happened continually and gradually or occasionally sped up, Solomon said.

Scientists will also be able to use variations in crater density across the surface to date the sequence of geological events. "The longer a surface sits out there, the more cratered" it becomes, Solomon explained, so more cratered surfaces should be older formations.

Above the surface
Messenger also used its flyby to investigate Mercury's magnetic field and magnetosphere, the region around a planet where the magnetic field influences other phenomena. Mercury is the only other planet in the inner solar system besides Earth that has a magnetic field.

NASA / JHUAPL / CIW Mercury's rugged, cratered landscape is illuminated obliquely by the sun.

During its flyby, Messenger took measurements of the magnetic field, and the results suggest that like Earth's field, Mercury's is largely dipolar (or like having a giant bar magnet stuck inside the planet).

But Mercury's magnetic field is much weaker than the one that surrounds our planet.

"Mercury's field is a smaller version of the Earth's," Solomon said.

Because the field is weak and Mercury is so close to the sun, the solar wind pushes the planet's magnetosphere very close to its surface on the side facing the sun, while on the side opposite the sun, the magnetosphere is very elongated.

In fact, "the solar wind gets very close to Mercury's surface some of the time," occasionally even hitting the surface, Solomon told Space.com.

When the solar wind hits Mercury's surface, it sputters particles off into the magnetosphere. Messenger detected these ionized atoms as it sailed through the magnetosphere; it found silicon, sodium, sulfur and even water ions surrounding the planet.

The impact of the solar wind also alters the chemistry of Mercury's surface, Solomon said, in a process called "space weathering." The chemical changes to weathered parts of the surface could help scientists determine which features are older, and which are fresh, newly exposed material.

Of course, this is just Messenger's first flyby, mission scientists noted. The second and third passes (to occur on Oct. 6 of this year and in September 2009) and the spacecraft's year-long orbital phase will likely yield even more information and shed further light on the history of the innermost planet.

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