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The newspaper of The Johns Hopkins University November 3, 2008 | Vol. 38 No. 10
MESSENGER Reveals More 'Hidden' Territory on Mercury

Spacecraft's second flyby yields more than 1,200 pictures of planet's surface

By Paulette Campbell
Applied Physics Laboratory

Gliding over the battered surface of Mercury for the second time this year, NASA's MESSENGER spacecraft has revealed even more previously unseen real estate on the innermost planet, sending home hundreds of photos and measurements of its surface, atmosphere and magnetic field.

The probe flew by Mercury shortly after 4:40 a.m. on Oct. 6, completing a critical gravity assist to keep it on course to orbit Mercury in 2011 and unveiling 30 percent of Mercury's surface never before seen by spacecraft.

"The region of Mercury's surface that we viewed at close range for the first time this month is bigger than the land area of South America," said Sean Solomon, the mission's principal investigator and director of the Department of Terrestrial Magnetism at the Carnegie Institution of Washington. "When combined with data from our first flyby and from Mariner 10, our latest coverage means that we have now seen about 95 percent of the planet."

The spacecraft's science instruments worked feverishly through the flyby; cameras snapped more than 1,200 pictures of the surface, while topography beneath the spacecraft was profiled with the laser altimeter. "We have completed an initial reconnaissance of the solar system's innermost planet, enabling us to gain a global view of Mercury's geological history and internal magnetic field geometry for the first time," Solomon said.

The comparison of magnetosphere observations from the probe's first flyby in January with data from its second pass has provided key new insight into the nature of the planet's internal magnetic field and revealed new features of Mercury's magnetosphere.

Deputy project scientist Brian Anderson, of Johns Hopkins' Applied Physics Laboratory, said, "The previous flybys by MESSENGER and Mariner 10 provided data only on Mercury's eastern hemisphere. The most recent flyby gave us our first measurements on Mercury's western hemisphere, and with them we discovered that the planet's magnetic field is highly symmetric.

"This seemingly simple result is significant for the planet's internal field because it implies that the dipole is even more closely aligned with the planet's rotation axis than we could conclude before the second flyby," Anderson said. "Even though the rigorous analyses of these data are ongoing, we expect that this result will allow us to limit the theories of planetary magnetic field generation to those that predict a strongly rotationally aligned moment."

The Mercury Atmospheric and Surface Composition Spectrometer, known as MASCS, observed the extended tail and night side and day side regions of Mercury's thin atmosphere — known as an exosphere — searching for emission from sodium, calcium, magnesium and hydrogen atoms.

Ron Vervack of APL, MESSENGER participating scientist, said, "The MASCS observations of magnesium are the first-ever detection of this species in Mercury's exosphere." Preliminary analysis of the sodium, calcium and magnesium observations suggests that the spatial distributions of these three species are different and that the distribution of sodium during the second flyby is noticeably different from that observed during the first flyby.

"The spatial distributions of sodium, calcium and magnesium are a reflection of the processes that release these species from Mercury's surface," Vervack said. "Now that we were finally able to measure them simultaneously, we have an unprecedented window into the interaction of Mercury's surface and exosphere."

The probe's Mercury Laser Altimeter measured the planet's topography, allowing scientists, for the first time, to correlate high-resolution topography measurements with high-resolution images.

"During the last flyby, the Mercury Laser Altimeter acquired a topographic profile in a hemisphere of the planet for which there were no spacecraft images," said Maria Zuber, the mission's co-investigator and head of the Department of Earth, Atmospheric and Planetary Sciences at MIT. "During the second flyby, in contrast, altimetry was collected in regions where images from MESSENGER and Mariner 10 are available, and new images were obtained of the region sampled by the altimeter in January. These topographic measurements now improve considerably the ability to interpret surface geology."

Now that the spacecraft's cameras have imaged more than 80 percent of Mercury, scientists say it is clear that, unlike the moon and Mars, the planet lacks hemispheric-scale geologic differences. "On the moon, dark volcanic plains are concentrated on the near side and are nearly absent from the far side," said co-investigator Mark Robinson, of Arizona State University. "On Mars, the southern hemisphere consists of older, cratered highlands, whereas the northern hemisphere consists of younger lowlands. Mercury's surface is more homogeneously ancient and heavily cratered, with large extents of younger volcanic plains lying within and between giant impact basins."

Color imaging also shows that Mercury's crust is compositionally heterogeneous. "Although definitive compositional interpretations cannot yet be made, the distribution of different components varies both across the surface and with depth. Mercury's crust is more analogous to a marbled cake than a layered cake," Robinson said. "Once MESSENGER's suite of science instruments returns a host of data from the orbital phase of the mission, compositions will be determined for the newly discovered color units."

Solomon said that the first two Mercury flybys have returned a rich dividend of new observations. "But some of the observations we are most eager to make — such as the chemical makeup of Mercury's surface and the nature of its enigmatic polar deposits — will not be possible until MESSENGER begins to orbit the innermost planet," he said. "Moreover, the very dynamic nature of Mercury's interaction with its interplanetary environment has taught us that continuous observations will be required before we can claim to understand our most sunward sister planet."

The MESSENGER spacecraft launched on Aug. 3, 2004, and after flybys of Earth, Venus and Mercury will start a yearlong study of Mercury, its target planet, in March 2011. The Applied Physics Laboratory built and operates the spacecraft and manages the Discovery-class mission for NASA.


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