03 November 2008
Second 2008 Flyby Reveals Surprises at Nearest Planet to Sun
Mercury mission to ask questions about geologic history, magnetic field
Washington — NASA’s Mercury Surface, Space Environment, Geochemistry and Ranging (Messenger) spacecraft flew past the planet nearest the sun in the early morning hours of October 6, completing a critical maneuver to keep it on course to orbit Mercury in 2011 and giving scientists a look at 30 percent of the planet’s surface that no one had seen before.
All seven instruments were operating during this second of three planned flybys — the first was January 21 and the last before the orbital phase is scheduled for September 29, 2009. The instruments returned 650 megabytes of data, 1,287 images and measurements of the surface of Mercury’s western hemisphere, atmosphere and magnetic field. (See “Scientists Find Active Magnetic Field, Volcanic Plains at Mercury.”)
Discoveries include a western hemisphere that is smoother than the eastern hemisphere; strong interactions between the solar wind and Mercury’s magnetosphere, the region around Mercury that is controlled by the planet’s magnetic field; the first detection of magnesium in the planet’s tenuous atmosphere, called an exosphere; and volcanism as an important process on the smallest and densest of the terrestrial planets.
“The thing that surprised me most was that one hemisphere of Mercury ... just along the equator where we’ve profiled, appears to be about 30 percent smoother than the other part,” Maria Zuber, Messenger co-investigator and head of the Department of Earth, Atmospheric and Planetary Sciences at the Massachusetts Institute of Technology, said during an October 29 briefing. “That’s something that we’re pretty excited to follow up on.”
QUESTIONS
Messenger is the second spacecraft to visit Mercury. Mariner 10 flew by the planet three times in 1974 and 1975, mapping 42 percent of its surface. The planet is too close to the sun to be imaged by the Hubble Space Telescope without risking damage to the telescope.
The latest mission, launched in 2004, is designed to answer these scientific questions:
• Why is Mercury so dense; what is its geologic history?
• What is the nature of its magnetic field?
• What is the structure of its core?
• What are the unusual materials at its poles?
• What volatiles – chemical elements that vaporize (boil) at relatively low temperatures — are important on the planet?
Comparing magnetosphere observations from Messenger’s first flyby in January with data from the probe’s second pass has given scientists new insight into the nature of Mercury’s internal magnetic field and revealed new features of its magnetosphere.
“The exciting thing about the second flyby from a magnetic field standpoint,” said Brian Anderson, deputy project scientist at the Johns Hopkins University Applied Physics Laboratory (APL) in Maryland, “is that we were able to obtain measurements for the first time from the western hemisphere of the planet.”
During the October 6 flyby, he added, “we saw numerous examples of strong interactions, strong dynamics between the solar wind and Mercury’s magnetic field. ... We weren’t expecting it to be that intense at all.”
CHANGING SYSTEM
The Mercury atmospheric and surface composition spectrometer, said Ronald Vervack, participating scientist at APL, “scans through the ultraviolet and visible portion of the wavelength spectrum and searches for emissions from atoms and molecules that make up the atmosphere.”
The instrument detected sodium and calcium, which had been detected on Mercury from Earth, and for the first time detected magnesium, which cannot be observed from Earth-based telescopes because Earth’s atmosphere blocks its wavelength emission.
“The atoms we see in the exosphere primarily originate from the surface,” Vervack said, “so the fact that we observed magnesium in the exosphere means that it is definitely a part of the Mercury surface. One of the important reasons for studying the exosphere is that it provides unique information about the surface material.”
Spacecraft images also are giving scientists new information about the planet’s history and composition.
“When we had information on Mercury only from Mariner 10, there was a fair amount of ambiguity about whether volcanism was even an important process on the planet,” Zuber said. “We had a lot of materials on the surface and we didn’t know their origin. From the first and second flybys of Mercury, we now have a better understanding that volcanism is quite an important process.”
Combining materials from the Mariner 10 and Messenger missions, images from Mercury now account for about 90 percent of the planet and show rugged terrain, craters and some cliffs, said Mark Robinson, co-investigator and professor at Arizona State University’s School of Earth and Space Exploration, who studies the Mercury surface.
“Impact craters are an important tool for us to get to the subsurface of Mercury,” Robinson said. “We don’t want to know just what’s on the surface; we want to know what’s in the subsurface. This tells us about the full range of compositions that occur in the crust but also in the mantle, which is much below the planet’s surface, because this is where most volcanic materials — we know this from the Earth — come up from the upper part of the mantle.”
More information about Messenger is available at the NASA Web site.
