A Messenger to Mercury

For his next cosmic field trip, Northwestern geologist Mark Robinson plans a visit to Mercury.

As a science team member for NASA's upcoming MESSENGER mission, Robinson will be part of only the second expedition to the innermost planet in the solar system. The Mercury Surface, Space Environment, Geochemistry and Ranging mission, scheduled for liftoff in 2004, will spend a year transmitting high-resolution images, atmospheric data and other information about Mercury, which has not been visited since the 1974–75 Mariner 10 mission.

"It's very complicated to go into orbit around Mercury," says Robinson. "You need a heat shield in the front to protect you from the planet and a heat shield in the back to protect you from the sun."

With temperatures on Mercury's surface alternating between 800 degrees and minus 300 degrees Fahrenheit, maintaining a stable operating environment for an orbiting spacecraft is challenging. To escape the worst of the heat, MESSENGER will be put into a highly elliptical orbit, minimizing the amount of time the probe spends close to the planet's surface. The variable orbit, however, will require Robinson and the imager team to operate two cameras — equipped with telephoto and wide-angle lenses — to produce a uniform global portrait of the planet.

Only distant Pluto has been less studied than Mercury, but Robinson has a better idea than most of what the alien world looks like. Four years ago Robinson, who was with the U.S. Geological Survey in Flagstaff, Ariz., at the time, reanalyzed Mariner 10 images taken more than two decades earlier and discerned details of the planet's surface never before observed. Enhancements in computer and image-processing technologies as well as a keener interpretation of how light reflects off the planet's surface led Robinson and colleague Paul Lucey at the University of Hawaii to the discovery of lava flows and deposits from volcanic eruptions across Mercury's face. The scientists also found Mercury had a different chemical composition from the other terrestrial planets of the inner solar system.

The new information only added to Mercury's mystique. Scientists don't know why the planet is the densest of any in the solar system, nor why it is the only planet besides Earth to have a global magnetic field. Mercury is a place of extremes, with temperatures that are among the hottest and coldest in the solar system. In addition to mapping the planet's topography, MESSENGER is being dispatched to study unusual deposits, found by Earth-based radar, that lie in permanently shadowed craters near the poles, to determine whether the planet has a liquid outer core and to measure the composition and structure of the planet's tenuous atmosphere.

The technical challenges will be matched by a test of the scientists' patience. NASA's new low-cost approach to space exploration will require MESSENGER to pick up speed for its journey by flying close to Venus and circling Mercury twice for gravitational boosts before going into orbit in 2009. Once science operations begin, however, time will pass quickly, relative to Mercury at least. The 12-month mission will last only two days in Mercury-time.

— I.B.