MESSENGER Settles Old Debates and Makes New Discoveries at Mercury

Scientists have argued about the origins of Mercury's smooth plains and the source of its magnetic field for over 30 years. Now, analyses of data from the January 2008 flyby of the planet by the MESSENGER spacecraft have shown that volcanoes were involved in plains formation and suggest that its magnetic field is actively produced in the planet's core and is not a frozen relic. Scientists additionally took their first look at the chemical composition the planet's surface material. The tiny craft probed the composition of Mercury's thin atmosphere, sampled charged particles (ions) near the planet, and demonstrated new links between both sets of observations and materials on Mercury's surface. The results are reported in a series of 11 papers published in a special section of the July 4 issue of Science magazine.

The controversy over the origin of Mercury's smooth plains began with the 1972 Apollo 16 Moon mission, which suggested that some lunar plains came from material that was ejected by large impacts and then formed smooth ‘ponds.' When Mariner 10 imaged similar formations on Mercury in 1975, some scientists believed that the same processes were at work. Others thought that Mercury's plains material came from erupted lavas, but the absence of volcanic vents or other volcanic features in images from that mission prevented a consensus.

Six of the papers in Science report on analyses of the planet's surface through its reflectance and color variation, surface chemistry, high-resolution imaging at different wavelengths, and altitude measurements. The researchers found evidence of volcanic vents along the margins of the Caloris basin, one of the Solar System's largest and youngest impact basins. They also found that Caloris has a much more complicated geologic history than previously believed.

"By combining Mariner 10 and MESSENGER data, the science team was able to reconstruct a comprehensive geologic history of the entire basin interior," explained James Head of Brown University, the lead author of one of the Science reports. "The Caloris basin was formed from an impact by an asteroid or comet during the heavy bombardment period in the first billion years of Solar System history. As with the lunar maria, a period of volcanic activity produced lava flows that filled the basin interior. This volcanism produced the comparatively light, red material of the interior plains intermingled with impact crater deposits. Subsidence caused the surface of the Caloris floor to shorten, producing what we call wrinkle-ridges. The large troughs, or graben, then formed as a result of later uplift, and more recent impacts yielded newer craters."

The first altitude measurements from any spacecraft at Mercury also found that craters on that planet are about a factor of two shallower than those on the Moon and they, too, show a complex geologic history.

Mariner 10 discovered Mercury's magnetic field. Earth is the only other terrestrial planet with a global magnetic field. In both cases the field produces a protective bubble called a magnetosphere, which generally shields the planet surface from the charged particles of the solar wind. Earth's magnetic field is generated by the churning, hot, liquid-iron core via a mechanism called a magnetic dynamo. Researchers have been puzzled by Mercury's field since its iron core should have cooled long ago and stopped generating magnetism. Some researchers have thought that the field may have been a relic of the past, frozen in the outer crust.

"MESSENGER's measurements did indicate that, like the Earth, Mercury's magnetic field is mostly dipolar, which means it has a north and south magnetic poles," stated lead author Brian Anderson of The Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Md. "The fact that it's dipolar, and that we did not find the ‘signature' shorter-wavelength anomalies that would signify patches of magnetized crust, supports the view that we're seeing a modern dynamo. We are eager for the October flyby and the year in orbit to see if this is the case elsewhere on the planet and confirm that the field comes from the core."

The flyby made the first-ever observations of the ionized particles in Mercury's unique exosphere. The exosphere is an ultrathin atmosphere where the molecules are so far apart they are more likely to collide with the surface than with each other. The planet's highly elliptical orbit, its slow rotation, and particle interactions with the magnetosphere, interplanetary medium, and solar wind result in strong seasonal and day-night differences in the way particles behave.

"MESSENGER was able to observe Mercury's exosphere in three areas—the dayside, the day/night line, or terminator, and its 25,000 mile-long (40,000 km) sodium tail," explained lead author Bill McClintock of the University of Colorado. "Atoms of hydrogen, helium, sodium, potassium, and calcium have been seen in the exosphere, and many other elements almost certainly exist there. When species escape from the surface they are accelerated by solar-radiation pressure and form a long tail of atoms flowing away from the Sun. But their abundances differ depending on whether it's day or night, effects from the magnetic field and solar wind, and possibly the latitude. Mercury's exosphere is remarkably active."

"Since Mariner 10's discovery of Mercury's magnetosphere, there's been speculation about its dynamics, ion composition, and how the solar wind interacts with the surface and exosphere," commented lead author Thomas Zurbuchen of the University of Michigan. "The planet's surface is the most space-weathered of any terrestrial planet, and the interaction of solar wind and micrometeoroid flux with the surface can inject both neutral and charged particles into the exosphere and space. The ion composition was not measured by Mariner 10 and MESSENGER once again provided a significant scientific surprise. The magnetosphere is full of many ionic species, both atomic and molecular and in a variety of charge states. 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. The abundances of silicon, sodium, and sulfur relative to oxygen in the solar wind are too low, and their charge states — ionization — are too high to account for the abundances we measured, so there is no doubt that this material came from the planet's surface. This observation means that this flyby got the first-ever look at surface composition."

Mercury's core makes up 60% of its mass, which is at least twice as large as any other planet. The flyby revealed that the magnetic field, originating in the outer core and powered by core cooling, drives very dynamic and complex interactions among the planet's interior, surface, exosphere, and magnetosphere.

Remarking on the importance of the core to surface geological structures, MESSENGER Principal Investigator Sean Solomon, at the Carnegie Institution of Washington, said: "The dominant tectonic landforms on Mercury, including areas imaged for the first time by MESSENGER, are features called lobate scarps, huge cliffs that mark the tops of crustal faults that formed during the contraction of the surrounding area. They tell us how important the cooling core has been to the evolution of the surface. After the end of the period of heavy bombardment, cooling of the planet's core not only fuels the magnetic dynamo, it also led to contraction of the entire planet. And the data from the flyby indicate that the total contraction is a least one third greater than we previously thought."

"When you look at the planet in the sky, it looks like a simple point of light," remarked MESSENGER Project Scientist Ralph McNutt, of APL. "But when you experience Mercury close-up through all of MESSENGER's ‘senses' seeing it at different wavelengths, feeling its magnetic properties, and touching its surface features and energetic particles, you perceive a complex system and not just a ball of rock and metal. We are all surprised by how active that planet is and at the dynamic interrelationships among its core, surface, exosphere, and magnetosphere."

"It's remarkable that this rich lode of data came from two days of imaging, just 30 minutes of sampling the planet's magnetosphere and exosphere, and less than ten minutes carrying out altimetry and collecting other data near the time of its closest approach 125 miles (200 kilometers) to the surface," offered Solomon. "MESSENGER's first flyby was a huge success, both in keeping us on target for the rest of our journey and in advancing our progress toward answering the science questions that have motivated this mission."

For copies of the papers contact SciPak at 202-326-6440 or scipak@aaas.org