December 10, 1997
The researchers, who presented their study today (Dec. 10) at the American Geophysical Union meeting in San Francisco, are conducting laboratory experiments to establish what quantity of electrically charged particles might be found near airless Mercury's surface and using those measurements as inputs to their computer simulations.
It is not well understood how Mercury, barely half again as big as Earth's moon, sustains a magnetic field. Furthermore, its proximity to the sun means Mercury is blasted by the charged particles and magnetic energy of the solar wind, whose forces distort and combine with the planet's malleable magnetic shield.
"Mercury is not a good candidate to have a magnetic field, but we know from the Mariner flybys in 1974 and 1975 that it does," said Los Alamos space scientist Rick Elphic. "Any mission that goes to Mercury in the future and hopes to understand the planet's magnetic field is going to have to untangle the effects of the solar wind."
Earth's magnetic field, which generates a protective bubble called the "magnetosphere," is created by the movement of molten metal at Earth's core. Earth's magnetosphere holds at bay most of the effects of the solar wind, but occasionally energy and charged particles from the solar wind wreak havoc with the magnetic shield and create displays such as the aurora or disturbances that can knock out satellites and electric power grids.
Mercury is too small to generate the heat needed to melt a standard interior of nickel and iron; scientists have proposed its interior could be enriched with sulfur or oxygen, which would lower the melting point of the metal.
On the other hand, Mercury is too hot to have a "frozen" magnetic field, as if it were a huge, solid bar magnet. It's sufficiently warm that the individual magnetic particles aren't constrained to align their magnetic forces with one another.
"Mercury is going to have a different kind of magnetosphere than Earth's," Elphic said. Mercury's magnetic field at the surface is 100 times weaker than Earth's field. At the same time, the solar wind pushes against Mercury's magnetosphere with about 10 times greater force than it does against Earth's magnetosphere.
"The solar wind pushes the sunward side of Mercury's magnetosphere very close to the planet's surface -- the wind could even hit the surface on occasion," Elphic said.
In addition to demonstrating the existence of a magnetic field on Mercury, the Mariner 10 mission also took measurements that hinted at the existence of "substorms" within Mercury's magnetosphere. Substorms in Earth's magnetosphere occur when some of the sun's magnetic field lines link with Earth's field lines, transferring energy from the solar wind into the magnetosphere and "creating a rapid reconfiguration of the magnetic field and the plasma trapped in the magnetosphere," Elphic said.
Standard theories of substorms require the existence of an ionosphere -- an electrically conducting atmospheric layer -- to immobilize the base of the magnetic field lines and allow the substorm to develop. Mercury, with no atmosphere, cannot apparently possess an ionosphere.
The researchers looked into whether a thin blanket of electrical charge -- a psuedoionosphere -- could be created by particles hitting Mercury's surface, and possibly help sustain a substorm. They used a soil mixture originally created to mimic the lunar highlands -- the best material available to represent Mercury's surface -- and bombarded the soil simulant with electrons to see how many secondary electrons they could generate and thus what amount of plasma they could expect Mercury's surface to produce.
The lab experiments showed that the process could generate plenty of secondary electrons, but the amount of plasma was still many times less than what Earth's ionosphere contains.
However, co-author Joachim Birn, who presented the paper at the AGU meeting, used the laboratory data to define boundary limits on his computer simulations. His results showed that "a highly conducting ionosphere is not all that important for the occurrence of substorms," Birn said. Some features of substorms seen at Earth are absent -- such as the deflection of electric currents from the magnetosphere, through the ionosphere and toward the ground -- but other features are retained. "The energization of charged particles by electric fields induced by the magnetic reconfiguration should occur quite similarly at Mercury to how it does at Earth," Birn said.
"Ultimately, by better understanding the characteristics of Mercury's magnetosphere we hope to help the people who model planetary interiors better understand the dynamo process that generates the magnetic field to begin with," Elphic said.
Other authors on the paper are Phil Barker, Joe Borovsky, John Bonnell, Sylvestre Maurice and Herb Funsten, all of Los Alamos, and Michael Hesse of NASA's Goddard Space Flight Center.
Los Alamos National Laboratory is operated by the University of California for the U.S. Department of Energy.