The moon has long since cooled off and the global magnetic field disappeared, but the brief burp nearly 4 billion years ago would explain the old, magnetized rocks picked up from the moon's surface during the Apollo missions 30 years ago.
"This 3-D convection model produces an elegant explanation for the magnetic field astronauts discovered on the moon," said UC Berkeley graduate student Dave Stegman, who developed the lunar model based on earlier and more general computer models simulating the dynamics of planetary interiors. "If this model is correct, this would be the first full understanding of the thermal history of any planet, including the Earth, and would be a cornerstone for understanding the histories of all the other planets, such as Mars and Earth."
The theoretical burp predicted by the computer model would also explain the lunar mare - seas of metal-rich volcanic rock, or basalt, that cover much of the near side of the moon but little of the far side.
Stegman; Mark Jellinek, a Miller postdoctoral fellow at UC Berkeley; Mark Richards, UC Berkeley professor of earth and planetary science; and John R. Baumgardner, of Los Alamos National Laboratory, report results of their modeling in the Jan. 9 issue of Nature. The late Stephen A. Zatman, a former post-doctoral fellow at UC Berkeley and most recently of the Department of Earth and Planetary Sciences at Washington University in St. Louis, also contributed significantly to the work.
"Unlike many previous models of planetary evolution, this one starts from a ball of goo and looks at how a likely magma ocean solidifies around a metallic core," said Jellinek, who will be joining the Department of Physics at the University of Toronto in April as an assistant professor. "One message from this research is that, if you want to look at planetary evolution properly, it's important to consider the initial conditions carefully."
A global magnetic field like the Earth's, strong enough to wrench a magnetized needle into north-south alignment, requires active convection within a molten iron core, akin to the convection in a boiling pot of water. The slowly cycling molten metal carries charged particles with it that, like any electric current, generate a magnetic field.
Convection, however, can only be sustained if heat flows out of the core at a high enough rate. The Earth's large core, for example, has presumably remained convective since its formation more than 4.5 billion years, thanks, in part, to the planet's active surface. Through volcanic eruptions and plate subduction, the Earth's tectonic surface efficiently cools the mantle and underlying core to maintain a high heat flux.
The problem with smaller bodies like the moon and Mars is that their cores may not be big enough and hot enough, and the cooling processes in the mantle efficient enough, to maintain a heat flux high enough to allow core convection. The solid crust of these single-plate planets seems to act as a blanket to keep heat from escaping the mantle, damping the heat flux in the core and quenching any convection. If the core heat flux drops below the level needed to sustain convection, any magnetic field disappears, usually leaving the only record of its existence in volcanic rocks erupted during that time.
How, then, could the moon have had a magnetic field from 3.9 to 3.6 billion years ago, as suggested by dating of lunar rocks? Some scientists have proposed that meteor impacts may have magnetized the surface briefly, creating the small fields we see today. Stegman hit upon the idea of a blanket of dense material that would briefly insulate and even heat the core before bobbing to the surface to allow a brief period of rapid heat flux and core convection. Others had proposed such a buoyant thermal blanket to explain the uneven distribution of dense basalts that covers the Earth-facing half of the moon, though support for this has come only from two-dimensional models of the moon's interior.
Stegman had at his disposal a three-dimensional, spherical convective model of planetary interiors originally developed by Baumgardner. Stegman, however, added a crucial component - the ability to account for different chemical elements in the interior. Since different chemicals heat and cool differently and have different densities, this makes a critical difference in what the model can predict.
"Modeling two-component fluid flow, what we call thermochemical convection, is much more difficult than modeling thermal convection alone," Richards said. "This was a technical challenge that Dave Stegman has solved by significant improvements to the computer model developed for the Earth."
Based on his model, Stegman proposes that, after the birth of the moon 4.5 billion years ago from the debris of a cataclysmic collision between the Earth and a Mars-sized object, the moon began to cool and solidify, with material separating into layers of different density. Iron intermixed with sulfur settled to the core, while less dense matter formed a thick mantle above the core. As the mantle solidified, however, the last liquid to freeze was at the top, producing a titanium and thorium-rich layer of rock. Because of the layer's density, however, it was unstable, and some of it eventually dripped through the mantle to form a blanket at the core-mantle boundary.
"Without this sinking, the moon would have cooled off very slowly," Stegman said. "This one event determined whether or not the moon had convection and thus allowed the planet to have an
interesting life."
This layer, rich in radioactive elements, eventually heated up and became buoyant, rising to the top in one or more burps, or superplumes. This removed the thermal blanket surrounding the core, allowing, for a brief time - about 300 million years - sufficiently rapid heat flux to start convection and generate a magnetic field. The lunar model shows that this scenario would create a lunar dynamo and a resultant surface magnetic field of about one-tenth of a Gauss - one-fifth the Earth's current field of one-half Gauss.
The burp would break through the surface over one hemisphere, not the whole surface, Stegman said, possibly explaining the mare of thorium-rich basalts - the dark feature we see as the "man on the moon."
Perhaps the most controversial aspect of the model is whether the early magnetism reported from the moon, based on analysis of moon rocks, is real.
"The paleomagnetism done on moon rocks is sketchy," Richards noted. "Dave's work is really motivating people to go back and reanalyze the samples from the Apollo missions."
This model of the moon's three-dimensional interior could also apply to the Earth, which appears to have a layer of dense material sitting at the core-mantle boundary. The model also could help explain the evolution of other planetary bodies, such as Mars, that have only one crustal plate. Stegman's next projects are to model the Martian interior as well as the dense rock layer at the base of Earth's mantle.
"We are inspired by this work on the moon to think that some similar kind of catastrophic overturn event may have occurred on Mars as well," Richards said.
