Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Moon’s early history may have been interrupted by big burp, geophysicists claim

09.01.2003


Using a state-of-the-art computer model of the lunar interior, geophysicists at the University of California, Berkeley, have shown that a mighty burp early in the moon’s history could account for some of its geologic mysteries.



The burp of hot rock, like a blob rising to the top of a lava lamp, would have lifted a blanket covering the moon’s core, allowing the core to cool quickly enough to produce a magnetic field.

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.


The research was funded by the Los Alamos National Laboratory, the National Aeronautics and Space Administration, the National Science Foundation and the Miller Institute for Basic Research in Science.

A color graphic showing a rising superplume in the moon’s mantle is at http://www.berkeley.edu/news/media/download/2003/01/moon600_2.tif.

Robert Sanders | EurekAlert!
Further information:
http://www.berkeley.edu/
http://www.berkeley.edu/news/media/download/2003/01/moon600_2.tif

More articles from Physics and Astronomy:

nachricht Hope to discover sure signs of life on Mars? New research says look for the element vanadium
22.09.2017 | University of Kansas

nachricht Calculating quietness
22.09.2017 | Forschungszentrum MATHEON ECMath

All articles from Physics and Astronomy >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: The pyrenoid is a carbon-fixing liquid droplet

Plants and algae use the enzyme Rubisco to fix carbon dioxide, removing it from the atmosphere and converting it into biomass. Algae have figured out a way to increase the efficiency of carbon fixation. They gather most of their Rubisco into a ball-shaped microcompartment called the pyrenoid, which they flood with a high local concentration of carbon dioxide. A team of scientists at Princeton University, the Carnegie Institution for Science, Stanford University and the Max Plank Institute of Biochemistry have unravelled the mysteries of how the pyrenoid is assembled. These insights can help to engineer crops that remove more carbon dioxide from the atmosphere while producing more food.

A warming planet

Im Focus: Highly precise wiring in the Cerebral Cortex

Our brains house extremely complex neuronal circuits, whose detailed structures are still largely unknown. This is especially true for the so-called cerebral cortex of mammals, where among other things vision, thoughts or spatial orientation are being computed. Here the rules by which nerve cells are connected to each other are only partly understood. A team of scientists around Moritz Helmstaedter at the Frankfiurt Max Planck Institute for Brain Research and Helene Schmidt (Humboldt University in Berlin) have now discovered a surprisingly precise nerve cell connectivity pattern in the part of the cerebral cortex that is responsible for orienting the individual animal or human in space.

The researchers report online in Nature (Schmidt et al., 2017. Axonal synapse sorting in medial entorhinal cortex, DOI: 10.1038/nature24005) that synapses in...

Im Focus: Tiny lasers from a gallery of whispers

New technique promises tunable laser devices

Whispering gallery mode (WGM) resonators are used to make tiny micro-lasers, sensors, switches, routers and other devices. These tiny structures rely on a...

Im Focus: Ultrafast snapshots of relaxing electrons in solids

Using ultrafast flashes of laser and x-ray radiation, scientists at the Max Planck Institute of Quantum Optics (Garching, Germany) took snapshots of the briefest electron motion inside a solid material to date. The electron motion lasted only 750 billionths of the billionth of a second before it fainted, setting a new record of human capability to capture ultrafast processes inside solids!

When x-rays shine onto solid materials or large molecules, an electron is pushed away from its original place near the nucleus of the atom, leaving a hole...

Im Focus: Quantum Sensors Decipher Magnetic Ordering in a New Semiconducting Material

For the first time, physicists have successfully imaged spiral magnetic ordering in a multiferroic material. These materials are considered highly promising candidates for future data storage media. The researchers were able to prove their findings using unique quantum sensors that were developed at Basel University and that can analyze electromagnetic fields on the nanometer scale. The results – obtained by scientists from the University of Basel’s Department of Physics, the Swiss Nanoscience Institute, the University of Montpellier and several laboratories from University Paris-Saclay – were recently published in the journal Nature.

Multiferroics are materials that simultaneously react to electric and magnetic fields. These two properties are rarely found together, and their combined...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

“Lasers in Composites Symposium” in Aachen – from Science to Application

19.09.2017 | Event News

I-ESA 2018 – Call for Papers

12.09.2017 | Event News

EMBO at Basel Life, a new conference on current and emerging life science research

06.09.2017 | Event News

 
Latest News

Rainbow colors reveal cell history: Uncovering β-cell heterogeneity

22.09.2017 | Life Sciences

Penn first in world to treat patient with new radiation technology

22.09.2017 | Medical Engineering

Calculating quietness

22.09.2017 | Physics and Astronomy

VideoLinks
B2B-VideoLinks
More VideoLinks >>>