Ancient lunar dynamo may explain magnetized moon rocks

ScienceDaily (Nov. 9, 2011) — The presence of magnetized rocks on the surface of the moon, which has no global magnetic field, has been a mystery since the days of the Apollo program. Now a team of scientists has proposed a novel mechanism that could have generated a magnetic field on the moon early in its history.

The "geodynamo" that generates Earth's magnetic field is powered by heat from the inner core, which drives complex fluid motions in the molten iron of the outer core. But the moon is too small to support that type of dynamo, according to Christina Dwyer, a graduate student in Earth and planetary sciences at the University of California, Santa Cruz. In the Nov. 10 issue of Nature, Dwyer and her coauthors--planetary scientists Francis Nimmo at UC Santa Cruz and David Stevenson at the California Institute of Technology--describe how an ancient lunar dynamo could have arisen from stirring of the moon's liquid core driven by the motion of the solid mantle above it.

"This is a very different way of powering a dynamo that involves physical stirring, like stirring a bowl with a giant spoon," Dwyer said.

Dwyer and her coauthors calculated the effects of differential motion between the moon's core and mantle. Early in its history, the moon orbited Earth at a much closer distance than it does today, and it continues to gradually recede from Earth. At close distances, tidal interactions between Earth and the moon caused the moon's mantle to rotate slightly differently than the core. This differential motion of the mantle relative to the core stirred the liquid core, creating fluid motions that, in theory, could give rise to a magnetic dynamo.

"The moon wobbles a bit as it spins--that's called precession--but the core is liquid, and it doesn't do exactly the same precession. So the mantle is moving back and forth across the core, and that stirs up the core, " explained Nimmo, a professor of Earth and planetary sciences at UCSC.

The researchers found that a lunar dynamo could have operated in this way for at least a billion years. Eventually, however, it would have stopped working as the moon got farther away from Earth. "The further out the moon moves, the slower the stirring, and at a certain point the lunar dynamo shuts off," Dwyer said.

Rocks can become magnetized from the shock of an impact, a mechanism some scientists have proposed to explain the magnetization of lunar samples. But recent paleomagnetic analyses of moon rocks, as well as orbital measurements of the magnetization of the lunar crust, suggest that there was a strong, long-lived magnetic field on the moon early in its history.

"One of the nice things about our model is that it explains how a lunar dynamo could have lasted for a billion years," Nimmo said. "It also makes predictions about how the strength of the field should have changed over the years, and that's potentially testable with enough paleomagnetic observations."

More detailed analysis is needed, however, to show that stirring of the core by the mantle would create the right kind of fluid motions to generate a magnetic field. "Only certain types of fluid motions give rise to magnetic dynamos," Dwyer said. "We calculated the power that's available to drive the dynamo and the magnetic field strengths that could be generated. But we really need the dynamo experts to take this model to the next level of detail and see if it works."

A working model of a lunar dynamo, combined with more detailed paleomagnetic analysis of moon rocks, could give scientists a powerful tool for investigating the history of the moon, Dwyer said. In addition, the study presents a novel mechanism for generating a magnetic field not only on the moon, but also on other small bodies, including large asteroids.

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The above story is reprinted from materials provided by University of California - Santa Cruz. The original article was written by Tim Stephens.

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Journal Reference:

C. A. Dwyer, D. J. Stevenson, F. Nimmo. A long-lived lunar dynamo driven by continuous mechanical stirring. Nature, 2011; 479 (7372): 212 DOI: 10.1038/nature10564

Note: If no author is given, the source is cited instead.

Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.

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Ancient lunar dynamo may explain magnetized moon rocks

ScienceDaily (Nov. 9, 2011) — The presence of magnetized rocks on the surface of the moon, which has no global magnetic field, has been a mystery since the days of the Apollo program. Now a team of scientists has proposed a novel mechanism that could have generated a magnetic field on the moon early in its history.

The "geodynamo" that generates Earth's magnetic field is powered by heat from the inner core, which drives complex fluid motions in the molten iron of the outer core. But the moon is too small to support that type of dynamo, according to Christina Dwyer, a graduate student in Earth and planetary sciences at the University of California, Santa Cruz. In the Nov. 10 issue of Nature, Dwyer and her coauthors--planetary scientists Francis Nimmo at UC Santa Cruz and David Stevenson at the California Institute of Technology--describe how an ancient lunar dynamo could have arisen from stirring of the moon's liquid core driven by the motion of the solid mantle above it.

"This is a very different way of powering a dynamo that involves physical stirring, like stirring a bowl with a giant spoon," Dwyer said.

Dwyer and her coauthors calculated the effects of differential motion between the moon's core and mantle. Early in its history, the moon orbited Earth at a much closer distance than it does today, and it continues to gradually recede from Earth. At close distances, tidal interactions between Earth and the moon caused the moon's mantle to rotate slightly differently than the core. This differential motion of the mantle relative to the core stirred the liquid core, creating fluid motions that, in theory, could give rise to a magnetic dynamo.

"The moon wobbles a bit as it spins--that's called precession--but the core is liquid, and it doesn't do exactly the same precession. So the mantle is moving back and forth across the core, and that stirs up the core, " explained Nimmo, a professor of Earth and planetary sciences at UCSC.

The researchers found that a lunar dynamo could have operated in this way for at least a billion years. Eventually, however, it would have stopped working as the moon got farther away from Earth. "The further out the moon moves, the slower the stirring, and at a certain point the lunar dynamo shuts off," Dwyer said.

Rocks can become magnetized from the shock of an impact, a mechanism some scientists have proposed to explain the magnetization of lunar samples. But recent paleomagnetic analyses of moon rocks, as well as orbital measurements of the magnetization of the lunar crust, suggest that there was a strong, long-lived magnetic field on the moon early in its history.

"One of the nice things about our model is that it explains how a lunar dynamo could have lasted for a billion years," Nimmo said. "It also makes predictions about how the strength of the field should have changed over the years, and that's potentially testable with enough paleomagnetic observations."

More detailed analysis is needed, however, to show that stirring of the core by the mantle would create the right kind of fluid motions to generate a magnetic field. "Only certain types of fluid motions give rise to magnetic dynamos," Dwyer said. "We calculated the power that's available to drive the dynamo and the magnetic field strengths that could be generated. But we really need the dynamo experts to take this model to the next level of detail and see if it works."

A working model of a lunar dynamo, combined with more detailed paleomagnetic analysis of moon rocks, could give scientists a powerful tool for investigating the history of the moon, Dwyer said. In addition, the study presents a novel mechanism for generating a magnetic field not only on the moon, but also on other small bodies, including large asteroids.

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Story Source:

The above story is reprinted from materials provided by University of California - Santa Cruz. The original article was written by Tim Stephens.

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Journal Reference:

C. A. Dwyer, D. J. Stevenson, F. Nimmo. A long-lived lunar dynamo driven by continuous mechanical stirring. Nature, 2011; 479 (7372): 212 DOI: 10.1038/nature10564

Note: If no author is given, the source is cited instead.

Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.

View the original article here

Ancient supernovas discovered: 10-billion-year-old exploding stars were a source of Earth's iron, researchers say

ScienceDaily (Oct. 7, 2011) — Supernovas -- stars in the process of exploding -- open a window onto the history of the elements of Earth's periodic table as well as the history of the universe. All of those heavier than oxygen were formed in nuclear reactions that occurred during these explosions.

The most ancient explosions, far enough away that their light is reaching us only now, can be difficult to spot. A project spearheaded by Tel Aviv University researchers has uncovered a record-breaking number of supernovas in the Subaru Deep Field, a patch of sky the size of a full moon. Out of the 150 supernovas observed, 12 were among the most distant and ancient ever seen.

The discovery sharpens our understanding of the nature of supernovas and their role in element formation, say study leaders Prof. Dan Maoz, Dr. Dovi Poznanski and Or Graur of TAU's Department of Astrophysics at the Raymond and Beverly Sackler School of Physics and Astronomy. These "thermonuclear" supernovas in particular are a major source of iron in the universe.

The research, which appears in the Monthly Notices of the Royal Astronomical Society this month, was done in collaboration with teams from a number of Japanese and American institutions, including the University of Tokyo, Kyoto University, the University of California Berkeley, and Lawrence Berkeley National Laboratory.

A key element of the universe

Supernovas are nature's "element factories." During these explosions, elements are both formed and flung into interstellar space, where they serve as raw materials for new generations of stars and planets. Closer to home, says Prof. Maoz, "these elements are the atoms that form the ground we stand on, our bodies, and the iron in the blood that flows through our veins." By tracking the frequency and types of supernova explosions back through cosmic time, astronomers can reconstruct the universe's history of element creation.

In order to observe the 150,000 galaxies of the Subaru Deep Field, the team used the Japanese Subaru Telescope in Hawaii, on the 14,000-foot summit of the extinct Mauna Kea volcano. The telescope's light-collecting power, sharp images, and wide field of view allowed the researchers to overcome the challenge of viewing such distant supernovas.

By "staring" with the telescope at the Subaru Deep Field, the faint light of the most distant galaxies and supernovas accumulated over several nights at a time, forming a long and deep exposure of the field. Over the course of observations, the team "caught" the supernovas in the act of exploding, identifying 150 supernovas in all.

Sourcing man's life-blood

According to the team's analysis, thermonuclear type supernovas, also called Type-la, were exploding about five times more frequently 10 billion years ago than they are today. These supernovas are a major source of iron in the universe, the main component of Earth's core and an essential ingredient of the blood in our bodies.

Scientists have long been aware of the "universal expansion," the fact that galaxies are receding from one another. Observations using Type-Ia supernovas as beacons have shown that the expansion is accelerating, apparently under the influence of a mysterious "dark energy" -- the 2011 Nobel Prize in Physics will be awarded to three astronomers for this work. However, the nature of the supernovas themselves is poorly understood. This study improves our understanding by revealing the range of the ages of the stars that explode as Type-Ia supernovas. Eventually, this will enhance their usefulness for studying dark energy and the universal expansion, the researchers explain.

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The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by American Friends of Tel Aviv University.

Journal Reference:

K. Maguire, M. Sullivan, R. C. Thomas, P. Nugent, D. A. Howell, A. Gal-Yam, I. Arcavi, S. Ben-Ami, S. Blake, J. Botyanszki, C. Buton, J. Cooke, R. S. Ellis, I. M. Hook, M. M. Kasliwal, Y.-C. Pan, R. Pereira, P. Podsiadlowski, A. Sternberg, N. Suzuki, D. Xu, O. Yaron, J. S. Bloom, S. B. Cenko, S. R. Kulkarni, N. Law, E. O. Ofek, D. Poznanski, R. M. Quimby. PTF10ops - a subluminous, normal-width light curve Type Ia supernova in the middle of nowhere. Monthly Notices of the Royal Astronomical Society, 2011; DOI: 10.1111/j.1365-2966.2011.19526.x

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