Another wrinkle in planetary internal convection induced dynamo theory

Posted: January 23, 2015 by tallbloke in Astrophysics, Geomagnetism

From Space.com. It’s worth reading between the lines of this one to see the perplexed furrows on the brows of the ‘surprised’  scientists.

dynamoThe magnetic fields of planetary building blocks lasted for a surprisingly long time in the solar system’s early days, a new study suggests.

The magnetic fields of these big asteroids were apparently generated by the same process that drives Earth’s global magnetic activity, and could have persisted for hundreds of millions of years after the objects’ formation, researchers said.
The study team analyzed pallasites, iron-and-nickel meteorites believed to originate from an ancient rocky body about 250 miles (400 kilometers) wide. The pallasites contain tiny particles of ‪tetrataenite — a mineral that records a magnetic history of the parent body going back billions of years.

The researchers probed this history using an X-ray electron microscope at the BESSY II synchrotron in Berlin, capturing the moment when the big asteroid’s global magnetic field died.

‪”We’re taking ancient magnetic field measurements in nanoscale materials to the highest-ever resolution in order to piece together the magnetic history of asteroids,” study lead author James Bryson, a Ph.D. student at Cambridge University in England, said in a statement. “It’s like a cosmic archaeological mission.”

The pallasites’ parent body was one of many relatively large, rocky objects that formed in the first few million years of solar system history. Radioactive decay heated up the interiors of these planetary building blocks, which segregated into molten metal cores surrounded by rocky mantles.

Scientists had thought that the global magnetic fields of these big asteroids were probably created by the circulation of heat energy within the core. This process would likely have petered out relatively quickly, after a few tens of millions of years at most, researchers said.

But the new X-ray observations, along with computer simulations performed by the study team, paint a different picture: The pallasite parent body’s magnetic field lasted for a long time — perhaps several hundred million years. Furthermore, this field was probably generated by the progressive solidification of the core — a phenomenon known as “compositional convection,” rather than thermal convection as previously presumed, researchers said. (When the core solidified completely, the magnetic field died.)

Compositional convection is also the primary process driving the creation of Earth’s magnetic field.

‪”It’s funny that we study other bodies in order to learn more about the Earth,” Bryson said. “Since asteroids are much smaller than the Earth, they cooled much more quickly, so these processes occur on shorter timescales, enabling us to study the whole process of core solidification.”

The new study has applications to other rocky bodies in the solar system as well, study team members said.

“These conclusions imply that a second epoch of dynamo activity [after thermal-convection-driven dynamos] across a potentially large fraction of small bodies occurred in the early solar system, and help to explain the long-lived magnetic activity observed for other bodies, such as the moon,” they wrote in the new study, which was published online today (Jan. 21) in the journal Nature.

Comments
  1. dscott says:

    The implicit assumption being that at no time was there another round of material ejected from the sun, i.e. everything was hot at once at the beginning.

    Ever wonder how much material the sun tosses into orbit from those solar prominences? Why are scientists surprised by the amount of material still running around the inner solar system?

  2. A. Ames says:

    Tallbloke:

    Do you have, or can you point to, a survey of the means by which we keep track of the planetary magnetic field? I don’t have a feeling for the sensors or networks that might be needed. I think in terms of giant loops scattered around, or maybe magnetic pendulums or bobs, but the only things I see routinely are compasses. The Weather Channel talks about lightning strikes, but never about magnetic field shifts even when flares are active, which suggests there is shortage of monitoring.

    Would there be any value in an amateur network of measurements?

    Thanks for this, and for your continued interest in magnetic phenomena.
    AA

  3. tallbloke says:

    A.Ames: Our friend Vukevic has a better handle on that than I do. I think NOAA hav links to the AA index and other terrestrial indices. http://www.swpc.noaa.gov/communities/space-weather-enthusiasts has the interplanetary stuff too.

  4. Centinel2012 says:

    Reblogged this on Centinel2012 and commented:
    Isn’t this something like the Alfven-Klein model of plasma flows?

  5. Steve C says:

    A. Ames – ‘Would there be any value in an amateur network of measurements?’

    People are already enthusiastically finding it. This site comes from one of my fellow radio amateurs. We do have rather a strong (ok, obsessive) interest in our electromagnetic environment …😉
    http://gw7eri.com/homebrew/hb3.htm

    I was encouraged to see on that page: “The magnetometer sensor … is so sensitive that it must be very carefully aligned on an east/west axis. The magnetic field of the earth’s north/south axis is too strong, and will swamp the sensor.” Details like that hint at very usable sensitivity and accuracy.

  6. oldbrew says:

    Asteroid 2004 BL86 which has its own mini-moon is ‘crossing’ Earth today.

    http://phys.org/news/2015-01-asteroid-earth-today-mini-moon.html

    It takes about 2 weeks less than Mars to orbit the sun.

  7. katesisco says:

    I have had a difficult time finding references to Kirschvink’s IIE. I have been unable to determine just what is proposed. Is it a crustal slide over the mantle? Is it an entire Earth bowing to a new weighty polar mass that demands to be placed on the Equator.
    Now that science has discovered an inner inner core the size of the Moon inside the inner core of a 90 different magnetic orientation of the inner core, does this promote the idea that our Earth has in the geologic past turned in its entirety to obey a new spin required by accommodating equatorial mass?
    I am thinking if that is so, then subsequent continental disruption of Gondwanaland and were disturbed because of this double (competing) field?
    I would also propose that the obvious collection of land masses to the North Pole may be the foundation for yet another ‘polar dump’ to the Equator as was the South Pole dump of Cambrian times. The sun’s hemispheric difference is the reason the continental mass gathering alternates between the N Pole and the S Pole.
    As to how the competing field originates, perhaps absence of material in the mantle close to the core allows solidification of a new magnetic shell over the old? Is it possible that a pending accumulation of N Pole mass would allow a new turn to 90 degrees? If so, would the core then reference the original inner inner magnetic field? Or would an entirely new magnetic field be created over the inner core, the same magnetic orientation of the first but itself a third?
    As all fields take up room, use materials in solidifying, would it be possible that Earth is on the way to becoming Mars?
    A solidified core which I have proposed in my paper 375 MILES TO MARS?