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Posted by: alan May 3 2018, 07:25 PM

Mars crustal magnetism as measured by Mars global Surveyor:

I noticed the resemblance of parts of it to satellite tracks, but they are somewhat off center.

So I tried NASA's GISS map projector, and playing a hunch used the antipodes of the Borealis Basin as the center:

Has anyone read anything explaining why the curves they make should be roughly centered at the basin's antipodes?

Posted by: mcaplinger May 3 2018, 07:55 PM

http://www.planetary.org/blogs/emily-lakdawalla/2008/1710.html

Posted by: serpens May 8 2018, 05:50 AM

Alternatively Sprenke and Baker provided a reasonably compelling explanation, taking into account true polar wander. The full paper can be found at Icarus Volume 147 Issue 1 (September 2000). Behind a paywall but a comprehensive abstract is here:

https://www.lpi.usra.edu/meetings/lpsc2000/pdf/1930.pdf

Posted by: ectoterrestrial Dec 1 2018, 12:26 AM

QUOTE (alan @ May 3 2018, 12:25 PM)

Mars crustal magnetism as measured by Mars global Surveyor:

I noticed the resemblance of parts of it to satellite tracks, but they are somewhat off center.

So I tried NASA's GISS map projector, and playing a hunch used the antipodes of the Borealis Basin as the center:

Has anyone read anything explaining why the curves they make should be roughly centered at the basin's antipodes?

You don't need a single hemisphere dynamo to form the crustal magnetic remnant.

-Start with a magnetized crust from a standard dynamo dipole.
-The Argyre and Hellas impacts shock demagnetize their respective hexants.
-The Borealis impact obliterates the now northern crust.
-Global relaxation of from the Borealis impact causes E-W aligned crustal fractures.
-These fractures of the crustal "magnets" give the observed linear pattern.

Posted by: serpens Dec 2 2018, 11:45 PM

Assuming that early Mars reflected Earth with a molten core dynamo, albeit smaller, then the magnetic stripes of alternating polarity seem to indicate reversals of the magnetic field combined with either plate tectonics, true polar wander or perhaps a combination of both. The dynamo shut down in the first billion years, possibly due to cooling and the residual, induced fields have been disrupted due to both large impact events and heating due to volcanic activity. It is a pity that we cannot measure the depth of the residual crustal magnetic field as this, combined with the results of Insights thermal determinations could allow us to pin down when the dynamo shut down.

Posted by: ectoterrestrial Dec 3 2018, 06:48 AM

QUOTE (serpens @ Dec 2 2018, 04:45 PM)

Assuming that early Mars reflected Earth with a molten core dynamo, albeit smaller, then the magnetic stripes of alternating polarity seem to indicate reversals of the magnetic field combined with either plate tectonics, true polar wander or perhaps a combination of both. The dynamo shut down in the first billion years, possibly due to cooling and the residual, induced fields have been disrupted due to both large impact events and heating due to volcanic activity. It is a pity that we cannot measure the depth of the residual crustal magnetic field as this, combined with the results of Insights thermal determinations could allow us to pin down when the dynamo shut down.

I remember photocopying the Icarus paper you mention in 2000. Its a neat idea but it was also a very early guess after the 1999 MGS-MAG data came out.

Look at the relationship between the impact basins and the surviving magnetic region. It's all you need. Continental drift is unnecessary in this case.

Some ground truth in this matter would make a huge difference.

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