[meteorite-list] The Magnetic Personality of Ancient Mars

From: Gerald Flaherty <grf2_at_meteoritecentral.com>
Date: Mon May 16 21:30:42 2005
Message-ID: <015a01c55a80$07e77450$2f01a8c0_at_Dell>

So revealing. Great research project! Thanks Ron. Jerry
----- Original Message -----
From: "Ron Baalke" <baalke_at_zagami.jpl.nasa.gov>
To: "Meteorite Mailing List" <meteorite-list_at_meteoritecentral.com>
Sent: Wednesday, May 11, 2005 4:33 PM
Subject: [meteorite-list] The Magnetic Personality of Ancient Mars


>
>
> http://www.space.com/scienceastronomy/050511_magnetic_mars.html
>
> The Magnetic Personality of Ancient Mars
> By Robert Roy Britt
> space.com
> 11 May 2005
>
> A new study of old rocks on Earth could force a revision of theories
> about Mars. The results suggest ancient Mars might have been more
> magnetic than thought, challenging basic assumptions about the evolution
> of the red planet.
>
> Unlike modern Earth, Mars has almost no magnetic field
> today. Evidence has suggested Mars didn't have a very strong magnetic
> field early on, either.
>
> Our planet's magnetism is created by the rubbing of a solid inner core
> against a liquid outer core, which rotate at different rates
> and act as a dynamo. The magnetic field helps deflect cosmic radiation
> and solar particles, making Earth comparatively more habitable.
>
> Fossil compass
>
> Magnetism is recorded in the structure of rocks. Superheated material,
> when it cools, takes on a structure parallel to the prevailing magnetic
> field at the time.
>
> A planet's magnetic activity changes over the eons, in part because a
> young planet cools and solidifies as it ages, so ancient bedrock can
> serve as a time capsule for magnetism, a sort of fossil compass.
>
> A study in 2003 found the core of Mars, at least the outer part, is
> liquid.
>
> Surveys in the 1990s of magnetic fields on Mars, by the orbiting Mars
> Global Surveyor, detected the signatures of relatively intense magnetism
> in some of the planet's more modern surfaces. But the fields were found
> to be very weak in two large and old impact basis, called Hellas
> and Argyre.
>
> Each basin, carved out by a colossal space rock, is more than 3 billion
> years old. The data implied that Mars had a weak magnetic field back then.
>
> That analysis has influenced theories of how Mars cooled after its
> formation and when its inner layers developed distinct boundaries.
>
> Up close
>
> The new research calls into question the validity of measuring magnetism
> from an orbital perch.
>
> A team led by Stuart Gilder of the Paris Earth Physics Institute found
> that rocks in the 2-billion-year-old Vredefort impact crater in South
> Africa -- the oldest such structure on Earth -- are highly magnetized,
> yet from above the magnetism appears weak. Two other ancient craters
> reveal similar differences.
>
> The basic reason is simple: While magnetism is strong in individual
> rocks, the direction varies from rock to rock in these impact craters,
> so when examined from a distance, they cancel each other out.
>
> The study is detailed in the May 12 issue of the journal Nature.
>
> "Meteorite craters can then seem to be magnetic or non-magnetic,
> depending on how close the magnetometer is to the source," writes David
> Dunlop, a University of Toronto researcher, in an accompanying analysis.
> "Viewed from satellite altitudes of 100-400 kilometers [60-250 miles],
> martian impact basins would appear magnetically featureless if the
> magnetic vectors of their source rocks vary in direction over distances
> of a few kilometers or less."
>
> Exactly why the rocks are magnetized randomly is more complicated.
>
> Based on differing mineral structures in the rocks, Gilder and his
> colleagues hypothesize that when a space rock hits, the shock of the
> event would briefly create intense localized magnetic fields. Rocks that
> cool during this initial period would be magnetized with orientation
> related to these temporary field. Other rocks would cool more slowly,
> and would take on the planet's magnetic orientation.
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Received on Mon 16 May 2005 09:30:37 PM PDT