Message-Id: <l03110708b35414447aca@[192.174.2.173]>
Date: Mon, 3 May 1999 19:05:54 -0800
To: pinhole@exploratorium.edu
From: Paul Doherty <pauld@exploratorium.edu>
Subject: Mars Magnetism with picture pointer
> Here is the NASA press release on the Martian Magnetic stripes.
You can see the image at
http://www.skypub.com/news/news.shtml
Paul Doherty
> MAGNETIC STRIPES PRESERVE RECORD OF ANCIENT MARS
>
> NASA's Mars Global Surveyor has discovered surprising
> evidence of past movement of the Martian crust, further evidence
> that ancient Mars was a more dynamic, Earth-like planet than it is
> today.
>
> Scientists using the spacecraft's magnetometer have
> discovered banded patterns of magnetic fields on the Martian
> surface. The adjacent magnetic bands point in opposite directions,
> giving these invisible stripes a striking similarity to patterns
> seen in the crust of Earth's sea floors. On the Earth, the sea
> floor spreads apart slowly at mid-oceanic ridges as new crust
> flows up from Earth's hot interior. Meanwhile, the direction of
> Earth's magnetic field reverses occasionally, resulting in
> alternating stripes in the new crust that carry a fossil record of
> the past hundreds of million years of Earth's magnetic history, a
> finding that validated the once-controversial theory of plate
> tectonics.
>
> "The discovery of this pattern on Mars could revolutionize
> current thinking of the red planet's evolution," said Dr. Jack
> Connerney of NASA's Goddard Space Flight Center, Greenbelt, MD, an
> investigator on the Global Surveyor's magnetometer team. "If the
> bands on Mars are an imprint of crustal spreading, they are a
> relic of an early era of plate tectonics on Mars. However, unlike
> on Earth, the implied plate tectonic activity on Mars is most
> likely extinct."
>
> Alternate explanations for the banded structure may involve
> the fracturing and breakup of an ancient, uniformly magnetized
> crust due to volcanic activity or tectonic stresses from the rise
> and fall of neighboring terrain.
>
> "Imagine a thin coat of dried paint on a balloon, where the
> paint is the crust of Mars," explained Dr. Mario Acu–a of Goddard,
> principal investigator on the Global Surveyor magnetometer. "If
> we inflate the balloon further, cracks can develop in the paint,
> and the edges of the cracks will automatically have opposite
> polarities, because nature does not allow there to be a positive
> pole without a negative counterpart."
>
> Peer-reviewed research based on the observations will be
> published in the April 30 issue of the journal Science.
>
> The observations of the so-called magnetic stripes were made
> possible because of Mars Global Surveyor's special aerobraking
> orbit. This process of dipping into the upper atmosphere of Mars
> to gradually shape the probe's orbit into a circle was extended
> due to a problem with a solar panel on the spacecraft. The lowest
> point of each elliptically shaped orbit curved below the planet's
> ionosphere, allowing the magnetometer to obtain better-than-
> planned regional measurements of Mars.
>
> "At its nominal orbit more than 200 miles high, the
> instruments face too much magnetic interference, and they do not
> have the resolution to detect these features," Acu–a noted. "We
> began with misfortune, and ended up winning the lottery."
>
> The bands of magnetized crust apparently formed in the
> distant past when Mars had an active dynamo, or hot core of molten
> metal, which generated a global magnetic field. Mars was
> geologically active, with molten rock rising from below cooling at
> the surface and forming new crust. As the new crust solidified,
> the magnetic field that permeated the rock was "frozen" in the
> crust. Periodically, conditions in the dynamo changed and the
> global magnetic field reversed direction. The oppositely directed
> magnetic field was then frozen into newer crust.
>
> "Like a Martian tape recorder, the crust has preserved a
> fossil record of the magnetic field directions that prevailed at
> different times in the ancient past," Connerney said. When the
> planet's hot core cooled, the dynamo ceased and the global
> magnetic field of Mars vanished. However, a record of the
> magnetic field was preserved in the crust and detected by the
> Global Surveyor instrument.
>
> The mission's map of Martian magnetic regions may help solve
> another mystery -- the origin of a striking difference in
> appearance between the smooth, sparsely cratered northern lowlands
> of Mars and the heavily cratered southern highlands. The map
> reveals that the northern regions are largely free of magnetism,
> indicating the northern crust formed after the dynamo died.
>
> "The dynamo likely died a few hundred million years after
> Mars' formation. One possibility is that later asteroid impacts
> followed by volcanic activity heated and shocked large areas of
> the northern crust, obliterating any local magnetic fields and
> smoothing the terrain," Acuna said. "When the crust cooled,
> there was no longer a global magnetic field to become frozen in
> again."
>
> The map also identifies an area in the southern highlands as
> the oldest surviving unmodified crust on Mars. This area on Mars
> is where the magnetic stripes are most prominent. The bands are
> oriented approximately east-to-west and are about 100 miles wide
> and 600 miles long, although the longest band stretches more than
> 1,200 miles.
>
> "The bands are wider than those on Earth, perhaps for a
> couple of reasons," Connerney said. "The Martian crust could have
> been generated at a greater rate, causing a given magnetic field
> to be imprinted over a wider area before it reversed direction.
> Second, the Martian magnetic field may have reversed direction
> less frequently, which would have given more time for any one
> field direction to imprint itself in the steadily moving crust,
> resulting in wider bands.
>
> "In order to call this pattern a crustal spreading center
> like that observed in the mid-oceanic ridges on Earth, we need to
> find a point of symmetry, where the pattern on one side matches
> the pattern on the other. We have not yet found evidence of this
> type of symmetry," Connerney added.
>
> Graphics of the magnetometer data, other supporting material
> and general information on the Global Surveyor mission may be
> found on the Internet at:
Paul "But it is more complicated than that!" Doherty,
Senior Staff Scientist, The Exploratorium.
pauld@exploratorium.edu, www.exo.net/~pauld