Douglas Isbell
Headquarters, Washington, DC               April 29, 1999
(Phone: 202/358-1753)

Bill Steigerwald
Goddard Space Flight Center, Greenbelt, MD
(Phone: 301/286-5017)

RELEASE: 99-56

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 Acuna 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," Acuna 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:

      FTP://PAO.GSFC.NASA.GOV/newsmedia/MARS/MAGNETIC/

       http://mpfwww.jpl.nasa.gov/mgs/index.html

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