Mars' patchwork magnetic field acts as array of umbrellas to protect planet's
atmosphere, according to new mapping study by Mars Global Surveyor
15
Dec 2000
By
Robert Sanders, Media Relations
Color
maps of the ionosphere and the surface magnetic fields are
available for download
Berkeley
- Though Mars lacks a global protective magnetic shield like
that of the Earth, strong localized magnetic fields embedded
in the crust appear to be a significant barrier to erosion
of the atmosphere by the solar wind.
This conclusion
by a researcher at the University of California, Berkeley,
emerges from a new map of the limits of the planet's ionosphere
obtained by the Mars Global Surveyor spacecraft, which was
launched in 1996 and reached the planet 10 months later. The
new data show that where localized surface magnetic fields
are strong, the ionosphere reaches to a higher altitude, indicating
that the solar wind is being kept at bay.
The findings
suggest that these crustal fields could have played an important
role in the past evolution of Mars' atmosphere. If, as some
Mars experts think, much of the planet's atmosphere was stripped
away by the solar wind, these maps show where the solar wind
did, and continues to do, the most damage.
"The ionosphere
is what shields the densest part of Mars' atmosphere from
being swept away by the solar wind," said David Mitchell,
a research physicist at UC Berkeley's Space Sciences Laboratory
who compiled the map from spacecraft data. "Our data show
for the first time that the crustal magnetic fields are a
major factor limiting erosion of the atmosphere in some regions.
These fields are like umbrellas scattered over the surface
protecting the atmosphere."
The map
of the ionosphere will be presented by Mitchell and his colleagues
on Saturday (Dec. 16) during a morning poster session at the
San Francisco meeting of the American Geophysical Union.
Mars at
one time presumably had an interior dynamo like that of the
Earth, which would have generated a global magnetic field
to shield the atmosphere from the solar wind.
Data reported
in 1998 from the same spacecraft indicated that Mars probably
lost its magnetosphere about four billion years ago, at which
time the atmosphere would have felt the full force of the
ionized particle sirocco from the sun. While at one time the
planet apparently had an atmosphere dense enough to allow
liquid water to flow on the surface, most of that has since
disappeared.
A crucial
finding was that ancient asteroid or comet impacts wiped out
part of the crustal magnetism, and those regions were not
subsequently remagnetized. Because these demagnetized craters
are some four billion years old, the dynamo must have wound
down at least that long ago.
"Finding
these demagnetized and very ancient crater sites helped us
date when the dynamo turned off, which was a big help, because
now we know when, in our models, to turn on erosion by the
solar wind," said Mitchell. "What I'm doing is trying to determine
where you can apply the erosion. So now we have the when and
the where, and we can estimate the how much."
Though
he hopes soon to use this information to estimate how quickly
the Martian atmosphere dissipated over time, "We're not at
the stage yet where we can apply this new information to modeling
the atmospheric loss," he said.
Mitchell's
colleagues include Robert P. Lin, professor of physics at
UC Berkeley and director of the Space Sciences Laboratory;
Henri Reme of the Centre d'Etude Spatiale des Rayonnements
(CESR) in Toulouse, France; Paul A. Cloutier of the Department
of Physics and Astronomy at Rice University; and J. E. P.
Connerney and Mario Acu–a of NASA's Goddard Space Flight Center.
The data
were obtained by an electron reflectometer aboard Mars Global
Surveyor, an instrument built at the Space Sciences Laboratory
and CESR to map surface magnetic fields. Mitchell used the
instrument to determine when the spacecraft, orbiting about
400 kilometers above the surface, was inside the planet's
ionosphere or outside in the gale of the solar wind. This
is possible because the energy spectrum of ionospheric electrons
is distinctly different from that of solar wind electrons.
Between
February 1999 and April 2000, the spacecraft mapped the position
of the ionopause - the boundary between the ionosphere and
the solar wind. The final map was an average over this time
period and over thousands of orbits, representing the probability
at any given point that the spacecraft was within or outside
the ionosphere.
When he
compared this map with a map of the surface magnetic fields
obtained by a magnetometer also aboard the spacecraft, he
found that the ionosphere extended to the highest altitudes
over the strong crustal magnetic fields. Over areas of weak
magnetic field, the ionopause rarely reached as high as the
spacecraft orbit, whereas over strong magnetic areas it nearly
always reached the spacecraft at 400 kilometers altitude,
and probably extended hundreds of kilometers higher.
"The correlation
is striking," Mitchell said. "When the spacecraft is flying
over the magnetic anomalies, it is almost always in the ionosphere,
whereas when it's over magnetically weak regions, 90 percent
of the time it is in the solar wind."
Mars' crustal
magnetic fields themselves are a mystery, because they are
nearly as strong at the surface as the Earth's magnetic field
- a few tenths of a Gauss, compared to a third of a Gauss
on Earth. Plus they are arrayed in east-west bands of alternating
polarity, extending for over 1,000 kilometers north to south
like a bar code across the planet's surface. Scientists still
do not know what materials produce this strong field, or why
it occurs in alternating bands.
Mitchell
said the crustal fields have been there for four billion years,
fending off the solar wind. Despite this protection over part
of the planet, however, the solar wind is still considered
the most likely cause of the loss of Mars' atmosphere.
The atmosphere
today extends hundreds of kilometers into space, where the
solar wind can ionize the atoms and sweep them away. At such
high altitudes, however, the density is very low. The ionosphere,
though it does not extend to such high altitudes, nevertheless
protects the densest part of the atmosphere from this type
of erosion.
The magnetic
field lines can be pictured as half cylinders lined up side
by side on the surface, oriented east and west. The place
where the half-cylinders touch are areas of strong vertical
magnetic field, where ionized hydrogen and helium of the solar
wind are able to funnel down to low altitudes. The tops of
the cylinders are areas of strong horizontal magnetic field,
which acts as an umbrella to shield the underlying atmosphere
from the solar wind.
"These
crustal magnetic anomalies form cylindrical magnetic objects
that shield the atmosphere much like the Earth's dipole field
does the entire Earth, except on Mars it is local," he said.
"You can see a very interesting pattern of shielded regions
and cusps where the solar wind funnels down in between."
The study
is supported by the National Aeronautics and Space Administration.
###
Color
maps of the ionosphere and the surface magnetic fields are
available for download
.
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