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Mars Global Surveyor Detects Magnetic Field on Mars

Douglas Isbell
Headquarters, Washington, DC.           Sept. 17, 1997
(Phone: 202/358-1753)

Diane Ainsworth
Jet Propulsion Laboratory, Pasadena, CA
(Phone: 818/354-5011)

Bill Steigerwald
Goddard Space Flight Center, Greenbelt, MD
(Phone: 301/286-7277)

RELEASE: 97-204

MARS GLOBAL SURVEYOR DETECTS MARTIAN MAGNETIC FIELD AS AEROBRAKING BEGINS

     Scientists have confirmed the existence of a planet-wide 
magnetic field at Mars using an instrument on-board NASA's Mars 
Global Surveyor orbiter, as the spacecraft began to circle and 
study the planet from a highly elliptical orbit.  

     "Mars Global Surveyor has been in orbit for only a few days, 
yet it already has returned an important discovery about the Red 
Planet," said Vice President Al Gore.  "This is another example of 
how NASA's commitment to faster, better, cheaper Mars exploration 
that began with Mars Pathfinder is going to help answer many 
fundamental questions about the history and environment of our 
neighboring planet, and the lessons it may hold for a better 
understanding of life on Earth."

     The spacecraft's magnetometer, which began making 
measurements of Mars' magnetic field after its capture into orbit 
on Sept. 11, detected the magnetic field on Sept. 15. The 
existence of a planetary magnetic field has important implications 
for the geological history of Mars and for the possible 
development and continued existence of life on Mars.

      "Preliminary evidence of a stronger than expected magnetic 
field of planetary origin was collected and is now under detailed 
study," said Dr. Mario H. Acuna, principal investigator for the 
magnetometer/electron reflectrometer instrument at NASA's Goddard 
Space Flight Center, Greenbelt, MD.  "This was the first 
opportunity in the mission to collect close-in magnetic field 
data. Much more additional data will be collected in upcoming 
orbits during the aerobraking phase of the mission to further 
characterize the strength and geometry of the field. The current 
observations suggest a field with a polarity similar to that of 
Earth's and opposite that of Jupiter, with a maximum strength not 
exceeding 1/800ths of the magnetic field at the Earth's surface."

     This result is the first conclusive evidence of a magnetic 
field at Mars. "More distant observations obtained previously by 
the Russian missions Mars 2, 3 and 5 and Phobos 1 and 2 were 
inconclusive regarding the presence or absence of a magnetic field 
of internal origin," said Acuna.

     The magnetic field has important implications for the 
evolution of Mars. Planets like Earth, Jupiter and Saturn generate 
their magnetic fields by means of a dynamo made up of moving 
molten metal at the core. This metal is a very good conductor of 
electricity, and the rotation of the planet creates electrical 
currents deep within the planet that give rise to the magnetic 
field. A molten interior suggests the existence of internal heat 
sources, which could give rise to volcanoes and a flowing crust 
responsible for moving continents over geologic time periods.

     "A magnetic field shields a planet from fast-moving, 
electrically charged particles from the Sun which may affect its 
atmosphere, as well as from cosmic rays, which are an impediment 
to life," Acuna said. "If Mars had a more active dynamo in its 
past, as we suspect from the existence of ancient volcanoes there, 
then it may have had a thicker atmosphere and liquid water on its 
surface."

     It is not known whether the current weaker field now results 
from a less active dynamo, or if the dynamo is now extinct and 
what the scientists are observing is really a remnant of an 
ancient magnetic field still detectable in the Martian crust. 

     "Whether this weak magnetic field implies that we are 
observing a fossil crustal magnetic field associated with a now 
extinct dynamo or merely a weak but active dynamo similar to that 
of Earth, Jupiter, Saturn, Uranus and Neptune remains to be seen," 
Acuna said.

     Mars Global Surveyor's magnetometer discovered the outermost 
boundary of the Martian magnetic field -- known as the bow shock -- 
during the inbound leg of its second orbit around the planet, and 
again on the outbound leg.

     The discovery came just before Mars Global Surveyor began its 
first aerobraking maneuver to lower and circularize its orbit 
around Mars, said Glenn Cunningham, Mars Global Surveyor project 
manager at NASA's Jet Propulsion Laboratory (JPL), Pasadena, CA.

     "This first 'step down' into the upper atmosphere was 
performed in two stages," Cunningham said. "On Sept. 16, during 
the farthest point in the spacecraft's orbit, called the apoapsis, 
the spacecraft fired its main engine for 6.5 seconds, slowing 
Global Surveyor's velocity by 9.8 miles per hour (4.41 meters per 
second).  This maneuver  lowered the spacecraft's orbit from 163 
miles (263 kilometers) to 93 miles (150 kilometers) above the 
surface of the planet.  

     At its closest approach to Mars this morning, known as the 
periapsis, the spacecraft dipped into the upper fringes of the 
Martian atmosphere for 27 seconds, allowing the drag on its solar 
panels to begin the long aerobraking process of circularizing its 
orbit."

     Mars Global Surveyor will continue aerobraking through the 
Martian atmosphere for the next four months, until its orbit has 
been circularized and it is flying about 234 miles (378 
kilometers) above the Martian surface.  All systems and science 
instruments onboard the spacecraft continue to perform normally 
after six days in orbit around the red planet. 

     Additional information about the magnetic field discovery and 
the Mars Global Surveyor mission is available on the World Wide 
Web by accessing the JPL home page at:

                  http://www.jpl.nasa.gov 

or at the Goddard Space Flight Center magnetometer site at: 

             http://mgs-mager.gsfc.nasa.gov

     Meanwhile, NASA's Hubble Space Telescope (HST) has continued 
monitoring the atmospheric conditions on Mars to help planning for 
the Mars Global Surveyor aerobraking activity.  The latest HST 
Mars image, taken Sept. 12 with the Wide Field Planetary Camera 2 
under the direction of Phil James of the University of Toledo and 
Steve Lee of the University of Colorado, is available on the 
Internet at the following URLs:

     http://oposite.stsci.edu/pubinfo/PR/gif/mars0609.gif (GIF),
    http://oposite.stsci.edu/pubinfo/PR/jpeg/mars0609.jpg (JPEG) 

                       and via links in:

          http://oposite.stsci.edu/pubinfo/PR/97/31.html

     Mars Global Surveyor is the first mission in a sustained 
program of robotic Mars exploration, known as the Mars Surveyor 
Program.  The mission is managed by the Jet Propulsion Laboratory 
for NASA's Office of Space Science, Washington, DC. JPL's 
industrial partner is Lockheed Martin Astronautics, Denver, CO, 
which developed and operates the spacecraft. JPL is a division of 
the California Institute of Technology, Pasadena, CA.

                              -end-