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Contact: Diane Ainsworth
FOR IMMEDIATE RELEASESeptember 17, 1997
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 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 4.41 meters
per second (9.8 miles per hour). This maneuver lowered the spacecraft's orbit from 263 kilometers (163 miles)
to 150 kilometers (93 miles) 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 378
kilometers (234 miles) 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:
or the Mars Global Surveyor home page at:
or at the Goddard Space Flight Center magnetometer site at:
Meanwhile, NASA’s Hubble Space Telescope has continued
monitoring the atmospheric conditions on Mars to help planning for the Mars Global Surveyor aerobraking
activity. The latest Hubble 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:
and via links in:
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.