MGS Press Conference, 2 October 1997
Speech given by
Dr. Jack
Connerney, Astrophysicist
Click on each picture for larger views.
This artist's rendition depicts the response of the solar wind
to the obstacle - the planet Mars - in it's path. A supersonic
"solar wind" consisting of electrically charged particles
(ions and electrons) streams off the Sun into space. It is
slowed to subsonic speeds in the vicinity of Mars at a parabolic
surface called a "bow shock" upstream of the planet. Here, the magnetic
field fluctuates wildly and the flow of the solar wind becomes
chaotic. Part of the orbital trajectory of the mars Global Surveyor
is indicated, with MGS approaching the planet just prior to over-
flight of the pole.
This figure shows a cross-section of the planet Mars revealing
an inner, high density core buried deep within the interior. Dipole
magnetic field lines are drawn in blue, showing the global scale
magnetic field that one associates with dynamo generation in the core.
Mars must have one day had such a field, but today it is not evident.
Perhaps the energy source that powered the early dynamo has shut down.
The differentiation of the planet interior - heavy elements like iron
sinking towards the center of the planet - can provide energy as can
the formation of a solid core from the liquid.
This image shows the orientation and magnitude of the magnetic field
measured by the MGS magnetometer as it sped over the surface of Mars
during an early aerobraking pass (Day of the year, 264; "P6" periapsis
pass). At each point along the spacecraft trajectory we've drawn vectors
in the direction of the magnetic field measured at that instant; the
length of the line is scaled to show the relative magnitude of the field.
Imagine traveling along with the MGS spacecraft, holding a string with
a magnetized needle on one end: this is esentially a compass with a
needle that is free to spin in all directions. As you pass over the surface
the needle would swing rapidly, first pointing towards the planet and then
rotating quickly towards "up" and back down again. All in a relatively
short span of time, say a minute or two, during which time the spacecraft
has traveled a couple of hundred miles. You've just passed over one of
many "magnetic anomalies" thus far detected near the surface of Mars.
A second major anomaly appears a little later along the spacecraft track,
about 1/4 the magnitude of the first - can you find it? The short scale
length of the magnetic field signature locates the source near the surface
of Mars, perhaps in the crust, a 10 to 75 kilometer thick outer shell of
the planet (radius 3397 km).
This is a simple schematic representation of localized magnetic sources
in the crust of Mars, buried beneath the surface, and revealed by observation
of the magnetic field (blue) extending up to satellite altitude (about 120
kilometers). Most of our close passes to date - for which we have data -
reveal the presence of one or more magnetic anomalies close to the path of
the spacecraft. Since the sources must be close to the path of the satellite,
we can only infer that the crust of Mars is strewn with similar magnetic
anomalies, awaiting discovery. Where we can obtain enough data - that is
to say, spaced more or less evenly in longitude with a spacing comparable
to our periapsis altitude - we can construct a detailed image of the magnetic
state of the Martian crust. We can then perhaps learn about the history
of the now-extinct early Mars dynamo and the evolution of the surface of Mars.
Contributors: David Brain, Sandy Kopman, Cisco Perin, and
Theresa Valentine
