Mars' Magnetic Field

Wake-up Question:

By measuring the heat flux out of a planet such as Mars we can estimate (assuming resonable heat conductivity K, for martian rocks) the rate of change of temperature with depth ~10 Kelvin per km. So, if the surface temperature is 225 Kelvin and the Curie temperature is about 700 Kelvin, then the Curie depth is (700-225)/10 = 47.5 km. This means that just ~48 km down from the surface the rock is too hot to hold magnetization. Any magnetic field has to be either generated in an internal dynamo or "frozen" into surface rocks.

How is a planet's magnetic field measured? - just in case you wondered....

So, now we get the the actual magnetic field of Mars...

I. MARTIAN MAGNETIC FIELD

As measured by the Mars Global Survey Magnetometer

WHAT DO YOU NOTICE?

1. No Global Field
2. Localized Magnetic Fields ("Ups" & "Downs")
3. Non-random Distribution of Local Fields
4. "Noise" (this is due to variable solar magnetic field)

Here is a map (showing the vector magnetic field) of the Earth's (roughly dipole - bar-magnet-like) magnetic field as it would be measured by a space craft orbiting Earth at 400 km

North positive, south negative - slight "wiggle" due to ~11 degree tilt of the Earth's magnetic axis with respect to the spin axis.

Now, look at Mars' vector magnetic field:

II. MARS HAS NO GLOBAL FIELD (TODAY)

• No Dynamo!

Why Not? Dynamo Requires:

2. Electrically conducting fluid
3. Energy for convection
4. Rotation

Mars is small than Earth -> probably lacks energy to sustain convection to drive the dynamo

III. MARS HAS LOCALIZED MAGNETIC FIELDS

• Mars’ sources much stronger than Earth’s sources

This implies:

2. Abundant Magnetic Materials
3. Large Background Field at the time of magnetization
4. Large Volumes of Coherent Magnetization

1. Clues about composition

More iron at Mars & probably an oxidizing environment (also makes Mars look red!)

Likely minerals: magnetite, hematite, maghemite, pyrohetite (all have Fe, O)

Some suggestions of aqueous alteration to produce strong magnetic field

2. Clues about Dynamo

One used to exist

3. Clues about Magnetization Mechanism

KISS principle: Big volumes of rock cooled below Curie Temperature in presence of ancient dynamo

Other mechanisms problematic

• POSSIBLE INTERIOR MODELS

Ways of making a magnetic field early in Mars' history - and then turning it off. (From David Stevenson's article in the Nature Insight collection on Mars).

IV. LOCALIZED MAGNETIC SOURCES ARE NOT RANDOMLY DISTRIBUTED

We have talked about Mars topography (from laser ranging measurements) and gravity measurements give estimates of the crustal thickness - plotted above.Now, look at the relationship of topography to magnetic field:

Notice: N/S asymmetry; north low and young; south higher, cratered and old; high Tharsis volcanic region (red); deep impact basins in south - Hellas and Argyre basins.

• Not correlated w/specific surface features or minerals=> buried magnetization
• Found in highest abundance/strength below oldest, thickest crust => very old

• Not found below big impacts => older than impacts

• Big impacts likely very old (~ 3.7 - 3.5 billion years) => Dynamo stopped quickly

Possible causes of Martian North-South dichotomy

1. Large impacts - several big impacts in north -> connecting basins.
2. Mantle convection - singe convection cell (see below) -> asymmetry
3. Plate tectonics - ceased at Pangea stage on Earth.

Single cell convection simulation.

V. SYNTHESIS: MARTIAN MAGNETIC HISTORY

1. Mars forms with lots of energy
2. Core formation, Differntiation, Convection -> Dynamo (& global magnetic field)
3. Large volumes of crust are magnetized
4. Dynamo shuts off
5. Big impacts & other mechanism "erase" magnetization in many regions

To see the movies of Mar's magnetic field here is David Brain's website.