Mississauga Astronomical Society

50th Meeting

Speaker’s’ Night

Day:                November 4, 2005

Speaker:     Professor David Dunlop

Magnetism of the Earth, Moon and Mars

David Dunlop, physics professor at U of T at Mississauga, studied at the University of Toronto and obtained his PhD there.  His research interest is geomagnetism and tectonics.  He spoke about the magnetism of the Earth, Moon and Mars.

Professor Dunlop first went over a number of basics regarding magnetism. Both electron and nuclear spins have a magnetic moment but the electron component  is 1000 times stronger than the nuclear.  With exchange coupling, the spontaneous magnetism, Ms, decreases as a substance is heated until at the Curie point it reaches zero and the particles are no longer alligned.  

Magnetic domains are regions of parallel spins and spontaneous magnetization. A few thousand atoms in each domain are lined up; as the temperature drops, new domains nucleate and grow.  Professor Dunlop explained that some bacteria, bees, pigeons use single domain particles to sense magnetic direction.   In rocks, single domain particles result in stable, strong fields. 

The commonest magnetic material on Earth is magnetite but it is of a small size (0.1 micron), whereas hematite has a very large single domain but is not as magnetic. Magnetic anomalies on Earth are commonest on the seafloor at the spreading mid-ocean ridges.  Reversals of the magnetic field cause linear magnetic anomalies.  Satellites can map magnetic anomalies on the Earth. 

Magnetism on the Moon was measured by magnetometers on the Apollo 15 and 16 missions.  There is no global lunar field; weak farside anomalies not over 1 nanotesla (nT) were recorded.  Magnetic anomalies on the Moon do not correspond to mascons (mass anomalies).  Lunar prospector (1998 – 99) measured anomalies as high as 25 nT at impact basins Crisium and opposite the Imbrium basin (Mare Ingenii).  It is hypothesized that ejecta traveled completely around the Moon.  The ancient Moon 4 billion years ago is thought to have had a field 20 to 100 microT; the present lunar magnetic field is negligible as is induced magnetism. 

Mars Global Surveyor (MGS) measured large linear magnetic anomalies an order of magnitude larger than over the Earth especially in the Southern Highlands crust (which is like the Earth’s precambrian shield) comprising 10% of the planet’s surface.  There is a complete lack of anomalies over the Northern Plains which have younger volcanoes, north of the dichotomy line.  Mars had an early dynamo in the first billion years, a hypothesis favoured by study of the Martian meteorite ALH 84001, and then the magnetic field seems to have died out.  Later on, impacts created basins. 

Anomalies over Terra Cimmeria and Terra Sirenum have a stripe-like pattern.  The stripes are hundreds of km wide and up to 2,000 km long.  Field magnitude at 100 to 200 km altitude is as high as 1,500 nT – close the Earth’s.  Question arise.  Are these magnetic strips ancient crustal spreading on Mars?  How can we explain such thick and strong magnetism?  Perhaps the ancient Martian magnetic field was strong.  This is thought unlikely.  Maybe Hellas and Argyre are holes in the magnetic crust, shocked by past impacts.  Professor Dunlop discussed possible magnetic minerals to account for Martian magnetism such as magnetite which is common and has a high Currie temperature, hemoilmenite in lamellar magnetism, pyrrhotite, and hematite which colours Mars red and is a likely candidate due to its large single domain size and abundance.  His own research compares hematite and magnetite as candidates for the magnetic field on Mars. 

Finally, Professor Dunlop touched upon the question of life on Mars.  Did life once exist on Mars and then perish?  Did it leapfrog to Earth?  Life needs protection from cosmic rays either in the form of an atmosphere which Mars has largely lost, or in a magnetic field which exists on the planet as a horizontal blanket in places at the Southern Highlands.  He described the findings from the Martian meteorite ALH 84001 which contains crystals of magnetite identical to biogenic magnetite on Earth.  Thin slices of the rock show quanta of magnetic fields suggesting that the interior had been heated to 40 degrees C. and therefore bacteria might have survived a journey to Earth even though the outside melted. 

Many questions ensued after this most interesting lecture. 

Submitted by Chris Malicki, Secretary  Chris Malicki, Secretary                               back to M.A.S. meeting reports page
Mississauga Astronomical Society