The Mystery of Mars' Magnetic Field

If you could pick up a rock from the surface of Mars, then the chances are it would be magnetic, even though doesn’t have a magnetic field coming from its core as does the Earth. rocks are clinging to the signal of an ancient magnetic field, dating back billions of years, to the times when the Red Planet had a magnetic field like Earth’s.

So how have these rocks hung onto their magnetic directions and what do they tell us about Mars? Oddly, the clues to that answer to these questions might be found right here on Earth.

Most continental rocks on Earth align their magnetic moments with the current magnetic field – they are said to have ‘induced’ magnetism.

“I consider induced rocks to have ‘Alzheimers’. These are the rocks that forgot where they were born and how to get home,” explains Suzanne McEnroe from the Geological Survey of Norway at a European Science Foundation, EuroMinScI conference near Nice, France this year.

However, not all of Earth’s continental rocks have an induced magnetization. Some rocks stubbornly refuse to swing with the latest magnetic field, and instead keep hold of the direction they were born with. These rocks are said to have a remanent magnetization.

McEnroe and her colleagues have been studying some of Earth’s strongest and oldest remanent magnetic rocks, to find out "why they have such good memories."

Understanding these remanent rocks may give us clues as to what kind of rocks lie on Mars.

One of their research projects is on rocks found in the Peculiar Knob Formation in South Australia, which are around 1 billion years old and have a strong magnetic remanence, more than 30 times larger than typically found in basaltic rocks.

By studying the samples under a powerful microscope and creating computer models of their magnetic properties, McEnroe was able to show that themineral  hematite was responsible for the strong magnetic field and that it was holding a remanent field from around 1 billion years ago. “We could see that the hematite contained small intergrowths that carried the magnetism."

A recent study on nearly billion-year-old rocks in Norway showed a remanent magnetic anomaly comparable in scale to those observed on Mars.  The remanent magnetic anomaly dominates the local magnetic field to such a degree that more than half the Earth's field is cancelled. It is nearly impossible to use a compass in the area, which cannot point correctly north because of the strong remanent magnetization in the rocks.

What they have found is that rocks containing nanometer scale intergrowths of ilmenite and hematite are better able to cling onto their original magnetization than those without such fine-scale features. “Placing a nanoparticle of ilmenite into the hematite host creates a strong and stable magnetic signal that can survive large changes in temperature and magnetic field over billions of years,” explains Harrison.

“These rocks are good analogues for the magnetic rocks we see on because of their strong magnetism and the length of time they have retained this memory,” says McEnroe.

“There is not going to be one mineral or one tectonic setting on Mars. There are going to be different reasons that enhance the signature in different places,” says McEnroe. The only way to definitively answer the question is to go and pick up some rocks from Mars.       

Posted by Casey Kazan.

Adapted from a European Science Foundation press release.