NASA MAVEN: How magnetic crustal fields affect planets - Video

Radiation environments on Earth and Mars. Credit: NASA JPL

If you are ever lost on the surface of Mars, don't count on a compass to help you get home.

On Mars, compasses don't work.

They don't work because there is not one magnetic field on Mars, rather there are dozens.

These small fields are powerful, concentrated in the crust, and scattered over the surface of the planet.

In their absence, compass needles would lie still; in their presence, they spin, pointing first at one bar magnet, then another.

How well these crustal fields protect the planet is a mystery, and one that may be solved soon by the MAVEN satellite, which is on its way to Mars right now.

What we do know is that if a compass ever worked well on Mars, it was over 3.5 billion years ago.

Before that time, Mars had a molten core, whose contents constantly churned upward towards the surface.

This process of convection permitted cooling of the interior, as well as active volcanism in the highlands and plains. Volcanoes brought iron to the surface, giving Mars its signature colour.

Iron in the core also moved electrons, which created a planetary dynamo: a device that converts mechanical energy into electric energy.

Electric fields generate magnetic fields. Large magnetic fields can provide protection from solar wind for any planet as long its interior maintains a steadfast supply of molten metal.

Large magnetic fields also decay unless maintained. After the first billion years or so, the Martian interior cooled to the point where convection halted. When the iron ceased to flow, the dynamo died. Volcanism declined.

The last iron deposits from the interior left their marks as pockets of magnetism, called crustal anomalies, largely sequestered in the southern hemisphere.


"Mars is on the interesting borderline of the magnetized and the unmagnetized objects," said Janet Luhmann, MAVEN's deputy principal investigator.

"We think that the weak magnetic field has been in place since it was about a billion years old."

We first learned about Mars' strange magnetic fields from the way they interacted with the solar wind.

In 1965 Mariner 4, one of America's first interplanetary probes, passed within 8,000 kilometers (4,971 miles) of Mars' surface.

At that distance, it failed to detect any magnetic field. By contrast, Earth's planet-wide field can be sensed by magnetometers within 60,000 kilometers (37,282 miles).

What Mariner did note was that the solar wind was being bent around the planet, widely in some places.

Position of magnetometers on the MAVEN spacecraft. Credit: NASAexplorer

In terms of solar storms, MAVEN will arrive on the downslope of a solar cycle, during the optimum window for observing one of the most powerful forces affecting atmospheric escape: coronal mass ejections.

"More CMEs (and larger CMEs) occur during the declining phase of the solar cycle, rather than at maximum," said Frank Eparvier, science lead on the Extreme Ultraviolet (EUV) Sensors attached to LPW.

"MAVEN will be at Mars during the declining phase of the current solar cycle, so we will actually be there at the right time to see CME impacts."