Skip to content

The lost field of Mars

Electricity and magnetism · level 6, lesson 5 of 6

A question to start

Mars has no planet-wide magnetic field today. So how do we know it once had one?

The idea

When hot rock with iron minerals cools, the minerals lock in the direction and strength of the magnetic field around them. The rock becomes a weak magnet, a record of the field at the time.

In the late 1990s, NASA's Mars Global Surveyor flew low over Mars with a magnetometer and found strongly magnetized crust, mostly in the ancient southern highlands. In places the magnetism runs in bands, with neighbouring bands pointing opposite ways. Younger regions have little, and the giant Hellas and Argyre impact basins almost none.

So Mars once had a global field, made by a dynamo in its core, strong enough to magnetize its early crust. The dynamo likely died a few hundred million years after Mars formed: rock made after that was not magnetized.

The InSight lander, which touched down in late 2018, carried the first magnetometer to measure the field on the surface. The field at its site was about 10 times stronger than the orbiters had predicted, coming from ancient magnetized rock between about 60 metres and several kilometres below ground.

Magnetic field Mars Earth
Planet-wide field today none 24 to 66 µT at the surface
Magnetized ancient crust strong, mostly in the southern highlands yes, including stripes on the ocean floor
Dynamo in the core stopped billions of years ago still running
magnetized bands in old crust, pointing opposite waysa record of an ancient field

Worked examples

1. Why the old crust. Rock that cooled while the dynamo ran was magnetized. Rock that formed later, like the younger lava plains, cooled with no field to record.

2. Impact basins. A giant impact heats the crust above the temperature where its minerals lose their magnetism. With no global field left, it cooled unmagnetized.

3. Why InSight saw more. An orbiter averages the field over a couple of hundred kilometres, smoothing out local patches. A lander measures right where it sits.

Try it

  1. How do rocks record a magnetic field?
  2. Where on Mars is the crust most strongly magnetized?
  3. What does the lack of magnetism in younger rocks tell us?
  4. Why did InSight measure a stronger field than the orbiters predicted?
Answers
  1. Iron minerals in cooling rock lock in the direction and strength of the field around them.
  2. In the ancient southern highlands.
  3. The global field had stopped by the time they formed.
  4. Orbiters average over a wide area; InSight measured the local field from rock right below it.

Common mistake

"Mars's magnetized rocks make a global field." They make only local fields, patchy and pointing different ways. A compass would swing about over them but would not find north.

Where it leads