For roughly the first 500 to 800 million years of its existence, Mars had a magnetic field generated by a natural internal dynamo. This was powered by convection currents in the planet’s molten iron core, which operated vigorously as they unloaded heat into the overlying mantle, where its own convection currents carried it up to the surface.
DRYING OUT THE MANTLE. As the Martian mantle gave up water and volatile gases to the surface, it became stiffer and less able to carry heat from the core. The color curves show the mantle's cooling rate given various amounts of water within it. After about a billion years, the heat flow dropped below the threshold to sustain a dynamo, as indicated by the heavy black line. (Image is Figure 3a from the paper.)
But the dynamo shut down for unknown reasons, and scientists have not understood why. Possible causes include disrupting the mantle’s convection currents by the heat from large impacts.
Constantin Sandu and Walter Kiefer (both Lunar and Planetary Institute) now propose in a paper in Geophysical Research Letters that driving water and volatile elements out of the mantle through volcanism could have been the culprit. In their scenario, the volcanic activity initially cooled the mantle, helping it to convey heat from the core, which enhanced the core’s heat flow and maintained the geodynamo.
But they note that as the mantle cooled and gave up its water and gases, it became stiffer, less convective, and less able to carry away heat. This in turn diminished the core’s ability to cool, making it less convective — and putting a damper on the dynamo that generated the magnetic field.
They write, “An initially wet mantle will promote strong convection, rapid core cooling, and dynamo activity.” But erupting water and volatiles onto the surface, plus overall planetary cooling, increased the mantle’s viscosity. This decreased the rate of core cooling and possibly shut down the dynamo activity.