Planetary Magnetic Fields : Terrestrial Planets

Cutaway views of the interiors of the terrestrial planets reproduced from Solarview. Image : Mulyukova and Bercovici 2021

The four innermost planets of our Solar System - Mercury, Venus, Earth and Mars - are classified as the terrestrial planets due to their similarities in structure. They are composed of metals or rocks and have a solid hard surface.

Starting with our planet Earth, it has the strongest magnetic field among these four. You must have heard that the Earth has a magnet inside it. Well, that’s not entirely right. The convecting motion of metals in the core produces a magnetic field that is similar to the field produced by a bar magnet. To make this dominant dipolar field easier to understand, the field is represented by a magnet inside. But, we also have non-dipolar fields from the core and other sources like the crust and the ionosphere. Equally interesting to study is the change in the magnetic field over time which also tells us that the field changes its polarity. So the compass you are using now will not show the same results to your descendants born after a reversal!

Image : Mouritsen 2015. It shows a representation of the Earth's magnetic field. The geographic and magnetic axes are not aligned but at an angle of about 12 degrees.

The smallest of the planets, Mercury, has a weak core field. The slow rotation of the planet is one of the reasons for its low magnitude. Another is it's not-so-hot core. But the field is still strong enough to have a magnetosphere. More magnetic data from upcoming missions will help to fully understand the planet.

Image : NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington, from results of the early phases of Messenger satellite. It is a depiction of the magnetosphere of Mercury with distortions of magnetic field (blue) from solar winds. 

Mars has been the hot topic lately. It has also had a bunch of satellites orbiting it and landers on it for some time now. So we can say assertively from those observations that it once had an active core field, i.e., a dynamo. It has remnant crustal fields of magnitudes higher than that observed on Earth that interact with the solar winds and produce mini-magnetospheres. So, surely it had a strong dynamo in the past, sometime near 3.7Ga or maybe 4.1Ga ago?

Images : (Coloured) Brain et al. 2015. The solar wind carries with it particles (yellow and dashed lines) that interact with the Martian crustal magnetic fields (orange). (Black and white) Zhang et al. 2008. Schematic of the induced magnetosphere of Venus.

Lastly, we have Venus. You can feel a little sad for the planet because no one talks much about it when it comes to magnetic fields. That’s because there has been no evidence of any core field there. But don't feel too sad, NASA and ESA have selected a total of three future missions to visit the planet!

Shivangi Sharan is a second year PhD student at the Laboratory of Planetology and Geodynamics in France. Her research focusses on the study of the magnetic field of Mars and to infer its internal structure from it. She is an active member of the IAGA Blog Team and can be contacted via e-mail here.



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