A planet’s global magnetic field arises from a so-called dynamo — a flow of molten metal within the planet’s core that produces an electrical current. On Earth, the dynamo is what makes compass needles point north. In a paper published in the journal Science Advances, a team of researchers from Canada, the United States and France presents new constraints on the timing and strength of the Martian dynamo from magnetic field data collected by NASA’s Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft.
![Martian dynamo timing scenarios: an early dynamo [a] predating Hellas, Isidis, and Argyre (1). The basin age range is shown according to the isochron (cyan) and N(50) (blue) age. Early dynamo termination by 4.13 billion years ago [b] is based on magnetic field signatures of a larger basin population. The age of magnetization of meteorite ALH 84001 (3.9 to 4.1 billion years ago) overlaps the early dynamo time frame [c]. A late dynamo [d] postdating the major basins. New constraints from MAVEN data (stars) over the Borealis basin, around the Utopia basin, and Lucus Planum that indicate a dynamo at 4.5 and 3.7 billion years ago. The timing of Utopia is uncertain (dotted line). The map displays Mars Observer Laser Altimeter topography with Borealis basin, Utopia, and Lucus Planum marked (stars). Image credit: M. Kornmesser / ESO / Mittelholz et al, doi: 10.1126/sciadv.aba0513.](https://cdn.sci.news/images/enlarge7/image_8394e-Mars-Dynamo.jpg)
Martian dynamo timing scenarios: an early dynamo [a] predating Hellas, Isidis, and Argyre (1). The basin age range is shown according to the isochron (cyan) and N(50) (blue) age. Early dynamo termination by 4.13 billion years ago [b] is based on magnetic field signatures of a larger basin population. The age of magnetization of meteorite ALH 84001 (3.9 to 4.1 billion years ago) overlaps the early dynamo time frame [c]. A late dynamo [d] postdating the major basins. New constraints from MAVEN data (stars) over the Borealis basin, around the Utopia basin, and Lucus Planum that indicate a dynamo at 4.5 and 3.7 billion years ago. The timing of Utopia is uncertain (dotted line). The map displays Mars Observer Laser Altimeter topography with Borealis basin, Utopia, and Lucus Planum marked (stars). Image credit: M. Kornmesser / ESO / Mittelholz et al, doi: 10.1126/sciadv.aba0513.
“Dynamo timing is a big part of a planet’s evolution, and what we find is very different from what we have thought so far.”
“The dynamo tells us something about the planet’s thermal history, its evolution, and how it got to where it is today, and it is unique for each of the terrestrial planets — Earth, Mars, Venus and Mercury.”
Magnetism in certain rocks on Mars’ surface indicate that the Martian dynamo was active between 4.3 and 4.2 billion years ago, but the absence of magnetism over three large basins — Hellas, Argyre, and Isidis — that formed 3.9 billion years ago has led most scientists to believe the dynamo was inactive by that time.
Dr. Mittelholz and her colleagues analyzed MAVEN data acquired globally at altitudes as low as 130 km at night.
They found clear evidence of a magnetic field coming from the Lucus Planum lava flow that formed less than 3.7 billion years ago — much later than the aforementioned basins.
They also detected low-intensity magnetic fields over the Borealis Basin in the planet’s northern hemisphere, which formed 4.5 billion years ago and is believed to be one of the oldest features on Mars.
“We have these two observations that point to a dynamo at the earliest known time in Mars’ history, and a dynamo that was present half a billion years after many people thought it had already switched off,” said study’s co-author Professor Catherine Johnson, from the Department of Earth, Ocean and Atmospheric Sciences at the University of British Columbia and the Planetary Science Institute.
“We offer two possible explanations for the absence of magnetic fields over the basins,” the researchers said.
“The dynamo may have stopped before the basins formed and then re-started before Lucus Planum formed, or the impacts that created the basins simply displaced the portion of crust containing minerals that can carry strong magnetism.”
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A. Mittelholz et al. 2020. Timing of the Martian dynamo: New constraints for a core field 4.5 and 3.7 Ga ago. Science Advances 6 (18): eaba0513; doi: 10.1126/sciadv.aba0513
