When and why did the Martian dynamo stop?
In plain words
Mars has strongly magnetized ancient crust but no global magnetic field today, so its core dynamo (the flowing-iron field generator) stopped long ago. When it stopped, and whether it ever restarted, decides how long Mars was shielded from the solar wind.
Precise statement
Weak magnetization over the large impact basins Hellas, Argyre and Isidis was taken to imply dynamo cessation near 4.1 Ga, but Mittelholz and coauthors (Science Advances 2020, https://doi.org/10.1126/sciadv.aba0513) reported evidence of a core field at about 4.5 Ga and at about 3.7 Ga. Determine the active interval or intervals of the dynamo and the cause of its end (core heat flow falling below the adiabatic value, no inner-core growth, changes in core-mantle boundary heat flux), consistent with InSight seismic constraints on core size and state. An answer is an onset and cessation chronology with a core thermal history that reproduces it.
What would settle it
Paleomagnetic measurements on dated Martian samples and magnetization maps of well-dated surfaces, combined with core thermal-evolution models.
Status in the literature
The 2020 evidence for a field at 3.7 Ga conflicts with the earlier inferred cessation near 4.1 Ga.