Did Earth have a magnetic field 4.2 billion years ago?
In plain words
Tiny crystals called zircons from the Jack Hills of Australia, some older than 4 billion years, contain magnetic grains that may record an ancient field. Researchers disagree on whether the magnetization formed with the crystals or was added much later.
Precise statement
Paleointensities from Jack Hills detrital zircons (Tarduno and coauthors, Science 2015, https://doi.org/10.1126/science.aaa9114) suggest a geodynamo from about $4.2\,\mathrm{Ga}$ with strength comparable to today's, whereas Borlina and coauthors (Science Advances 2020, https://doi.org/10.1126/sciadv.aav9634) argued that the magnetic carriers are secondary and the record unreliable. Determine whether a geodynamo existed between about $4.2 \text{ and } 3.5\,\mathrm{Ga}$, and its intensity. An answer is a primary paleointensity record older than $3.5\,\mathrm{Ga}$ whose magnetic carriers are shown by in situ dating and micromagnetic imaging to be as old as the host crystal.
What would settle it
Paleomagnetic measurements on Hadean and Eoarchean zircons whose magnetic inclusions are demonstrated to be primary.
Status in the literature
Unverified note
Tarduno and coauthors defended a primary record (PNAS 2020, https://doi.org/10.1073/pnas.1916553117) against Borlina and coauthors 2020; no decisive test was found in a 2026 literature check.