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Accepted · first revision · record

Title: Did Earth have a magnetic field 4.2 billion years ago?
Plain: 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: 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.
Settled by: Paleomagnetic measurements on Hadean and Eoarchean zircons whose magnetic inclusions are demonstrated to be primary.
Kind: well-posed
Answer type: yes-no
References: (empty)
Literature status: contested
Status 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.
Posed since: 2015