Was there a late heavy bombardment spike about 3.9 billion years ago?
In plain words
Moon rocks brought back by Apollo showed many impact melts about 3.9 billion years old, suggesting a sudden burst of impacts. It may instead be the tail of a steadily declining impact rate, sampled unevenly.
Precise statement
Determine whether the inner Solar System impact flux between 4.5 and 3.8 Gyr ago had a transient increase (a terminal cataclysm about 3.9 Gyr ago) or declined monotonically, accounting for the dominance of Imbrium ejecta in Apollo samples and for impact-age resetting. An answer is an impact-flux history with uncertainties from lunar, meteoritic and Martian chronology.
What would settle it
Secure radiometric ages for several intermediate-age basins (for example Nectaris and Serenitatis) with confirmed sample provenance, which together with the 4.25 Gyr South Pole-Aitken age fix the shape of the basin-formation rate.
Status in the literature
Unverified note
Chang'e-6 samples (returned 2024) date the South Pole-Aitken basin to about 4.25 Gyr (Xu and Chen 2025, National Science Review), which leaves about 350 Myr between the oldest and youngest large basins and argues against a single cataclysm about 3.9 Gyr ago; the shape of the 4.25 to 3.8 Gyr flux, including any late uptick, remains debated.