Stabilized impact flux on the Moon at 2.8 Ga constrained by Chang'e-6 low-Ti basalts
Current knowledge of lunar impact flux
Impact craters are the most ubiquitous landforms on terrestrial planetary surfaces, recording crucial information about planetary evolution. Due to its minimal weathering rates, the Moon preserves the most complete impact record, serving as the primary reference for reconstructing the impact history in the solar system.1 Impact craters formed by extralunar impactors (i.e., primaries) since the emplacement of a geological unit constitute the production population, and their size-frequency distribution (SFD) can be mathematically described by the production function (PF).1 In combination with crater spatial densities, the radiometric ages of samples returned from the Moon provide a baseline to construct the lunar impact flux, which is termed the crater chronology function (CF).1 The CF and PF enable the construction of crater density isochrons at diameters of at least 10 m for arbitrary model ages. Vice versa, the model ages of specific crater populations can be derived by fitting their SFD against the CF and PF.
The first-order reliability of the lunar crater chronology has been repeatedly verified. For example, the current impact flux on the Moon, derived from newly formed impact craters, is generally consistent with the annual production rate predicted using the canonical Neukum crater chronology (cf. Xiao et al.2); the model ages of the Chang’e-5 landing area, derived from crater statistics, align well with the radioisotope ages of returned local basalts (cf. Xiao et al.2). However, significant uncertainties persist in the canonical crater chronology model, especially in the age interval of ∼1.0–3.2 Ga, which was only recently anchored by Chang’e-5 basalts.2 Enhancing the reliability and precision of the lunar crater chronology remains a key objective of ongoing and future lunar exploration missions.
Despite being calibrated using the same anchor points, different versions of CF exist due to uncertainties in sample ages and crater densities (cf. Xiao et al.2). All available CF models suggest a similar overall trend of lunar impact flux, which features an early sharp decline and a quasi-constant rate at later times. However, the timing of this transition varies from ∼3.7 to 3.0 Ga according to different CF models (cf. Xiao et al.2). In addition, recent studies have revealed episodic short-term spikes of impact flux over the past ∼1 billion years (cf. Xiao et al.2), though their potential effects on the overall stability of impact flux remain unresolved. Deciphering the post-3.7 Ga changes of lunar impact flux will require new samples formed within this time frame.
