What the argon counts
40Ar/39Ar geochronology never sees an impact; it reads the moment the impact melt last cooled, from the clock that argon keeps inside the rock. In the melt rocks Chang'e-6 returned from the Moon's far side, that clock gives ages running from about 4.33 billion years ago to 1.13 billion years ago. An interval of more than three billion years comes out of one space mission's small bag of samples.[1]
That describes the measurement's limit as much as its power. What we hold is a set of ages belonging to individual fragments, and not a curve of bombardment intensity against time. Moving to the second requires an assumption about how representative the fragments are: that melts collected at a single landing site sample the impacts that struck it without bias. The assumption may be reasonable; it has not been tested.[1]
One model weakens, another gains
Two pictures compete. One posits a short, intense period of impacts early in the Moon's history; the other, a steady and slow decline since the planets formed. The spread of the ages across three billion years strengthens the second picture and weakens the first. Weakening does not amount to refutation: a dense period compressed into a narrow window could also produce a similar distribution if its melts were reheated by later impacts, or if they fell outside this particular sample set.[1]
That is also why we look at the far side. As Fred Jourdan of Curtin University puts it, the moon is like a time capsule: it carries a record of events that erosion and plate tectonics erased on Earth. Every question we ask about Earth's first billion years is in practice asked of the Moon. What would move this argument forward is therefore not a stronger reading, and not a bolder one, but an independent set of fragments from a second landing site on the far side, dated by the same method. A distribution measured once does not yet count as a history.[1]