Credit: © NASA/GSFC/ARIZONA STATE UNIV./LUNAR RECONNAISSANCE ORBITER

Reconstructing the history of terrestrial asteroid impact is more complex than simply cataloguing crater formation. For a start, craters offer little in the way of accurate temporal information, as they are obscured by geological and meteorological interference. Instead, the most useful information may be contained in the spherule layers that settle from the plume of vapour that results upon collision — as two studies now demonstrate.

The period during which many young lunar basins were created is generally thought to have ended nearly four billion years ago, around the time that the Orientale basin (pictured) was formed. But the Earth-bound evidence for impacts of similar size ties these events to more recent times — some as late as two billion years ago. In search of a plausible source for such bombardment, William Bottke and co-workers tracked asteroid trajectories using a model that implicates giant-planet migration as the disturbance that nudged asteroids from a prominent belt into planet-crossing orbits (Nature http://dx.doi.org/10.1038/nature10967; 2012).

The study revealed that the blitz responsible for creating the lunar basins lasted much longer than previously thought. By extrapolating data from the survivors of the belt featured in the model, Bottke et al. were able to infer the probable ages of lunar and terrestrial basins. Their inferences are consistent with both known spherule-bed data and the characteristics of lunar craters.

In terms of probing the details of these large ancient impact events, spherule layers have yet more to offer. In another study, Brandon Johnson and Jay Melosh have characterized the details of impactors using the thickness of the spherule layers themselves (Nature http://dx.doi.org/10.1038/nature10982; 2012). They derived an equation relating layer thickness to the size of culprit asteroids and tested its accuracy by comparing their results with existing methods for determining impactor size. They found that they could also determine impact velocity from the average size of spherules — and reconstruct a chronology that documents the decline of impactor flux over the period on which Bottke et al. focus.

Together, the studies point towards an enriched understanding of the ancient history of asteroid impact, which makes good use of the clues that the blasts left behind.