February 13, 2012
One of the hottest topics in planetary science is the nature of the Moon’s early impact history. So it was not unexpected that the Early Impact Bombardment of the Solar System workshop, recently held at the Lunar and Planetary Institute in Houston, generated some interesting discussion. More than 30 years ago, when researchers at Caltech and elsewhere noted that many lunar highland impact generated rocks had very similar ages, they advanced the idea that the early Moon underwent an extremely large increase (peak) in the rate of asteroid and comet bombardment 3.9 billion years ago (600 million years after the Moon had formed). They called this late bombardment the lunar “cataclysm.” Work on lunar samples continued after this proposal, and while that apparent peak in impact ages around 3.9 billion years was considered remarkable, many investigators resisted the idea of a cataclysm. Part of their resistance was because such a late barrage of impacts is difficult to explain dynamically.
All the planets form by the accretion (addition by impact) of smaller particles. Much work with meteorites (samples from the early Solar System, including lunar and other planetary materials) has shown that Earth and Moon had mostly finished assembling (i.e., grown to their present mass) by 4.5 billion years ago. This means that the debris clouds orbiting the Sun had been largely swept clean by that time. How could the Moon then experience another intense bombardment 600 million years later, at 3.9 billion years? Many complex ideas were examined, including the break up of other, previously existing planets or the incursion of a large number of objects from the shadowy Oort cloud of comets orbiting the Sun in the deep cold and darkness far beyond the orbit of Pluto.
What if there was no cataclysm? Perhaps the plethora of ages at 3.9 billion years is more apparent than real (a possibility that has been studied in detail). Part of the problem is that most of our lunar samples come from the “Apollo zone,” a small polygon centered on the lunar near side. Most of whose landing sites seem to be somehow related to the enormous Imbrium basin, one of the youngest of the many large, multi-ring impact craters that cover the entire Moon. Perhaps the reason this age turns up so often is that it is really just reflecting one age – that of the Imbrium basin. Indeed, our best estimate for the age of that feature is 3.85 billion years, very close to the age of the supposed “cataclysm.” Moreover, some argue that because early cratering rates were so high, they would have destroyed the very evidence we seek – the older impact rocks would have been ground up into an unrecognizable powder, the so-called “stonewall effect.”
Lest you think that this is merely some esoteric debate among lunar scientists and only of interest to them, comparable to arguments between medieval theologians about the number of angels on the head of a pin, you should know that much of our current alleged understanding of the early history of the Earth and other planets comes from our interpretation of the well preserved geological record of the Moon. Mass accretion followed by global melting and then an impact bombardment is the received wisdom for the early story of all the planets. We did not make up this narrative; it was dictated by the lunar record. If we have the story of the Moon wrong, perhaps we are wrong about all the other planets as well. This era of time is important to Earth history in a special way – it is the time when we suspect that life may have arisen. The bombardment rate is a crucial variable in that story, as too high a rate will produce too many sterilizing impacts, thereby stopping life in its yet-to-be-made-because-there-are-not-yet-feet tracks.
So which model for early lunar history is correct and how might we decide that? One possibility is to study samples derived far from the Apollo sites, ones that may not have been as heavily influenced by the dynamics of the Imbrium basin. We have additional samples of the Moon in the form of meteorites (blasted off the Moon by impact); over 100 are presently known. Assuming that they come from random places on the Moon (and there is no reason to assume otherwise), many of them could come from areas of the Moon not affected by Imbrium, such as the lunar far side. There is ongoing study of these objects but at first glance, although they contain impact melts from post-heavy bombardment times, they do not appear to have impact melts much older than what is found in the Apollo collections. This relation suggests that the absence of impact melts older than 3.9 billion years is a global phenomena and not a sampling bias reflecting the effects of the single Imbrium impact event. Such a relation could be produced via a cataclysm or the stonewall effect.
So this result leaves the question of the early bombardment unresolved. The discussion at the workshop tried to imagine new ways to solve this problem. Most agreed that the best way to document or refute the cataclysm was to absolutely date some older basin whose relative age is well known and see if it is close in time to Imbrium or not. We can absolutely date rocks through isotopic methods, but getting the right rocks is the challenge. Humans can intelligently select samples, but no humans are going to the Moon in the near future. Robotic spacecraft can collect rocks and soil, so perhaps a robotic sample return mission could provide the samples to resolve this problem. But where would we send such a robot?
For the last decade, attention has been focused on the extremely large (2500 k diameter) South Pole-Aitken basin, the oldest visible impact feature on the Moon, centered on the southern far side. The problem is that this feature is so old, much has occurred since its formation and although “grab samples” could be obtained by a robot probe, what might these rocks represent – basin impact melt or some other, post-basin deposit? Moreover, even if we could somehow convince ourselves that SPA melt had been obtained, the only way these samples resolve the issue is if they are the same age as Imbrium, in which case there was a cataclysm of epic proportions. If the age of SPA is much older, it could have formed early, leaving the Moon with little subsequent activity, and then a cataclysm at 3.9 billion years ago. An old age would provide no information on that possibility.
At the workshop, the man who first discovered the multi-ring nature of lunar impact basins over 50 years ago, Bill Hartmann, suggested a different approach. Bill advanced the notion that we should not sample the oldest basin, but one in the middle of the sequence – the Nectaris basin. First, because it is much younger than SPA and we are more likely to find a basin melt sheet. Second, it is on the near side of the Moon, which simplifies the mission requirements and allows direct communication with Earth. Finally and most importantly, determining the age of Nectaris resolves the cataclysm because it is old enough to be distinct from Imbrium, yet young enough to let us resolve the intermediate cratering history of the Moon. If Nectaris is 3.9 billion years old, there was a cataclysm. If it is significantly older (say 4.1 billion years old) there was not one. It’s not often we get the possibility of such a clear-cut answer in science.
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