New Research Casts Doubt on Late Heavy Bombardment
Was the early Solar System bombarded with lots of big impacts? This is a question that has puzzled scientists for over 35 years. And it’s not just an academic one. We know from rocks on Earth that life began to evolve very early on, at least 3.8 billion years ago. If the Earth was being pummeled by large impacts at this time, this would certainly have affected the evolution of life. So, did the Solar System go through what is known as the Late Heavy Bombardment (LHB)? Exciting new research, using data from the Lunar Reconnaissance Orbiter Camera (LROC) may cast some doubt on the popular LHB theory.
It’s actually quite a heated debate, one that has polarized the science community for quite some time. In one camp are those that believe the Solar System experienced a cataclysm of large impacts about 3.8 billion years ago. In the other camp are those that think such impacts were spread more evenly over the time of the early Solar System from approximately 4.3 to 3.8 billion years ago.
The controversy revolves around two large impact basins, which are found fairly close to each other on the Moon. The Imbrium basin is one of the youngest basins on the near side of the Moon, while the Serenetatis basin is thought to be one of the oldest. Both are flooded with volcanic basalts and are big enough to be seen from Earth with the naked eye.
Scientists know the relative ages of such lunar basins because of a concept called superposition. Basically, superposition states that what is what is on top must be younger than what is beneath. Using such relationships, scientists can determine which basins are older and which are younger.
To get an absolute age, though, scientists need actual bits of rock, so they can use radiometric dating techniques. The lunar samples returned by the Apollo program provided exactly that. But, the Apollo samples suggest that the Imbrium and Serenitatis basins are barely 50 million years apart.
Relative age dating tells us there are over 30 other basins that formed within that time frame. This means that roughly one major impact occurred every 1.5 million years! Now, 1.5 million years may sound like a long time. But consider the last large impact that happened on Earth, the Chicxulub event 65 million years ago, which is thought to have exterminated the dinosaurs. Imagine another 40 dinosaur-killing impacts occurring since then. It would be surprising if any life survived such a barrage!
This is why a team of researchers, led by Dr. Paul Spudis of the Lunar and Planetary Institute, is looking very carefully at this question. Their research is using the principle of superposition to show that several of the areas visited by the Apollo program were blanketed by material from the Imbrium impact. This could mean that many of the collected Apollo materials may be sampling the same event.
Dr. Spudis’s research focuses on the Montes Taurus area, between the Serenitatis and Crisium basins, not far from the Apollo 17 landing site. This is a region dominated by sculpted hills that have been interpreted to be ejected material from the adjacent Serenitatis basin impact. But, Dr. Spudis and his team have found that, instead, this sculpted material comes from the Imbrium basin some 600 kilometers away.
Artist impression of NASA’s Lunar Reconnaissance Orbiter. Credit: NASA
Previous data of this area, from the Lunar Orbiter IV camera, hadn’t shown this because a fog on the camera lens made the details difficult to see (this fog problem was eventually resolved, and Lunar Orbiter IV provided a lot of useful data on other parts of the Moon).The new LROC data, however, shows that the sculpted terrain seen at Apollo 17 is very widespread, extending far beyond the Montes Taurus region. Furthermore, the grooves and lineated features of this terrain point to the Imbrium basin, not the Serenitatis basin, and line up with similar features in the Alpes and Fra Mauro Formations, which are known to be ejecta from the Imbrium impact. In the north of Serenitatis, these Imbrium formations even seem to transform into the Montes Taurus, confirming that the sculpted hills do, in fact, originate from the Imbrium impact.
If the sculpted hills are Imbruim ejecta, then it is possible that Apollo 17 sampled Imbrium and not Serenitatis materials. That casts suspicion on the very close radiometric ages of these two basins. Perhaps these ages are so close because we effectively measured the same material. In that case, the age of Serenitatis could be much older than the 3.87 billion years the Apollo 17 samples suggest. If true, this would mean that there was no Late Heavy Bombardment at the time life was forming on the early Earth, leaving life to evolve with relatively few impact-related interruptions.
Editor’s note: Paul Spudis contacted us with an amendment to this news article:
I thank Irene for her cogent and well written summary of our paper. I want
to clarify a bit more precisely exactly what we concluded from our work.
Our principal result is that the Sculptured Hills of the Taurus highlands
are not ejecta from the Serenitatis basin and are most likely composed of
distal ejecta from the Imbrium basin. Because this unit drapes the
mountains near the Apollo 17 site, we contend that the Serenitatis basin
provenance of Apollo 17 impact melts may no longer be assumed.
We did NOT conclude that there was no lunar “cataclysm” (the lunar version
of the LHB) — we offered two alternative scenarios, each with slightly
different implications for lunar history. If the Apollo 17 rocks are from
Imbrium, then we did not directly date the Serenitatis impact and with 25
basins and over 2000 large craters stratigraphically sandwiched between
Serenitatis and Imbrium, it makes the “cataclysm” less necessary to explain
lunar rock ages. It also implies that given the variations we see in the
Apollo 17 impact melts, we do not fully understand this process for
large-scale impact events.
On the other hand, if the Apollo 17 impact melts ARE from Serenitatis (and
there is no independent evidence to confirm this one way or the other), then
not only WAS there a cataclysm, but it would have been a cataclysm in one of
its most extreme forms — creating more than half of the observed lunar
basins and highland craters all within a very narrow 30 million year time
Please note well that we did NOT select one alternative over the other, nor
should the reader infer that we have any preference for one over the other.
We are simply stating that this geological issue — once thought to be a
firm constraint to lunar history — is not as clear-cut as we had previously
thought. In fact, there are two broad impact melt compositional groups at
the Apollo 17 site and it is tempting to equate one (the so-called
“poikilitic melt sheet”) with Serenitatis and the other (the “aphanites”)
with Imbrium (the adjectives refer to their appearance under a rock
microscope). But we do NOT so equate them because there are at present no
objective grounds to do so.
I am happy to send interested readers a PDF reprint of our full paper if
they will e-mail me at firstname.lastname@example.org