Map of the Serenitatis basin area of the Moon

New Research Casts Doubt on the Late Heavy Bombardment

5 Jan , 2012 by

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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.


Map of the Serenitatis basin area of the Moon

What if the Apollo 17 samples didn't come from the Serenitatis basin, where the astronauts collected them, but rather from the Imbrium basin, located some 600 km away? Studies from the new Lunar Reconnaissance Orbiter Camera suggest this may be the case. If true, this means Serenitatis is much older than the Imbrium basin and a solar system-wide impact catastrophe is not needed to explain the uncannily close ages of the Imbrium and Serenitatis basins. Image credit: NASA  Click on the image to download the full map and explore it in more detail.

Scientists know the relative ages of such lunar basins because of a concept called superposition. Basically, superposition states that 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.

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.

LROC Data of Serenitatis basin area on the Moon

Recent high quality data from the Lunar Reconnaissance Orbiter Camera shows that the sculpted terrain, which is present at the Apollo 17 landing site, is related to material that is known to be from the Imbruim impact. This means that Apollo 17 may have sampled Imbrium and not Serenitatis material. This could explain the unusually close ages of these two basins, suggested by the Apollo samples. If so, the Serenitatis impact may have occurred much earlier than previously thought, meaning that a barrage of frequent bombardments did not occur, and life on Earth could have evolved without being molested by too many impact events. Image credit: NASA/GSFC/Arizona State University  Click on the image to explore the LROC data in greater detail.

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.

Source:
Spudis et al., 2011, Journal of Geophysical Research, V116, E00H03

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Ron 1958
Member
Ron 1958
January 5, 2012 2:06 PM

“Basically, superposition states that what is on top must be older than what’s underneath.”

This is backwards. Superposition states that what is on top must beyounger than what is beneath.

Steve Rollins
Guest
Steve Rollins
January 5, 2012 2:30 PM

And here I was trying to figure out what I was missing after reading that, and coming to the conclusion that I must be slightly more logically inept than I thought.

Tim McDaniel
Member
Tim McDaniel
January 5, 2012 3:08 PM

I was in WTF mode staring at that, wondering how that could be. If that’s indeed backwards, could Dr. Antonenko or someone please fix it?

Irene Antonenko
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Irene Antonenko
January 5, 2012 3:23 PM

Thank you so much Joseph Yates for catching that! I can’t believe I missed it.

Dan Sanderson
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Dan Sanderson
January 5, 2012 4:32 PM

Forgive me if you fixed already but I think you had a copy and paste moment :

“Basically, superposition states that what is what is on top must be younger than what is beneath.”

ie, what is x2

gopher65
Member
gopher65
January 6, 2012 3:14 PM

It was indeed fixed, thanks to his postsmile.

Jason
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Jason
January 5, 2012 5:21 PM

That’s what she said.

Ivan3man_At_Large
Member
Ivan3man_At_Large
January 5, 2012 2:07 PM

It’s actually quite a heated debate, one that has polarized the science community for quite some time.

It’s something like this. wink

.

Matt Hickman
Guest
January 5, 2012 3:14 PM

You have both sides of the argument having the LHB ending approx 3.8 bya. Other researchers feel that the bombardment may have been spread even further and lasted as long as to 2.4 bya. http://youtu.be/4MMYTzb0L_s

Torbjorn Larsson OM
Member
Torbjorn Larsson OM
January 6, 2012 12:47 PM

As I added to my comment, I forgot to check the updated thread before posting a similar reference after yours. My apologies! Great minds and all that, I suppose.

Torbjorn Larsson OM
Member
Torbjorn Larsson OM
January 5, 2012 4:01 PM
W00t! Universe Today starts the new year with a big bang. This interests me a lot as an astrobiology student, so with all due respect to the discipline of planetary geology I am going to take this in two passes. The astrobiology first: If the Earth was being pummeled by large impacts at this time, this would certainly have affected the evolution of life. Yes, whether or not the Late Heavy Bombardment (LHB) happened is important in many hypotheses, from aspects of supplementing Earth volatile supply over when life arose to the effects it had on first life. First it must be noted that people assumed for a very long time that the LHB would be difficult to… Read more »
Olaf
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Olaf
January 5, 2012 5:32 PM

I actually don’t care what people believe.
I want to see evidence and facts.
I have no doubt to change camps if the evidence leads over there.

Rob Hemmings
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Rob Hemmings
January 6, 2012 5:53 AM

Yes, so we must send men back to the Moon, or *very* capable robotic rovers. Lets have some of the far side this time, too.

HeadAroundU
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HeadAroundU
January 5, 2012 8:13 PM

Good stuff, Torbjorn.

Let me guess what happened. Why not? grin From 4.5 billion years ago big boulders had continually been falling on planets till 3.8 billion years ago. grin And they keep falling till now, but not so much. Pluto is not a planet and LHB didn’t happen. grin It was ECB, early continual bombardment. grin Give me Nobel prize. :d

Torbjorn Larsson OM
Member
Torbjorn Larsson OM
January 6, 2012 12:53 PM

Thank you!

Well, the dating issues and the recent dynamics is exciting. A few years ago, it was all “~ 3.8 Ga bp and then the rock record disappears”.

Now there are implications on everything from early water, organics and tectonics (from zircons) to early crust and mantle compositions and dating (various rocks & isotope ratios) to early life (the changing and argued fossil record & gene family histories).

And the Moon is both a player and a bench sitter with records on some of these things. I honestly thinks it adds to the romance of the “night light” when I see it.

Torbjorn Larsson OM
Member
Torbjorn Larsson OM
January 5, 2012 5:08 PM
And, a little more sheepishly seeing the length of the first comment and the subject of planetary geology, on to the latter: I don’t have access to Spudis et al, but I think this is a work in a series that criticizes the Apollo 17 samples. That is all good and well, but there are much more things up in the air that may or may not problematize datings on the Moon: – As reported here on Universe Today: Moon volatile content is now on par with the Earth’s which supports a Earth-Moon impactor. While at the same time the datings of crustal FAN rocks may rewrite early lunar history. – The Moon nearside/farside dichotomy has solutions affecting… Read more »
Torbjorn Larsson OM
Member
Torbjorn Larsson OM
January 5, 2012 5:08 PM
And, a little more sheepishly seeing the length of the first comment and the subject of planetary geology, on to the latter: I don’t have access to Spudis et al, but I think this is a work in a series that criticizes the Apollo 17 samples. That is all good and well, but there are much more things up in the air that may or may not problematize datings on the Moon: – As reported here on Universe Today: Moon volatile content is now on par with the Earth’s which supports a Earth-Moon impactor. While at the same time the datings of crustal FAN rocks may rewrite early lunar history. – The Moon nearside/farside dichotomy has solutions affecting… Read more »
Anonymous
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Anonymous
January 6, 2012 11:13 AM
“In one camp are those that believe the solar system experienced a cataclysm of large impacts about 3.8 billion years ago.” ( As one holding to an alternate view of star-planet creation, I find this very interesting in context. Where else is supposed evidence, clearly engraved or subtly etched, for one model actually camouflaged proof of another, either overlooked, or actually readable – but misinterpreted? ) ____________________________________________________________________________ If I understand the revision born in this new insight ( not a surprising to me ), there was ONE massive bombardment episode – or “cataclysm” – which stroke the Solar System in the distant past. In a different frame of view, this would be a sudden storm-front emergence, breaking-out and… Read more »
Torbjorn Larsson OM
Member
Torbjorn Larsson OM
January 6, 2012 4:32 PM
I think this merit some comment. The astrobiology first: – We have to distinguish between rebound, which in turn can be repopulation and/or diversification, and emergence. Here we are mostly discussing repopulation as I commented on below. Gene family data makes clear that diversification happened later. – What actually happened during LHB if Abramov et al are correct is not a global repopulation but a process of local exponential population growth such as in a petri dish. A recently sterilized area was an excellent locale for invading cells as there was little competition in the beginning. And chemical evolution theories notes that impactors supply both energy and organics, quite likely making the locale improved relative to before. Some… Read more »
Anonymous
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Anonymous
January 7, 2012 5:11 PM
My apology, but much you wrote to respond to, and a 10-minute reply was insufficient for me, time was wanting, and thoughtful reply was needful. ___________________________________________________________________ “The astrobiology first:” ______________________________ I would say in simple response to this section of your reply, you can be as brilliant as Einstein, understand the finest-point workings of chemistry, or the cellular complexities of biology, and still be blind ( or you can be dense as a rock, and dull as a board, and be just as blind ). Well, I hope I am not a rock, heavy as some of your thoughts can be to decipher. My use of “rebound”, regarding the recuperation of a planets life, was not meant to… Read more »
Torbjorn Larsson OM
Member
Torbjorn Larsson OM
January 6, 2012 4:32 PM
I think this merit some comment. The astrobiology first: – We have to distinguish between rebound, which in turn can be repopulation and/or diversification, and emergence. Here we are mostly discussing repopulation as I commented on below. Gene family data makes clear that diversification happened later. – What actually happened during LHB if Abramov et al are correct is not a global repopulation but a process of local exponential population growth such as in a petri dish. A recently sterilized area was an excellent locale for invading cells as there was little competition in the beginning. And chemical evolution theories notes that impactors supply both energy and organics, quite likely making the locale improved relative to before. Some… Read more »
Torbjorn Larsson OM
Member
Torbjorn Larsson OM
January 6, 2012 4:32 PM
I think this merit some comment. The astrobiology first: – We have to distinguish between rebound, which in turn can be repopulation and/or diversification, and emergence. Here we are mostly discussing repopulation as I commented on below. Gene family data makes clear that diversification happened later. – What actually happened during LHB if Abramov et al are correct is not a global repopulation but a process of local exponential population growth such as in a petri dish. A recently sterilized area was an excellent locale for invading cells as there was little competition in the beginning. And chemical evolution theories notes that impactors supply both energy and organics, quite likely making the locale improved relative to before. Some… Read more »
Torbjorn Larsson OM
Member
Torbjorn Larsson OM
January 6, 2012 4:32 PM
I think this merit some comment. The astrobiology first: – We have to distinguish between rebound, which in turn can be repopulation and/or diversification, and emergence. Here we are mostly discussing repopulation as I commented on below. Gene family data makes clear that diversification happened later. – What actually happened during LHB if Abramov et al are correct is not a global repopulation but a process of local exponential population growth such as in a petri dish. A recently sterilized area was an excellent locale for invading cells as there was little competition in the beginning. And chemical evolution theories notes that impactors supply both energy and organics, quite likely making the locale improved relative to before. Some… Read more »
Paul Spudis
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Paul Spudis
January 6, 2012 1:28 PM
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… Read more »
Torbjorn Larsson OM
Member
Torbjorn Larsson OM
January 6, 2012 2:55 PM

I would like to thank Paul Spudis for taking the time of making a very clarifying comment, as well as offering copies of the paper!

And I would add that I believe this is the sort of public outreach that adds much value to modern science and its social effects.

gopher65
Member
gopher65
January 6, 2012 3:13 PM

I agree completely Torbjörn Larssonsmile, This is exactly the sort of outreach that I think is not only beneficial for the individual researchers in question, but necessary to increase the public’s understanding of the scientific process.

vagueofgodalming
Member
January 7, 2012 12:42 PM

Paul Spudis’s blog is worth following too:

http://blogs.airspacemag.com/moon/

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