Moon is younger than first thought

Mosaic of the near side of the moon as taken by the Clementine star trackers. The images were taken on March 15, 1994. Credit: NASA

Improved age data for the Moon suggests that it is much younger than previously believed according to scientists presenting at a Royal Society discussion meeting entitled Origins of the Moon this week (23 September). Professor Richard Carlson of the Carnegie Institution of Washington will say that Earth's Moon is more likely between 4.4 and 4.45 billion years old rather than 4.56 billion years old, as previously thought.

The young age for the Moon implies an origin by a late giant impact into Earth with potentially big consequences for the Earth too.

Scientists have long studied lunar crustal rocks to try and estimate the age of the Moon. In the past obtaining accurate ages for lunar crustal rocks hasn't been easy for technical reasons. However as methods have improved, the ages of lunar crustal rock have begun to cluster not near 4.568 billion years, the precisely determined start time of Solar System formation, but between 4.36 and 4.45 billion years. Looking then at the Earth returns less clearly defined ages for Earth formation, but again, the ages tend to be less than 4.5 billion years.

Current models for assemble dust in the pretty quickly - where "pretty quickly" means a couple of million years. When assembled at this rate, the energy from violent collisions between planetesimals (small thought to fuse and form planets) and the heating caused by decay of causes even small planetesimals to undergo large-scale or complete melting. Through this melting process, iron metal segregates to the centre of the planetesimal and most of the volatile elements move to the atmosphere. When this chemical occurs on a small planetesimal, the planetesimal does not have enough gravity to hold on to its atmosphere, so it escapes into space. Earth is very depleted in compared to the average composition of the Solar System, likely because it formed from differentiated planetesimals that had already lost their atmospheres.

Professor Carlson uses the example of the asteroid Vesta to explain the variety of approaches scientists have taken to estimating the Moon's age:

"If you asked the question 'How old is the asteroid Vesta?' the answer would be 4.565 ± 0.001 billion years. Scientists can state this so precisely because the global melting of the asteroid Vesta, as sampled by a group of meteorites known as eucrites, happened so quickly that the age was frozen in precisely in the rocks formed during this event. Furthermore, no later significant geologic events happened to disturb the age recorded by the rocks because Vesta is too small to retain enough interior heat to allow further melting/volcanism.

However, ask the same question of the Earth or Moon and you don't get a very precise answer. Earth likely took longer to grow to full size compared to a small asteroid like Vesta and every step in its growth tends to erase, or at least cloud, the memory of earlier events."

By comparing planetary ages in this way, scientists have concluded that Moon formation, which many believe to be the result of a very large impact into the proto-Earth, did not occur until about 4.4 to 4.45 billion years ago. The giant impact set the "age of the Moon" but also reset most (but not all) older ages that can be used to estimate the "age of the Earth'.

The most precisely determined age for the type of lunar crustal rock that is believed to form directly from the magma ocean that occurred during Moon formation is 4.360 ± 0.003 billion years. Over the last decade or so, two areas have been found on Earth that have crustal rocks/minerals with ages approaching this date. The first is an area where a few zircon grains were found in much younger sediment in Western Australia. The other is a group of rocks found along the shores of Hudson's Bay in Canada (the Nuvvuagittuq terrane). Other regions of very old Earth rocks (Isua Greenland, and the Acasta rocks in central Canada) are also beginning to show evidence of a major differentiation event on Earth around 4.45 billion years ago, so the possibility exists that we are now seeing the first crusts formed on both Earth and Moon after the giant impact.

Professor Carlson says:

"There are several important implications of this late Moon formation that have not yet been worked out, for example, if the Earth was already differentiated prior to the giant impact, would the impact have blown off the primordial atmosphere that formed from this earlier epoch of Earth history?"

Scientists will discuss a number of different forming theories at the Royal Society meeting with other topics including 'how does ongoing exploration of Mercury inform our understanding of the Moon?" and "Are the Earth and Moon isotopic twins?".

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User comments

Sep 25, 2013
Before the moon could have been formed by an impact, both the proto-Earth and the impactor ("Theia") had to have been formed separately. Start the clock at 4.65 GY.
They likely formed as co-orbiting bodies at 1.0 AU (this would account for the relatively low-energy impact), possible with Theia in a horseshoe orbit.
After the impact it would take time for the debris to settle into a ring within the Earth's Roché limit, and then re-coalesce into the Moon as tidal forces caused the debris to move away from earth beyond that limit.
100 MY sounds fair for this elaborate process.

Sep 25, 2013
I looked at the web site and they plan to have audio recordings of the talks at that conference, which took place last Mon. and Tues. I hope they have them up soon!

Sep 25, 2013
100 MY sounds fair for this elaborate process.

Do you have any actual basis for this claim, or did you just 'Crank' it outta yer arse?

Sep 25, 2013
@verkle - there are numerous papers on this subject, many not behind paywalls. A Google search for:
earth collision moon formation physics
turns up roughly 6 million hits, a few percent of which will refer to papers have the calculations you seek.

Sep 26, 2013
I thought a redating was possible. It is nice, because the oldest zircons are ~ 4.4 billion years, and those that shows a global water supply are ~ 4.25 Ga bp (IIRC). Meaning the samples are nearing completion, and we get a better handle on conditions for early life.

If the Isua BIFs becomes accepted as early trace fossils (of Fe reducers), they show life > 3.8 Ga bp. Diverse molecular clocks predicts life started 4.1 - 4.0 Ga bp, which means life likely started within 150 - 600 million years of habitable conditions.

@verkle: No one disputes that an impact event is the main hypothesis. It is so accepted that there is a proposition in the geophysical society on datings, where it is taken as basis to define the time period between bodies in the disk (first geological event) to the Hadean.

And as all such successful theories it amasses a lot of passed tests as time passes. A few months ago, improved oxygen data showed Earth = Moon material yet again, say.

Sep 26, 2013
The sad thing is that it now becomes likely both Tellus (pre-collision Earth) and Theia had life - the best current models IMO put them equally massed and consistent with the scenario tadchem related. (Low velocity collision.) Such large bodies had volatiles and ~ 150 million years to form life, plenty of time. (As comparison, Mars coalesced in ~ 3 million years)

And post-impact, Earth had time to evolve life yet again before the start of the late bombardment ~ 4.1 Ga bp. It is likely life survived that, but the uncertainties in the clocks means we don't know.

So there may be up to 3 periods with up to 4 independent biospheres, erased and starting over until life finally 'took'.

Sep 26, 2013
@verkle: Actually, the link will likely become a good source for anyone that is genuinely asking questions instead of trolling. (Since you usually troll, I'm not encouraged to think you are interested in knowledge.)

They ask your questions, and answer them hopefully with the current data, and it is claimed that the material will come up eventually. Else it will be printed (see the link).

Sep 26, 2013
A pleasure to read comments such as yours Torbjorn.

Sep 26, 2013
@Verkle -
Small impactors are messy because they fragment in the atmosphere, and the extent to which they do this is determined by their composition as well as size and their velocity.

Mid-sized impactors are the simplest, and the physics for was worked out in the 1970s by firing high-speed projectiles at rock. The kinetic energy of the impactor is not disipated but remains in the impactor and the rock it passes through as the impactor penetrates the crust. Basically nothing has time to get out of the way.
The energy produces extremely dense hot rock vapor that explodes as the dense, hot gas from dynamite would, so the physics is much the same as for large explosions. This is why the moon's craters are round in spite of the various angles that the impactors that created them hit with.

- continued -

Sep 26, 2013
With huge impactors the collision is more of a 'splat' because of the scale involved. The collision occurs at the mutual escape velocity plus the impactor's initial relative speed, so there is plenty of energy to drive lot of material to reach orbit (which takes less energy than to reach escape velocity).
In the simplest head-on impacts the material directly between the centers comes to a stop and is very hot due to the energy, but due to the scale this takes time (minutes). Material to the sides of the impact is squished outwards, during this time, and so is already moving outward (~90 degrees to the line of impact) when the central material stops. This causes the dense-hot-rock-vapor explosion to 'burst out' in a ring around the line of impact.

In a head-on collision the ejected material has no net angular momentum, and so does not stay in orbit (some escapes, some falls back to the new merged body). So the impact has to be off-center.


Sep 26, 2013
And in a high-speed impact (such as seen with comets), too much material escaped and not enough remains in orbit.

Scientists therefore run a variety of starting masses, initial velocities, impact angles and even compositions and temperatures through calculations in which the earth and the impactor are each represented by thousands of blobs of material with the appropriate properties (cores as blobs of iron, crusts as blobs of rock). More recent calculations include shock waves as well as kinetic and thermal energy and gravitational force.

The simulations are then categorized by how well they match the moon's mass, composition and angular momentum, and, if anything new is learned, a paper is written on the results.

Purdue University has such a simulator on line, although I don't know if it is sophisticated enough to handle Mars-sized impactors accurately.

Sep 29, 2013
I'm still waiting to see a scientific basis for debris or mass to start orbiting the earth after a collision with some object. Can someone please provide some physics equations, even using the most rosy scenario, that can show this happen?

Someone, please take up the challenge.

I think the simple answer is that angular momentum was conserved.

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