
The Moon is older than previously believed - dnetesn
https://phys.org/news/2019-07-moon-older-previously-believed.html
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cbkeller
Geologist here -- this has been an active topic lately!

Just a few weeks ago there was a paper proposing a 4.336 +/\- 0.031 Ga age for
the Lunar Magma Ocean [1], which is often equated with (esp. in public press
[e.g., 2], though not really the same as) the age of the Moon. For comparison,
the "40 to 60 Ma after solar system formation" in the new paper [3] discussed
here translates into roughly 4.517 +/\- 0.010 Ga.

This new paper agrees almost perfectly with one I was a coauthor on a couple
years ago though [4], which also gave an age of 4.51 (+/\- 0.01) Ga, so maybe
we're starting to get slightly closer to some consensus.

[1]
[https://doi.org/10.1016/j.epsl.2019.07.008](https://doi.org/10.1016/j.epsl.2019.07.008)

[2] [https://www.newscientist.com/article/mg21128265-100-moon-
may...](https://www.newscientist.com/article/mg21128265-100-moon-may-
be-200-million-years-younger-than-thought/)

[3]
[https://www.nature.com/articles/s41561-019-0398-3](https://www.nature.com/articles/s41561-019-0398-3)

[4]
[https://advances.sciencemag.org/content/3/1/e1602365](https://advances.sciencemag.org/content/3/1/e1602365)

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xefer
How is the time of "solar system formation" defined?

Presumably a system slowly forms out of gas and dust but at what point does
the accumulating material cross some threshold to where it is considered a new
system? Solar ignition?

~~~
xefer
I found this reference in which the abstract provides this definition: "The
age of the Solar System can be defined as the time of formation of the first
solid grains in the nebular disc surrounding the proto-Sun"

[https://doi.org/10.1038/ngeo941](https://doi.org/10.1038/ngeo941)

"The age of the Solar System can be defined as the time of formation of the
first solid grains in the nebular disc surrounding the proto-Sun. This age is
estimated by dating calcium–aluminium-rich inclusions in meteorites. These
inclusions are considered as the earliest formed solids in the solar nebula.
Their formation marks the beginning for several long- and short-lived
radiogenic clocks that are used to precisely define the timescales of Solar
System events, such as the formation and evolution of planetary bodies1,2,3.
Here we present the 207Pb–206Pb isotope systematics in a calcium–aluminium-
rich inclusion from the Northwest Africa 2364 CV3-group chondritic meteorite,
which indicate that the inclusion formed 4,568.2 million years ago."

~~~
cbkeller
Yeah, that's it -- since the first solid material also means the first
material we can determine an age of!

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mirimir
> The new study has thus determined that the Moon is significantly older than
> previously believed

True enough, I suppose: ~50 million years vs ~150 million years after solar
system formation. But on the scale of ~4.5 billion years ago, that's not such
a huge difference. And conversely, 50-100 million years is a _long_ time in
the context of stuff with solar orbits on the scale of 1-100 years.

I'm guessing that formation of the Sun and solar system was a strongly
exponential process. And the Moon probably formed during the final flurry of
adjustments among major bodies.

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squirrelicus
Hahahaha. "Adjustments" is quite a way to describe it. I wonder how many large
bodies exist today because an impact sightly nudged them from catastrophe to
orbital stability. My arm chair astrophysics training tells me the answer is
"literally all of them"

~~~
kijin
Yeah, "Adjustments" would be an understatement of the billennium. According to
recent theories I've read about, Jupiter may have veered inward within 1.5 AU
of the Sun, crashing into everything and creating a massive debris field that
is the asteroid belt. Meanwhile, Uranus and Neptune may have switched
positions at some point. The solar system was a huge mess back then.

~~~
mirimir
True.

So was it likely that it was this Jupiter incursion that sent the "Mars-size
planet" to the Earth, creating the Moon?

Also, I read somewhere, a few years ago, that the core of said "Mars-size
planet" survived the collision, and ended up as Mercury. As I recall, part of
the argument was that by composition, Mercury looks like a core, not a planet
(not even a near-solar planet). And there perhaps was some isotopic evidence.
Maybe also some compositional mass-balance argument like "'proto Earth' \+
'Mars-size planet' <> Earth + Moon". There being missing iron etc.

~~~
kijin
The jumping Jupiter scenario is usually associated with the Late Heavy
Bombardment, which is half a billion years too late for the age of the Moon
estimated by OP. But of course there are disagreements about when Jupiter did
its thing.

There's a lot of variance in "Mars-sized", so I wouldn't count on an easily
noticeable mass-balance match. Mars, Mercury, and the Moon are all very small
compared to Earth. Earth is more massive than all of the other rocky planets
and the Moon _combined_ , and yes, that includes Venus.

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slg
This is only tangentially related, but if the subject of these moon rocks
interests you I would highly recommend a recent video [1] Destin of Smarter
Every Day did on the NASA facility that stores the moon rocks, catalogues
them, breaks them apart, and loans them to researchers for studies like this.

[1] -
[https://www.youtube.com/watch?v=QxZ_iPldGtI](https://www.youtube.com/watch?v=QxZ_iPldGtI)

~~~
lunaru
Also tangentially related, but apparently, moon rocks were once fed to
cockroaches and injected into mice: [https://www.cnn.com/2019/07/29/us/nasa-
lunar-samples-testing...](https://www.cnn.com/2019/07/29/us/nasa-lunar-
samples-testing-scn-trnd/index.html)

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tectonic
This would seem to lend more evidence against the Late Heavy Bombardment.
[https://en.m.wikipedia.org/wiki/Late_Heavy_Bombardment](https://en.m.wikipedia.org/wiki/Late_Heavy_Bombardment)

~~~
garmaine
The LHB (and its connection to the Nice model) goes quite far in explaining
the size of Mars, the water content of Earth and Venus, the distribution of
the asteroids ... what about all that evidence?

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cbkeller
Not my exact area of expertise, but my two cents would be that the model space
is sufficiently vast and underconstrained that there are likely an infinitude
of equally allowable models that could also explain these features without an
LHB. Since the Nice model was specifically developed to explain the LHB [1],
using it as evidence for the LHB seems a bit circular.

[1]
[https://www.nature.com/articles/nature03676](https://www.nature.com/articles/nature03676)

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yummybear
So when they say "[...] this radioactive decay only lasted for the first 70
million years of the solar system", when is the start of the solar system
calculated from?

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forgotmypwd123
Solar ignition?

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yummybear
This might be a lack of understanding, but aren't the radioactive elements
created in the supernova preceding solar ignition. If so, isn't there a period
between element creation and solar system formation? How does this factor in
to the decay calculation.

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chrisbro
Will this have implications for the giant impact hypothesis, or not at all?
Can't tell if shortening the time window in which that could've happened would
change much. [https://en.m.wikipedia.org/wiki/Giant-
impact_hypothesis](https://en.m.wikipedia.org/wiki/Giant-impact_hypothesis)

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luxuryballs
Yo if it was a magma ocean does that mean some of the craters could have been
huge magma bubbles that rapidly cooled and then were eventually broken and
eroded down to look like craters? I sure hope so.

