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.
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?
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"
"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."
> 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.
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"
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.
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.
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.
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.
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?
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.
Yeah, even though the two aren't necessarily mutually exclusive, among folks I know who work on this there's a pretty good (subjective) correlation between favoring an old age for the moon and being skeptical of the LHB.
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?
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.
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
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.
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
[2] https://www.newscientist.com/article/mg21128265-100-moon-may...
[3] https://www.nature.com/articles/s41561-019-0398-3
[4] https://advances.sciencemag.org/content/3/1/e1602365