Study questions dates for cataclysms on early moon, Earth

October 16, 2015
The deformed lunar zircon at center was brought from the moon by Apollo astronauts. The fractures characteristic of meteorite impact are not seen in most lunar zircons, so the ages they record probably reflect heating by molten rock, not impact. Credit: Apollo 17/Nicholas E. Timms

Phenomenally durable crystals called zircons are used to date some of the earliest and most dramatic cataclysms of the solar system. One is the super-duty collision that ejected material from Earth to form the moon roughly 50 million years after Earth formed. Another is the late heavy bombardment, a wave of impacts that may have created hellish surface conditions on the young Earth, about 4 billion years ago.

Both events are widely accepted but unproven, so geoscientists are eager for more details and better dates. Many of those dates come from zircons retrieved from the moon during NASA's Apollo voyages in the 1970s.

A study of zircons from a gigantic meteorite impact in South Africa, now online in the journal Geology, casts doubt on the methods used to date lunar impacts. The critical problem, says lead author Aaron Cavosie, a visiting professor of geoscience and member of the NASA Astrobiology Institute at the University of Wisconsin-Madison, is the fact that lunar zircons are "ex situ," meaning removed from the rock in which they formed, which deprives geoscientists of corroborating evidence of impact.

"While zircon is one of the best isotopic clocks for dating many geological processes," Cavosie says, "our results show that it is very challenging to use ex situ zircon to date a large impact of known age."

Although many of their zircons show evidence of shock, "once separated from host rocks, ex situ shocked zircons lose critical contextual information," Cavosie says.

This highly shocked zircon, from the Vredefort Dome in South Africa, shows thin, red bands that are a hallmark of meteorite impact. Uranium-lead dating from this zircon matched the age of the rocks exposed at Vredefort, not the more recent age of impact (2 billion years). Credit: Aaron Cavosie

The "clock" in a zircon occurs as lead isotopes accumulate during radioactive decay of uranium. With precise measurements of isotopes scientists can calculate, based on the half life of uranium, how long lead has been accumulating.

If all lead was driven off during asteroid impact, the clock was reset, and the amount of accumulated lead should record exactly how long ago the impact occurred.

Studies of lunar zircons have followed this procedure to produce dates from 4.3 billion to 3.9 billion years ago for the late heavy bombardment.

To evaluate the assumption of clock-resetting by impact, Cavosie and colleagues gathered zircons near Earth's largest impact, located in South Africa and known to have occurred 2 billion years ago. The Vredefort impact structure is deeply eroded, and approximately 90 kilometers across, says Cavosie, who is also in the Department of Applied Geology at Curtin University in Perth, Australia. "The original size, estimated at 300 kilometers diameter, is modeled to result from an impactor 14 kilometers in diameter," he says.

The researchers searched for features within the zircons that are considered evidence of impact, and concluded that most of the ages reflect when the zircons formed in magma. The zircons from South Africa are "out of place grains that contain definitive evidence of shock deformation from the Vredefort impact," Cavosie says. "However, most of the shocked grains do not record the age of the impact but rather the age of the rocks they formed in, which are about 1 billion years older."

The story is different on Earth, says zircon expert John Valley, a professor of geoscience at UW-Madison. "Most zircons on Earth are found in granite, and they formed in the same process that formed the granite. This has led people to assume that all the zircons were reset by impact, so the ages they get from the Moon are impact ages. Aaron is saying to know that, you have to apply strict criteria, and that's not what people have been doing."

The accuracy of zircon dating affects our view of Earth's early history. The poorly understood late heavy bombardment, for example, likely influenced when life arose, so dating the bombardment topped a priority list of the National Academy of Sciences for lunar studies. Did the giant craters on the moon form during a brief wave or a steady rain of impacts? "It would be nice to know which," Valley says.

"The question of what resets the zircon clock has always been very complicated. For a long time people have been saying if zircon is really involved in a major impact shock, its age will be reset, so you can date the impact. Aaron has been saying, 'Yes, sometimes, but often what people see as a reset age may not really be reset.' Zircons are the gift that keep on giving, and this will not change that, but we need to be a lot more careful in analyzing what that gift is telling us."

Explore further: Ancient minerals on Earth can help explain the early solar system

Related Stories

How did the Late Heavy Bombardment affect Earth's crust?

February 24, 2014

Astrobiologists supported by the NASA Astrobiology Institute have assessed the effects of impacts on the crust of the early Earth. The research could help determine whether or not evidence of such violent events in our planet's ...

Oldest bit of crust firms up idea of a cool early Earth

February 23, 2014

With the help of a tiny fragment of zircon extracted from a remote rock outcrop in Australia, the picture of how our planet became habitable to life about 4.4 billion years ago is coming into sharper focus.

Recommended for you

Mars rover Opportunity on walkabout near rim

June 23, 2017

NASA's senior Mars rover, Opportunity, is examining rocks at the edge of Endeavour Crater for signs that they may have been either transported by a flood or eroded in place by wind.

CHESS mission will check out the space between stars

June 23, 2017

Deep in space between distant stars, space is not empty. Instead, there drifts vast clouds of neutral atoms and molecules, as well as charged plasma particles called the interstellar medium—that may, over millions of years, ...

Dutch astronomers discover recipe to make cosmic glycerol

June 23, 2017

A team of laboratory astrophysicists from Leiden University (the Netherlands) managed to make glycerol under conditions comparable to those in dark interstellar clouds. They allowed carbon monoxide ice to react with hydrogen ...

Scientists uncover origins of the Sun's swirling spicules

June 22, 2017

At any given moment, as many as 10 million wild jets of solar material burst from the sun's surface. They erupt as fast as 60 miles per second, and can reach lengths of 6,000 miles before collapsing. These are spicules, and ...

21 comments

Adjust slider to filter visible comments by rank

Display comments: newest first

SuperThunder
3.3 / 5 (14) Oct 16, 2015
Whichever country goes the Moon next, check out that big gigantic crater in the Lunar South for zircons. Compare the reset age to zircons found elsewhere on the Moon. Compare those to Earth's.

Or put up a giant pressurized bouncy castle and live stream a naked astronaut bounce-house party from the Moon. Both are acceptable to my sense of science.
Shootist
5 / 5 (3) Oct 16, 2015
Whichever country goes the Moon next, check out that big gigantic crater in the Lunar South for zircons. Compare the reset age to zircons found elsewhere on the Moon. Compare those to Earth's.



South Pole - Aitken appears to predate the Terminal Lunar Cataclysm. What we need to do is date Utopia and Hellas basins on Mars and see how they compare with Imbrium and Orientale on the Moon.
Returners
1.4 / 5 (10) Oct 17, 2015
casts doubt on the methods used to date lunar impacts.

Any mixing of material during the event would change the isotopic ratios of all material in itself, therefore it is mostly unreliable. You would need to un-earth (un-moon?) a large, well preserved inclusion rock and then take a core sample of that rock in order to date at least the moon's crust formation, which could be several thousand or millions of years younger than the moon itself, but such an inclusion of rock would give you a minimum and maximum range which is more reliable than heavily mixed or shocked material.

once separated from host rocks, ex situ shocked zircons lose critical contextual information,


Of course. You can't tell when and where the Zircon formed if there isn't obvious compositional signatures. It could have formed in a prior impact involving the meteor in space before it hit the Earth or the Moon.

Context and contamination is a huge problem for all radio-dating.
Returners
1.4 / 5 (10) Oct 17, 2015
South Pole - Aitken appears to predate the Terminal Lunar Cataclysm. What we need to do is date Utopia and Hellas basins on Mars and see how they compare with Imbrium and Orientale on the Moon.


I think it's a mistake to assume that all of the largest craters came from the same time period. there are still some very big ones visible on Earth which are not weathered away yet, even though Earth has like 1000 times the erosion potential of Mars and pretty much infinitely more than the Moon.

You are right in that most of the formations will need to be dated independently, and hopefully using at least 2 or 3 unrelated techniques for maximum confirmation.

We know there is cross-contamination between Earth and Mars, and Earth and the Moon, and Apparently between Earth and Mercury. I doubt any Venus rocks made it to Earth, because passing through Venus atmosphere and Earth's atmosphere al in one trip seems impossible..
Returners
1.4 / 5 (10) Oct 17, 2015
The goal would be to find Earth rocks on the Moon and date those Earth rocks. This still won't tell you exactly when the Earth rock was ejected from Earth, as the rock will be older than the impactor, but it will tell you a maximum time range...but it would be difficult to use helium isotopes from beta decay to date such a rock, because it will have been heated to high temperatures and out-gassed during the ejection. You'd have to rely on dating the heavier daughter product from the decay, and hope that it isn't effected by out-gassing.

But ideally you'd want to be able to date each crater on each planet or moon directly, and then hopefully for confirmation, data a cross-contaminated rock on the other bodies, which were likely ejected in that event.

I think it is easily possible that Luna would have a lot of rocks from Earth, Mars, and mercury because they won't have been weathered away...assumes Late Bombardment significantly after Moon formation.
Returners
1.4 / 5 (10) Oct 17, 2015
I guess I'm saying the "Late Heavy Bombardment" could have been more than one series of events. It may have happened in 2 or 3 waves, explaining why a few of the largest craters on Earth have never weathered away yet.

Perhaps they are younger than the craters on the Moon and Mars. With 3 times the gravity, and 100 times the atmosphere, and all the water Earth has, the erosion potential on Earth dwarfs mars. So our craters are probably misleading and are younger than the craters on the Moon, Mars, and perhaps mercury.

Most of our oldest craters would be subducted by the ocean plates long, long ago, or shredded in continental divides and collisions. Essentially on Earth, if Weather doesn't destroy a Crater, then volcanism and plate tectonics eventually will.

The "Dark" Side of the Moon is heavily cratered, as the "light" side facing Earth was apparently shielded from something pretty bad, but Earth rotates, and most of "evidence" was subducted in the ocean.
Returners
1.4 / 5 (10) Oct 17, 2015
It might be worth a shot to use computer modeling to try to calculate the most likely Latitude, the most likely landing places are on the Moon and Mars for rocks ejected from Earth and Mercury, etc. This would presumably help plan any landing missions to look for such rocks, but even if the model were good, even very good, it could be off by 10 or 20 degrees due to differences in relative axis angle or orbital inclination, or things like that as the events happened..

I don't know how useful it is in total, because the model would need to take into account SS scale effects such as planetary drift caused by Gas Giants, which is not very well understood at all.
jim_xanara
3.8 / 5 (13) Oct 17, 2015
How are we going to whack this Philistine? OK, you've won the biggest boor on the planet prize. Before you show us your skills at slinging your own feces would you just please go away forever?
AGreatWhopper
4.2 / 5 (5) Oct 17, 2015
"Most zircons on Earth are found in granite, and they formed in the same process that formed the granite. This has led people to assume that all the zircons were reset by impact, so the ages they get from the Moon are impact ages. Aaron is saying to know that, you have to apply strict criteria, and that's not what people have been doing."


This point is too often taken for granite. :-)
ogg_ogg
2 / 5 (4) Oct 17, 2015
"The poorly understood late heavy bombardment, for example, likely influenced when life arose..." No, it likely influenced when life was last mostly extinguished, since life, if any, which developed earlier would be unlikely to survive (although both deep underground and ejecta in orbit might have provided sanctuary). I wonder what magical circumstances would have forced the loss of "all" of the lead? "All" is one of those words that should only be used with great caution in a scientific discussion. I note that the LHB is, as article says clearly, NOT "established science", but rather the "consensus (main-stream) view", a competing viewpoint is that impact activity didn't "peak" but ebbed and flowed as the Gas Giants settled into their orbits and gradually diminished...
ogg_ogg
2.3 / 5 (6) Oct 17, 2015
Seems to me that if Phys.org used an exponentially increasing exclusion time window, say 4 minutes, 16 minutes, 256 minutes, ... then while some back and forth would clearly be lost, some of the worst multiple-ploppers would find somebody else to bother. They could if they wanted.
Returners
1.6 / 5 (7) Oct 17, 2015
Seems to me that if Phys.org used an exponentially increasing exclusion time window, say 4 minutes, 16 minutes, 256 minutes, ... then while some back and forth would clearly be lost, some of the worst multiple-ploppers would find somebody else to bother. They could if they wanted.


It is my duty to point out an apparently un-noticed flaw in the science, and offer a much more viable alternative, albeit difficult to obtain without a full geological, seismology survey of the bodies in question: the Moon and Mars.

Surface rocks and crater rocks are too easily contaminated by other events.

The type of rock you need to find for the most reliable date for the maximum range is very, very rare, and will likely be tens or hundreds of meters below the surface, if it exists at all...

You get maximum range from the rock itself, and you get minimum range from whatever melt body or sediment layer it is included within.
wduckss
2.3 / 5 (6) Oct 17, 2015
Withhold the basic things. Research shows that they Australian zircons formed before 4404 ± 0.008 billion years ago, and the conditions which are then prevailed were cold and wet.
See discussion on:http://www.unexpl...c=286309
or https://en.wikipe...ed_rocks Oldest terrestrial material.
Torbjorn_Larsson_OM
3.7 / 5 (3) Oct 17, 2015
Interesting. Cavosie is the scientist who finds no shock impact evidence of the usual kind here on Earth from the late bombardment. (There is another putative kind, damaged zircon edges, but that isn't found much later from impacts such as Vredefort. Cavosie thinks - correctly I now think - that it is not from impact.)

The hypothesis that the late bombardment was spiked (cataclysmic) in impact frequency comes mainly from dating Moon craters I think. So Cavosie has showed a couple of observations around the late bormbardment that is in tension.

That said, the press release has some problems. The Moon forming impact is well tested (not "proven") and the late bombardment is an outcome of the Nice model that also predicts a lot of solar system features. The ~ 50 Myrs date for the Moon forming impact is Valley's I think, the astrophysicists have dating methods that gets ~ 90 Myrs. (And the possible window is up to 150 Myrs.) Finally, life may not have been much affected.
Torbjorn_Larsson_OM
3.3 / 5 (3) Oct 17, 2015
@Returners: "think it's a mistake to assume that all of the largest craters came from the same time period."

No one does. [Say, Vredefort in the article shows that.] It is impossible to point out all errors in your lengthy comments, so I'll let this be the proxy.

@ogg_ogg: Abramov et al shows that everything thermophile to mesophile would easily survive 10 times the impact flow of the old Nice 1.0 model and 30 times the current Nice 2.0 flow. They repopulate the crust faster than the sterilizing flow can keep up. [ http://www.nature...015.html ] Thermophily of the UCA lineage is more likely remains from its alkaline hydrothermal vent ancestry.

"I wonder what magical circumstances would have forced the loss of "all" of the lead?"

No magic in nature. Impact heat as they suggest, and it isn't all atoms but all the amount that would mess with a radiological U/Pb clock.
Torbjorn_Larsson_OM
4 / 5 (4) Oct 17, 2015
@wduckss:

I think the original claim that the _oldest_ zircon, not the youngest of course (see the article on the Vredesort ~ 3 billion year old ones), was that age. FYI Valley has since then revised that date somewhat lower, the oldest dateable zircon JH 4.4 is now ~ 4.374 billion years old. [ http://www.minsoc...1355.pdf ]

Still crust is ~ 4.4 Ga bp old (and ocean before 4.3 Ga bp), see fig. 17 in the link. That makes the Nuvvuagittuq the oldest known rock, with a 4.406 +0.014/-0.017 Ga bp age. [ http://www.resear...one_Belt ] O'Neil thinks the parent rock was _too young_ (never subducted) to have formed zircons, else you would find any. He has convinced me that his is the best explanation for the data they have. [NASA posted webinar.]
Vietvet
4 / 5 (4) Oct 17, 2015
How are we going to whack this Philistine? OK, you've won the biggest boor on the planet prize. Before you show us your skills at slinging your own feces would you just please go away forever?


As a despicable feces slinging anti-Semite you are no better than Returners and you both need to go away.
wduckss
2.3 / 5 (6) Oct 18, 2015
@wduckss:

I think the original claim that the _oldest_ zircon, not the youngest of course (see the article on the Vredesort ~ 3 billion year old ones), was that age. FYI Valley has since then revised that date somewhat lower, the oldest dateable zircon JH 4.4 is now ~ 4.374 billion years old. [ http://www.minsoc...1355.pdf ] O'Neil thinks the parent rock was _too young_ (never subducted) to have formed zircons, else you would find any. He has convinced me that his is the best explanation for the data they have. [NASA posted webinar.]


The data on zircons I looked within the framework age of the Earth.
Air before 4.4 billion years was a cold and there was a lot of water.
Fantastic claims about 4.5 billion year and manner of emergence is now eliminated (this for some time I'm trying to explain), however, article returns old the consumed hypothesis, biblical (dreadful) origin.
jim_xanara
1 / 5 (4) Oct 18, 2015
How are we going to whack this Philistine? OK, you've won the biggest boor on the planet prize. Before you show us your skills at slinging your own feces would you just please go away forever?


As a despicable feces slinging anti-Semite you are no better than Returners and you both need to go away.


OK. I have no ego involved in this. Followed him from another site here anyway. If he goes, I'll gladly go. Deal? Seriously.
Moltvic
3 / 5 (2) Oct 20, 2015
Whichever country goes the Moon next, check out that big gigantic crater in the Lunar South for zircons. Compare the reset age to zircons found elsewhere on the Moon. Compare those to Earth's.

Or put up a giant pressurized bouncy castle and live stream a naked astronaut bounce-house party from the Moon. Both are acceptable to my sense of science.


He's just saying what we're all thinking.
jim_xanara
1 / 5 (3) Oct 20, 2015
Well, vietvet, you could have had it your own way and couldn't be bothered. You can't whinge in the future.

I'm curious what makes it anti-Semitic, though.

The word "Zionist"? You love to quote UN resolutions and claim that only a Luddite would dismiss them out of hand, as with A/RES/63/281. That's what you're doing. Don't shove UN resolutions down our throat on climate change and then conveniently ignore ones like 3379. "Zionism is a form of racism and racial discrimination". That establishes, by a criterion you love to invoke,
a) Zionism exists;
b) It is a threat to society.

Are you a denier? If you accept those two points, do Zionists do nothing all day? Does their racism have no effect on society? That's all established fact. As a climate skeptic why is it vile to point out that many of the movers and shakers behind promoting AGW are Zionists? Would you fail to note if Nazis were well represented among skeptics?

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.