New fossils push back the indisputable origin of life

[lee_merrill]

The first fossils of life are now at 3.7 billion years ago, about 220 million years earlier than the previous early fossils.

Source: Nature

Here we report evidence for ancient life from a newly exposed outcrop of 3,700-Myr-old metacarbonate rocks in the ISB that contain 1-4-cm-high stromatolites-macroscopically layered structures produced by microbial communities. The ISB stromatolites grew in a shallow marine environment, as indicated by seawater-like rare-earth element plus yttrium trace element signatures of the metacarbonates, and by interlayered detrital sedimentary rocks with cross-lamination and storm-wave generated breccias. The ISB stromatolites predate by 220 Myr the previous most convincing and generally accepted multidisciplinary evidence for oldest life remains in the 3,480-Myr-old Dresser Formation of the Pilbara Craton, Australia. The presence of the ISB stromatolites demonstrates the establishment of shallow marine carbonate production with biotic CO2 sequestration by 3,700 million years ago (Ma), near the start of Earth's sedimentary record.

Source

© Copyright Original Source

Blessings,
Lee

[shunyadragon]

The first fossils of life are now at 3.7 billion years ago, about 220 million years earlier than the previous early fossils.

Source: Nature

Here we report evidence for ancient life from a newly exposed outcrop of 3,700-Myr-old metacarbonate rocks in the ISB that contain 1-4-cm-high stromatolites-macroscopically layered structures produced by microbial communities. The ISB stromatolites grew in a shallow marine environment, as indicated by seawater-like rare-earth element plus yttrium trace element signatures of the metacarbonates, and by interlayered detrital sedimentary rocks with cross-lamination and storm-wave generated breccias. The ISB stromatolites predate by 220 Myr the previous most convincing and generally accepted multidisciplinary evidence for oldest life remains in the 3,480-Myr-old Dresser Formation of the Pilbara Craton, Australia. The presence of the ISB stromatolites demonstrates the establishment of shallow marine carbonate production with biotic CO2 sequestration by 3,700 million years ago (Ma), near the start of Earth's sedimentary record.

Source

© Copyright Original Source

Blessings,
Lee

OK. Good science. This fits the beginning of life with the formation of spreading zones, continental drift, early continents, oceans and sediment deposits. Now what?

[lee_merrill]

OK. Good science. This fits the beginning of life with the formation of spreading zones, continental drift, early continents, oceans and sediment deposits. Now what?

Well, they say now that the origin of life probably happened in the Hadean era:

Source: Nature

A sophistication of life by 3,700 Ma is in accord with genetic molecular clock studies placing life's origin in the Hadean eon (>4,000 Ma).

© Copyright Original Source

So the origin of life would probably be in the oceans (near smokers?), where the large quantity of water would tend to dilute any reactions.

Source: Reasons to Believe

Yet the latest discovery by the Australian scientists doesn’t fit this scenario. The Isua stromatolites formed at the earth’s surface in a shallow water environment. In fact, the research team generated data that effectively ruled out stromatolite formation near hydrothermal vents. But if the refugium model has validity, the Isua fossils should have formed in a high-temperature milieu.

Source

© Copyright Original Source

Also, we may note that reducing the timeframe for the origin of life by 220 million years doesn't leave much time for the origin of life! And having life originate in the Hadean era is going to be more difficult.

Blessings,
Lee

[rogue06]

Of course there are competing theories about life first arising from shallow ponds and hot springs, IIRC, largely because such locations would be rich in nitrogen which ancient seas might have been low on.

For instance, here's the abstract from a paper on just that subject Nitrogen Oxide Concentrations in Natural Waters on Early Earth (the entire paper is available by clicking on the hyperlink) from 2019.

Abstract

A key challenge in origins-of-life studies is estimating the abundances of species relevant to the chemical pathways proposed to have contributed to the emergence of life on early Earth. Dissolved nitrogen oxide anions ( ), in particular nitrate ( ) and nitrite ( ), have been invoked in diverse origins-of-life chemistry, from the oligomerization of RNA to the emergence of protometabolism. Recent work has calculated the supply of from the prebiotic atmosphere to the ocean and reported steady state [ ] to be high across all plausible parameter space. These findings rest on the assumption that is stable in natural waters unless processed at a hydrothermal vent. Here, we show that is unstable in the reducing environment of early Earth. Sinks due to ultraviolet photolysis and reactions with reduced iron (Fe²⁺) suppress [ ] by several orders of magnitude relative to past predictions. For pH = 6.5–8 and T = 0–50 °C, we find that it is most probable that [ ] <1μM in the prebiotic ocean. On the other hand, prebiotic ponds with favorable drainage characteristics may have sustained [ ] ≥1μM. As on modern Earth, most on prebiotic Earth should have been present as , due to its much greater stability. These findings inform the kind of prebiotic chemistries that would have been possible on early Earth. We discuss the implications for proposed prebiotic chemistries and highlight the need for further studies of kinetics to reduce the considerable uncertainties in predicting [ ] on early Earth.

And here's a nice piece on both the history and current state OOL research from Nature last year: How the first life on Earth survived its biggest threat -- water: Living things depend on water, but it breaks down DNA and other key molecules. So how did the earliest cells deal with the water paradox? where the researchers posit even shallower water:

The emerging evidence has caused many researchers to abandon the idea that life emerged in the oceans and instead focus on land environments, in places that were alternately wet and dry. The shift is hardly unanimous, but scientists who support the idea of a terrestrial beginning say it offers a solution to a long-recognized paradox: that although water is essential for life, it is also destructive to life’s core components.

So be careful of doing battle with the very sort of evidence that caused the shift from deep sea vents as if this were something new and unexplained. You can quickly find yourself combatting straw men if you end up battling an idea that is losing support.

[shunyadragon]

Well, they say now that the origin of life probably happened in the Hadean era:

Source: Nature

A sophistication of life by 3,700 Ma is in accord with genetic molecular clock studies placing life's origin in the Hadean eon (>4,000 Ma).

© Copyright Original Source

So the origin of life would probably be in the oceans (near smokers?), where the large quantity of water would tend to dilute any reactions.

Source: Reasons to Believe

Yet the latest discovery by the Australian scientists doesn’t fit this scenario. The Isua stromatolites formed at the earth’s surface in a shallow water environment. In fact, the research team generated data that effectively ruled out stromatolite formation near hydrothermal vents. But if the refugium model has validity, the Isua fossils should have formed in a high-temperature milieu.

Source

© Copyright Original Source

Also, we may note that reducing the timeframe for the origin of life by 220 million years doesn't leave much time for the origin of life! And having life originate in the Hadean era is going to be more difficult.

Blessings,
Lee

This speculation does not address the facts of abiogenesis origins of life. When continental formation began with crustal spreading zones, oceans forming and sedimentary deposits the conditions were right for abiogenesis to take place. There were millions of years in this time frame for life to begin. The only criteria is the environmental conditions must be present. The boundary between the Hadean and the Archean is not that exact. The conditions described are representative of the Archean.

There are no known rock formation dated in the Hasean. The formations described in this research are Archean.

As with other threads there is a problem with your speculation to justify an ID Creationist belief not science.

[lee_merrill]

Of course there are competing theories about life first arising from shallow ponds and hot springs, IIRC, largely because such locations would be rich in nitrogen which ancient seas might have been low on.
...

So be careful of doing battle with the very sort of evidence that caused the shift from deep sea vents as if this were something new and unexplained. You can quickly find yourself combatting straw men if you end up battling an idea that is losing support.

I agree that it's unlikely that life could originate near a hydrothermal vent, and yet if life originated in the Hadean era, it's unlikely to have developed on land! Given all the meteorite bombardment that occurred shortly afterward.

Source: Reasons to Believe

Around 3.8 billion years ago, a gravitational perturbation in the early solar system sent asteroids towards Earth. Some estimates have the earth experiencing over 17,000 impact events during this time. This event, called the late heavy bombardment (LHB), was originally regarded as a sterilization event. If so, then any life present on Earth prior to the LHB would have been obliterated. That being the case, again, it appears as if complex microbial ecologies appeared on Earth suddenly, within a geological instant.

Recently, some planetary scientists have challenged the notion that the LHB was a sterilization event. They argue that life on the planet’s surface would have been destroyed, but life in some environments, such as hydrothermal vents, could have survived. In other words, there would have been refugiums on Earth that served as “safe houses” for life, ushering it through the LHB.

Yet the latest discovery by the Australian scientists doesn’t fit this scenario. The Isua stromatolites formed at the earth’s surface in a shallow water environment. In fact, the research team generated data that effectively ruled out stromatolite formation near hydrothermal vents. But if the refugium model has validity, the Isua fossils should have formed in a high-temperature milieu.

Source

© Copyright Original Source

Blessings,
Lee

[rogue06]

I agree that it's unlikely that life could originate near a hydrothermal vent, and yet if life originated in the Hadean era, it's unlikely to have developed on land! Given all the meteorite bombardment that occurred shortly afterward.

Cosmic impacts take place at sea as well.

[lee_merrill]

This speculation does not address the facts of abiogenesis origins of life. When continental formation began with crustal spreading zones, oceans forming and sedimentary deposits the conditions were right for abiogenesis to take place. There were millions of years in this time frame for life to begin. The only criteria is the environmental conditions must be present.

But the Late Heavy Bombardment presents a problem to this scenario! See my response to rogue06...

Blessings,
Lee

[shunyadragon]

But the Late Heavy Bombardment presents a problem to this scenario! See my response to rogue06...

Blessings,
Lee

More Creationist speculation. There is absolutely no evidence that any of the bombardments destroyed everything from the Archean to Proterozoic. There is still evidence for life in these periods,

Can you present evidence that your speculative Creationist speculations are true.

Still waiting . . .

[shunyadragon]

I agree that it's unlikely that life could originate near a hydrothermal vent, and yet if life originated in the Hadean era, it's unlikely to have developed on land! Given all the meteorite bombardment that occurred shortly afterward.

Source: Reasons to Believe

Around 3.8 billion years ago, a gravitational perturbation in the early solar system sent asteroids towards Earth. Some estimates have the earth experiencing over 17,000 impact events during this time. This event, called the late heavy bombardment (LHB), was originally regarded as a sterilization event. If so, then any life present on Earth prior to the LHB would have been obliterated. That being the case, again, it appears as if complex microbial ecologies appeared on Earth suddenly, within a geological instant.

Recently, some planetary scientists have challenged the notion that the LHB was a sterilization event. They argue that life on the planet’s surface would have been destroyed, but life in some environments, such as hydrothermal vents, could have survived. In other words, there would have been refugiums on Earth that served as “safe houses” for life, ushering it through the LHB.

Yet the latest discovery by the Australian scientists doesn’t fit this scenario. The Isua stromatolites formed at the earth’s surface in a shallow water environment. In fact, the research team generated data that effectively ruled out stromatolite formation near hydrothermal vents. But if the refugium model has validity, the Isua fossils should have formed in a high-temperature milieu.

Source

© Copyright Original Source

Blessings,
Lee

There is no evidence that life began in the Hadean Period, because The evidence for life is in the Archean where there are spreading zones, initial evidence of contenents and sedimentary rocks. Stop looking for rabbits in Hadean rocks, which do not exist. Despite the meteorite bombbardments life did eventually form by natural processes by the evidence.

[shunyadragon]

Well, they say now that the origin of life probably happened in the Hadean era:

Source: Nature

A sophistication of life by 3,700 Ma is in accord with genetic molecular clock studies placing life's origin in the Hadean eon (>4,000 Ma).

© Copyright Original Source

So the origin of life would probably be in the oceans (near smokers?), where the large quantity of water would tend to dilute any reactions.

Source: Reasons to Believe

Yet the latest discovery by the Australian scientists doesn’t fit this scenario. The Isua stromatolites formed at the earth’s surface in a shallow water environment. In fact, the research team generated data that effectively ruled out stromatolite formation near hydrothermal vents. But if the refugium model has validity, the Isua fossils should have formed in a high-temperature milieu.

Source

© Copyright Original Source

Also, we may note that reducing the timeframe for the origin of life by 220 million years doesn't leave much time for the origin of life! And having life originate in the Hadean era is going to be more difficult.

Blessings,
Lee

Let's go with the evidence of what is found in the Archean. There is evidence of bacterial life in the beginning of the Archain, given a range of ~.2 billion years one way or the other). Therefore there was no complete oblideration of the beginning of life.in the early Archean. This research just pushes back the beginning date of the Archean. The bottomline is there are no known rocks dating to the Hadean.

The various meteorite bombbardments occured throughout the Archean and Proterozoic. Nonetheless fossil evidence of life is found throughout this period.

Source: https://ucmp.berkeley.edu/precambrian/archean_hadean.php

The Archean Eon and the Hadean

The Archean eon, which preceded the Proterozoic eon, spanned about 1.5 billion years and is subdivided into four eras: the Neoarchean (2.8 to 2.5 billion years ago), Mesoarchean (3.2 to 2.8 billion years ago), Paleoarchean (3.6 to 3.2 billion years ago), and Eoarchean (4 to 3.6 billion years ago).*

If you were able to travel back to visit the Earth during the Archean, you would likely not recognize it as the same planet we inhabit today. The atmosphere was very different from what we breathe today; at that time, it was likely a reducing atmosphere of methane, ammonia, and other gases which would be toxic to most life on our planet today. Also during this time, the Earth's crust cooled enough that rocks and continental plates began to form.

It was early in the Archean that life first appeared on Earth. Our oldest fossils date to roughly 3.5 billion years ago, and consist of bacteria microfossils. In fact, all life during the more than one billion years of the Archean was bacterial. The Archean coast was home to mounded colonies of photosynthetic bacteria called stromatolites. Stromatolites have been found as fossils in early Archean rocks of South Africa and western Australia. Stromatolites increased in abundance throughout the Archean, but began to decline during the Proterozoic. They are not common today, but they are doing well in Shark Bay,

The Hadean

Hadean time (4.6 to 4 billion years ago)* is not a geological period as such. No rocks on the Earth are this old, except for meteorites. During Hadean time, the Solar System was forming, probably within a large cloud of gas and dust around the sun, called an accretion disc. The relative abundance of heavier elements in the Solar System suggests that this gas and dust was derived from a supernova, or supernovas — the explosion of an old, massive star. Heavier elements are generated within stars by nuclear fusion of hydrogen, and are otherwise uncommon. We can see similar processes taking place today in so-called diffuse nebulae in this and other galaxies, such as the Nebula M16, below left.

The sun formed within such a cloud of gas and dust, shrinking in on itself by gravitational compaction until it began to undergo nuclear fusion and give off light and heat. Surrounding particles began to coalesce by gravity into larger lumps, or planetesimals, which continued to aggregate into planets. "Left-over" material formed asteroids and comets, like asteroid Ida, above right.

Because collisions between large planetesimals release a lot of heat, the Earth and other planets would have been molten at the beginning of their histories. Solidification of the molten material into rock happened as the Earth cooled. The oldest meteorites and lunar rocks are about 4.5 billion years old, but the oldest Earth rocks currently known are 3.8 billion years. Sometime during the first 800 million or so years of its history, the surface of the Earth changed from liquid to solid. Once solid rock formed on the Earth, its geological history began. This most likely happened prior to 3.8 billion years, but hard evidence for this is lacking. Erosion and plate tectonics has probably destroyed all of the solid rocks that were older than 3.8 billion years. The advent of a rock record roughly marks the beginning of the Archean eon.

© Copyright Original Source

The timing of the beginning of life in the Archean is not definitely established, but nonetheless the beginning of the Archean, given .2 billion years one way or the other is the beginning of the possibility of life beginning when the continents began forming, sedimentary rocks appeared, and evidence of oceans and mid-ocean ridges formed.

[lee_merrill]

Cosmic impacts take place at sea as well.

Certainly, which would make the origin of life in the sea more difficult too. Though I'm not sure what point you're making here...

Blessings,
Lee

[lee_merrill]

Let's go with the evidence of what is found in the Archean. There is evidence of bacterial life in the beginning of the Archain, given a range of ~.2 billion years one way or the other). Therefore there was no complete oblideration of the beginning of life.in the early Archean. This research just pushes back the beginning date of the Archean. The bottomline is there are no known rocks dating to the Hadean.

So the earliest fossils are at 3.7 billion years ago, and the late heavy bombardment (LHB) was at 3.8 billion years ago. The LHB has been considered a sterilization event, if this is correct, that leaves only 100 million years for life to originate. Not much time! Which is why I would expect people to want to put the origin of life back in the Hadean era, along with (as was mentioned) genetic molecular clock estimates.

Blessings,
Lee

[shunyadragon]

So the earliest fossils are at 3.7 billion years ago, and the late heavy bombardment (LHB) was at 3.8 billion years ago. The LHB has been considered a sterilization event, if this is correct, that leaves only 100 million years for life to originate. Not much time! Which is why I would expect people to want to put the origin of life back in the Hadean era, along with (as was mentioned) genetic molecular clock estimates.

Blessings,
Lee

The geologic evidence speaks for itself. Life began and persisted throughout the Archean and Proterozoic despite the comet and meteorite bombardment. The fact that the environment necessary for the beginning of life existed and the evidence of life exists is enough to justify that life began regardless of your hypothetical limits on time frame. The research you cited simply pushed back the beginning of life and the beginning of the Archean. ~0.2 billion years, and the beginning of the Archean. So what? Please not the (~) in these estimates of dates.

The evidence in history of life on earth is that both abiogenesis and evolution is environmentally driven.

[rogue06]

Two relatively recent articles to chew on dealing with the whole idea of a Late Heavy Bombardment and when it might have taken place:

Source: Bashing holes in the tale of Earth’s troubled youth

New analyses undermine a popular theory about an intense asteroid storm 4 billion years ago.



Early in Earth’s history, roughly half a billion years after the planet formed, all hell broke loose in the inner Solar System. A barrage of asteroids — some the size of Hong Kong — pummelled the globe intensely enough to melt large parts of its surface. This incendiary spree around 4 billion years ago vaporized most of Earth’s water and perhaps even sterilized its exterior, killing off any life that might have started to emerge. Only after this storm of impacts passed did the planet become safe enough for hardy organisms to take firm root and eventually give rise to all later life.

That horrific episode, known as the Late Heavy Bombardment (LHB), has been an integral part of Earth’s origin story for decades, ever since geologists did a systematic study of samples brought back from the Moon by NASA Apollo missions. But now, the once-popular theory has come under attack, and mounting evidence is causing many researchers to abandon it. A growing community of planetary scientists thinks that things quietened down relatively quickly, with a steadily decreasing rain of asteroids that ended a few hundred million years after Earth and the Moon formed.

Settling the debate could have major ramifications for some of the biggest questions in geoscience: when did life emerge and what were conditions like on early Earth? But some researchers think that fresh samples will be needed to finally put this conundrum to rest. They are looking with hope at the United States’ recent pledge to send astronauts back to the Moon — although no timeline has yet been set. In the meantime, the community is grappling with the fact that a key chapter of Solar System history might be vanishing before their eyes.

“The Late Heavy Bombardment was seen as one of the great triumphs of the Apollo era,” says geochemist Mark Harrison of the University of California, Los Angeles. “There’s no question that something has happened in the past few years that has profoundly upset the apple cart.”

The Solar System formed some 4.6 billion years ago, after the centre of a massive cloud of gas and dust collapsed into a dense sphere that became our Sun. Pebbles in a dusty disk orbiting the star continuously collided and sometimes stuck together. After tens of millions of years, these agglomerations had built up into planetesimals — the beginnings of the planets. Other rocky fragments remained, crashing into their larger kin and leaving deep craters. Over time, the Solar System thinned out, leaving something like the configuration we see today.

Most of the evidence of this violent history has been erased on Earth by the churning of tectonic plates. But the scarred surface of the Moon, long inert, retains a lengthy record of impacts. Some of that record — roughly 382 kilograms of lunar rock and soil — was collected by Apollo astronauts and carried back to scientists eager to see what the samples might reveal about the Moon’s history. In 1973, the year after the last Apollo landing, a group at Sheffield University, UK, reported a curious pattern in samples from four separate Apollo missions as well as a Soviet Luna mission. Radiometric dating of each one returned the same age: 3.95 billion years¹. A team at the California Institute of Technology (Caltech) in Pasadena corroborated the findings the same year².

Curious chronology

The confluence of ages suggested that a flurry of objects struck the Moon in a narrow 50-million-year window, leaving behind countless impact craters — including as many as a dozen of the Texas-sized basins that scar the surface. Because it seemed to represent a final surge of pandemonium after the Solar System’s chaotic genesis, the Caltech team named the event the terminal lunar cataclysm, although it later became more popularly known as the LHB.

The idea was immediately divisive, in large part because of ambiguity in the rock dating. This was done primarily by measuring the rocks’ ratio of argon-40 atoms to radioactive potassium-40. ⁴⁰K decays into ⁴⁰Ar with a half-life of 1.25 billion years. At high temperatures, that ⁴⁰Ar can leak out of minerals. That makes the ratio of these two isotopes a kind of clock: the more time that has elapsed since a rock was hot, the more ⁴⁰Ar should be present. But making sense of the argon and potassium concentrations can be difficult because the same ratio could have been caused by a concentrated barrage that heated the rocks and released ⁴⁰Ar some 3.95 billion years ago, or by a long, dwindling asteroid torrent that released it in fits and starts before fizzling out at about the same time.

The first really new data arrived in 2000. Planetary scientist David Kring, cosmochemist Timothy Swindle and planetary scientist Barbara Cohen, all then at the University of Arizona in Tucson, collected lunar meteorites that had fallen to Earth after being blasted from the Moon’s surface by asteroid strikes. They hoped such rocks would provide a more random sample of the Moon’s crust than those from Apollo, which represent at most 4% of the lunar surface. But when the results came back, they showed a curious, and familiar, pattern.

“Frankly, I thought we’d measure a bunch of these and have ages running back to 4.3 and 4.4 [billion years] and prove once and for all that this whole idea was wrong,” says Swindle. Instead, they found no evidence of impacts before the hypothesized time of the LHB³. “That kind of pushed me to a different side of the fence,” he says.

But researchers still wondered how a bombardment could come so long after the Solar System formed. By the half-billion-year mark, most of the leftover debris should either have been cast out or have settled into stable zones such as the main asteroid belt, which sits between Mars and Jupiter, or the Kuiper belt beyond Neptune. Nobody could come up with a physical reason for the unexpected drama at such a late date. “Where did you have the bodies in the Solar System that could hang around for 600 million years and then come screaming in and hit the Moon?” asks Cohen, who is now at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.

A potential answer arrived in 2005, with the emergence of what came to be known as the Nice model, after the French city where it was conceived. Originally proposed to explain odd orbital behaviour by distant icy objects in the Kuiper belt, the conjecture asserted that the Solar System’s outer planets had formed much closer to one another than they are now. Computer simulations showed⁴ how the massive gravitational pull of Jupiter and Saturn could have created an instability that ultimately bumped Uranus and Neptune into more distant orbits, knocked comets out of remote reservoirs and kicked asteroids out of the main belt.

The Nice model offered huge support for the LHB. “I think this helped cement this idea,” says physicist Nicolle Zellner of Albion College in Michigan. Geologist Marc Norman of the Australian National University in Canberra agrees. “That was the next real turning point,” he says.

Cataclysmic confusion

Yet just when the idea of the LHB finally seemed unimpeachable, holes began to appear. Apollo data and ‘crater counting’, which estimates the order in which craters were laid down on the basis of how they overlap, had indicated that three of the largest crater basins on the Moon’s near side — Imbrium, Nectaris and Serenitatis — might all be about 3.95 billion years old (see ‘Sampling the Moon’). But high-resolution maps from NASA’s Lunar Reconnaissance Orbiter, which started circling the Moon in 2009, spotted rays of debris extending from Imbrium⁵. This suggested that the impact that formed the crater might have knocked rocks into nearby Serenitatis, contaminating the Apollo samples picked up there. In 2010, a reanalysis of rocks thought to have been ejected from Nectaris indicated that they were also chemically and geologically similar to Imbrium material⁶. “We started realizing that maybe we were sampling Imbrium over and over,” says Zellner.

Clickenate on imagification
d41586-018-01074-6_15407032.jpg
to enbiggenate

The data from lunar meteorites didn’t necessarily help. Although none of the samples seemed to be older than 4 billion years, some were billions of years younger than that³, with no obvious spike around 3.95 billion years. And the Apollo samples held other surprises. Since 2012, detailed study⁷ of microscopic regions in the rocks has turned up ages of as much as 4.2 billion years, much older than any seen before, suggesting that there had been significant impacts earlier than the proposed spike.

Prodded in part by these revelations, some researchers proposed⁸ a longer-lasting LHB that began around 4.1 billion or 4.2 billion years ago. But that idea had one major strike against it: some of the most ancient crystals on Earth, from the Jack Hills range in Australia, suggest⁹ that the planet was a fairly clement place then, with relatively low temperatures and ample water.

Hot topic

Others are still scrutinizing the original Apollo evidence. To determine the samples’ ages, researchers heated the rocks to release argon, slowly ramping up the temperature. But as far back as 1991, Harrison had pointed out that the process won’t work well for rocks containing multiple minerals. Different minerals will release their argon at different temperatures. A sample heated to 400 °C might provide an age of 2 billion years; to 500 °C, an age of 2.5 billion. Researchers have tried to extrapolate from this behaviour, but Harrison says the complex patterns often lead them to pick essentially arbitrary ages. “This is quackery,” he says. “There’s no physical basis for it.”

Swindle says the argon heating situation is not necessarily as bad as Harrison makes it out to be; Apollo samples can be found whose ages don’t change significantly with temperature, and their dates — whether they refer to one or multiple impacts — still cluster around 3.95 billion years. Cohen says that other chronometers, such as those using radioactive isotopes of rubidium and uranium, corroborate the argon ages (although Harrison counters that the dates can differ by as much as 600 million years).

Such back and forth underscores how difficult it can be to tease small clues out of extremely ancient rocks. “Sherlock Holmes was good at resolving mysteries that happened last year,” says David Nesvorný, a planetary scientist at the Southwest Research Institute in Boulder, Colorado. “This all happened 4 billion years ago.”

Meanwhile, the Nice model has proved less helpful to the idea of an LHB than it once seemed. More-advanced simulations of the early Solar System’s gravitational interactions indicate that the planetary reshuffling probably happened shortly after formation, not with a delay of hundreds of millions of years¹⁰. Nesvorný likens delaying the reshuffling — and so keeping the Solar System hovering on the edge of instability — to trying to balance a pencil on its tip. “It’s really hard to put the pencil there in such a way that it falls in an hour,” he says.

One of the original architects of the Nice model, astronomer Alessandro Morbidelli of the Cote d’Azur Observatory in Nice, admits that the first versions took fine-tuning to get the reshuffling to occur so late. He no longer believes in the LHB, and sees many others in the field trading in the idea of a sudden asteroid deluge for that of a long, declining tail of bombardment. “My prediction is people will abandon the cataclysm,” he says.

Even those who remain tied to the LHB have had to modify their ideas. Planetary scientist William Bottke of the Southwest Research Institute agrees that there is no longer much support for a single, short spike. He says the best reading of the evidence, including samples from ancient Earth and radiometric dates in meteorite rocks, is a more drawn-out surge of bombardment that began around 4.1 billion or 4 billion years ago, with a relative lull before that, consistent with the existence of surface water in that period.

Astronomer William Hartmann, a visiting scientist at the International Space Science Institute in Bern, thinks the current situation proves that the idea of a cataclysm was never particularly robust. Various research communities “kind of had the impression that the other community had really solved this”, he says. “A paradigm structure was built up from supporting evidence, none of which was actually conclusive in itself.”

If an LHB did not happen, that could make it easier to explain how life emerged. Evidence of microbial life has been found in rocks that are around 3.5 billion years old. But those fossils seem quite complex, suggesting that they had been evolving from earlier forms for at least a few hundred million years, during the originally hypothesized time of the LHB. Without the cataclysm, such an ancient genesis might make more sense. Then again, some evidence suggests that the microbes at the base of the tree of life were hyperthermophiles — that is, organisms that thrived in extreme heat. The intense conditions created by a rain of asteroids could have resulted in a number of pockets where life might have emerged.

So far, efforts to clinch the LHB debate with evidence from other likely victims — Mercury, Venus, Mars and objects from the asteroid belt — have proved inconclusive. Each camp accuses the other of cherry-picking favourable data and not looking at the total picture. “It’s a Rorschach test,” says Norman. “People see what they want to see and disregard the rest.”

The only thing that researchers say will substantially move the needle is new samples from the Moon. Kring, now at the Lunar and Planetary Institute in Houston, Texas, has developed some concepts for sample-return missions, including one that would see astronauts collecting rocks from the South Pole–Aitken basin, the largest and oldest impact crater on the Moon. However, the next human mission to the Moon is still a long way off. The first new lunar rocks to be carried back to Earth may come from China’s Chang’e-5, a robotic mission currently planned for 2019. It aims to collect samples from the volcanic Mons Rümker formation, an area younger than those explored by Apollo astronauts.

Although no single exploration effort is likely to end the dispute, researchers’ improved understanding of the Moon and how to determine the ages of samples should provide greater confidence in the results.

However things eventually shake out, the new evidence will shift careers and rewrite textbooks. Yet, perhaps because of the long-lived nature of this debate, those trying to make sense of the LHB remain flexible, sceptical and surprisingly lighthearted.

“We are close friends and therefore we disagree all the time and then go drink a beer together,” says Bottke. “One should carry models lightly and be prepared to drop them if something better comes along, because it happens all the time.”


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and

Source: New date for 'Late Heavy Bombardment' may change life's timeline on Earth

Our planet may have been last pummeled by asteroid impacts longer ago than previously thought, explaining why life began to form almost 4 billion years ago.

The solar system once experienced a meteor shower of epic proportions: Asteroids whizzed around the inner planets, crashing down in a rain of fire that left their surfaces scarred for billions of years. Astronomers typically call this period the Late Heavy Bombardment.

But exactly when that fiery assault happened has been a matter of intense debate. The answer has big implications for the evolution of the solar system as a whole, and even for the timeline of life on Earth.

Finding evidence of such a bombardment here on Earth is difficult. Our planet regularly melts and recycles its crust, destroying detailed evidence that might give us a concrete age for the period of heavy meteor impacts. Farther off, on Mercury, Mars, and the rocky or icy moons of the outer solar system, scientists are left to count craters, an imprecise dating method. The other option is to use an objective dating method – radiometric rock dating, for instance – on bodies that have kept cleaner records than Earth. The Moon and asteroids – or the meteorite pieces of them that fall to Earth – are the most accessible.

When astronauts first brought back samples of the Moon, 50 years ago this summer, scientists found that they all showed evidence of massive and intense impacts at about 3.9 billion years ago. Later lunar missions returned more samples, and all agreed: some disaster occurred on the Moon that indicated a massive slew of impacts less than 4 billion years ago. For decades, scientists sought to explain what might have caused a sudden influx of asteroids and comets into the inner solar system.

But more recent evidence has hinted that Earth might have had liquid surface water before this period. It’s hard to reconcile how our planet maintained a surface cool enough to host water while undergoing a massive cataclysm. And dates from meteorites never agreed with the 3.9 billion-years-ago date from lunar rocks.

Now, astronomers led by Stephen Mojzsis from the University of Colorado, Boulder, have shown that the bombardment may have happened much earlier: 4.48 billion years ago. That would leave plenty of time for Earth to cool and life to emerge. They published their findings August 12 in the Astrophysical Journal.

Resetting the Clock

Most researchers think the Late Heavy Bombardment was caused by the giant planets moving around, orbiting closer to and farther from the sun and pushing lots of smaller solar system objects like asteroids along with them. But Mojzsis points out that there’s no timeline inherently attached to such a reshuffling.

“So look to the asteroid belt,” Mojzsis suggests. “The asteroids predate the planets, by definition. And we have 60,000 meteorites from the asteroids.”

His study, he says, is the first to consider the ages of all those meteorites on Earth. “And we find no uptick at 3.9 billion,” the time of the proposed Late Heavy Bombardment, he says. But his team did see that most of the rocks had been “reset” — basically melted to such an extent that it restarts the radiometric clocks researchers use to figure out a rock’s age. That melting is a sign of massive impacts, and they found the clocks reset at 4.48 billion years ago, only 80 million years after the start of the solar system. “The best explanation is that’s when giant planet migration occurred,” Mojzsis says.

Instead of one big spike, this earlier period of asteroid bombardment would have been a slow tapering-off from the early days when the solar system was little more than rocks crashing into each other. In Mojzsis’ timeline, the giant planets still migrated, but much earlier than previous theories suggested. This means there was no giant spike of meteors, but rather a flux of incoming asteroids that blended into the general chaos of the young solar system.

The best – and only real – argument for a more recent spike of impactors comes from lunar samples, which do show signs of some cataclysm occurring 3.9 billion years ago.

But, as Mojzsis explains, “If you look at the bombardment record from craters from Mercury, the Moon, Mars, satellites of the outer solar system, none of them show an uptick in bombardment. It’s only the lunar samples, which were all collected and returned to the Earth from a small patch of the Moon, just some 12 percent of the lunar surface, and all collected near Mare Imbrium [Crater].”

That geographical clustering, more than anything else, hurts the reliability of the lunar samples. It’s clear something catastrophic happened in the Mare Imbrium area, but it’s not as obvious that it must have been a Moon-wide — let alone solar system-wide — event.

If NASA – or any of the other actors in the increasingly crowded race back to the Moon – succeeds in visiting the Moon’s more remote South Pole-Aitkin Basin and returning samples from that oldest known crater, it would “complete the puzzle,” according to Mojzsis.

But in the meantime, he thinks his results lay to rest the idea of a Late Heavy Bombardment. Instead, Mojzsis prefers a history where the influx of asteroids and comets slowly wound down from the solar system’s wild earlier days to a gentle drift of space dust and the occasional stray impactor that still occurs today.

Letting Life Flourish

Mojzsis’ work doesn’t only depend on measuring meteorite ages. Because there’s a lot of evidence for a migration of giant planets, Mojzsis’ team modeled what it would look like if the event had happened early enough to explain the 4.48 billion years ago date he saw in the meteorites.

“We dynamically modeled what we’d analyzed geochemically. If this is correct, can this predict the reset ages we see on the Earth, Moon, and Mars? And it does. At 4.48 [billion years ago],” he says.

And that in turn pushes back the age of a hospitable Earth. If space more or less ceased pelting our planet with asteroids by 4.48 billion years ago, that allows the Earth to cool and form water. The oldest rocks scientists have found on Earth come from zircons, and these indicate Earth had water some 4 billion years ago. The first hints of life appear at 3.8 billion to 3.9 billion years ago, an age hard to reconcile with the idea of a massive meteor bombardment happening at the same time.

Earth would have still suffered the occasional asteroid blow – we see them even today, and we have strong evidence that one killed off the dinosaurs. But Mojzsis’ work means that Earth wouldn’t have suffered the kind of strikes that would boil away entire oceans and liquefy the whole surface more recently than 4.48 billion years ago.

“I think this resolves the conversation,” Mojzsis says of his work.



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The full paper for the latter, Onset of Giant Planet Migration before 4480 Million Years Ago is available by clicking on the hyperlink provided, although here is the abstract from it

Abstract

Soon after their formation, the terrestrial planets experienced intense impact bombardment by comets, leftover planetesimals from primary accretion, and asteroids. This temporal interval in solar system evolution, termed late accretion, thermally and chemically modified solid planetary surfaces and may have impeded life's emergence on the Hadean (pre-3850 Ma) Earth. The sources and tempo of bombardment, however, remain obscure. Here we present a timeline that relates variably retentive radiometric ages documented from asteroidal meteorites to new dynamical models that invoke an early episode of planetesimal-driven giant planet migration after the dispersal of the protoplanetary disk. Reconciliation of geochronological data with dynamical models shows that such giant planet migration should lead to an intense ~30 Myr influx of comets to the entire solar system manifested in radiometric age data. The absence of wholesale crustal reset ages after ~4450 Ma for the most resilient chronometers from Earth, Moon, Mars, 4 Vesta, and various meteorite parent bodies confines the onset of giant planet migration to ca. 4480 Ma. Waning impacts continue to strike the inner planets through a protracted monotonic decline in impactor flux, in agreement with predictions from crater chronology. New global 3D thermal analytical bombardment models derived from our revised impact mass-production functions show also that persistent niches for prebiotic chemistry leading to the emergence of life on the early Hadean Earth could endure late accretion since at least about 4400 million years ago.