It might seem unlikely, but tiny grains of minerals can help tell the story of early Earth. And researchers studying those grains say that 4.4 billion years ago, Earth was a barren, mountainless place, and almost everything was under water. Only a handful of islands poked above the surface.
Scientists at the Australian National University are behind this study, led by researcher Dr. Antony Burnham. The mineral grains in the study are the oldest rocks ever found. They’re 4.4 billion year old zircon mineral grains from the Jack Hills of Western Australia, where they were preserved in sandstone formations.
4.4 billion years ago, the Earth was in what is called the ‘Hadean Eon’. This time period is poorly understood, because there is no rock record dating from that time. This is where the zircon mineral grains come in.
“The history of the Earth is like a book with its first chapter ripped out with no surviving rocks from the very early period, but we’ve used these trace elements of zircon to build a profile of the world at that time.” – Lead Researcher Dr. Antony Burnham, Australian National University
In a sense, these zircon grains are the missing pages.
Zircon’s chemical name is zirconium silicate, and it’s found almost everywhere in the Earth’s crust. This study focuses on what’s called detrital zircon, which is formed in igneous rocks, but then survives over time until deposited in sedimentary rocks. Zircon grains are typically very small, about 0.1 to 0.3 mm in size.
Zircon commonly contains traces of thorium and uranium, which means they can be easily dated by modern dating methods. Zircon can survive a lot of geologic processes like metamorphism, which explains their usefulness in studying the early Earth. They’re not easily destroyed, and they have a story to tell us.
The zircon grains were crystallized in magma about 500 million years after the Earth formed. As a result, they offer rare insights into conditions on Earth at the time. The zircon grains in question are referred to as detritus, because they formed in magma but were deposited in the geologically-important sandstone at Jack Hills, Australia. The detritus grains were compared to a second type of zircon grain which were formed directly in the sedimentary formations.
By distinguishing between the two types, researchers gain a new geochemical tool to understand early Earth.
“We used the granites of southeast Australia to decipher the link between zircon composition and magma type, and built a picture of what those missing rocks were,” he said.
“Our research indicates there were no mountains and continental collisions during Earth’s first 700 million years or more of existence – it was a much more quiet and dull place,” says Dr. Burnham from the ANU Research School of Earth Sciences.
“Our findings also showed that there are strong similarities with zircon from the types of rocks that predominated for the following 1.5 billion years, suggesting that it took the Earth a long time to evolve into the planet that we know today.” The team’s results are published in the journal Nature Geoscience, and is titled “Formation of Hadean granites by melting of igneous crust.”
This is not the first time that ancient, detrital zircons have been at the center of scientific studies of early Earth. In a previous study, they were used to conclude that Earth had a significant hydrosphere 4.3 billion years ago.
Fossilized remains are a fascinating thing. For paleontologists, these natural relics offer a glimpse into the past and a chance to understand what kind of lifeforms lurked there. But for astronomers, fossils are a way of ascertaining precisely when it was that life first began here on our planet – and perhaps even the Solar System.
And thanks to a team of Australian scientists, the oldest fossils to date have been uncovered. These fossilized remains have been dated to 3.7 billion years of age, and were of a community of microbes that lived on the ancient seafloor. In addition to making scientists reevaluate their theories of when life emerged on Earth, they could also tell us if there was ancient life on Mars.
The fossil find was made in what is known as the Isua Supracrustal Belt (ISB), an area in southwest Greenland that recently became accessible due to the ice melting in the area. According to the team, these fossils – basically tiny humps in rock measuring between one and four centimeters (0.4 and 1.6 inches) tall – are stromatolites, which are layers of sediment packed together by ancient, water-based bacterial colonies.
According to the team’s research paper, which appeared recently in Nature Communications, the fossilized microbes grew in a shallow marine environment, which is indicated by the seawater-like rare-earth elements and samples of sedimentary rock that were found with them.
They are also similar to colonies of microbes that can be found today, in shallow salt-water environments ranging from Bermuda to Australia. But of course, what makes this find especially interesting is just how old it is. Basically, the stone in the ISB is dated back to the early Archean Era, which took place between 4 and 3.6 billion years ago.
Based on their isotopic signatures, the team dated the fossils to 3.7 billion years of age, which makes them 220 million years older than remains that had been previously uncovered in the Pilbara Craton in north-western Australia. At the time of their discovery, those remains were widely believed to be the earliest fossil evidence of life on Earth.
As such, scientists are now reconsidering their estimates on when microbial life first emerged on planet Earth. Prior to this discovery, it was believed that Earth was a hellish environment 3.7 billion years ago. This was roughly 300 million years after the planet had finished cooling, and scientists believed it would take at least half a billion years for life to form after this point.
But with this new evidence, it now appears that life could have emerged faster than that. As Allen P. Nutman – a professor from the University of Wallongong, Australia, and the study’s lead author – said in a university press release:
“The significance of stromatolites is that not only do they provide obvious evidence of ancient life that is visible with the naked eye, but that they are complex ecosystems. This indicates that as long as 3.7 billion years ago microbial life was already diverse. This diversity shows that life emerged within the first few hundred millions years of Earth’s existence, which is in keeping with biologists’ calculations showing the great antiquity of life’s genetic code.”
When life emerged is a major factor when it comes to Earth’s chemical cycles. Essentially, Earth’s atmosphere during the Hadean was believed to be composed of heavy concentrations of CO² atmosphere, hydrogen and water vapor, which would be toxic to most life forms today. During the following Archean era, this primordial atmosphere slowly began to be converted into a breathable mix of oxygen and nitrogen, and the protective ozone layer was formed.
The emergence of microbial life played a tremendous role in this transformation, allowing for the sequestration of CO² and the creation of oxygen gas through photosynthesis. Therefore, when it comes to Earth’s evolution, the question of when life arose and began to affect the chemical cycles of the planet has always been paramount.
“This discovery turns the study of planetary habitability on its head,” said associate Professor Bennett, one of the study’s co-authors. “Rather than speculating about potential early environments, for the first time we have rocks that we know record the conditions and environments that sustained early life. Our research will provide new insights into chemical cycles and rock-water-microbe interactions on a young planet.”
The find has also inspired some to speculation that similar life structures could be found on Mars. Thanks to the ongoing efforts of Martian rovers, landers and orbiters, scientists now know with a fair degree of certainty that roughly 3.7 billion years ago, Mars had a warmer, wetter environment.
“The structures and geochemistry from newly exposed outcrops in Greenland display all of the features used in younger rocks to argue for a biological origin. This discovery represents a new benchmark for the oldest preserved evidence of life on Earth. It points to a rapid emergence of life on Earth and supports the search for life in similarly ancient rocks on Mars.”
Another thing to keep in mind is that compared to Earth, Mars experiences far less movement in its crust. As such, any microbial life that existed on Mars roughly 3.7 billion years ago would likely be easier to find.
This is certainly good news for NASA, since one of the main objectives of their Mars 2020 rover is to find evidence of past microbial life. I for one am looking forward to seeing what it leaves for us to pickup in its cache of sample tubes!
Just how did the Earth — our home and the place where life as we know it evolved — come to be created in the first place? In some fiery furnace atop a great mountain? On some divine forge with the hammer of the gods shaping it out of pure ether? How about from a great ocean known as Chaos, where something was created out of nothing and then filled with all living creatures?
If any of those accounts sound familiar, they are some of the ancient legends that have been handed down through the years that attempt to describe how our world came to be. And interestingly enough, some of these ancient creation stories contain an element of scientific fact to them.
When it comes to how the Earth was formed, forces that can only be described as fiery, chaotic, and indeed godlike, were involved. However, in the past few centuries, research and refinements made in what is today known as Earth Sciences have allowed scientists to assemble a more empirical and scientific understanding of how our world was formed.
Basically, scientists have ascertained that several billion years ago our Solar System was nothing but a cloud of cold dust particles swirling through empty space. This cloud of gas and dust was disturbed, perhaps by the explosion of a nearby star (a supernova), and the cloud of gas and dust started to collapse as gravity pulled everything together, forming a solar nebula — a huge spinning disk. As it spun, the disk separated into rings and the furious motion made the particles white-hot.
The center of the disk accreted to become the Sun, and the particles in the outer rings turned into large fiery balls of gas and molten-liquid that cooled and condensed to take on solid form. About 4.5 billion years ago, they began to turn into the planets that we know today as Earth, Mars, Venus, Mercury, and the outer planets.
The first era in which the Earth existed is what is known as the Hadean Eon. This name comes from the Greek word “Hades” (underworld), which refers to the condition of the planet at the time. This consisted of the Earth’s surface being under a continuous bombardment by meteorites and intense volcanism, which is believed to have been severe due to the large heat flow and geothermal gradient dated to this era.
Outgassing and volcanic activity produced the primordial atmosphere, and evidence exists that liquid water existed at this time, despite the conditions on the surface. Condensing water vapor, augmented by ice delivered by comets, accumulated in the atmosphere and cooled the molten exterior of the planet to form a solid crust and produced the oceans.
It was also during this eon – roughly 4.48 billion years ago (or 70–110 million years after the start of the Solar System) – that the Earth’s only satellite, the Moon, was formed. The most common theory, known as the “giant impact hypothesis” proposes that the Moon originated after a body the size of Mars (sometimes named Theia) struck the proto-Earth a glancing blow.
The collision was enough to vaporize some of the Earth’s outer layers and melt both bodies, and a portion of the mantle material was ejected into orbit around the Earth. The ejecta in orbit around the Earth condensed, and under the influence of its own gravity, became a more spherical body: the Moon.
The Hadean Eon ended roughly 3.8 billion years ago with the onset of the Archean age. Much like the Hadean, this eon takes it name from a ancient Greek word, which in this case means “beginning” or “origin.” This refers to the fact that it was during this period that the Earth had cooled significantly and life forms began to evolve.
Most life forms today could not have survived in the Archean atmosphere, which lacked oxygen and an ozone layer. Nevertheless, it is widely understood that it was during this time that most primordial life began to take form, though some scientists argue that many lifeforms may have occurred even sooner during the late Hadean.
At the beginning of this Eon, the mantle was much hotter than it is today, possibly as high as 1600 °C (2900 °F). As a result, the planet was much more geologically active, processes like convection and plate tectonics occurred much faster, and subduction zones were more common. Nevertheless, the presence of sedimentary rock date to this period indicates an abundance of rivers and oceans.
The first larger pieces of continental crust are also dated to the late Hadean/early Achean Eons. What is left of these first small continents are called cratons, and these pieces of crust form the cores around which today’s continents grew. As the surface continually reshaped itself over the course of the ensuing eons, continents formed and broke up.
The continents migrated across the surface, occasionally combining to form a supercontinent. Roughly 750 million years ago, the earliest-known supercontinent called Rodinia began to break apart, then recombined 600 – 540 million years ago to form Pannotia, then finally Pangaea. This latest supercontinent broke apart 180 million years ago, eventually settling on the configuration that we know today. (See graphics from Geology.com here)
Since that time, a mere blip on the geological time scale, all the events that we consider to be “recent history” took place. The dinosaurs ruled and then died, mammals achieved ascendancy, hominids began to slowly evolve into the species we know as homo sapiens, and civilization emerged. And it all began with a lot of dust, fire, and some serious impacts. From this, the Sun, Moon, Earth, and life as we know it were all created.
During the Hadean Eon, some 4.5 billion years ago, the world was a much different place than it is today. As the name Hades would suggest (Greek for “underworld”), it was a hellish period for Earth, marked by intense volcanism and intense meteoric impacts. It was also during this time that outgassing and volcanic activity produced the primordial atmosphere composed of carbon dioxide, hydrogen and water vapor.
Little of this primordial atmosphere remains, and geothermal evidence suggests that the Earth’s atmosphere may have been completely obliterated at least twice since its formation more than 4 billion years ago. Until recently, scientists were uncertain as to what could have caused this loss.
But a new study from MIT, Hebrew Univeristy, and Caltech indicates that the intense bombardment of meteorites in this period may have been responsible.
This meteoric bombardment would have taken place at around the same time that the Moon was formed. The intense bombardment of space rocks would have kicked up clouds of gas with enough force to permanent eject the atmosphere into space. Such impacts may have also blasted other planets, and even peeled away the atmospheres of Venus and Mars.
In fact, the researchers found that small planetesimals may be much more effective than large impactors – such as Theia, whose collision with Earth is believed to have formed the Moon – in driving atmospheric loss. Based on their calculations, it would take a giant impact to disperse most of the atmosphere; but taken together, many small impacts would have the same effect.
Hilke Schlichting, an assistant professor in MIT’s Department of Earth, Atmospheric and Planetary Sciences, says understanding the drivers of Earth’s ancient atmosphere may help scientists to identify the early planetary conditions that encouraged life to form.
“[This finding] sets a very different initial condition for what the early Earth’s atmosphere was most likely like,” Schlichting says. “It gives us a new starting point for trying to understand what was the composition of the atmosphere, and what were the conditions for developing life.”
What’s more, the group examined how much atmosphere was retained and lost following impacts with giant, Mars-sized and larger bodies and with smaller impactors measuring 25 kilometers or less.
What they found was that a collision with an impactor as massive as Mars would have the necessary effect of generating a massive a shockwave through the Earth’s interior and potentially ejecting a significant fraction of the planet’s atmosphere.
However, the researchers determined that such an impact was not likely to have occurred, since it would have turned Earth’s interior into a homogenous slurry. Given the appearance of diverse elements observed within the Earth’s interior, such an event does not appear to have happened in the past.
A series of smaller impactors, by contrast, would generate an explosion of sorts, releasing a plume of debris and gas. The largest of these impactors would be forceful enough to eject all gas from the atmosphere immediately above the impact zone. Only a fraction of this atmosphere would be lost following smaller impacts, but the team estimates that tens of thousands of small impactors could have pulled it off.
Such a scenario did likely occur 4.5 billion years ago during the Hadean Eon. This period was one of galactic chaos, as hundreds of thousands of space rocks whirled around the solar system and many are believed to have collided with Earth.
“For sure, we did have all these smaller impactors back then,” Schlichting says. “One small impact cannot get rid of most of the atmosphere, but collectively, they’re much more efficient than giant impacts, and could easily eject all the Earth’s atmosphere.”
However, Schlichting and her team realized that the sum effect of small impacts may be too efficient at driving atmospheric loss. Other scientists have measured the atmospheric composition of Earth compared with Venus and Mars; and compared to Venus, Earth’s noble gases have been depleted 100-fold. If these planets had been exposed to the same blitz of small impactors in their early history, then Venus would have no atmosphere today.
She and her colleagues went back over the small-impactor scenario to try and account for this difference in planetary atmospheres. Based on further calculations, the team identified an interesting effect: Once half a planet’s atmosphere has been lost, it becomes much easier for small impactors to eject the rest of the gas.
The researchers calculated that Venus’ atmosphere would only have to start out slightly more massive than Earth’s in order for small impactors to erode the first half of the Earth’s atmosphere, while keeping Venus’ intact. From that point, Schlichting describes the phenomenon as a “runaway process — once you manage to get rid of the first half, the second half is even easier.”
This gave rise to another important question: What eventually replaced Earth’s atmosphere? Upon further calculations, Schlichting and her team found the same impactors that ejected gas also may have introduced new gases, or volatiles.
“When an impact happens, it melts the planetesimal, and its volatiles can go into the atmosphere,” Schlichting says. “They not only can deplete, but replenish part of the atmosphere.”
The group calculated the amount of volatiles that may be released by a rock of a given composition and mass, and found that a significant portion of the atmosphere may have been replenished by the impact of tens of thousands of space rocks.
“Our numbers are realistic, given what we know about the volatile content of the different rocks we have,” Schlichting notes.
Jay Melosh, a professor of earth, atmospheric, and planetary sciences at Purdue University, says Schlichting’s conclusion is a surprising one, as most scientists have assumed the Earth’s atmosphere was obliterated by a single, giant impact. Other theories, he says, invoke a strong flux of ultraviolet radiation from the sun, as well as an “unusually active solar wind.”
“How the Earth lost its primordial atmosphere has been a longstanding problem, and this paper goes a long way toward solving this enigma,” says Melosh, who did not contribute to the research. “Life got started on Earth about this time, and so answering the question about how the atmosphere was lost tells us about what might have kicked off the origin of life.”
Going forward, Schlichting hopes to examine more closely the conditions underlying Earth’s early formation, including the interplay between the release of volatiles from small impactors and from Earth’s ancient magma ocean.
“We want to connect these geophysical processes to determine what was the most likely composition of the atmosphere at time zero, when the Earth just formed, and hopefully identify conditions for the evolution of life,” Schlichting says.
Schlichting and her colleagues have published their results in the February edition of the journal Icarus.