Early Earth: A Battered, Hellish World with Water Oases for Life

[Bethany Beightol]

Early Earth: A Battered, Hellish World with Water Oases for Life
By Charles Q. Choi, Space.com Contributor   |   July 30, 2014 01:01pm ET
Early Earth-moon System After Bombardment
[Pin It] An artistic conception of the early Earth-moon system showing the Earth's surface after being bombarded with large impacts, causing magma extrusion on the surface, though some liquid water was retained. Image released on July 30, 2014.
Credit: Simone Marchi
View full size image

Asteroids and comets that repeatedly smashed into the early Earth covered the planet's surface with molten rock during its earliest days, but still may have left oases of water that could have supported the evolution of life, scientists say.

The new study reveals that during the planet's infancy, the surface of the Earth was a hellish environment, but perhaps not as hellish as often thought, scientists added.

Earth formed about 4.5 billion years ago. The first 500 million years of its life are known as the Hadean Eon. Although this time amounts to more than 10 percent of Earth's history, little is known about it, since few rocks are known that are older than 3.8 billion years old.

http://i.space.com/images/i/000/041/013/iFF/early-earth-moon-system-art-1.jpg?1406737814

[Bethany Beightol]

An artistic conception of the early Earth-moon system showing the Earth's surface after being bombarded with large impacts, causing magma extrusion on the surface, though some liquid water was retained. Image released on July 30, 2014.


Credit: Simone Marchi

[Bethany Beightol]

October 23, 2014
Early Earth: "Life Could Have Reseeded the Surface Multiple Times During Bombardment by Comets and Asteroids"

 

 

Image_1527e-Early-Earth

 

No-one knows when life first established a firm foothold on Earth. Ask around in the scientific community, though, and you’ll probably hear that the surface of early Earth, before about 3.8 billion years ago, was too hostile an environment for even a lowly microbe to set up shop. But new research by NASA Astrobiologists shows that life appeared to have been flourishing during the period of the Late Heavy Bombardment, at a time when Earth’s surface was thought to be uninhabitable.
The evidence appears to be a banded iron rock formation, or BIF, from Akilia Island in West Greenland. BIFs were deposited on Earth’s ocean floors during the first 2 billion years of the planet’s history. Iron and oxygen present in the oceans combined to form rust, which settled to the sea floor in layered sediments. Movements of the Earth’s crust later pushed some of these sediments to the surface, where they can now be studied.

It’s not possible to date the Akilia BIF sediments directly because they have undergone metamorphism – pressure cooking – so that traditional radioactive dating techniques cannot be used to determine their age. But jutting into these sediments are younger igneous rocks that can be accurately dated with these techniques.

An earlier analysis of this igneous rock, performed by a group headed by Dr. Allen Nutman of the Australian National University, put its age at 3.85 billion years. And because the igneous rock intrudes into the banded iron formation, it must have formed later than the sediments did. So the Akilia sediments must have been formed at least 3.85 billion years ago. Exactly how old they are, there is no way to know. But they are more ancient than any other sedimentary rocks found so far on Earth.

Rocks this old are rare on Earth because tectonic recycling action has crushed, buried and melted all of the material that formed the Earth’s crust during its first half-billion years of existence. Finding sedimentary rocks this old is important to geologists because they provide invaluable clues about what Earth was like in its early years.

Few people dispute the notion that between 4.1 and 3.8 billion years ago, our planet was heavily bombarded by debris from space, a period known as the Late Heavy Bombardment. If you look up at a full moon on a clear night, you see that its surface is riddled with impact craters. Scientists who study the size and distribution of those craters see clear evidence that the Moon underwent an intense period of impacts between 4.1 and 3.8 billion years ago. Although no craters from this time remain on Earth, because the Earth and Moon are so near each other, the assumption is that Earth suffered a similar fate.

[Bethany Beightol]

Ariel D. Anbar, an Assistant Professor in the Department of Earth and Environmental Sciences at the University of Rochester, working with Gail L. Arnold, a graduate student, decided to look for traces of this bombardment in the Akilia sediments. Comets and asteroids contain greater quantities of the chemical element iridium than does the Earth’s crust. So Anbar and Arnold, members of the NASA Astrobiology Institute (NAI), probed the Akilia sediments for abnormally high traces of iridium.

They didn’t find them. “Our naive expectation going in,” explained Anbar, was that “these sediments date from this bombardment period, so we should see evidence of the bombardment in them, right? So we looked for iridium in these rocks and didn’t find any. They were clean as a whistle.”

But earlier study of the Akilia sediments by one of Anbar’s collaborators, Steve Mojzsis, had turned up a very different type of signature in the Akilia formation – a signature of biological activity. Mojzsis, also a member of the NAI, performed his analysis of the Akilia sediments while at the University of California San Diego.

Carbon atoms come in two distinct forms, or isotopes. Carbon-12 atoms, the lighter of the two, contain 6 neutrons; carbon-13 atoms contain 7 neutrons. Microorganisms that take in carbon dioxide prefer to use the lighter carbon-12 atoms to construct the organic building blocks of which they are made.

When ancient ocean-dwelling organisms died, the carbon that was formerly part of their living tissue settled to the ocean floor, becoming part of the sedimentary material deposited there. When Mojzsis found that the Akilia sedimentary rock samples contained higher-than-normal quantities of carbon-12, he concluded that biological activity must have been taking place at the time the sediments were formed – at least 3.85 billion years ago.

So Anbar, Arnold and Mojzsis were faced with seemingly contradictory evidence. Life appeared to have been flourishing during the period of the Late Heavy Bombardment, at a time when Earth’s surface was thought to be uninhabitable. And traces of the bombardment were nowhere to be found in the Akilia rocks.

[Bethany Beightol]

Ariel D. Anbar, an Assistant Professor in the Department of Earth and Environmental Sciences at the University of Rochester, working with Gail L. Arnold, a graduate student, decided to look for traces of this bombardment in the Akilia sediments. Comets and asteroids contain greater quantities of the chemical element iridium than does the Earth’s crust. So Anbar and Arnold, members of the NASA Astrobiology Institute (NAI), probed the Akilia sediments for abnormally high traces of iridium.

They didn’t find them. “Our naive expectation going in,” explained Anbar, was that “these sediments date from this bombardment period, so we should see evidence of the bombardment in them, right? So we looked for iridium in these rocks and didn’t find any. They were clean as a whistle.”

But earlier study of the Akilia sediments by one of Anbar’s collaborators, Steve Mojzsis, had turned up a very different type of signature in the Akilia formation – a signature of biological activity. Mojzsis, also a member of the NAI, performed his analysis of the Akilia sediments while at the University of California San Diego.

Carbon atoms come in two distinct forms, or isotopes. Carbon-12 atoms, the lighter of the two, contain 6 neutrons; carbon-13 atoms contain 7 neutrons. Microorganisms that take in carbon dioxide prefer to use the lighter carbon-12 atoms to construct the organic building blocks of which they are made.

When ancient ocean-dwelling organisms died, the carbon that was formerly part of their living tissue settled to the ocean floor, becoming part of the sedimentary material deposited there. When Mojzsis found that the Akilia sedimentary rock samples contained higher-than-normal quantities of carbon-12, he concluded that biological activity must have been taking place at the time the sediments were formed – at least 3.85 billion years ago.

So Anbar, Arnold and Mojzsis were faced with seemingly contradictory evidence. Life appeared to have been flourishing during the period of the Late Heavy Bombardment, at a time when Earth’s surface was thought to be uninhabitable. And traces of the bombardment were nowhere to be found in the Akilia rocks.

[Bethany Beightol]

The solution lay in quantifying more carefully the effects of bombardment, using models developed by NAI member Kevin Zahnle at the NASA Ames Research Center. The essence of these models is that they treat the bombardment as a series of impact episodes, rather than assuming continuous pummeling of the Earth. They also take into account that smaller impact events are far more common than larger ones.

The Akilia sediments would not be expected to contain telltale traces of extraterrestrial iridium unless a massive asteroid had slammed into the Earth, spewing iridium into the global environment, precisely during the period when the Akilia formation was being deposited. Zahnle’s models indicate, however, that even during the Late Heavy Bombardment, such massive impacts were rare – too rare for there to be much chance of seeing their signs in sediments like those found on Akilia Island. So it made perfect sense that the sediments didn’t contain elevated levels of iridium.

Anbar and his colleagues reason that if the bombardment had a smaller-than-expected effect on the composition of sediments, it may also have had a smaller-than-expected effect on early life. Although small impacts were more common during the Late Heavy Bombardment than at any time since then, each such impact would destroy life at the surface only in one small area, not globally. Only the rarest, most massive impacts had the potential to wipe out all life on the planet’s surface.

Zahnle’s models indicate such impacts occurred only once every ten to one hundred million years. Moreover, the worst of their effects lasted for only about ten thousand years, after which time conditions on the Earth’s surface returned to normal. “So during most of this violent period of Earth’s history,” says Anbar, “the Earth’s surface – if you’re a microbe, anyway – was a perfectly balmy place to be. Which runs contrary to this picture that is out there that this was a very inhospitable period of time for life.”

That still leaves open one important question: Where could life hang out in safety during those rare, massive impact events that caused the surface literally to boil away? One suggestion is that hydrothermal vents might have filled that role. If life migrated down to these vents – or perhaps even began there – it could have continued on during major impact events, oblivious to what was going on the surface. And when the environment topside returned to habitability, life could have moved back up and recolonized the surface.

“So as long as microorganisms had places on the Earth where they’d be sheltered from really massive impact events,” concluded Anbar, “there’s no reason that they couldn’t have repopulated the surface multiple times. And therefore there’s no reason not to expect to find evidence of life if you find sediments from the earth’s surface during the period of heavy bombardment.”

The Daily Galaxy via Astrobiology/NASA

Image credit: Peter Sawyer / Smithsonian Institution

[Bethany Beightol]

http://www.dailygalaxy.com/my_weblog/2014/10/early-earth-life-could-have-reseeded-the-surface-multiple-times-during-bombardment-by-comets-and-ast.html

[Bethany Beightol]

Early Earth less hellish than previously thought

by David Salisbury | Posted on Monday, Sep. 15, 2014 — 11:34 AM
rugged landscape



Artist's illustration of what a cool early Earth looked like. (Artwork by Don Dixon, cosmographica.com)

Conditions on Earth for the first 500 million years after it formed may have been surprisingly similar to the present day, complete with oceans, continents and active crustal plates.

This alternate view of Earth’s first geologic eon, called the Hadean, has gained substantial new support from the first detailed comparison of zircon crystals that formed more than 4 billion years ago with those formed contemporaneously in Iceland, which has been proposed as a possible geological analog for early Earth.
Calvin Miller standing on a hilly landscape




Professor Calvin Miller (Vanderbilt University)

The study was conducted by a team of geologists directed by Calvin Miller, the William R. Kenan Jr. Professor of Earth and Environmental Sciences at Vanderbilt University, and published online this weekend by the journal Earth and Planetary Science Letters in a paper titled, “Iceland is not a magmatic analog for the Hadean: Evidence from the zircon record.”

From the early 20th century up through the 1980’s, geologists generally agreed that conditions during the Hadean period were utterly hostile to life. Inability to find rock formations from the period led them to conclude that early Earth was hellishly hot, either entirely molten or subject to such intense asteroid bombardment that any rocks that formed were rapidly remelted. As a result, they pictured the surface of the Earth as covered by a giant “magma ocean.”

This perception began to change about 30 years ago when geologists discovered zircon crystals (a mineral typically associated with granite) with ages exceeding 4 billion years old preserved in younger sandstones. These ancient zircons opened the door for exploration of the Earth’s earliest crust. In addition to the radiometric dating techniques that revealed the ages of these ancient zircons, geologists used other analytical techniques to extract information about the environment in which the crystals formed, including the temperature and whether water was present.

Since then zircon studies have revealed that the Hadean Earth was not the uniformly hellish place previously imagined, but during some periods possessed an established crust cool enough so that surface water could form – possibly on the scale of oceans.

Accepting that the early Earth had a solid crust and liquid water (at least at times), scientists have continued to debate the nature of that crust and the processes that were active at that time: How similar was the Hadean Earth to what we see today?
Panoramic photo of Miller standing on a hilltop

Calvin Miller at the Kerlingarfjoll volcano in central Iceland. Some geologists have proposed that the early Earth may have resembled regions like this. (Tamara Carley / Vanderbilt)

Two schools of thought have emerged: One argues that Hadean Earth was surprisingly similar to the present day. The other maintains that, although it was less hostile than formerly believed, early Earth was nonetheless a foreign-seeming and formidable place, similar to the hottest, most extreme, geologic environments of today. A popular analog is Iceland, where substantial amounts of crust are forming from basaltic magma that is much hotter than the magmas that built most of Earth’s current continental crust.

“We reasoned that the only concrete evidence for what the Hadean was like came from the only known survivors: zircon crystals – and yet no one had investigated Icelandic zircon to compare their telltale compositions to those that are more than 4 billion years old, or with zircon from other modern environments,” said Miller.

[Bethany Beightol]

In 2009, Vanderbilt doctoral student Tamara Carley, who has just accepted the position of assistant professor at Layfayette College, began collecting samples from volcanoes and sands derived from erosion of Icelandic volcanoes. She separated thousands of zircon crystals from the samples, which cover the island’s regional diversity and represent its 18 million year history.

Working with Miller and doctoral student Abraham Padilla at Vanderbilt, Joe Wooden at Stanford University, Axel Schmitt and Rita Economos from UCLA, Ilya Bindeman at the University of Oregon and Brennan Jordan at the University of South Dakota, Carley analyzed about 1,000 zircon crystals for their age and elemental and isotopic compositions. She then searched the literature for all comparable analyses of Hadean zircon and for representative analyses of zircon from other modern environments

[Bethany Beightol]

“We discovered that Icelandic zircons are quite distinctive from crystals formed in other locations on modern Earth. We also found that they formed in magmas that are remarkably different from those in which the Hadean zircons grew,” said Carley.
Tiny crystals on black background

Images of a collection of Icelandic zircons taken with a scanning electron microscope. They range in size from a tenth of a millimeter to a few thousands of a millimeter. (Tamara Carley / Vanderbilt)

Most importantly, their analysis found that Icelandic zircons grew from much hotter magmas than Hadean zircons. Although surface water played an important role in the generation of both Icelandic and Hadean crystals, in the Icelandic case the water was extremely hot when it interacted with the source rocks while the Hadean water-rock interactions were at significantly lower temperatures.

“Our conclusion is counterintuitive,” said Miller. “Hadean zircons grew from magmas rather similar to those formed in modern subduction zones, but apparently even ‘cooler’ and ‘wetter’ than those being produced today.”

The study was supported by National Science Foundation grants EAR-1220523, EAR- CAREER-0844772 and DGE-0909667, and research grants from the National Geographic Society and the Keck Geology Consortium.

Contact:
David Salisbury, (615) 322-NEWS
david.salisbury@vanderbilt.edu
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[Bethany Beightol]

http://news.vanderbilt.edu/2014/09/early-earth-less-hellish/

[Bethany Beightol]

How Was Earth Formed?

http://www.space.com/19175-how-was-earth-formed.html

[Bethany Beightol]

 Earth's violent youth

For much of the Hadean, Earth and its sister worlds in the inner solar system were pummeled with an extraordinary number of cosmic impacts.

"It was thought that because of these asteroids and comets flying around colliding with Earth, conditions on early Earth may have been hellish," said lead study author Simone Marchi, a planetary scientist at the Southwest Research Institute in Boulder, Colorado. This imagined hellishness gave the eon its name — Hadean comes from Hades, the lord of the underworld in Greek mythology.

However, in the past dozen years or so, a radically different picture of the Hadean began to emerge. Analysis of minerals trapped within microscopic zircon crystals dating from this eon "suggested there was liquid water on the surface of the Earth back then, clashing with the previous picture that the Hadean was hellish," Marchi said. This could explain why the evidence of the earliest life on Earth appears during the Hadean — maybe the planet was less inhospitable during that eon than previously thought.

[Bethany Beightol]

This artist's illustration shows a close-up of the early Earth, revealing magma extrusion on the surface and the scars from severe cosmic bombardment. Image released on July 30, 2014.
Credit: Simone Marchi

[Bethany Beightol]

 Cosmic bombardment history

The exact timing and magnitude of the impacts that smashed Earth during the Hadean are unknown. To get an idea of the effects of this bombardment, Marchi and his colleagues looked at the moon, whose heavily cratered surface helped model the battering that its close neighbor Earth must have experienced back then.

"We also looked at highly siderophile elements (elements that bind tightly to iron), such as gold, delivered to Earth as a result of these early collisions, and the amounts of these elements tells us the total mass accreted by Earth as the result of these collisions," Marchi said. Prior research suggests these impacts probably contributed less than 0.5 percent of the Earth's present-day mass.

The researchers discovered that "the surface of the Earth during the Hadean was heavily affected by very large collisions, by impactors larger than 100 kilometers (60 miles) or so — really, really big impactors," Marchi said. "When Earth has a collision with an object that big, that melts a large volume of the Earth's crust and mantle, covering a large fraction of the surface," Marchi added.

These findings suggest that Earth's surface was buried over and over again by large volumes of molten rock — enough to cover the surface of the Earth several times. This helps explain why so few rocks survive from the Hadean, the researchers said.

However, although these findings might suggest that the Hadean was a hellish eon, the researchers found that "there were time gaps between these large collisions," Marchi said. "Generally speaking, there may have been something on the order of 20 or 30 impactors larger than 200 km (120 miles) across during the 500 million years of the Hadean, so the time between such impactors was relatively long," Marchi said.

Any water vaporized near these impacts "would rain down again," Marchi said, and "there may have been quiet tranquil times between collisions — there could have been liquid water on the surface."

The researchers suggested that life emerging during the Hadean was probably resistant to the high temperatures of the time. Marchi and his colleagues detailed their findings in the July 31 issue of the journal Nature.

Follow us @Spacedotcom, Facebook and Google+. Original article on Space.com.

http://www.space.com/26685-early-earth-bombardment-water-oasis.html