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.
Writing in the journal Nature Geoscience, an international team of researchers led by University of Wisconsin-Madison geoscience Professor John Valley reveals data that confirm the Earth’s crust first formed at least 4.4 billion years ago, just 160 million years after the formation of our solar system. The work shows, Valley says, that the time when our planet was a fiery ball covered in a magma ocean came earlier.
This period of Earth history is known as the Hadean eon, named for ancient Greek god of the underworld Hades because of hellish conditions including meteorite bombardment and a molten surface.
“This confirms our view of how the Earth cooled and became habitable,” says Valley, a geochemist whose studies of zircons, the oldest known terrestrial materials, have helped portray how the Earth’s crust formed during the first geologic eon of the planet. “This may also help us understand how other habitable planets would form.”
The new study confirms that zircon crystals from Western Australia’s Jack Hills region crystallized 4.4 billion years ago, building on earlier studies that used lead isotopes to date the Australian zircons and identify them as the oldest bits of the Earth’s crust. The microscopic zircon crystal used by Valley and his group in the current study is now confirmed to be the oldest known material of any kind formed on Earth.
The study, according to Valley, strengthens the theory of a “cool early Earth,” where temperatures were low enough for liquid water, oceans and a hydrosphere not long after the planet’s crust congealed from a sea of molten rock. “The study reinforces our conclusion that Earth had a hydrosphere before 4.3 billion years ago,” and possibly life not long after, says Valley.
The study was conducted using a new technique called atom-probe tomography that, in conjunction with secondary ion mass spectrometry, permitted the scientists to accurately establish the age and thermal history of the zircon by determining the mass of individual atoms of lead in the sample. Instead of being randomly distributed in the sample, as predicted, lead atoms in the zircon were clumped together, like “raisins in a pudding,” notes Valley.
The clusters of lead atoms formed 1 billion years after crystallization of the zircon, by which time the radioactive decay of uranium had formed the lead atoms that then diffused into clusters during reheating.
“The zircon formed 4.4 billion years ago, and at 3.4 billion years, all the lead that existed at that time was concentrated in these hotspots,” Valley says. “This allows us to read a new page of the thermal history recorded by these tiny zircon time capsules.”
The formation, isotope ratio and size of the clumps — less than 50 atoms in diameter — become, in effect, a clock, says Valley, and verify that existing geochronology methods provide reliable and accurate estimates of the sample’s age. In addition, Valley and his group measured oxygen isotope ratios, which give evidence of early homogenization and later cooling of the Earth.
“The Earth was assembled from a lot of heterogeneous material from the solar system,” Valley explains, noting that the early Earth experienced intense bombardment by meteors, including a collision with a Mars-sized object about 4.5 billion years ago “that formed our moon, and melted and homogenized the Earth. Our samples formed after the magma oceans cooled and prove that these events were very early.”
The new study was supported by grants from the National Science Foundation, Department of Energy and the NASA Astrobiology Institute.
The zircon crystal has given scientists valuable information about how the earth became habitable, but does not tell us how it may remain so.
Research by Andrew Rushby and his team at the University of East Anglia has concluded that the Earth will not be able to sustain life indefinitely. They predict that the sun will increase in size and heat intensity and will eventually render the Earth uninhabitable. Initially it will be us humans and other complex life forms who will falter in the scorching conditions, but our demise will be succeeded by all forms of known cellular life which will gradually disappear over a period of around 1.75 to 3.25 billion years.
The timescale for our exit from Earth is not yet clear, as global warming and many other factors will also play their part, but it seems certain that our days here are ultimately numbered.
The study looked at the variable factors that dictate whether Earth-like planets remain in the 'habitable zone' (HZ) of their host stars.
Generally stars adhere to an order of events that begins with a star becoming dense enough to fuse hydrogen into helium; as time passes, these stars become hotter until they run out of hydrogen. As the star heats up, it also heats up nearby planets until eventually they become too hot to support life, usually because water supplies evaporate in the intense heat.
The time to panic is not yet here; it is estimated that our host star, the Sun, will nudge our home planet our of the HZ in approximately 1.75 billion years, though scientists believe that living conditions will become extremely challenging long before that.
”Of course conditions for humans and other complex life will become impossible much sooner – and this is being accelerated by anthropogenic climate change,” says Rushby. “Humans would be in trouble with even a small increase in temperature, and near the end only microbes in niche environments would be able to endure the heat.”
Hope may not be lost for our species however: assuming we can develop the necessary technology, there are other planets that we could feasibly decamp to, such as the newly discovered exoplanets Kepler 22b an Gliese 581d. These earth-like planets are both thought to be within their host star's HZ and could therefore have the potential to host life, though Kepler 22b's lifespan is predicted to be the same as Earth's. Gliese 581d looks more hopeful as an alternative future home planet, as its lifespan is thought to be around 55 billion years.
If Gliese 581d proves to be unsuitable, then do not despair: data from the Kepler space telescope suggests that there could be up to 50 sextillion other Earth-like planets out in the universe, so, subject to our ability to develop the necessary technology to evacuate the Earth, we should have plenty of choice for a new location.
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