According to new research, some of Earth’s atmosphere was brought to the planet by comets billions of years ago.
This artwork shows a rocky planet being bombarded by comets. Image credit: NASA / JPL-Caltech.
The mystery of how the Earth’s atmosphere was formed has long baffled scientists. Some think comets might have originally brought some of the water, organic and atmospheric molecules to Earth that now make up its life.
Now a new study, led by Dr. Guillaume Avice of Caltech, CRPG-CNRS and the University of Lorraine, has found evidence to back up the theory.
Dr. Avice and co-authors have been analyzing tiny samples of ancient air trapped in water bubbles found in the mineral, quartz, which dates back more than 3 billion years.
The researchers found that the air in the rocks is partly made up of a primordial component labeled ‘U-Xe.’
“Xenon (Xe) is a noble gas which, being chemically inert and having nine isotopes, is an ideal element to reveal the xenon isotopic composition in the Earth’s primary atmosphere. This also makes it an ideal way of finding out where the atmosphere came from,” Dr. Avice said.
“U-Xe has solar-like composition for the light isotopes of xenon (124-130Xe), and is depleted in heavy Xe isotopes,” the authors explained.
“It isn’t usually found on Earth. The component is not present in the Earth’s mantle, nor is it found in meteorites.”
Therefore, the scientists believe that the U-Xe must have been added to the Earth after a primordial atmosphere had developed.
Simply put, comets are the best candidates for carrying the U-Xe to the planet.
“The Earth formed too close to the Sun for volatile elements, such as U-Xe, to easily condense and they would have rapidly boiled off the surface and been lost to space,” said co-author Prof. Ray Burgess, from the University of Manchester, UK.
“The reason that oceans and an atmosphere exist at all is because volatiles were still being added after the Earth formed.”
“The puzzle is in identifying where the volatiles came from and what objects carried them to the early Earth.”
“The difficulty is that many of the different volatile ingredients that were originally added have been thoroughly mixed together by geological processes during Earth’s long geological history.”
To combat this ‘mixing’ issue the team used tiny samples of ancient air trapped in water bubbles found in quartz in drill cores from the Barberton area of South Africa.
The rocks from this region of the continent are extremely old and very well preserved.
The researchers found that, in the Barberton quartz, 3.3-billion-year-old U-Xe has a composition very different from the xenon found in the Earth’s atmosphere today.
“We measured the amount and isotopic abundance of xenon in the 3.3 billion year old air with unparalleled precision,” Dr. Avice said.
“Our study reveals that 3 billion years ago there was already a xenon component in the Earth’s atmosphere different from solar gases and in asteroids. One possibility is that this xenon was from comets,” said co-author Prof. Bernard Marty, from CRPG-CNRS and the University of Lorraine.
But the discovery also shows the research possibilities of studying gases found trapped deep in the Earth.
“The study of gases trapped in ancient rocks opens new perspectives in our understanding of the origin and evolution of Earth’s volatile elements which are key factors for our planet’s habitability,” Dr. Avice said.
The research is published in the journal Nature Communications.
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Guillaume Avice et al. 2017. The origin and degassing history of the Earth’s atmosphere revealed by Archean xenon. Nature Communications 8, article number: 15455; doi: 10.1038/ncomms15455
