Early Earth atmosphere favourable to life: study

Waterloo, ON – A new study by researchers at the University of Colorado at Boulder and the University of Waterloo indicates that Earth in its infancy probably had substantial quantities of hydrogen in its atmosphere, a surprising finding that may alter the way many scientists think about how life began on the planet.

Published in the April 7 issue of Science Express, the online edition of Science Magazine, the simulation study concludes that traditional models estimating hydrogen escape from Earth’s atmosphere several billions of years ago are flawed.

The new study indicates that up to 40% of the early atmosphere was hydrogen, implying a more favourable climate for the production of pre-biotic organic compounds like amino acids, and ultimately, life.

The paper was authored by doctoral student Feng Tian, Dr Owen Toon and research associate Dr Alexander Pavlov of CU-Boulder’s Laboratory for Atmospheric and Space Physics, and by Dr Hans De Sterk of Waterloo’s applied mathematics department. The study was supported by the NASA Institute of Astrobiology and NASA’s exobiology program.

“I didn’t expect this result when we began the study,” says Mr Tian, a doctoral student in CU-Boulder’s Astrobiology Center at LASP and chief author of the paper. “If Earth’s atmosphere was hydrogen-rich as we have shown, organic compounds could easily have been produced.”

Earth was formed about 4.6 billion years ago, and geologic evidence indicates that life may have begun on Earth roughly a billion years later.

“This study indicates that the carbon dioxide-rich, hydrogen-poor Mars and Venus-like model of Earth’s early atmosphere that scientists have been working with for the last 25 years is incorrect,” says Dr Toon. In such atmospheres, organic molecules are not produced by photochemical reactions or electrical discharges.

He says the premise that early Earth had a CO2-dominated atmosphere long after its formation has caused many scientists to look for clues to the origin of life in hydrothermal vents in the sea, fresh-water hot springs, or organic molecules delivered to Earth from space via meteorites or dust.

The team concluded that even if the atmospheric CO2 concentrations were large, the hydrogen concentrations would have been larger. “In that case, the production of organic compounds with the help of electrical discharge or photochemical reactions may have been efficient,” says Dr Toon.

Amino acids that likely formed from organic materials in the hydrogen-rich environment may have accumulated in the oceans or in bays, lakes and swamps, enhancing potential birthplaces for life, the team reports.

The new study indicates that the escape of hydrogen from Earth’s early atmosphere was probably two orders of magnitude, or 100 times, slower than scientists previously believed, says Mr Tian. The lower escape rate is based in part on new estimates for past temperatures in the highest reaches of Earth’s atmosphere some 8000 km in altitude where it meets the space environment.

While previous calculations assumed Earth’s temperature at the top of the atmosphere to be well over 800 degrees C several billion years ago, the new mathematical models show that temperatures would have been twice as cool back then. According to the study, the new calculations involve supersonic flows of gas escaping from Earth’s upper atmosphere as a planetary wind, in analogy with the solar wind.

“These simulation results are not easy to obtain because, mathematically, the flow exhibits a singularity point where the flow velocity makes a transition from subsonic to supersonic speeds,” says Dr De Sterck, who was recently hired as a faculty member for UW’s new interdisciplinary computational mathematics program.

“There seems to have been a blind assumption for years that atmospheric hydrogen was escaping from Earth three or four billion years ago as efficiently as it is today,” says Dr Pavlov. “We show that the escape was limited considerably back then by low temperatures in the upper atmosphere and the supply of energy from the sun.”

Despite somewhat higher ultraviolet radiation levels from the sun in Earth’s infancy, the escape rate of hydrogen would have remained low. The escaping hydrogen would have been balanced by hydrogen being vented by Earth’s volcanoes several billion years ago, making it a major component of the atmosphere.

In 1953, University of Chicago graduate student Stanley Miller sent an electrical current through a chamber containing methane, ammonia, hydrogen and water, yielding amino acids, considered to be the building blocks of life.

“I think this study makes the experiments by Miller and others relevant again,” says Dr Toon. “In this new scenario, organics can be produced efficiently in the early atmosphere, leading us back to the organic-rich soup-in-the-ocean concept.”

In the new CU-Boulder and UW scenario, it is a hydrogen- and CO2-dominated atmosphere that leads to the production of organic molecules, not the methane and ammonia atmosphere used in Miller’s experiment.

This summer Dr De Sterck will work on the supersonic flow modeling part of the project at Waterloo with the help of undergraduate students funded by NSERC.