Earth plays host to life thanks to numerous unique planetary characteristics, one of which is the presence of abundant liquid water. When life first emerged on Earth more than 3.5 billion years ago it was in large part thanks to water, evidence for which is well documented in the geological record. However, in 1972 prominent astronomers Carl Sagan and George Mullen described the “faint young Sun paradox”; 4 billion years ago the Sun was smaller and weaker, and would emit only 70% of the energy we receive on Earth today .
Sagan and Mullen calculated that all other things being equal (i.e. atmospheric composition), 4 billion years ago there would be insufficient sunlight reaching Earth to prevent the oceans freezing over. But, they reasoned, if Earth’s atmosphere were different, a stronger greenhouse effect might prevent such a massive, life-inhibiting, glaciation event.
Listen to Bernard Marty on the Science Magazine podcast (20 September 2013).
One hypothesis put forward states that if the concentration of nitrogen in the Archean atmosphere were to approach three times what it is today, the greenhouse effect of other gases, such as carbon dioxide, would be amplified, thus warming Earth . In a recent paper published in Science, DCO scientists from CRPG-CNRS Université de Lorraine (France), in collaboration with the University of Manchester (UK) and the Institut de Physique du Globe de Paris, show that this is unlikely to be the solution to the faint young Sun paradox .
"We were looking for rocks that might have preserved a record of the ancient atmospheric composition, and found quartz crystals that contained minute, micron-sized fluid inclusions that had trapped and preserved 3.5 billion-year-old fossilized water,” said lead author Bernard Marty of CRPG-CNRS. “Analyzing extracted gases by mass spectrometry allowed us to identify ancient atmospheric gases, and to derive the atmospheric pressure of nitrogen (N2) at that time was similar to, or even lower that, that of the present-day.”
Other theories that might resolve the paradox remain, including increased levels of carbon dioxide, ammonia, and methane in the early atmosphere. But fossilized soil data suggest that carbon dioxide levels have not changed dramatically since the Archean, and the susceptibility of ammonia and methane to ultra-violet mediated breakdown make these gases less likely to have driven a stable greenhouse effect.
The current study also confirmed that Earth’s geomagnetic field was already strong in the Archean Eon. Isotopic ratios of nitrogen have not changed in the last 3 billion years therefore, unlike planets like Mars, Earth has maintained its thick, insulating atmosphere.
“This measurement shows that Earth's environment did not change drastically over the last several billion years, a stability that was necessary on our planet to permit life to flourish,” said Marty. “Carbon has been pivotal to maintain such clement environmental conditions at Earth's surface for several billion years.”
Photo: Magali Pujol: Fluid inclusion in sample PI-02. This 10 micron sized inclusion has trapped what is probably the most ancient water preserved in a terrestrial sample. Atmospheric gases from the Archean atmosphere dissolved in this water allowed the authors of this study to determine the partial pressure and isotopic composition of Archean atmospheric nitrogen.