Methane present in the early Martian atmosphere, together with carbon dioxide and hydrogen may have created a greenhouse effect on the planet 3.5-4.5 billion years ago, according to a Harvard University-led team of researchers.
Mosaic of the Valles Marineris hemisphere of Mars projected into point perspective, a view similar to that which one would see from a spacecraft. The distance is 1,550 miles (2,500 km) from the surface of the planet. The mosaic is composed of 102 Viking Orbiter images of Mars. The center of the scene shows the entire Valles Marineris canyon system, over 1,240 miles (2,000 km) long and up to 5 miles (8 km) deep, extending form Noctis Labyrinthus, the arcuate system of graben to the west, to the chaotic terrain to the east. Many huge ancient river channels begin from the chaotic terrain from north-central canyons and run north. The three Tharsis volcanoes (dark red spots), each about 15.5 miles (25 km) high, are visible to the west. South of Valles Marineris is very ancient terrain covered by many impact craters. Image credit: NASA / JPL-Caltech.
“Early Mars is unique in the sense that it’s the one planetary environment, outside Earth, where we can say with confidence that there were at least episodic periods where life could have flourished,” said Dr. Robin Wordsworth, assistant professor of environmental science and engineering at the Harvard John A. Paulson School of Engineering and Applied Science.
“If we understand how early Mars operated, it could tell us something about the potential for finding life on other planets outside the Solar System.”
Around 4 billion years ago, the Sun was about 30% fainter than today and significantly less solar radiation reached the surface of the Red Planet.
The scant radiation that did reach the planet was trapped by the atmosphere, resulting in warm, wet periods.
For years, planetary researchers have struggled to model exactly how early Mars was insulated.
The obvious culprit is carbon dioxide, which makes up 95% of today’s Martian atmosphere and is the most well-known greenhouse gas on Earth.
“You can do climate calculations where you add carbon dioxide and build up to hundreds of times the present day atmospheric pressure on Mars and you still never get to temperatures that are even close to the melting point,” Dr. Wordsworth said.
There must have been something else in the Martian atmosphere that contributed to a greenhouse effect.
The atmospheres of terrestrial planets lose lighter gases, such as hydrogen, to space over time.
Dr. Wordsworth and co-authors looked to these long-lost gases — known as reducing gases — to provide a possible explanation for Mars’ early climate.
In particular, the researchers looked at methane, which today is not abundant in the Martian atmosphere.
Billions of years ago, however, geological processes could have been releasing significantly more methane into the atmosphere.
This methane would have been slowly converted to hydrogen and other gases, in a process similar to that occurring today on Titan, Saturn’s largest moon.
To understand how this early Martian atmosphere may have behaved, the authors needed to understand the fundamental properties of these molecules.
“When you’re looking at exotic atmospheres, you can’t compare them to Earth’s atmosphere,” Dr. Wordsworth said.
“You have to start from first principles. So we looked at what happens when methane, hydrogen and carbon dioxide collide and how they interact with photons.”
“We found that this combination results in very strong absorption of radiation.”
Carl Sagan first speculated that hydrogen warming could have been important on early Mars back in 1977, but this is the first time scientists have been able to calculate its greenhouse effect accurately.
It is also the first time that methane has been shown to be an effective greenhouse gas on early Mars.
“This research shows that the warming effects of both methane and hydrogen have been underestimated by a significant amount,” Dr. Wordsworth said.
“We discovered that methane and hydrogen, and their interaction with carbon dioxide, were much better at warming early Mars than had previously been believed.”
The findings were published online Jan. 21, 2017 in the journal Geophysical Research Letters.
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R. Wordsworth et al. Transient reducing greenhouse warming on early Mars. Geophysical Research Letters, published online January 21, 2017; doi: 10.1002/2016GL071766
