This Is The Biggest Unsolved Paradox Of Mars

“It’s a paradox, an unresolved paradox of Mars,” says Kevin Zahnle, a NASA scientist. “On the one hand, some people say that it looked warmish and wettish, at least occasionally. On another hand, nobody can figure out how it could have been warmish and wettish.”

Since its epic touch down in 2012, NASA's rover Curiosity has discovered that Mars' 96-mile-wide Gale Crater harbored a series of lakes around 3.5 billion years ago, revealing environments that would have been potentially habitable for Earth-like life.

Because of its history, 96-mile wide Gale Crater crater with its strangely sculpted Mount Sharp --three times higher than the Grand Canyon is deep--was the ideal place for Curiosity to conduct its mission of exploration into the Red Planet's past. NASAS researchers used Curiosity to study layers in the mountain that hold evidence about wet environments of early Mars.

"This may be one of the thickest exposed sections of layered sedimentary rocks in the solar system," said Joy Crisp, MSL Deputy Project Scientist from NASA's Jet Propulsion Laboratory. "The rock record preserved in those layers holds stories that are billions of years old -- stories about whether, when, and for how long Mars might have been habitable."

Today the Red Planet is a radiation-drenched, bitterly cold, bleak world. Enormous dust storms explode across the barren landscape and darken Martian skies for months at a time. Recent data suggests that Mars once hosted vast lakes and flowing rivers.

 "Gale Crater and its mountain tell this intriguing story," said Matthew Golombek, Mars Exploration Program Landing Site Scientist from JPL. "The layers there chronicle Mars' environmental history.

The presence of water on ancient Mars is a paradox. There’s plenty of geographical evidence that rivers periodically flowed across the planet’s surface. Yet in the time period when these waters are supposed to have run — three to four billion years ago — Mars should have been too cold to support liquid water.  So how did it stay so warm?

Researchers from the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) suggest that early Mars may have been warmed intermittently by a powerful greenhouse effect. In a paper published in Geophysical Research Letters, researchers found that interactions between methane, carbon dioxide and hydrogen in the early Martian atmosphere may have created warm periods when the planet could support liquid water on the surface.

“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 Robin Wordsworth, assistant professor of environmental science and engineering at SEAS, and first author of the paper. “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.”

Four billion years ago, the Sun was about 30 percent fainter than today and significantly less solar radiation — a.k.a. heat — reached the Martian surface. The scant radiation that did reach the planet was trapped by the atmosphere, resulting in warm, wet periods. For decades, researchers have struggled to model exactly how the planet was insulated.

The obvious culprit is CO2. Carbon dioxide makes up 95 percent of today’s Martian atmosphere and is the most well-known and abundant greenhouse gas on Earth. But CO2 alone does not account for Mars’ early temperatures.

“You can do climate calculations where you add CO2 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,” said Wordsworth.

There must have been something else in Mars’ atmosphere that contributed to a greenhouse effect.

The atmospheres of rocky planets lose lighter gases, such as hydrogen, to space over time. (In fact, the oxidation that gives Mars its distinctive hue is a direct result of the loss of hydrogen.)

Wordsworth and his collaborators looked to these long-lost gases — known as reducing gases — to provide a possible explanation for Mars’ early climate. In particular, the team 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 Saturn's moon, Titan.

To understand how this early Martian atmosphere may have behaved, the team 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,” said Wordsworth. “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,” said Wordsworth. “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 researchers hope that future missions to Mars will shed light on the geological processes that produced methane billions of years ago.

“One of the reasons early Mars is so fascinating is that life needs complex chemistry to emerge,” said Wordsworth. “These episodes of reducing gas emission followed by planetary oxidation could have created favorable conditions for life on Mars.”

Mars' wettest period was likely the first billion years of its 4.6 billion-year life, the Noachian period, when it had a thicker atmosphere that would have been better able to keep liquid water stable on the planet's surface.

"Although the climate was relatively cold compared to Earth, there is evidence that liquid water flowed in streams and rivers, formed alluvial fans and deltas, and ponded in big lakes and possibly seas," Alberto Fairen of the Centro de Astrobiologia in Spain and Cornell University.

The Noachian Epoch was followed by the 600 million-year Hesperian period, when Mars morphed from a cold, wet world to a cold, icy one, as the protective atmosphere thinned and the planet's interior cooled. The next 3 billion years until now are known as the Amazonian period, during which Mars became the harsh, cold, dry planet we see today.

"Previous hypotheses have struggled to explain lake-forming climates that are both rare and long-lasting," Fairen wrote. "For example, volcanism and impacts can produce episodes of climate warming, but not of sufficiently long duration."

This evidence of liquid water suggests that Mars might have experienced periodic warm spells during an otherwise icy period — but so far, no one has been able to figure out exactly how. Now, Edwin Kite, a planetary scientist the University of Chicago, and his colleagues say that after running climate models they've come up with an explanation for the intermitent wet epochs of the Red Planet --"global warming produced by massive quantities of methane in the atmosphere."

Earlier in 2017, research led by Robin Wordsworth at Harvard University revealed that methane — when mixed with carbon dioxide — could have been a much stronger greenhouse gas on Mars than previously thought, strengthening the theory Kite and planetary scientist Peter Gao were building for methane’s role in warming up the Red Planet.

For this methane to be released into the atmosphere, the ice had to thaw. That could have happened when Mars wobbled on its axis, according to Kite’s models — tilting so that the Sun hit more of the planet’s surface.  triggering enough warming to thaw some of the ice beneath the surface, releasing the methane trapped inside — kicking off a bout of global warming that could last hundreds of thousands of years, ending when the Sun’s light disintegrated the methane molecules.
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