Could the greenhouse effect have made early Mars habitable?

Like Goldilocks’s third bowl of porridge, Earth’s just-right environment sits between that of frigid Mars and boiling Venus, largely thanks to our well balanced not-too-weak not-too-strong greenhouse effect. Now, a Harvard team suggests that methane may have warmed early Mars in a similar way.

Little doubt remains that ancient Mars was warmer and wetter than the frozen red world we see today. Signs of water erosion, riverbeds, lake basins, and even hints of a huge ocean all but confirm that at some point Martian temperatures regularly exceeded 32 degrees, which puts it in an unusual cosmic position.

"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 Harvard, in a press release.

The mystery is how that could be possible. Earthbound scientists are all too familiar with the mechanisms that warm our home planet, but despite accounting for 95 percent of what passes on Mars for an atmosphere, famous greenhouse gas carbon dioxide isn’t up to the task.

According to the Harvard press release, even if scientists crank up the atmospheric pressure to hundreds of times its current levels, Martian models just won’t melt. One challenge to liquid water was the fact that the young sun was almost a third less bright than it is today.

Dr. Wordsworth's team's insight was to consider other greenhouse gases, specifically methane. While only present at trace the level of a handful of parts per billion today, places such as Martian moon Titan have an abundance of the organic molecule.

By modeling the ways methane, hydrogen, and carbon dioxide behave together when warmed by sunlight, the researchers found that such an atmosphere could have indeed warmed the planet enough to maintain liquid water. Their findings were published Tuesday in the journal Geophysical Research Letters.

"This research shows that the warming effects of both methane and hydrogen have been underestimated by a significant amount," said Wordsworth in the press release. "We discovered that methane and hydrogen, and their interaction with carbon dioxide, were much better at warming early Mars than had previously been believed."

As for direct evidence, further on-the-ground investigation is needed. Nevertheless, some recent discoveries provide promising hints. In 2013, NASA rover Curiosity caught a whiff of methane when the atmospheric concentration spiked to ten times its normal level, which scientists said could have been the result of either biological or geological activity.

More recently, researchers crushed bits of Martian meteorites on Earth, and found they released surprisingly large amounts of methane gas. The team concluded that underground methane could support extremophile bacteria, Space.com reports.

Even if it turns out that early Mars was methane-poor, the Harvard scientists’ findings could have consequences that extend outside our solar system. The team explained in the paper: "Our results also suggest that inhabited exoplanets could retain surface liquid water at significant distances from their host stars."

While exoplanets are constantly expanding our understanding of how and where planets can form, exobiologists are particularly interested in those that exist in what’s sometimes called the "Goldilocks Zone," where surface temperatures are just right for water. If methane, hydrogen, or other common gasses can warm planets more easily than carbon dioxide can alone, that habitable zone may be wider than we thought.

"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," said Wordsworth.