A combination of three gases could have created a greenhouse effect on Mars 3.8 billion years ago that made the planet warm enough for liquid water to flow across the surface.
In a new study published in Nature Geoscience, a research team uses a climate model to show that an atmosphere with sufficient carbon dioxide, water, and molecular hydrogen could have made the surface temperatures of Mars warm to above freezing.
That flowing water could have formed the ancient valley networks, such as Nanedi Valles, much the way sections of the Grand Canyon snake across the western United States today.
Previous efforts to produce temperatures warm enough to allow for liquid water used climate models that include only carbon dioxide and water and were unsuccessful.
“This is exciting because explaining how early Mars could have been warm and wet enough to form the ancient valleys had scientists scratching their heads for the past 30 years,” says Ramses M. Ramirez, a doctoral student working with James Kasting, a professor of geosciences at Penn State.
“We think we may have a credible solution to this great mystery.”
Volcanoes, not meteorites
The researchers note that one alternative theory is that the Martian valleys formed after large meteorites bombarded the planet, generating steam atmospheres that then rained out. But this mechanism cannot produce the large volumes of water thought necessary to carve the valleys.
“We think that there is no way to form the ancient valleys with any of the alternate cold early Mars models,” says Ramirez. “However, the problem with selling a warm early Mars is that nobody had been able to put forth a feasible mechanism in the past three decades. So, we hope that our results will get people to reconsider their positions.”
Ramirez and post-doctoral researcher Ravi Kopparapu co-developed a one-dimensional climate model to demonstrate the possibility that the gas levels from volcanic activity could have created enough hydrogen and carbon dioxide to form a greenhouse and raise temperatures sufficiently to allow for liquid water.
Once they developed the model, Ramirez ran the model using new hydrogen absorption data and used it to recreate the conditions on early Mars, a time when the sun was about 30 percent less bright than it is today.
“It’s kind of surprising to think that Mars could have been warm and wet because at the time the sun was much dimmer,” Ramirez says.
Mars’ mantle appears to be more reduced than Earth’s, based on evidence from Shergotty, Nahkla, and Chassigny meteorites, Martian meteorites named for the towns near which they were found. A more reduced mantle outgasses more hydrogen relative to water, thus bolstering the hydrogen greenhouse effect.
“The hydrogen molecule is symmetric and appears to be quite boring by itself,” says Ramirez. “However, other background gases, such as carbon dioxide, can perturb it and get it to function as a powerful greenhouse gas at wavelengths where carbon dioxide and water don’t absorb too strongly. So, hydrogen fills in the gaps left by the other two greenhouse gases.”
Researchers on the project include Michael E. Zugger, senior research engineer, Applied Research Laboratory, Penn State; Tyler D. Robinson, University of Washington; and Richard Freedman, SETI Institute.
NASA Astrobiology Institute’s Virtual Planetary Laboratory supported the project.
Source: Penn State