Mars is cold today but once had water lakes and flowing rivers. The early warm climate cannot be explained by basic models of the early Mars greenhouse effect — involving only carbon dioxide and water vapor — because these predict climates that are too cold. In new research, a team of planetary scientists from the University of Chicago, NASA’s Jet Propulsion Laboratory and Aeolis Research evaluated the water ice cloud greenhouse hypothesis for warming early Mars.
Hubble photo of Mars taken when the planet was 50 million miles (80 million km) from Earth on May 12. The photo reveals details as small as 20 – 30 miles (32.2 – 48.3 km) across. Image credit: NASA / ESA / Hubble Heritage Team / STScI / AURA / J. Bell, ASU / M. Wolff, Space Science Institute.
“There’s been an embarrassing disconnect between our evidence, and our ability to explain it in terms of physics and chemistry. This hypothesis goes a long way toward closing that gap,” said Dr. Edwin Kite, a planetary researcher in the Department of the Geophysical Sciences at the University of Chicago.
Of the multiple explanations planetary scientists had previously put forward, none have ever quite worked.
Dr. Kite and colleagues wanted to revisit the water ice cloud greenhouse hypothesis.
“Even a small amount of clouds in the atmosphere can significantly raise a planet’s temperature, a greenhouse effect similar to carbon dioxide in the atmosphere,” Dr. Kite said.
“The idea had first been proposed in 2013, but it had largely been set aside because.”
“It was argued that it would only work if the clouds had implausible properties.”
“For example, the models suggested that water would have to linger for a long time in the atmosphere — much longer than it typically does on Earth — so the whole prospect seemed unlikely.”
Using a 3D model of the entire planet’s atmosphere, the researchers went to work.
The missing piece, they found, was the amount of ice on the ground. If there was ice covering large portions of Mars, that would create surface humidity that favors low-altitude clouds, which aren’t thought to warm planets very much.
But if there are only patches of ice, such as at the poles and at the tops of mountains, the air on the ground becomes much drier. Those conditions favor a high layer of clouds that tend to warm planets more easily.
The results showed that the scientists may have to discard some crucial assumptions based on our own particular planet.
“In the model, these clouds behave in a very un-Earth-like way,” Dr. Kite said.
“Building models on Earth-based intuition just won’t work, because this is not at all similar to Earth’s water cycle, which moves water quickly between the atmosphere and the surface.”
Here on Earth, where water covers almost three-quarters of the surface, water moves quickly and unevenly between ocean and atmosphere and land — moving in swirls and eddies that mean some places are mostly dry and others are drenched.
In contrast, even at the peak of its habitability, Mars had much less water on its surface. When water vapor winds up in the atmosphere, in the team’s model, it lingers.
“Our model suggests that once water moved into the early Martian atmosphere, it would stay there for quite a long time — closer to a year — and that creates the conditions for long-lived high-altitude clouds,” Dr. Kite said.
A paper on the findings was published in the Proceedings of the National Academy of Sciences.
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Edwin S. Kite et al. 2021. Warm early Mars surface enabled by high-altitude water ice clouds. PNAS 118 (18): e2101959118; doi: 10.1073/pnas.2101959118
