The picture from NASA’s Perseverance rover, which recently landed on Mars, succinctly summarizes one of the great mysteries of modern space science: Today, Mars is a desert planet, but the rover is sitting right next to a historic river delta.
For decades, scientists have been perplexed by the seeming contradiction, especially because, at the time when Mars had flowing rivers, it received less than a third of the sunlight that we get today on Earth.
However, a new study led by Edwin Kite, an assistant professor of geophysical sciences at the University of Chicago and an expert on the climates of other worlds, uses a computer model to propose a promising explanation: Mars could have had a thin layer of icy, high-altitude clouds that caused a greenhouse effect.
“There’s been an embarrassing disconnect between our evidence, and our ability to explain it in terms of physics and chemistry,” said Kite. “This hypothesis goes a long way toward closing that gap.”
None of the various explanations proposed by scientists in the past have ever quite worked. Some speculated that a massive asteroid colliding with the planet could have produced enough kinetic energy to warm the globe.
However, some calculations indicated that this effect would last only a year or two, while the tracks of ancient rivers and lakes indicate that the warming likely lasted hundreds of years.
In the model, these clouds behave in a very un-Earth-like way. 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.
Edwin Kite
Kite and his colleagues sought to look at another possibility: Clouds that form at high altitudes, such as cirrus on Earth. Even a little amount of clouds in the atmosphere, similar to carbon dioxide in the atmosphere, can drastically elevate a planet’s temperature.
The idea had first been proposed in 2013, but it had largely been set aside because, Kite said, “It was argued that it would only work if the clouds had implausible properties.”
For example, the simulations predicted that water would have to remain in the atmosphere for a lengthy time, considerably longer than it occurs on Earth, making the whole scenario seem improbable. Kite and his crew got to work on a 3D model of the entire planet’s atmosphere.
They discovered that the missing element was the amount of ice on the ground. If significant areas of Mars were covered in ice, the surface dampness would favor low-altitude clouds, which aren’t considered to warm planets very much (or can even cool them, because clouds reflect sunlight away from the planet.)
However, if there is only a thin layer of ice, such as at the poles or on the peaks of mountains, the air on the ground becomes much drier. These conditions support a thick layer of clouds, which are more likely to warm planets.
Scientists may have to abandon certain key assumptions based on our own planet, according to the model’s findings.
“In the model, these clouds behave in a very un-Earth-like way,” said Kite. “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.”
Water travels swiftly and unevenly between ocean and atmosphere and land on Earth, resulting in swirls and eddies that leave some locations mostly dry (the Sahara) and others soaked (the Amazon).
Mars, on the other hand, had significantly less water on its surface even at the height of its habitability. In Kite’s model, when water vapor reaches the atmosphere, 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,” said Kite.
The newly landed Perseverance rover from NASA should be able to put this theory to the test in a variety of ways, including studying pebbles to recreate past air pressure on Mars.
The scientists believe that knowing the complete narrative of how Mars earned and lost its warmth and atmosphere would aid in the hunt for potential livable worlds.
“Mars is important because it’s the only planet we know of that had the ability to support life and then lost it,” Kite said. “Earth’s long-term climate stability is remarkable. We want to understand all the ways in which a planet’s long-term climate stability can break down and all of the ways (not just Earth’s way) that it can be maintained. This quest defines the new field of comparative planetary habitability.”
Former UChicago postdoctoral researcher Liam Steele, who is now with the Jet Propulsion Laboratory, Michael Mischna of the Jet Propulsion Laboratory, and Mark Richardson of Aeolis Research were co-authors on the work. The University of Chicago Research Computing Center was used for some of the analysis.
