Mars is a dry and cold world today but billions of years ago it had flowing rivers, despite receiving less than one-third of the sunlight our planet gets today. So how was Mars able to be warm enough to keep liquid water? A new study suggests that it could be due to some special icy clouds.
As reported in the Proceedings of the National Academy of Sciences, a thin layer of high-altitude long-lived icy clouds could be enough to generate a greenhouse effect. This would keep the Red Planet warm enough, even taking into account the reduced sunlight it experienced in the past.
"There's been an embarrassing disconnect between our evidence, and our ability to explain it in terms of physics and chemistry," lead author professor Edwin Kite from the University of Chicago said in a statement. "This hypothesis goes a long way toward closing that gap."
This model has been proposed before but it had been discarded because it doesn’t work if the water cycle happens rapidly like on Earth. Kite and colleagues argue that Mars, even at its wettest, had a lot less water than Earth has today and that its water cycle probably allowed for water particles to remain in the air for up to a year.
"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," Kite explained. "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."
According to the models, the resulting climate from icy-cloud-induced global warming is arid, with a relative humidity of around 25 percent. The lakes in such an environment would be sourced from ice melting or from groundwater.
"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."
NASA's Perseverence is expected to be able to test this hypothesis in multiple ways over the coming years.