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In 2008, NASA’s Phoenix lander directly confirmed the presence of water ice on Mars. It was not exactly a surprise. Satellite images had previously suggested that the red planet was flush with lakes and rivers about 4 billion years ago.

But what has long puzzled scientists is how water evolved on what is now a cold, arid planet. To support those ancient lakes and rivers, Mars would have needed an atmosphere that caused enough warming from global warming. The planet’s atmosphere is too thin today to cause such a warming.

One hypothesis for how Mars once supported water holds that an asteroid collided with the planet and the resulting heat made liquid water possible. But some researchers have noted that this heating effect would have lasted only a few years. That wouldn’t have been long enough for water to leave the visible geological evidence of lakes and rivers we see today.

A new hypothesis for water on Mars

New research published in PNAS explores another hypothesis: Mars once had high-altitude icy clouds, similar to cirrus clouds on Earth, that created a greenhouse effect capable of supporting a multi-forming climate.

This explanation was first proposed in 2013 and has been criticized for allegedly having clouds with unusual properties. In particular, water should stay in clouds for much longer compared to Earth’s water cycle. The recent study sheds new light on how these unusual clouds could have shaped and warmed the planet.

Credit: NASA / JPL-Caltech / USGS

In earlier versions of the cloud greenhouse hypothesis, researchers had assumed that large areas of Mars’ surface were covered with ice. Such conditions would have prevented cloud formation at high altitudes. But if the surface had had less ice, a layer of icy clouds could have formed at high altitudes.

Lead study author Edwin Kite explained this process to Big Think:

“The distribution of surface water affects the height of the clouds. If there is surface water anywhere on the planet, then the relative humidity is ~ 1 in updraft, and clouds will form at a low level in that updraft.” only found in cold places, most of the surface is warmer than the cold traps, so no clouds can form on most of the surface (higher temperatures -> lower relative humidity -> no condensation -> no clouds) High in the atmosphere, the temperatures are lower and so clouds can form. ”

Clouds are complicated

To investigate how different amounts of surface water and clouds would have affected the planet, the researchers made a computer model of early Mars. The model suggested a planet that was mostly dry, but with ice flecks in places, such as on mountain peaks and at the poles of the planet. Clouds are said to have formed at low altitudes above these ‘cold traps’.

But above the rest of the planet’s warmer and drier regions, the researchers noted that “ clouds are only found at high altitudes ” because the increase the condensation level (LCL) is high. (LCL refers to the height at which an air particle has cooled enough to saturate and form clouds. Compared to air near cold traps, air must rise higher near warm surfaces to cool enough to form clouds, so it has a higher LCL.)

So why does cloud height matter in terms of global warming?

Kite et al.

“Clouds absorb infrared emitted from the ground and then emit it back into space (purple arrows; greenhouse effect),” Kite told Big Think. “Planetary energy balance requires that energy in (absorbed sunlight) equals energy out (infrared emitted to space). If the clouds have the correct particle size and thickness to absorb infrared effectively, it means that the top temperature of the cloud is constant for a given amount of sunlight absorbed. “

“If the cloud-top temperature is constant with the cloud height, why does the surface temperature depend on the cloud height? This is because below the clouds the temperature always drops with the altitude in the atmosphere. So if the clouds are higher, then the temperature difference between the cloud tops and the surface must be greater – which implies a warmer surface. ”

While the model fits scientists’ current understanding of ancient Mars, the researchers said the results do not definitively rule out the collision hypothesis. But NASA’s Perseverance rover could quickly settle the debate by analyzing samples of Martian rocks, giving scientists an insight into the atmosphere of early Mars and, more generally, what makes planets habitable.

“Mars is important because it is the only planet we know of that had the ability to support life – and then lost it,” Kite said in a statement. press release. “The long-term climate stability of the Earth is remarkable. We want to understand all the ways a planet’s climate stability can collapse in the long term – and all the ways it can be maintained (not just Earth’s). defines the new field of comparative planetary habitability. ”

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