Ancient Mars may have icy clouds to thank for its liquid water
High-altitude icy clouds may have allowed ancient Mars to host rivers and lakes by warming the planet via a greenhouse effect, according to the results of a new study. The theory could be tested by NASA’s Perseverance rover, which is currently actively exploring the surface of the Red Planet.
One of the greatest unsolved mysteries in our Solar System is the question of how Mars transformed from a world capable of harboring vast lakes – and possibly life – to the inhospitable barren planet that we know today.
Scientists know for a fact that ancient Mars once hosted an abundance of liquid water because the crevices, delta remains and rock sediments carved out and transported by the flowing liquid are still extremely visible on the Red Planet in satellite imagery. The remains of these once-watery sites are now considered some of the most promising places to discover clues of past microbial life that we know of.
But despite the many theories that have been put forward, scientists are still uncertain as to how Mars was able to maintain liquid water despite the fact that, given its relatively distant orbit, it received around a third the amount of sunlight from our star as Earth does today.
The results of the new research suggest that the secret to Mars’ watery past may be rooted in its atmospheric history. The study made use of advanced 3D computer simulations of the Red Planet to model what the ancient Martian climate may have looked like. More specifically, the scientists were trying to determine whether the presence of high-altitude icy clouds could have had a significant warming effect on the planet’s atmosphere.
This theory had originally been proposed back in 2013, but was rejected by some members of the scientific community. This was partially due to the fact that the clouds would have to maintain their cohesion for much longer than their closest Earthly equivalents – known as cirrus clouds – were capable of.
However, the new simulations carried out by the researchers revealed that it was indeed possible for the unusual clouds to persevere, and that the key to their longevity revolved around the amount of water ice present on the Martian surface.
In the digitized version of Mars used by the team, it was shown that, in instances where there was a widespread ground covering of ice, it would lead to a surface humidity that would likely trigger the creation of low-altitude clouds.
However, when Mars was given overall less ice coverage – with the frozen water remaining in the polar regions and at the tops of mountains, for example – it was found that the air near the surface was much drier. This in turn led to the creation of high-altitude clouds that could last for up to a year before losing their cohesion.
"In the model, these clouds behave in a very un-Earth-like way," said Edwin Kite, lead author of the new study and Assistant Professor of geophysical sciences at the University of Chicago. "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."
These icy high-altitude clouds cause a greenhouse effect, wherein heat from the Sun is trapped in the atmosphere, further warming the planet, and allowing for the presence of liquid water on the surface.
Further in-situ studies of the Martian landscape by the Perseverance rover could help test the team’s model.
"Mars is important because it's the only planet we know of that had the ability to support life – and then lost it," explained Kite. "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."
A paper on the study has been published in the journal Proceedings of the National Academy of Sciences.
Source: University of Chicago