The view from NASA's Perseverance rover, which recently landed on Mars, neatly summarizes one of the great mysteries of modern space science: Today, Mars is a desert world, but the rover is seated right next to an ancient river delta.
Martian Waters Mystery
For decades, scientists have been perplexed by the obvious paradox, notably because, at the time when Mars had running rivers, it received less than a third of the sunlight that we have today on Earth.
However, recent research led by Edwin Kite, an assistant professor of geophysical sciences at the University of Chicago and an authority on the climates of other planets, uses a computer model to propose a good explanation. Mars may have had a thin layer of frozen, high altitude icy clouds that produced a greenhouse effect.
"There's been a humiliating gap between evidence and willingness to justify it in terms of physics and chemistry," Kite said. "This theory contributes significantly to closing the void."
None of the many theories proposed by scientists in the past have ever quite succeeded. Some speculated that a massive asteroid colliding with the Earth may have released enough kinetic energy to warm the planet. However, some estimates indicated that this impact would last just a year or two, although the tracks of ancient rivers and lakes suggest that the warmth actually prevailed hundreds of years.
Related Article: New Study Disputes Conventional Theory on Mars' Water
Kite and his colleagues decided to look at another possibility: Clouds that form at high altitudes, such as cirrus on Earth. Even a tiny amount of clouds in the atmosphere, equivalent to carbon dioxide in the atmosphere, will dramatically increase a planet's temperature.
The concept was first suggested in 2013, but it was mainly dismissed because "it was claimed that it would only succeed if the clouds had implausible properties," according to Kite. For example, the simulations indicated that water would have to remain in the atmosphere for a long time, far longer than it does on Earth, making the entire scenario appear impossible.
Kite and his colleagues got to work on a 3D model of the entire planet's atmosphere. They discovered that the missing piece was the amount of ice on the ground. If vast areas of Mars were covered with frost, the surface humidity would favor low-altitude clouds, which aren't thought to warm planets too well (or can even cool them because clouds reflect sunlight away from the planet.)
However, where there is just a thin layer of frost. The poles, the peaks of mountains, the weather on the land remains even drier. These conditions prefer a thick layer of icy clouds, which warm worlds more quickly.
According to the model's findings, scientists will have to abandon specific main hypotheses based on our own world.
"These clouds behave in a very un-Earth-like manner in the model," Kite said. "Building models based on Earth-based intuition would fail because there is nothing like Earth's water cycle, which transfers water rapidly between the atmosphere and the soil."
Water flows rapidly and unevenly between seas, atmosphere, and land on Earth, covering almost three-quarters of the surface and flowing in swirls and eddies that leave some areas completely dry (the Sahara) and others drenched (the Amazon). On the other hand, Mars had far less water on the surface, even at the height of its habitability. In Kite's model, as water vapor reaches the atmosphere, it lingers.
"Our model predicts that once water enters the early Martian atmosphere, it will remain there for a long time - closer to a year - allowing long-lived altitude clouds to form," Kite said.
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