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Where did little Mars get its warmth from? High ice clouds could have a critical effect on early Mars climate

“Impossible” warmth: Researchers were able to solve a big mystery about the red planets – why Mars had a temperate climate when sunlight and atmospheric pressure were, in fact, insufficient. Accordingly, high ice clouds can act as a heat cover due to the greenhouse effect. They heated the surface of Mars just below zero degrees – enough to make rivers and lakes possible at least temporarily.

Mars used to be much warmer and wetter than it is today – it had rivers Visible And maybe even the ocean. However, how this was possible has so far been disputed. Because according to the models, a young Martian received about a third of the solar radiation that strikes Earth today. Its atmosphere was a little denser than it is today, but it may not have reached much more than that 0.5 bus 1.3 bar There are also lost items Carbon deposits. But this meant that the air on Mars was so thin that temperatures could not reach freezing point or just above the greenhouse effect.

“There is an almost shameful gap between the observations and our ability to explain them physically and chemically,” says first author Edwin Kate of the University of Chicago.

The greenhouse effect of high ice clouds?

Kate and his team may now have found a possible solution to this problem. To do so, they examined a common hypothesis again. After that, high clouds can appear Frozen carbon dioxide Or the water ice caused the necessary greenhouse effect on Mars. These clouds over 30 kilometers high allow most of the incoming sunlight to pass through, but they block the long wave radiation returning from the surface and thus heat the atmosphere and the surface.

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“Calculations have shown that even a small amount of water of 0.01 kilograms per square meter in the form of such ice clouds can increase the planet’s temperature by 50 K,” the researchers explained. However, previous studies have shown that such clouds must have unrealistic properties in order to achieve the necessary thermal effect. The water in Martian clouds should stay a hundred times longer than that on Earth – this was previously considered physically implausible.

A heating blanket for little mars

For their study, Kate and his team recreate the tiny planet Mars and its atmosphere in a model simulation. “We are looking at a cool, dry planet at first, where surface water (ice) is initially confined to Antarctica and at altitudes of more than four kilometers,” the researchers explain. Additionally, they assumed that 80% of the sun’s luminosity today and the pressure of Mars’ atmosphere are 0.6 bar.

It turns out that enough water vapor reached the Martian atmosphere in a relatively short time to allow the first thin clouds to condense at high altitudes. This starts a self-reinforcing process as the surface of Mars warms, more water evaporates and more high clouds form. Eventually a state of equilibrium is reached as the young Mars is surrounded by an almost universal veil of thin, high ice clouds.

An increase in average annual temperatures on the young planet due to ice clouds. Regions of at least 100 degrees on Mars are outlined in green. © Kate et al. / PNAS

Warm enough for ice-covered lakes

This had consequences for the Martian climate: The global warming effect of clouds raised average global temperatures to about 265 K – equivalent to about minus eight degrees Celsius. “In nearly all known locations with Paleo Lakes, temperatures in excess of five degrees Celsius below zero prevailed during the day for several hours at values ​​close to zero,” according to a report by Kate and his team. “This is well above the threshold that allows for large ice-covered lakes to form in the dry valleys of Antarctica.”

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Thus this process could explain how a young Mars could develop a temperate climate despite the lack of sunlight and a thin atmosphere. The warming effect of high ice clouds has warmed its surface enough to make lakes and rivers possible at least in places and for a certain period of time. “This scenario can bridge the gap between observation and theory,” says Kate.

Indicators as to whether this model is correct could be Perseverance Marsrover You find. Rock samples he collected and brought back to Earth from a future mission could provide chemical clues to these mechanisms, the researchers explain.

It only works on dry Martian surface

The team also provides an explanation as to why previous models were unable to reproduce their scenario: The prerequisite for formation of high ice clouds is that the surface of Mars is fairly dry and not rich in ice. Because when large areas of water ice are covered, the evaporation of this water creates low dense clouds with little greenhouse effect.

This scenario for a temperate but dry Martian climate works well Previous studiesAnd then, many Mars rivers may only carry water temporarily – similar to valleys and riverbeds in the dry regions of Earth today. Under these conditions, the water exchange between the atmosphere and the surface will be less pronounced than on Earth, and the water bound in ice clouds will last longer in them – a good year instead of just ten days as it is on Earth.

“The cycle on Mars differs from that on Earth, as water is exchanged very quickly between the atmosphere and the surface,” Kate says. “This creates the conditions for high, long-lived clouds.” (Proceedings of the National Academy of Sciences, 2021; Doi: 10.1073 / pnas.2101959118)

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Coyle: University of Chicago