Mars used to be warmer and wetter than it is today – that seems pretty obvious. Until now, however, it has been disputed as to where our neighboring planet got its heat, because the density of sunlight and atmosphere was inadequate for this. Now researchers offer a possible explanation: high ice clouds in Mars’ atmosphere could have been enough to warm the planet with their greenhouse effect. As a result, the surface of Mars can reach temperatures below zero degrees despite less solar radiation. This would have been enough to make liquid water and lakes possible in places.
Today Mars is a cold, dry planet, but it was not always so: the recording and measurement data of rovers and orbital probes suggest that our neighboring planet may have had rivers and lakes three billion years ago. Therefore, the Martian climate must have been sufficiently hot and moist at that time to allow liquid water for at least a time. But how this was possible is debatable. Because according to models, Mars receives only about a third of the solar radiation that hits Earth today. Its atmosphere was slightly denser than today, but according to recent calculations it cannot reach more than 0.5 to 1.3 times. This would be too low to bring the average temperature to freezing point or just above it through the greenhouse effect. .
Snow Clouds as a Thermal Blanket?
“We leave an almost embarrassing difference between what we see and our ability to explain it physically and chemically,” says Edwin Kight, the first author of the University of Chicago. He and his team have now examined a hypothesis discussed in more detail. Subsequently, high water ice clouds could produce the necessary greenhouse effect. “Calculations have shown that even small amounts of 0.01 kg of water per square meter such as snow clouds can increase the temperature of the planet by Kelvin,” the researchers understood. However, previous studies have shown that such clouds must have unrealistic properties to achieve the required heat effect. Water in these clouds would last them a hundred times longer than earthly clouds, previously thought to be physically incurable.
Kite and his team have now once again examined in a model simulation whether there could have been enough snow clouds on Mars and what effect they would have. Unlike previous studies, they also included interactions between high clouds and water or ice reservoirs on the surface of Mars. In particular, in his model he considered the pressure of today’s 80 percent solar lighting system and 0.6 bar Martian atmosphere. “We are initially looking at a cold, dry planet, on which surface water (ice) is initially confined to the South Polar region and altitudes of more than four kilometers,” the researchers explained. After a short time, however, enough water vapor reaches the Martian atmosphere to allow the first thin clouds to condense at higher altitudes. Kite and his colleagues said, “At low latitudes, these phenomena are nearly half-shattered at the surface, but mostly visible on long-wave radiation that returns and is reflected by high, cold clouds.”
Warm enough for snow-capped lakes
In the simulation, this process soon produced a greenhouse effect, which raised the global average temperature to around 265 Kelvin – minus eight degrees Celsius. “With Paleo lakes at almost all known locations, we find temperatures above zero to five degrees Celsius and many hours during the day with values around zero degrees,” the team reports. “It is significantly warmer than the threshold that allows for the creation of large, perennial ice-covered lakes in the Antarctic dry valleys.” The climate of this early Mars would therefore be overall dry, but at least temporarily warm enough for liquid water. However, the condition for this is that the Martian surface is not initially very watery, as the researchers insist. Because even though large areas of water ice fall into the plains and close to the equator, evaporation of this water creates thicker, less cloudy ones with only a minor greenhouse effect. In addition, the water cycle of Mars will still be very active and will be Earth-like to sustain high snow clouds for a long time.
Kite says, “In our model, these Mars clouds behave quite uncontrollably.” “The cycle on Mars is different from that on Earth, in which water is very rapidly exchanged between the atmosphere and the surface.” On early, rather dry marshes, however, this exchange is restricted: once the water vapor rises and crystallizes. Clouds of high snow, it lasts for at least a year from cold polar regions and high altitudes – on Earth it is only around ten days. The reason for this is the high temperature difference between the cold clouds and the warm air below. As a result, snow particles falling from the cloud can re-neutralize the downstream and become water vapor – and thus enrich the cloud bottom with new moisture. “This creates conditions for long-lasting altitude clouds,” Kite says. These youngsters, in turn, can significantly reduce the climate of Mars, at least in some areas and for some time, to make liquid water possible despite low solar radiation and a thin atmosphere. The Mars Rover may strongly indicate that this model is correct. As researchers point out, the rock samples they collected from a future mission and returned to Earth could provide chemical evidence of these mechanisms.
Quail: Edwin S. Kite (University of Chicago) et al., Proceedings of the National Academy of Sciences, doi: 10.1073 / pnas.2101959118
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