Mars used to be warmer and wetter than it is today – that much seems clear. So far, however, it has been disputed where our neighboring planet got its warmth from, because the sunlight and the density of the atmosphere were actually insufficient for this. Now researchers present a possible explanation: high ice clouds in the atmosphere of Mars could have been enough to heat up the planet with their greenhouse effect. As a result, the surface of Mars could have reached mean temperatures of just under zero degrees despite low solar radiation. That would have been enough to make liquid water and lakes possible in places.
Today Mars is a cold, dry planet, but that was not always the case: Recordings and measurement data from rovers and orbital probes show that our neighboring planet must have had rivers and lakes more than three billion years ago. Therefore, the Martian climate would have to have been warm and humid enough at that time to allow liquid water at least for a time. But how this was possible is debatable. Because according to models, Mars received only around a third of the solar radiation that hits the earth today. Its atmosphere was a little denser than it is today, but according to recent calculations it may not have reached much more than 0.5 to 1.3 bar. But that would have been too little to bring the mean temperatures to freezing point or just above using the greenhouse effect.
Ice clouds as a thermal blanket?
“This leaves an almost shameful gap between what we observe and our ability to explain it physically and chemically,” says first author Edwin Kite of the University of Chicago. He and his team have now examined one of the hypotheses discussed in more detail. After this, high water ice clouds could have created the necessary greenhouse effect. “Calculations have shown that even the small amount of 0.01 kilograms of water per square meter in the form of such ice clouds could have increased the planetary temperature by 50 Kelvin,” the researchers explain. However, previous studies have shown that such clouds would have to have unrealistic properties in order to achieve the necessary heat effect. The water in these clouds would have to stay in them a hundred times longer than in earthly clouds, which was previously considered to be physically implausible.
Kite and his team have now investigated once again in a model simulation whether there could have been enough ice clouds on Mars and what effect they would have had. In contrast to previous studies, they also included the interaction between the high clouds and water or ice reservoirs on the Martian surface. Specifically, in their model they assumed 80 percent of today’s solar luminosity and a pressure of the Martian atmosphere of 0.6 bar. “We are looking at an initially cold, dry planet on which superficial water (ice) is initially limited to the south polar region and at altitudes of more than four kilometers,” the researchers explain. After a short time, however, enough water vapor reaches the Martian atmosphere to allow the first thin clouds to condense at high altitudes. “At low latitudes, these scatter around half of the incident sunlight onto the surface, but most of the long-wave radiation that returns is stopped and reflected by the high, cold clouds,” said Kite and his colleagues.
Warm enough for ice-covered lakes
In the simulation, this process soon led to a greenhouse effect that raised global mean temperatures to around 265 Kelvin – around minus eight degrees Celsius. “At almost all known locations with paleo lakes, we find temperatures of more than minus five degrees Celsius and several hours during the day with values around zero degrees,” reports the team. “That is significantly warmer than the threshold that allows the formation of large, perennial ice-covered lakes in the Antarctic dry valleys.” The climate of this early Mars would therefore be rather dry overall, but warm enough for at least temporarily liquid water. The prerequisite for this, however, is that the Martian surface is initially not too rich in water, as the researchers emphasize. Because even if large areas of water ice are covered in the plains and closer to the equator, the evaporation of this water creates thick, low clouds with only a minor greenhouse effect. In addition, the water cycle of Mars would then be too active and earth-like to maintain the high ice clouds for a long time.
“In our model, these Martian clouds behave quite unearthly,” says Kite. “The cycle on Mars is different from that on Earth, in which water is exchanged very quickly between the atmosphere and the surface.” On the early, rather dry Mars, however, this exchange is restricted: once water vapor has risen and crystallized into high ice clouds , it remains there for at least a year away from the cold polar regions and high altitudes – on earth it is only around ten days. The reason is the high temperature difference between the cold clouds and the warmer air below. As a result, ice particles falling from the cloud can sublime again just below and become water vapor – and thus enrich the underside of the cloud with new moisture. “This creates the conditions for long-lasting altitude clouds,” says Kite. These in turn could have heated up the climate of young Mars enough, at least in some regions and for some time, to make liquid water possible despite the low solar radiation and thin atmosphere. The Mars rover Perseverance could find indications that this model is correct. Because, as the researchers explain, the rock samples he collected and returned to Earth from a future mission could provide chemical evidence of these mechanisms.
Source: Edwin S. Kite (University of Chicago) et al., Proceedings of the National Academy of Sciences, doi: 10.1073 / pnas.2101959118