Though scientists have long observed signs of water on ancient Mars, they could not explain how it came to exist in the frigid climate of those times. But in a paper published in “Icarus,” scientists have recently developed a model of ancient Mars that reconciles this conflict and explains the effects of seasonal temperature variation on the climate.
There are a number of examples of evidence that indicate the presence of water on early Mars, such as lakes and valley networks, said Ashley Palumbo GS, co-lead author of the paper. There are also phyllosilicates — clay substances in the Martian surface — that required water to form, said Robin Wordsworth, assistant professor of environmental science and engineering at Harvard and co-lead author of the paper. Despite this, there had not been a consensus as to how liquid water could have survived in the extremely cold environment. The general acceptance among scientists was that the temperature nearly four billion years ago, when the water would have existed, must have been warm enough to cause rainfall and surface runoff, Palumbo said.
For their research, Palumbo and Wordsworth, along with Jim Head, professor of geological sciences and co-lead author of the paper, developed a model of Mars to test for the climate where water could exist while remaining within the constraints of known ancient Martian climate knowledge, Wordsworth said. They used the Laboratoire de Météorologie Dynamique global climate model, Palumbo said. Incorporating atmospheric and surface physics, the model was able to accurately recreate climate dynamics and generate a plot of where the valley networks occur. They also used long-term ecological data from Antarctica, which has flowing water despite its frigid terrain. This model offered the team a window into the Mars of 3.8 billion years ago — when life was just forming on Earth — to see water on a completely different planet, Wordsworth said.
The team found that average annual temperatures could have been far below the freezing point of water, and liquid water still could have persisted in the warmest hours of the summer season, Palumbo said. Seasonal changes could have caused the temperature to briefly increase enough to cause melting of surface ice, and the resulting water could accumulate to create the different geological features. This explanation reconciles the previous conflicts of water existing in the frozen climate without any sort of external force, such as volcanic eruptions or impact cratering, she said.
This “excellent bit of modelling” allowed the team to show there were temporal climates on an annual basis on Mars, said John Mustard, professor of earth, environmental and planetary sciences.
Scientists do not have definitive knowledge of Martian temperatures from the past, Wordsworth said. What they do know, however, is constraints on the luminosity of the sun and on carbon dioxide gas in the atmosphere going back in time. Using these two constraints, as well as a few other factors, scientists defined a range of possible surface temperatures and subsequently found which state of climate is the most plausible to allow for the melting of ice on Mars.
Creating a model that allows for the formation of water despite the coldness of the surface has been a longstanding challenge, Mustard said. Prior to the study, no model had been able to recreate the continuous warm and wet climate of Mars that would explain the continuous presence of liquid water; models could only recreate the icy climate, Palumbo said.
This led the team to test if seasonal variations would be sufficient to cause the necessary warmth. Mars is currently very cold, with temperatures far below the freezing point of water, because it has a very thin atmosphere, Palumbo said. Mars lost its atmosphere through time, though scientists are not exactly sure how much; however, there are currently projects in operation, such as the Mars Atmosphere and Volatile Evolution mission, that are attempting to discover the atmospheric loss throughout time. By constraining the thickness of the atmosphere further, climate models can better describe the early climate.
By using Earth as a Mars analog, this research can help scientists expand their knowledge of Earth, Palumbo said. Since Antarctica and early Mars appear to be climactically similar, the positing of such temperature variation in frigid Martian climates could serve to help uncover more facts regarding implications of temperature variation in the coldest part of Earth. Liquid water activity still takes place in the Antarctic and some basic forms of life persist there, which has implications for possible life on Mars, she added.
Wordsworth said this project has potential for studies off of Mars in that it could help explain early developmental processes in the solar system. It may also assist in work regarding the habitability and observation of planets further from the sun than Earth, such as exoplanets.