History of Mars: from warm and wet to cold and dry
New research explains why Mars’ atmosphere is thin and composed mainly of carbon dioxide. This has been puzzling scientists for a long time, as evidence from the planet’s surface rather indicates that the Red Planet was once warmer and wetter, thus in possession of a much thicker and more complex atmosphere.
The study, published last March in the journal Science, is the results of data collected by the Mars Atmosphere and Volatile Evolution (MAVEN) mission, which launched in 2013. Senior investigator Professor Bruce Jakovsky, from University of Colorado (Boulder, US), and collaborators measured the abundances of argon isotopes at different heights in the Red Planet’s atmosphere. They found that 66% of Mars’ argon had disappeared since the planet formation, as well as that the leftover argon was mainly made of heavier isotopes.
The two pieces of evidence are a strong indication of a process called ‘sputtering’. Because argon could not have reacted chemically with anything else, the loss must have occurred due to solar wind accelerating ion gases and ejecting them from the atmosphere (light isotopes are more susceptible to this mechanism than heavier ones). Based on the 66% loss and the nature of the mechanism behind it, the team of scientists was then able to determine the amount of oxygen and carbon dioxide lost over the same time.
“The geological evidence points to there having been liquid water flowing over the surface [of Mars] in its early history, but it’s a cold and dry planet today,” Professor Jakosky said. “Something changed. The best explanation is that Mars had a thicker atmosphere that trapped the heat and warmed the planet, and that this thicker atmosphere was lost. We think that CO2 is the best gas for this […]. Now the evidence points to the gas as having been lost to space […]. If this is the explanation of where the early atmosphere went, it explains how Mars went from a planet with liquid water at the surface to one with only very limited liquid water.”
Interestingly, this means that Mars could have once supported life – at least microbial – and also that the sputtering process – or better the absence of it – may be a key element in determining the habitability of a planet. Earth did not suffer from the same fate only because its magnetic field protects us from the solar wind.
The study unfortunately shuts down the possibility, recently advanced at NASA, to build a magnetic shield to protect Mars. It would be too little too late. Shutting down the now low level of gas removal would take billions of years before affecting ever so slightly the thickness of Mars’ atmosphere. “This magnetic shield would be an interesting experiment, but is not a way to replenish the atmosphere,” Professor Jakosky concluded.
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