A large portion of Mars's atmospheric gases have been literally lost in space, according to a new study.
The discovery helps paint a clearer picture of the red planet's atmospheric history and how it evolved geologically from a wet, warm planet to today's cold, arid place.
It also casts doubt on a NASA plan to build a protective magnetic shield around the planet to help replenish its atmosphere and make it more hospitable for crewed missions to go there.
The findings, published today in Science, are based on data collected by the Mars Atmosphere and Volatile Evolution (MAVEN) mission, launched in 2013 to investigate the upper Martian atmosphere.
Principal investigator Professor Bruce Jakosky, from the University of Colorado, Boulder, and colleagues, made the finding by detailing variations in the abundance of two argon isotopes in Mars's atmosphere.
He said the team focused on the gas argon as it did not react chemically with anything else and so, because of its weight, could only be removed from the atmosphere by a physical process known as sputtering.
"In this process, ions which have been created in the Mars upper atmosphere, and extended corona of gas surrounding the planet, are grabbed or 'picked-up' by the solar wind and accelerated to high velocities," Professor Jakosky said.
"Some of these pick-up ions will be slammed back into the planet, where they collide with atmospheric gases and can knock them out into space."
Professor Jakosky said an abundance of heavier argon isotopes in the atmosphere was a strong indicator of gas loss to space because the lighter argon isotope was more susceptible to being ejected through sputtering.
Based on measurements taken by the MAVEN spacecraft, the researchers showed more than 66 per cent of Mars's argon (two-thirds) had disappeared from the atmosphere since the planet's formation.
Oxygen and carbon dioxide loss
The team then used its measurements of argon loss to determine the amount of oxygen atoms that had also been knocked into space through sputtering.
Professor Jakosky said because oxygen was derived from both water (H2O) and carbon dioxide (CO2), this enabled them to also work out how much CO2 had been lost.
"This … tells us that a large fraction of the CO2 has also been lost to space by this same sputtering process," he said.
However carbon dioxide could also be lost through other processes — such as pickup by solar wind and photochemical processes that can kick off a carbon or an oxygen atom.
"As a result, we know that the loss by sputtering represents just a small fraction of the total amount of CO2 that has been lost to space," Professor Jakosky said.
Findings explain loss of water on Mars
Professor Jakosky said the findings were important because they helped answer "one of the real puzzles about Mars".
"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, but where did the CO2 go?
"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.
"With surface water and warmer temperatures early on, Mars could have supported life — at least, microbial life. With the disappearance of liquid water, any life would either have had to migrate underground or die out.
"Thus, this process may be an important one controlling the habitability of the surface of a planet."
NASA protective shield would be 'too little too late'
Professor Jakosky said Earth's atmosphere did not suffer the same fate as Mars's because Earth has a magnetic field that protects it from solar wind.
While Mars did have a magnetic field early in its history, it disappeared, he said, setting up the conditions for the stripping of the atmosphere.
A panel of researchers led by the director of NASA's Planetary Science Division, Dr Jim Green, recently proposed building a magnetic shield to protect Mars' atmosphere from the solar wind.
However, Professor Jakosky said the proposal would be a case of too little, too late.
While a magnetic shield would stop solar wind from hitting Mars, the rate of gas loss today was extremely small.
He said shutting off the low level of ongoing removal of gas would "take literally billions of years to have even a small effect" on the thickness of the atmosphere.
