Solar wind and radiation are responsible for stripping the Martian atmosphere, transforming Mars from a planet that could have supported life billions of years ago into a frigid desert world, according to new results from NASA's MAVEN spacecraft. 

"We've determined that most of the gas ever present in the Mars atmosphere has been lost to space," said Bruce Jakosky of the University of Colorado, Boulder, principal investigator for the Mars Atmosphere and Volatile Evolution Mission, or MAVEN. "Loss of gas to space was a major process in the changing Mars climate through time." Jakosky is lead author of a paper on this research, published in Science this week. 

According to Roger Yelle, a professor in the University of Arizona's Lunar and Planetary Laboratory who has served as a co-investigator on the MAVEN mission since its inception, this scenario of Mars' losing battle was expected, but the new results provide the first measurements proving the process indeed has been happening that way. 

The research team obtained the result by using measurements of the atmosphere today to determine how much gas has been lost to space over time. Instruments on MAVEN measured the atmospheric abundance of two different isotopes of argon gas. Isotopes are atoms that have the same elemental composition but different masses. Because the lighter of the two isotopes escapes to space more readily, it will leave the gas remaining behind enriched in the heavier isotope. The team used this enrichment together with how it varied with altitude in the atmosphere to estimate what fraction of the atmospheric gas has been lost to space. 

"Because the two isotopes have quite similar masses, these measurements are not easy to do," said Yelle, adding that NASA's Viking probes, whose landings in 1976 marked the first touchdown of a man-made object on Mars, were not able to make those measurements. "MAVEN has given us the first opportunity to make these measurements as a function of altitude, which we think are a good proxy for other gases in the Martian atmosphere."

Liquid water, essential for life, is not stable on Mars' surface today because the atmosphere is too cold and thin to support it. However, evidence such as features resembling dry riverbeds and minerals that only form in the presence of liquid water indicates the ancient Martian climate was much different — warm enough for water to flow on the surface for extended periods. 

There are many ways a planet can lose some of its atmosphere. For example, chemical reactions can lock gas away in surface rocks, or atmosphere can be eroded by radiation and wind from the planet's parent star. The new result reveals that solar wind and radiation were responsible for most of the atmospheric loss on Mars, and that the depletion was enough to transform the Martian climate.

"Since young stars have far more intense ultraviolet radiation and winds, atmospheric loss by these processes was likely much greater early in Mars' history, and these processes may have been the dominant ones controlling the Martian climate," Jakosky said. "Since temperatures at the surface have been so strongly affected by loss to space, these processes may also have determined the history of the habitability of Mars by microbes. It's possible that life could have existed at the surface early in Mars' history. As the planet cooled off and dried up, any life — if there ever was any — could have been driven underground or forced into occasional or rare surface oases."

As a "noble gas," argon cannot react chemically with anything; the only process that can remove it to space is a physical process called "sputtering" by the solar wind. The solar wind is a thin stream of electrically conducting gas that blows from the surface of the sun into space at about a million miles per hour. In the sputtering process, electrically charged particles (ions) in the Martian upper atmosphere get "picked up" by the solar wind, and some of them are slammed into the upper atmosphere at high speed. These high-speed ions can then knock atoms (such as argon) into space.

The team tracked argon because it is a great indicator of loss by the sputtering process. Most gases can be removed by several different processes. Deuterium, for example, a water isotope, is known to condense and accumulate at the polar ice caps on Mars, Yelle explained. Because argon doesn't react chemically with anything, only sputtering can remove it from the atmosphere. Once they determined the amount of argon lost by sputtering, they could use the efficiency of sputtering to determine the sputtering loss of other atoms and molecules, including carbon dioxide (CO2).  

CO2 is of interest because it is the major constituent of Mars' atmosphere and because it's an excellent greenhouse gas that can retain the planet's heat. "We determined that the majority of the planet's CO2 also has been lost to space by sputtering," Jakosky said. "There are other processes that can remove CO2, so this gives the minimum amount of CO2 that's been lost to space."

The team made its estimate using data on the Martian upper atmosphere from MAVEN's Neutral Gas and Ion Mass Spectrometer, or NGIMS, instrument supported by measurements from the Martian surface made by NASA's Sample Analysis at Mars, or SAM, instrument on board the Curiosity rover. 

"The combined measurements enable a better determination of how much Martian argon has been lost to space over billions of years," said Paul Mahaffy of NASA's Goddard Space Flight Center in Greenbelt, Maryland. "While SAM on the surface precisely measures the average composition of the atmosphere, NGIMS in orbit allows us to see how the lighter gases separate from the heavier gases in the very thin upper atmosphere. Using measurements from both spacecraft points to the value of having multiple spacecraft that measure complementary things." 

"We did the argon measurements first because argon is not involved in chemistry and doesn't accumulate in ice," Yelle said. "Next, we will look at the other isotopes in the Martian atmosphere — hydrogen, carbon, nitrogen — and see if we can understand those in terms of atmospheric loss. It's more complicated, but we think we can do it." 

Yelle explained the main reason Mars can't hold on to its atmosphere is its magnetic field, which is a feeble remnant of its former self. Early in its history, Mars had a substantial magnetic field similar to that of Earth, but geologic evidence showed that it all but disappeared around 4 billion years ago. Even though Earth's magnetic field provides an effective shield against the brunt of the solar wind, our planet, too, slowly loses atmospheric gases to space — mostly hydrogen and, to a lesser degree, oxygen.   

The research was funded by the MAVEN mission. MAVEN's principal investigator is based at the University of Colorado's Laboratory for Atmospheric and Space Physics, Boulder, and NASA Goddard manages the MAVEN project. MSL/Curiosity is managed by NASA's Jet Propulsion Laboratory, Pasadena, California.