Most of Mars’ atmosphere lost to space, scientists say
According to a new study by scientists working with NASA’s MAVEN spacecraft, the Martian atmosphere was mostly stripped away by solar wind and radiation, changing Mars from a world that could have supported life billions of years ago into the frigid desert planet it is today.
“We’ve determined that most of the gas ever present in the Mars atmosphere has been lost to space,” said Bruce Jakosky, principal investigator for the Mars Atmosphere and Volatile Evolution Mission (MAVEN), University of Colorado in Boulder.
The team made this determination from the latest results, which reveal that about 65 percent of the argon that was ever in the atmosphere has been lost to space. Jakosky is the lead author of a paper on this research published March 31, 2017, in Science.
In 2015, members of the MAVEN team announced findings that indicated atmospheric gas is being lost into space today and described how it is being stripped away. The new study used measurements of the current Martian atmosphere to estimate how much gas has been lost over time.
Liquid water, which is essential to sustain life, is not stable on the Martian surface today because the atmosphere is too thin and cold to support it. Evidence found on Mars, such as features resembling dry riverbeds and minerals that form only in the presence of liquid water, suggest the ancient Martian environment was once warm enough for water to flow on the surface for an extended period.
“This discovery is a significant step toward unraveling the mystery of Mars’ past environments,” said Elsayed Talaat, MAVEN Program Scientist, at NASA Headquarters in Washington. “In a broader context, this information teaches us about the processes that can change a planet’s habitability over time.”
There are several ways a planet could lose part of its atmosphere. Chemical reactions could lock gas within surface rocks or the atmosphere could be eroded by radiation and the stellar wind of the star it orbits.
This new study indicates solar wind, a thin stream of electrically conducting gas blowing out from the surface of the Sun, and radiation were responsible for most of the atmospheric loss on Mars.
Jakosky and his colleagues measured the atmospheric abundance of two different isotopes of argon gas. Isotopes are atoms of the same elements with different masses. Because the lighter of the two isotopes escapes into space more readily, it leaves the gas remaining behind enriched in the heavier isotope.
The researchers used the relative abundance of the two argon isotopes measured in the upper atmosphere and the surface to estimate the fraction of the atmosphere that had been lost into space.
Argon is a “noble gas” and cannot react chemically and cannot be sequestrated in rocks. Only a physical process called “sputtering” can remove argon into space.
In sputtering, ions picked up by solar wind can impact Mars at high speeds and physically knock atmospheric gas into space. The scientists tracked argon because it could only be removed from the atmosphere by sputtering.
After determining the amount of argon lost due to sputtering, the researchers were able to use this information to determine the sputtering loss of other gasses, including carbon dioxide.
Carbon dioxide is of interest to the researchers because it is the most plentiful gas in the Martian atmosphere and because it is a greenhouse gas that can retain heat and warm the planet.
“We determined that the majority of the planet’s [carbon dioxide] was also lost to space by sputtering,” said Jakosky. “There are other processes that can remove [carbon dioxide], so this gives the minimum amount of [carbon dioxide] that’s been lost to space.”
The team based its estimate on upper atmosphere data collected by MAVEN’s Neutral Gas and Ion Mass Spectrometer (NGIMS) and surface measurements made by NASA’s Sample Analysis at Mars (SAM) 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. “Using measurements from both platforms points to the value of having multiple missions that make complementary measurements.”
Mahaffy, a co-author of the paper, is the principal investigator on the SAM instrument and lead on the NGIMS instrument, both of which were developed at NASA Goddard.
Video courtesy of NASA
Jim Sharkey is a lab assistant, writer and general science enthusiast who grew up in Enid, Oklahoma, the hometown of Skylab and Shuttle astronaut Owen K. Garriott. As a young Star Trek fan he participated in the letter-writing campaign which resulted in the space shuttle prototype being named Enterprise. While his academic studies have ranged from psychology and archaeology to biology, he has never lost his passion for space exploration. Jim began blogging about science, science fiction and futurism in 2004. Jim resides in the San Francisco Bay area and has attended NASA Socials for the Mars Science Laboratory Curiosity rover landing and the NASA LADEE lunar orbiter launch.