The solar wind has removed most of the Martian atmosphere

According to new research, the solar wind and radiation are responsible for removing the reddening atmosphere and turning Mars from a planet that could have supported life billions of years ago into a frozen desert world

The solar wind and radiation are responsible for removing the atmosphere on Mars, and turning Mars from a planet that could have supported life billions of years ago into a cold desert world, according to a study based on new data provided by NASA's MAVEN spacecraft led by the University of Colorado at Boulder.

"We were able to demonstrate that most of the gas that was ever in the Martian atmosphere was lost to space," said Bruce Jakowski, principal investigator of the MAVEN spacecraft and professor at the Laboratory for Atmospheric and Space Physics (LASP). "The team made this determination thanks to recent findings revealed by the MAVEN spacecraft, according to which about 65% of the air that was ever in the atmosphere of Mars has been lost in space."

Jakowski is the lead author of the paper on this research published in the journal Nature on March 24 along with doctoral student Mark Slipsky.

Members of the MAVEN team previously announced measurements showing that the gases in the atmosphere were lost to space, and described the processes by which the atmosphere was ripped apart. The current analysis uses measurements of today's atmosphere to estimate the amount of gas removed.

Liquid water, essential to all living things, is unstable on the surface of Mars today because the atmosphere is too cold and thin to support this molecule. However, evidence such as features resembling dry lakebeds and minerals that can only form in the presence of liquid water suggest that the ancient Martian climate was much different and warm enough for water to flow on the surface for extended periods.

There are many ways in which a planet can lose some of its atmosphere. For example, chemical reactions can trap gas in rocks or the surface or atmosphere is eroded by radiation and the solar wind (of that planet). The new findings reveal that the solar wind and radiation were responsible for most of the loss of the Martian atmosphere and that the thinning alone was enough to change the Martian climate. The solar wind is a thin stream of electrically conductive gas constantly emitted from the face of the sun.

Young stars like our Sun as it was 4 billion years ago, emit ultraviolet radiation and return a stronger solar wind, and thus the loss of atmosphere caused by these processes was much greater at the beginning of the history of Mars, and these processes may have been the processes that control the climate of the planet until today. It is possible that microbial life could have existed on the surface early in the history of Mars. But as the planet cooled and dried out, life forms had to hide underground, or be forced to occasionally come to the surface.

Jakowski and his team reached this result by measuring the proportion in the atmosphere of two different isotopes of the gas argon. Isotopes are atoms of the same element but with different masses. Since the lighter of the two isotopes escapes into space more easily, it will leave and the gas left behind will be enriched in the heavy isotope. The team repeated the test at different heights of the atmosphere to examine the changing concentration of the gas at different heights of the atmosphere, in order to estimate what part of the atmospheric gas was lost to space.

Since argon is a "noble gas" it cannot react chemically or be released from rocks. The only process that can remove it from the atmosphere and bring it into space is a physical process called "shaking" by the solar wind. In this process, the ions are lifted by the solar wind hitting Mars at high speeds and blowing atmospheric gas into space. The team tracked the organization that is only emitted naturally during volcanic eruptions. Once they determined the amount of argon lost in the shaking process, they could use the shaking efficiency to determine the loss of other atoms and molecules, including carbon dioxide (CO2).
“CO2 is particularly interesting because it is the main component of the Martian atmosphere, carbon dioxide is also an effective greenhouse gas that can keep the planet warm. We determined that most of the CO2 was also lost to space." Jakowski said.

The team evaluated the estimate using data on the upper atmosphere of Mars from the Neutral Gas and Ion Mass Spectrometer (NGIMS) instrument on the MAVEN spacecraft, and verified by measurements of the Martian surface made by NASA's SAM instrument analysis on the Mars rover Curiosity.