MAVEN Finds New Evidence that Most of Martian Atmosphere Was Lost to Space

According to an analysis of new data from NASA’s MAVEN (Mars Atmosphere and Volatile Evolution) spacecraft, solar wind and radiation are responsible for stripping the atmosphere of Mars. By measuring light and heavy isotopes of argon in the planet’s atmosphere, the authors have determined that the majority of the planet’s air and water were removed to space by a physical process called ‘sputtering.’

This image shows ancient Mars capable of supporting liquid water on its surface. Image credit: Michael Lentz / NASA Goddard Conceptual Image Lab.

“We’ve determined that most of the gas ever present in the Mars atmosphere has been lost to space,” said MAVEN principal investigator Dr. Bruce Jakosky, a researcher at the University of Colorado in Boulder and lead author of a paper published today in the journal Science.

Dr. Jakosky and co-authors made this determination from the latest results, which reveal that about 65% of the argon that was ever in the atmosphere has been lost to space.

In 2015, MAVEN scientists announced results that showed atmospheric gas is being lost to space today and described how atmosphere is stripped away.

The present analysis uses measurements of today’s atmosphere for the first estimate of how much gas was lost through time.

Liquid water, essential for life, is not stable on the Martian 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.

“This discovery is a significant step toward unraveling the mystery of Mars’ past environments. In a broader context, this information teaches us about the processes that can change a planet’s habitability over time,” said MAVEN program scientist Dr. Elsayed Talaat, of NASA Headquarters in Washington.

This infographic shows how Mars lost argon and other gasses over time due to ‘sputtering.’ Image credit: NASA’s Goddard Space Flight Center.

There are many ways a planet can lose some of its atmosphere. For example, chemical reactions can lock gas away in surface rocks, or an atmosphere can be eroded by radiation and a stellar wind from a planet’s parent star.

The new result reveals that solar wind and radiation were responsible for most of the atmospheric loss on Mars, and the depletion was enough to transform the Martian climate.

The solar wind is a thin stream of electrically conducting gas constantly blowing out from the surface of the Sun. The early Sun had far more intense UV radiation and solar wind, so atmospheric loss by these processes was likely much greater in Mars’ history.

These processes, according to the researchers, may have been the dominant ones controlling the planet’s climate and habitability.

Its possible microbial life could have existed at the surface early in Mars’ history. As the planet cooled off and dried up, any life could have been driven underground or forced into rare surface oases.

The scientists got the new result by measuring the atmospheric abundance of two different isotopes of argon gas.

Isotopes are atoms of the same element with different masses. Since 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 the relative abundance of the two isotopes measured in the upper atmosphere and at the surface to estimate the fraction of the atmospheric gas that has been lost to space.

As a ‘noble gas’ argon cannot react chemically, so it cannot be sequestered in rocks; the only process that can remove noble gases into space is a process called ‘sputtering’ by the solar wind.

In sputtering, ions picked up by the solar wind can impact Mars at high speeds and physically knock atmospheric gas into space.

The team tracked argon because it can be removed only by sputtering.

Once they determined the amount of argon lost by sputtering, they could use this information to determine the sputtering loss of other atoms and molecules, including carbon dioxide.

Carbon dioxide is of interest because it is the major constituent of Mars’ atmosphere and because it’s an efficient 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. There are other processes that can remove carbon dioxide, so this gives the minimum amount of carbon dioxide that’s been lost to space,” Dr. Jakosky said.

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B.M. Jakosky et al. 2017. Mars’ atmospheric history derived from upper-atmosphere measurements of ³⁸Ar/³⁶Ar. Science 355 (6332): 1408-1410; doi: 10.1126/science.aai7721

This article is based on a press-release from the National Aeronautics and Space Administration.