Mars would be much more clement if it still possessed a beefy atmosphere. Thanks to new measurements from NASA’s MAVEN spacecraft, we now have a definitive answer on why Mars evolved from relatively warm and benign place to today’s frigid desert where nighttime temperatures routinely plunge to –100°F (–73°C). Blame it on the sun!
Solar wind and radiation are responsible for stripping away the Martian atmosphere, transforming Mars from a planet that could have supported life billions of years ago into one that for the most part appears hostile to it, according to new results from MAVEN.
“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 studied the latest atmospheric measurements made by MAVEN from Mars orbit and found that about 65% of the argon that was ever in the atmosphere has been lost to space. Argon is a chemically inert element, one that doesn’t like attaching itself to others. A loner.
Liquid water, essential for life, is not stable on Mars’ surface today because the atmosphere is too cold and thin to support it. Leave a cup of it out in the open air, and it would boil away. Or freeze first, depending on the temperature. Yet evidence of water is everywhere in dry riverbeds and in minerals discovered by the Curiosity and Opportunity rovers that could only form in the presence of liquid water. Clearly, the Martian climate was once warm enough and air thick enough for water to flow on the surface for long periods of time.
If you’re a regular reader of this blog, you know that the solar wind is a stream of electrically conducting gas entwined with magnetic fields constantly blowing out from the sun.
Astronomers theorize that the early Sun emitted more intense ultraviolet light and a more vigorous solar wind, so the Martian atmosphere really took a pounding assuming the planet lost its protective magnetic field early in its history. Magnetic fields arise from electrical currents within a small planet’s iron-nickel core. Earth’s core is still dynamic — a good thing or our atmosphere might have been stripped away, too.
According to the team, atmospheric stripping may have been the main reason for the planet’s worst-case scenario climate change. Microbial life could have existed at the surface early in Mars’ history, but as the planet cooled off and dried up, any life could have been driven underground or forced into rare surface oases.
Jakosky and his team 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. On an atomic level, isotopes have the same number of protons as their parent element but a different number of neutral particles called neutrons.
Since the lighter of the two argon isotopes — the one with fewer neutrons — escapes to space more readily, it leaves the gas remaining behind enriched in the heavier isotope. Lighter gases escape a planet’s atmosphere more readily because they’re light and they zip around so fast, they’re relatively immune to gravity’s grasp. Especially if you’re a small planet like Earth or Mars with weak gravity to begin with.
The team used the relative abundance of the two argon isotopes measured in the upper atmosphere and at the surface to estimate the fraction of the atmospheric gas that has been lost to space. Since argon is one of the “noble gases” that doesn’t combine with other elements to make minerals or other compounds, it just hangs around as pure argon. The only way to get rid of it is to remove it by literally knocking it out of the atmosphere through “sputtering.”
Mars Atmosphere Loss Through Sputtering
First, that powerful UV light I mentioned earlier strikes atoms in Mars’ upper atmosphere and ionizes them by knocking off one of their electrons. Once ionized, an atom (now called an ion) can be dragged away by the magnetic field embedded in the solar wind. Second, here comes the solar wind. It blows by Mars, picks up some of those ions and flings them across the upper atmosphere where they strike other atoms and send them flying off into space. That’s the sputtering part.
The team tracked argon because it can be removed only by sputtering not by combining with something else. It’s noble after all! Once they determined the amount of argon lost by sputtering, they used the information to determine the sputtering loss of other atoms and molecules, including carbon dioxide.
CO2 interests scientists because it’s still the most abundant gas in Mars’ atmosphere (96%) to this day and an efficient greenhouse gas that can retain heat and warm the planet.
“We determined that the majority of the planet’s CO2 was also lost to space by sputtering,” said Jakosky. The team made its estimate using data from the Martian upper atmosphere, which was collected by MAVEN’s Neutral Gas and Ion Mass Spectrometer (NGIMS). The surface measurements were made by the Curiosity Rover.
That’s why we send spacecraft to other planets. We gather information right there in situ, something impossible from Earth, and use it to fill in a few more pieces of the big puzzle.