NASA Explains How The Sun Ripped Away Ancient Mars Atmosphere

Why did Earth avoid the fate of its neighboring planet?

The Lunar and Planetary Institute NASA\'s MAVEN mission
Mar 30, 2017 at 3:49 PM ET

Billions of years ago, Mars had an atmosphere, oceans of liquid water, and quite possibly microbial life. But the red planet has since become a frigid, lifeless world, with a wispy thin atmosphere and ice mostly frozen underground. Now new findings from a NASA probe have given scientists our best understanding yet of just why Mars died, and the ancient sun appears to be the main culprit.

According to the scientists leading the Mars Atmosphere and Volatile Evolution mission, or Maven, the red planet’s atmosphere was likely the victim of intense solar radiation and wind. Charged particles from the sun collided with those in the atmosphere Mars, knocking them out into space. The radiation from young stars is especially fierce, so it’s likely that within the first billion years of the solar system’s existence most of the atmosphere was stripped away.

But why did Mars in particular suffer this fate? After all, solar wind ought to be an equal opportunity offender, with just as much headed toward neighboring planets Earth and Venus — both of which still have thick atmospheres. The fact that those planets are closer to the sun means they should have faced even worse effects, yet they escaped unscathed. The difference between Mars and Earth is simple enough to explain.

“On Earth, we have a global magnetic field that causes the solar wind to stand off at a great distance from the planet,” Maven principal investigator Bruce Jakosky told Vocativ. “So there’s no direct interaction with the atmosphere.”

Of the four rocky inner planets, only Earth and (somewhat unexpectedly) Mercury have those magnetic fields. Mars did have a magnetic field when it first formed, but it appears to have disappeared about 4.1 billion years ago, only a few hundred million years after the planet first formed. “We think the turn off of the magnetic field allowed the turn on of the stripping of the atmosphere.”

That still leaves Venus, which is similarly without a magnetic field. There, it’s more question of quantity over quality: Venus probably is losing some of its gases to space, but when its atmosphere is about 100 times thicker than Earth’s, that doesn’t make any noticeable difference.

The researchers looked at the concentration of both the lighter and heavier forms of the gas argon. Solar radiation would only eject the lighter isotope from the atmosphere, so seeing how much more of the heavier isotope was left let Jakosky and his colleagues work out how much was lost. The Maven observations suggest Mars has lost about two thirds of its atmosphere since it first formed, though looking at argon can only reveal one of several possible ways for Mars to lose its atmosphere, meaning the air on ancient Mars could have been even thicker than this study suggests it once was.