NASA solves the mystery of Mars’ missing atmosphere

By Joel Hruska 

After decades of work, NASA believes it has solved one of the most fundamental questions of how Mars became the dusty, barren planet that it is today. Over the past 15 years, rovers like Spirit, Opportunity, and Curiosity have uncovered compelling evidence that Mars was once covered by a liquid ocean. This, in turn, implies that the planet was much warmer, possibly with an oxygen-rich atmosphere.

The problem is, today’s Mars bears little resemblance to the primordial planet it must have been. Mars’ atmosphere is a fraction the density of Earth’s — air pressure at the summit of Mount Everest (the highest point on Earth) is 4.89 PSI, while the air pressure at the bottom of Mars’ Hellas Planitia (a 23,465-foot deep crater) is just 0.168 PSI. In other words, the air pressure at the highest point on our planet is 29x higher than the air pressure at the lowest point on Mars.

There have been three main hypotheses for how Mars might have lost its atmosphere: It’s possible that Mars’ atmosphere was eroded by solar wind, that much of the atmosphere was torn away by a cataclysmic impact, or that the low gravity of the planet allowed atmosphere to blow off and dissipate over time. These conditions are not mutually exclusive, and it’s possible that all three of them played a part, but NASA believes it has found sufficient evidence to assign a primary cause. The culprit? Solar wind — particularly the types of energetic blasts emitted by the sun during periods of unrest.

Blowing in the wind

In 2014, NASA’s Maven (Mars Atmosphere and Volatile Evolution Mission) arrived at Mars and began taking samples of the Red Planet’s atmosphere. The probe found that Mars continues to lose significant amounts of its atmosphere even today, roughly one-quarter pound per second. The solar wind is comprised of highly charged particles moving at a million miles an hour or more. When those particles strike the Martian atmosphere, they create an electric field, supercharging ions in the Martian atmosphere and sending them rocketing into space.

This same process occurs on Earth as well (and we’ve written about the devastating impact a massive solar storm could have on our planet’s infrastructure), but ordinary solar activity doesn’t cause modern life much trouble. That’s thanks to the magnetic field that sheaths our planet. Mars, in contrast, has only the remnants of a magnetic field, as shown in the image below:

The blue and red areas are the spots on Mars still shielded by a partial magnetic field.

Maven observed that periods of increased solar activity and solar storms sharply increased the rate at which Mars lost atmosphere. The video below shows how Mars loses atmosphere — 25% is lost from a polar plume, while 75% of the loss occurs at the long tail.

The sun has been a main sequence star for billions of years, but we know that it can undergo periods of greater and lesser activity. A Carrington-level event that struck Mars instead of Earth could have done colossal damage. Given that such events occur frequently on stellar time scales (a Carrington-level event hits Earth roughly every 500 years), Mars’ atmosphere wasn’t just diluted — it was violently wrenched away.

These findings could sharpen our own search for planets likely to harbor life. Strong, planet-wide magnetic fields appear nearly essential for protecting the environment required to sustain life. The remains of Mars’ magnetic field were nowhere near strong enough to shield the planet, and without that protection, planets may not be able to hold on to the atmosphere required to maintain temperatures above freezing. It could also have implications for any plan to terraform the planet — without a magnetic field in place, it could prove difficult to generate a sustainable atmosphere. Given that any long-term terraforming project of this nature would take millennia, however, scientists should have plenty of time to consider the problem.