What Happened to Mars' Atmosphere?

After a year spent observing the Martian atmosphere from orbit, NASA's MAVEN (Mars Atmosphere and Volatile Evolution) space probe has finally gathered enough data to determine exactly how quickly Mars is currently losing its atmosphere to solar wind.

Most of it was lost billions of years ago, but what's left could yet remain for billions more.

Illustration of the solar wind stripping away Martian atmosphere.
Image Credit: NASA/GSFC

Researchers have calculated that approximately 100 grams of gas is being stripped from Mars' atmosphere every second. That equates to 6,000 tons of material lost per Martian year (which is about 687 Earth days long), or the weight equivalent of 33 adult blue whales leaving the red planet every Martian year. Atmospheric space whales. 

Solar wind is composed of charged particles ejected from the sun at speeds as high as 3 million km/hr, or 900 kilometers per second. These charged particles slam into free-floating particles in the Martian atmosphere and, like magic, are ejected from the atmosphere at escape velocity. 

During solar storms, Mars' rate of atmospheric loss can increase by a factor of 10 or 20. Since solar storms were more intense and common early on in the solar system's history, Mars appears to have lost the majority of its atmosphere a long time ago. And when the atmosphere disappeared, the water followed. 


Mars once had an abundance of water

From geological evidence gathered by Curiosity and other exploratory Mars rovers, scientists know that water was abundant and active on the red planet between 4.3 to 3.7 billion years ago. During this period, Mars harbored a large liquid ocean and a dense atmosphere composed primarily of carbon-dioxide—a vastly different environment from the Mars of today. 

A rendering of what a Martian ocean may have looked like.
Image Credit: NASA/GSFC

According to MAVEN principal investigator Bruce Jakosky, it's during this same early period in the solar system that solar storms would have been much more common and intense. It's thought that, at some point around 4.2 billion years ago, Mars' magnetic field started to shut down.

Without the protection of a magnetic field to deflect charged solar particles, Mars' atmosphere was left exposed to the full force of severe solar winds. Over a period lasting hundreds of millions of years, Mars' atmosphere would have started bleeding away at a rate 100 to 1000 times faster than today. As the atmosphere was gradually whittled down, its oceans slowly dried up and its surface began to look more like the red planet we see today.

Planetary scientists aren't certain what triggered the sudden demise of Mars' magnetic field, but it's hypothesized that Mars' smaller size and greater distance from the sun than Earth caused its core to cool more rapidly. As its molten metallic core began to cool, the planet's magnetic field could have weakened to the point where the solar wind could begin stripping particles off of the upper atmosphere. 


Mars was once more habitable

Scientists now know that, at some point in the past, the surface of Mars met all of the conditions necessary for life to exist. This then begs the question as to whether or not life ever existed on the red planet, and if so, if life could still exist in the present day. 

Present-day Mars. Less blue, more red.
Image Credit: NASA/JPL-Caltech

If we combine the facts that Mars once had an abundance of liquid water and that Mars currently has occasional liquid water flowing on its surface, an interesting thought experiment emerges. 

Imagine that life did arise in Mars' distant past. That 4 billion years ago, water-based microbes were floating in Martian oceans. Then the magnetic field shuts down. Due to being submerged in water, anything alive in Mars' oceans would be shielded from a significant amount of solar radiation and life would simply continue on as usual. 

Gradually, Mars' oceans would evaporate into the atmosphere and a significant portion of water vapor would then be stripped away by violent solar winds. With this collapse of a sustainable water cycle, the amount of water present on the surface of Mars would gradually decrease. With water scarcity, any lifeforms that had been thriving in Martian oceans would be forced to adapt in order to survive. 

Part of that adaptation could have been moving into Martian soil. Scientists now know that every cubic meter of Martian soil contains roughly 30 liters of water, so it's plausible that some Martian extremophile lifeforms could have found a way to survive by moving below the surface. In adopting Martian soil as their new habitat, these lifeforms would be protected from the hazards of low atmospheric pressure and would be able to sustain themselves on a permanent subsurface water supply. 


Could Martian life have endured?

We know that Mars experienced a 600 million year period in which there was an abundance of fresh, drinkable water. This presence of water created an ideal environment for life (as we know it) to exist. And if life existed then, it's plausible that life still exists there now.

Mars has enjoyed a 4.3 billion year unbroken chain of liquid water existing on its surface, ranging in abundance from the vast oceans of ancient Mars to the occasional water flows of present-day Mars. This unbroken chain represents the best opportunity for discovering life beyond Earth—we just need to dig down and look for it.

After all, how often do we find earthworms wriggling across the surface of the dirt rather than burrowing along beneath it?

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