Four billion years ago, Mars and Earth were like twins. Water flowed on the Martian surface beneath an atmosphere rich in carbon dioxide, oxygen, methane and water vapour. Today the Martian atmosphere is vanishingly thin, just a hundredth the density of Earth’s, and its surface water has disappeared.
Where did it all go? To find out NASA sent MAVEN – the Mars Atmosphere and Volatile Evolution spacecraft – all decked out with sensitive new instruments. It’s been orbiting the planet since last September and this November it finally answered the riddle. The solar wind blew away the Martian atmosphere. This result was the highlight of a landslide of papers published in November using data collected by MAVEN – four in Science and 40 in Geophysical Research Letters.
The first hints water used to flow on Mars came from NASA’s Viking missions in the 1970s. The orbiters beamed back pictures of valleys that looked like they’d been carved by ancient rivers. More recent landers showed fossilised ripples of lakebeds and streams known as mudstone. And just in September, instruments on the Mars Reconnaissance Orbiter detected the signatures of hydrated salts streaking down crater edges. The briny residue showed water may still be found occasionally on the surface of Mars. But it’s a drop in the ocean compared to the bodies of water that resided in the ancient lakes some four billion years ago.
So where did the water go? Some thought it was locked away in subsurface ice deposits. And as for the atmosphere, carbon dioxide and other gases might have chemically reacted with rocks over hundreds of millions of years, and become locked away inside Mars’ geology – similar to the way carbon dioxide in Earth’s atmosphere can get locked away as limestone.
The other possibility was that both had been lost to space: first the atmosphere, then the water, which in the thin air would simply have evaporated away. If this theory was right, the real question was, why did the atmosphere vanish in the first place? It shouldn’t have: Mars’ gravity, a third of Earth’s, is sufficiently strong to keep its atmosphere.
“Like the theft of a few coins from a cash register every day,
the loss becomes significant over time”
First off, MAVEN established that the Martian atmosphere was indeed vanishing into space. Dipping in and out of the Red Planet’s upper atmosphere, it detected wisps of ionised air escaping at the rate of about 100 grams each second. “Like the theft of a few coins from a cash register every day, the loss becomes significant over time,” says Bruce Jakosky, MAVEN principal investigator at the University of Colorado, Boulder.
MAVEN was also present when a solar storm hit Mars in March 2015.
The rate of atmospheric loss increased up to 20-fold when the storm struck. The storm was the result of a coronal mass ejection by the Sun, which hurled billions of tonnes of superhot material into space. Unlike the constant, steady stream of particles of the solar wind, these events are far more energetic and damaging. With each direct hit, more of the Martian atmosphere is lost.
The Sun had been caught in the act of planetary vandalism. MAVEN’s data showed the long suspected culprit, the solar wind (and its sometime partner in crime, solar storms), was easily capable of removing an atmosphere. While the Sun is still at work shearing away the Martian atmosphere today, four billion years ago a youthful Sun was even more tempestuous with storms that were more frequent and powerful than those of today.
So why was Earth spared this fate? Our planet is blessed with a magnetic shield that deflects the charged solar particles; Mars is not. A magnetic shield is created by a churning liquid iron core, which Earth has. Mars once had a molten core too but around four billion years ago, it cooled and solidified. Just why we have been spared this fate is not entirely understood – perhaps it is simply because Mars is smaller and lost heat more quickly. The same fate undoubtedly awaits Earth too, but not for many millions (if not billions) of years yet.
Without a liquid iron core, Mars’ magnetic field faded away. The solar wind then ripped away most of the atmosphere, leaving the oceans to evaporate into space. But the removal of its atmosphere would have taken place over a few hundred million years, so any life that existed had time to adapt to living underground; the Sun’s ultraviolet radiation would be fatal to life on the surface.
This is good news for scientists hoping to find life on Mars, but bad news for human colonists.
Some had hoped the gases that made up the atmosphere might still be present beneath the surface, awaiting our arrival to unlock all that carbon dioxide, begin to grow plants and terraform the Red Planet. Not so: at least as far as the atmosphere goes, colonisers will need to bring their own.
Alan Duffy is an astrophysicist at Swinburne University of Technology, Melbourne. He was Lead Scientist of The Royal Institution of Australia from 2017 to 2021. Twitter | @astroduff
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