Every bit of research that’s ever been conducted on Mars has in some ways been an autopsy. It’s now well-established that Mars was once a warm, wet place—about half as big as Earth but in most other respects every bit as fertile-looking. If all of the water Mars once had were spread evenly over its face, it would have formed a global ocean 0.3 mi. (0.5 km) deep. The water wasn’t distributed that way, of course. Instead it pooled in great oceans, rushed in deep rivers, spread out in giant alluvial fans like the Nile or Mississippi deltas.
But all that changed about 3.7 billion years ago, when Mars and Earth were both only 800 million years old, and all of the Martian water begin vaporizing into space or freezing into ice caps. And the reason the planet lost its water: it first lost its air, getting by today with an atmosphere that’s only about 1% as dense as Earth’s.
The longstanding question has always been, Why? What destroyed the Martian atmosphere and left the planet the cadaver it is today? Now, NASA‘s MAVEN spacecraft, set for launch on Nov. 18, may find out, studying the remains of Mars’s air in a way it never has been before and finally determining what became of the rest of it.
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The simplest explanation for the sad state of Mars today is that it was just too small to hold onto its air. Not only is Mars smaller than Earth by size, but also by mass and density, which gives it a gravity just 38% of ours. That would have made it easy for the upper reaches of the atmosphere to have simply trickled away into space. Far more damage would have been done by meteor strikes during what’s known as the late heavy bombardment period, from 4.1 to 3.8 billion years ago, which would have blasted atmosphere away in great, one-way gusts. Earth, of course, got clobbered too, and while it surely lost some of its atmosphere, it hung onto much more.
But Mars may have been subjected to a more insidious atmosphere-stripping force. Solar wind, the storm of charged particles that flow outward from the sun at about 1 million mph (1.6 million k/h), would have collided with the planet’s atmosphere, causing some of its atoms and molecules to sputter into space and changing the electromagnetic properties of others, sending them drifting off as well. Again, Earth would have felt the same solar fury—indeed, its tighter orbit would have made the blasts even worse—but our planet also has a magnetic field that protects us from the solar plasma. Mars once had one too but it shut down early in the planet’s history—and with that the planet’s air and water were doomed.
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“Studies of the remnant magnetic field by NASA’s Mars Global Surveyor mission, set the disappearance of the planet’s…magnetic field at about 3.7 billion years ago, leaving the Red Planet vulnerable to solar winds,” said MAVEN project scientist Joseph Grebowsky in a NASA statement.
But all of those mechanisms are only theories. MAVEN (a somewhat labored acronym for Mars Atmosphere and Volatile EvolutioN—and no, that capital N isn’t a typo) will attempt to settle them. The spacecraft is a comparatively lean machine, measuring 37.5 ft (11.4 m) from solar wingtip to solar wingtip and weighing 1,991 lbs (903 kg). But that small body will be stuffed with eight different instruments to measure atmospheric composition, solar wind ionization, planetary magnetism and more.
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In order to get both a big-picture look at the planet in ultraviolet wavelengths and some close-up tastes of the atmosphere, the ship will fly an exceedingly eccentric orbit, from a high of 3,278 mi. (6,000 km), to a low of 93 mi. (150 km). It will also make a few deep dives down to just 77 mi. (125 km). The goal in all of these studies is, in a sense, to reverse engineer the planet: understand the atmosphere and magnetism that are left, the continued battering by the solar wind and what’s that doing to the remaining air, and you can infer how things played out more than 3 billion years ago.
“MAVEN has been designed to measure the escape rate for all the applicable processes and will be able to single out the most prominent,” says Grebowsky. And that, in turn, will paint a fuller picture than we’ve ever had of Mars’s past.
Nothing, of course, will heal Mars. The planet that could have been Earth’s verdant little sister will remain the desert it is. If microbial or other life ever took hold there, it might still hang on under the soil or within caves or fissures. The most ancient signs of life on Earth date back to 3.8 billion years ago, or shortly before Mars began to go dry, so if biology got started here, it could have similarly arisen there. It will take landers, rovers and perhaps an eventual human mission to settle that question for sure. Until then, MAVEN will help explain what snuffed out the planet’s chances of being much more than it became.
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