One of the biggest challenges about flying to Mars is remembering why you went there in the first place. The Curiosity rover has been on the Red Planet for almost a year now, and the landing itself — an outrageous feat achieved by a stationary hovercraft that lowered the 1-ton Mars car to the surface by cables — was a global television event. But once the wheels touched the soil and all of the high fives had been exchanged, most people outside of the space community turned away.
Curiosity, however, went to Mars to work, and if its sister rovers Spirit and Opportunity — both of which arrived in 2004 and one of which is still chugging — are any indication, it should be at it for a long time. In the past year, Curiosity has already made some intriguing discoveries about the mineralogy of Mars and the planet’s watery past, and this week it delivered again. In a pair of papers published in the journal Science, investigators announced new findings from the spacecraft about one of Mars’ most long-standing mysteries: how it lost its atmosphere, and why.
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Mars’ modern atmosphere is only 1% the density of Earth‘s, but the planet’s watery phase is believed to have lasted for the first billion of its 4.5 billion years, which means its air must have been around that long too. But things were never likely to stay that way. Mars has only half Earth’s diameter, 11% its mass and 38% its gravity, making it easy for upper layers of the original atmosphere to have boiled away into the vacuum of space and been blasted out by meteor hits. And that cycle would build on itself: the thinner the air became, the easier it would be for space rocks to hit the ground, unleashing still more explosive energy and, in effect, blowing still more holes in the sky.
But that’s only one mechanism. Planets can lose their air not just from the top up but also from the bottom down, as elements of the atmosphere bond with — and retreat into — the soil. Martian meteorites that landed on Earth have often been found to include gas bubbles from the Martian sky, evidence that this commingling was going on.
Curiosity scientists sought to settle the matter with the help of the rover’s Sample Analysis at Mars (SAM) instruments, a collection of sensors that sniff the air for its chemical makeup — particularly its mix of isotopes. Elements don’t come in just one form, but in different sizes and weights — such as carbon 12 and carbon 13 — determined by the number of neutrons in the nucleus. That weight issue is critical in atmospheric studies, because just as heavier metals sink downward and lighter ones rise as a molten planet is forming, so do gases stratify themselves in the atmosphere by weight.
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Earlier measurements of Mars’ current atmosphere had always shown a high concentration of the heavy isotopes of carbon and oxygen — convenient elements to measure because Mars’ atmosphere is overwhelmingly made of carbon dioxide. Those findings differ from the isotopic makeup of the sun and the early solar system as a whole, in which lighter isotopes were more evenly represented. Mars, like Earth and all of the other planets, would have started out with that same relatively even mix. The fact that the heavy isotopes dominate the remaining Martian air means its lighter, high-altitude gases bled away first — supporting the top-down theory.
“As atmosphere was lost, the signature of the process was embedded in the isotopic ratio,” said NASA‘s Paul Mahaffy, principal investigator for the SAM team, in a statement. That was the theory anyway, but it took a suite of instruments like SAM to sample the air with enough sensitivity to prove the heavy-isotope imbalance. As the Science paper revealed, Curiosity indeed sealed that deal.
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The findings are considered particularly reliable because Curiosity used two different instruments to do its work: the tunable laser spectrometer, which analyzes how Martian air pumped into a chamber reflects two different frequencies of infrared laser; and the mass spectrometer, which, as its name suggests, measures the entire spectrum of elements present in an air sample according to their mass. “Getting the same results with two very different techniques increased our confidence that there’s no known systematic error,” said NASA’s Chris Weber, lead author of one of the new papers.
Mars’ lost air is never coming back, but the little bit it does have still makes the planet a chemically active place — and plays a major role in the combination of parachutes and braking-rockets spacecraft from Earth rely on to reach the surface safely. But change is a constant everywhere in the universe, and even today, the Red Planet’s atmospheric loss is thought to be continuing. How fast that’s happening will not be known until the arrival of NASA’s next Mars probe, the Mars Atmosphere and Volatile Evolution (MAVEN) mission, which is set for launch in November. The already harsh Mars, MAVEN may find, is fast becoming harsher still — one more reason to appreciate the improbably verdant Earth.