James Wray is an assistant professor of Earth and atmospheric sciences at the Georgia Institute of Technology in Atlanta. He is a collaborator on the Curiosity rover and the Mars Reconnaissance Orbiter science teams. His research explores the chemistry, mineralogy and geology of Martian rocks as records of environmental conditions throughout the planet’s history.
In less than a month, the Opportunity rover will begin her 10th year on the surface of Mars. She has already outlived her 90-day warranty 35 times over, like a human living 2,500 years instead of 70 – an astonishing engineering achievement.
But how has Mars science advanced during this period?
Opportunity and her twin sister, Spirit, went to Mars to determine whether, where, and how liquid water ever flowed across the Martian surface. Before their missions, we knew Mars had dry river valleys, but how could we be sure that water carved them? Where were the minerals that liquid water leaves behind: the clays that dominate our tropical soils on Earth, or salts deposited after evaporation?
Opportunity landed on Mars and opened her robotic eyes to a paradise of salt-rich rocks, with the frozen ripples of 3-billion-year-old ponds confirming that water once was there. But as the years passed on, like any Eden, the paradise felt more like a prison, and a heretical plan emerged to journey a seemingly impossible distance in pursuit of new knowledge.
Meanwhile, an international orbital armada was beaming back crucial new insights. Infrared spectrometers—first on the European Mars Express, and later the American Mars Reconnaissance Orbiter—showed that sulfate salt deposits like those found by Opportunity are widespread across the Martian tropics. NASA’s veteran Mars Odyssey found that chloride (table salt?) flats were widespread across other regions, implying that the short-lived Martian water was diverse in its chemistry.
And perhaps most importantly, the orbiters found clay minerals, which on Earth typically form when water interacts with rocks over millennia or longer. These were found in the oldest terrains of Mars, such as the rim of Endeavour crater, a “mere” dozen miles from where Opportunity landed. So we set out for that far horizon, making “landfall” over a year ago.
Opportunity probably sits atop the clays right now, but her two instruments that could directly confirm their presence are now defunct. Ironically, a study published this month suggests that clays may also have been present in the sulfate-rich rocks that Opportunity was so eager to escape. If they were there all along, yet went undetected, then how can we avoid such an outcome in the future?
NASA’s newest rover, Curiosity, was sent to Gale crater partly because clay minerals are inferred there, too. Curiosity can identify clays, but only if they are well crystallized; if they are instead non-crystalline or amorphous, as up to half of Martian soils appear to be, then Curiosity’s Chemistry & Mineralogy instrument will provide few new constraints.
But Europe is planning another Mars rover to launch in 2018, and NASA recently announced a new Mars rover set to launch two years later. Europe’s rover will (and NASA’s rover should) carry an infrared spectrometer able to locate clays in the same way they are identified from orbit: by analyzing how much light is reflected (not absorbed) by the surface. The relative proportions of each color that are reflected constitute a unique fingerprint for clays and for many other minerals.
Ultimately, our mission is not only to explore strange new worlds, but to seek out new life. Curiosity is searching for life’s organic building blocks, and clay minerals seem a promising target: They could not be there if Martian water had been too scarce or too acidic, and organics naturally bind to their “sticky” surfaces. An infrared spectrometer that looks even farther beyond the visible range of colors could detect both clays and organics.
But to seek evidence of life itself, for now the consensus is that we’d do best by bringing the Mars rocks back to Earth, where we can unleash all manner of sophisticated lab experiments on them for negligible cost. The first step toward sample return would be for the 2020 rover to store a piece of each auspicious rock it encounters in a “cache,” a bucket that could one day be picked up by another mission—one with the unprecedented capability to not only visit Mars, but to launch back off its surface and return to Earth.
Over the past decade we have witnessed truly astronomical progress in our understanding of Mars, from wondering whether it was ever wet, to detailed characterization of the chemistry and habitability of water that we know was there over long time periods. We must not forget that potentially habitable worlds await our attention in the outer solar system and beyond. But for now, it is time to cash (or cache?) in on our decade-long investment in Mars.