Chromatically speaking, Mars is a one-trick pony. Known as the "Red Planet," the ruddy world is, unsurprisingly, red. But in recent observations from NASA's Curiosity rover, you'd be forgiven in thinking your computer monitor needs a refresh.
Seen here from the slopes of Mount Sharp in the center of Gale Crater is a typically rocky scene from our planetary neighbor. However, the fact that the slabs in the foreground seem to have a purple hue will likely come as a visual shock to anyone who has spent any time surveying the rusty landscapes surrounding the tenacious Mars rover since landing in 2012. Though visually stunning, these rocks' purple hue isn't down to some alien residue, it's the minerals in these rocks that are showing their true colors.
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"The purple tone of the foreground rocks has been seen in other rocks where Curiosity's Chemical and Mineralogy (CheMin) instrument has detected hematite," NASA wrote in a statement. "Winds and windblown sand in this part of Curiosity's traverse and in this season tend to keep rocks relatively free of dust, which otherwise can cloak rocks' color."
This scene was captured in three separate photographs taken by the rover's Mastcam instrument on Nov. 10, 2016, on the 1,516th Martian day (or sol) of Curiosity's mission. The image was color-adjusted for how the landscape would look in daytime light conditions on Earth. This adjustment helps scientists distinguish the different colors of rocks without the red tinge created by sunlight filtering through the Martian atmosphere.
Mars' general redness is caused by iron oxide-rich (think rust) dust that blankets the planet. Iron oxide has been forming on Mars for billions of years - the weathering of abundant iron on the surface with atmospheric oxygen (and atmospheric water in the planet's more habitable years) created a ruddy regolith. Regolith, often referred to as "Martian soil", is fine powdered rock produced after eons of meteorite impacts on the red planet. Though it's not "soil" in the terrestrial sense, it is hoped that future Mars colonists will be able to remediate the regolith of toxic chemicals and enrich it with organic compounds so it can be used as the raw ingredients for agriculture.
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Mars is awash with a variety of minerals that reveal a complex and interesting geological history of Mars' evolution. One of those minerals, hematite (another iron oxide compound), is possibly responsible for the purple hue of the rocks in this latest view of the Martian landscape. Hematite can be formed through liquid water processes, so its presence on the Martian surface has been a key piece of evidence that points to the Red Planet was once a warmer, wetter and therefore more habitable place than it is now. The mineral has been found in several locations by orbiting missions since 2001, which helped NASA determine where to land Mars Exploration Rover Opportunity in 2004. Opportunity continues to explore Meridiani Planum to this day, and has found the area rich in hematite deposits. Curiosity, too, has sampled samples at Gale Crater and confirmed the presence of the mineral.
What minerals these purple rocks actually contain remains to be seen, but their color tells us a lot about Mars' fascinating geological history.
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