Curiosity's self portrait during before analyzing the silica-rich rocks. NASA/JPL-Caltech/MSSS

Three and a half billion years ago Mars was a completely different place. Water was flowing, the atmosphere was thicker, and maybe some basic forms of life had evolved. Curiosity’s latest findings provide more evidence for flowing water, but create new questions about the Red Planet.

The rocks analyzed by Curiosity in the last seven months have much higher concentrations of silica than any other terrain visited by the rover since its arrival in 2012.  Silica, which is a chemical made of silicon and oxygen, makes up 90 percent of the composition of some of those rocks.

"These high-silica compositions are a puzzle. You can boost the concentration of silica either by leaching away other ingredients while leaving the silica behind, or by bringing in silica from somewhere else," said Albert Yen, a Curiosity science team member, in a statement. "Either of those processes involve water. If we can determine which happened, we'll learn more about other conditions in those ancient wet environments."

If the origin of the silica is sedimentary, water must have flowed abundantly on Mars. The other alternative is that it formed through leaching, a type of rock weathering due to acidic water. While many minerals in rocks would dissolve, silica would not be affected by the acidic water. NASA’s rover Spirit previously discovered traces of sulfuric and hydrochloric acidity, which could favour the leaching hypothesis, but the team is still considering both scenarios until more evidence is found.

Another puzzling discovery is the presence of tridymite, a very rare silica (at least on Earth) formed by volcanoes. Researchers are curious about a potential magmatic past on Mars, but they are also testing if there are other ways for this mineral to form.  

Curiosity is currently climbing Mount Sharp, a feature within Gale crater where it landed 40 months ago. Mount Sharp formed because surrounded terrain eroded away, so as Curiosity climbs higher and higher, it encounters younger and younger terrain. This will give us an indication of how Mars went from a wet to arid environment.

"What we're seeing on Mount Sharp is dramatically different from what we saw in the first two years of the mission," said Curiosity project scientist Ashwin Vasavada.

"There's so much variability within relatively short distances. The silica is one indicator of how the chemistry changed. It's such a multifaceted and curious discovery, we're going to take a while figuring it out."

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