Rocky discoveries on Mount Sharp are puzzling

As NASA’s Curiosity rover ascends Mount Sharp — the 3-mile-high (5 kilometers) pile of layered sedimentary rock inside Mars’ Gale Crater
— it continues to surprise scientists.
In mid-December, Curiosity’s science team announced the probe’s discovery of huge concentrations of silica, a rock-forming mineral made of silicon and oxygen that on Earth often appears as quartz. Some rocks contain up to 90 percent silica, dwarfing the levels seen on the mountain’s lower slopes.

“These high-silica compositions are a puzzle,” says team member Albert Yen of NASA’s Jet Propulsion Laboratory in Pasadena, California. “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. [Both] of those processes involve water.” The findings were such a surprise that scientists sent Curiosity back to the area to study it in greater detail.

ROCK ON. NASA’s Curiosity rover has discovered silica-rich rocks in the Marias Pass region of Mars’ Mount Sharp. In this view of the
pass, the lighter area at center is an older section that abuts an overlying layer of sandstone. NASA/JPL-CALTECH/MSSS

Unraveling the silica mystery will forge a better understanding of Gale Crater’s history. Does the mineral’s presence signify a flow of acidic water, which would carry away other compounds and leave silica behind? Or is it a marker for neutral or alkaline water, which could transport the dissolved mineral into the area and then deposit it?

Curiosity drilled into one rock that adds an intriguing piece to the puzzle. The rock contained tridymite, a type of silica rare on Earth that had never been seen before on Mars. On our planet, tridymite forms at high temperatures and often in explosive volcanic eruptions, raising the possibility that Gale Crater experienced volcanic activity in addition to flowing water