Over seven years ago, NASA’s Curiosity was drilling into one of the intriguing rocks of Gale Crater, which was once upon a time the site of a large lake. The drilling left behind a grayish powder and a mineral that scientists did not expect to see on Mars: tridymite. This substance, associated with explosive eruptions, is a type of quartz that forms at high temperatures and low pressures. It is rare on Earth, and it was not exactly clear how it could have been found on the Red Planet.
New research, published in Earth and Planetary Science Letters, set out to explain how the mineral got there and how the water in the lake and the volcanism on Mars might have come to create a concentration of tridymite.
“The discovery of tridymite in a mudstone in Gale Crater is one of the most surprising observations that the Curiosity rover has made in 10 years of exploring Mars,” co-author Dr Kirsten Siebach, from Rice University, said in a statement. “Tridymite is usually associated with quartz-forming, explosive, evolved volcanic systems on Earth, but we found it in the bottom of an ancient lake on Mars, where most of the volcanoes are very primitive.”
The scenario they propose can explain all the peculiarities of the discovery. Martian magma stayed in its chamber longer than usual, where it experienced fractional crystallization until extra silicon was accumulated. It was then spewed out in a vast ash cloud containing tridymite, which then deposited itself in Gale Crater lake and surrounding rivers.
“It’s actually a straightforward evolution of other volcanic rocks we found in the crater,” Siebach said. “We argue that because we only saw this mineral once, and it was highly concentrated in a single layer, the volcano probably erupted at the same time the lake was there. Although the specific sample we analyzed was not exclusively volcanic ash, it was ash that had been weathered and sorted by water.”
The findings suggest that explosive volcanism was happening on Mars over 3 billion years ago as the planet was changing from a wetter world of the past to the dry, frigid desert of today.
“There’s ample evidence of basaltic volcanic eruptions on Mars, but this is a more evolved chemistry,” she said. “This work suggests that Mars may have a more complex and intriguing volcanic history than we would have imagined before Curiosity.”