Gale Crater was chosen as Curiosity’s landing site because of the likelihood that it once held liquid water, and indeed the rover found evidence confirming that it was actually a lake as recently as 1 billion years ago years.
This meant that finding tridymite in the mudstone of the crater was surprising because the mineral is usually associated with explosive and evolved volcanic systems on Earth, which are different from primitive volcanoes on Mars.
In their search for answers, the researchers re-evaluated the data from each find of tridymite on Earth. They also reviewed volcanic materials from models of Martian volcanism and reexamined sedimentary evidence from Gale Crater Lake.
They then came up with a new scenario that matched all the evidence: Martian magma sat longer than usual in a chamber beneath a volcano, undergoing a partial cooling process called fractional crystallization that concentrated silicon. In a massive eruption, the volcano spewed ash containing the extra silicon in the form of tridymite into Gale Crater Lake and surrounding rivers. Water helped break down the ash through natural processes of chemical weathering, and water also helped sort the minerals produced by weathering.
The scenario would have concentrated tridymite, producing minerals consistent with the 2016 find. It would also explain other geochemical evidence Curiosity found in the sample, including opaline silicates and reduced concentrations of aluminum oxide.
“It’s actually a simple evolution of other volcanic rocks we found in the crater,” study co-author Kirsten Siebach said in a media release. “We argue that because we only saw this mineral once and it was very 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 ash volcanic, but ashes that had been weathered and classified by water”.
If a volcanic eruption like the one in the scenario occurred when Gale Crater contained a lake, it would mean that explosive volcanism occurred more than 3 billion years ago, while Mars was transitioning from a wetter world and perhaps warmer on the dry and arid planet. it’s today
“There is a lot of evidence for basaltic volcanic eruptions on Mars, but this is a more evolved chemistry,” Siebach said. “This work suggests that Mars may have a more complex and intriguing volcanic history than we would have imagined before Curiosity.”