Tridymite is a high-temperature, low-pressure form of quartz that is extremely rare on Earth, and it wasn’t clear how a chunk of it ended up in Gale Crater on Mars, the home of NASA’s Curiosity rover.
Gale Crater, which is at least 3.8 billion years old, was chosen as Curiosity’s landing site due to the likelihood that it once held liquid water, and the rover found evidence that confirmed Gale Crater was a lake as recently as 1 billion years ago.
In July 2015, Curiosity drilled into a rock at the ‘Buckskin’ target location in Gale Crater, producing rock powder.
The X-ray diffraction analysis of the sample inside the rover’s Chemistry and Mineralogy (CheMin) instrument revealed the presence of a silica-containing mineral known as 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,” said Dr. Kirsten Siebach, a researcher in the Department of Earth, Environmental, and Planetary Sciences at Rice University.
“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.”
In the study, Dr. Siebach and colleagues reevaluated data from every reported find of tridymite on Earth.
They also reviewed volcanic materials from models of Mars volcanism and reexamined sedimentary evidence from the Gale Crater lake.
They then came up with a new scenario that matched all the evidence: Martian magma sat for longer than usual in a chamber below a volcano, undergoing a process of partial cooling called fractional crystallization that concentrated silicon.
In a massive eruption, the volcano spewed ash containing the extra silicon in the form of tridymite into the 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 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 straightforward evolution of other volcanic rocks we found in the crater,” Dr. 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.”
If a volcanic eruption like the one in the scenario did occur when Gale Crater contained a lake, it would mean explosive volcanism occurred more than 3 billion years ago, while Mars was transitioning from a wetter and perhaps warmer world to the dry and barren planet it is today.
“There’s ample evidence of basaltic volcanic eruptions on Mars, but this is a more evolved chemistry,” Dr. Siebach said.
“This work suggests that Mars may have a more complex and intriguing volcanic history than we would have imagined before Curiosity.”
The study was published in the journal Earth and Planetary Science Letters.
V. Payre et al. 2022. Tridymite in a lacustrine mudstone in Gale Crater, Mars: Evidence for an explosive silicic eruption during the Hesperian. Earth and Planetary Science Letters 594: 117694; doi: 10.1016/j.epsl.2022.117694