Rice University, NASA’s Johnson Space Center, and the California Institute of Technology have solved a mystery that has baffled the Mars research community since NASA’s Curiosity rover discovered tridymite in Gale Crater in 2016.
Tridymite is a high-temperature, low-pressure form of quartz that is extremely rare on Earth, and how a concentrated chunk of it ended up in the crater was not immediately clear. Curiosity chose Gale Crater as its landing site due to the possibility that it once held liquid water, and Curiosity discovered evidence that confirmed Gale Crater was a lake as recently as 1 billion years ago.
“The discovery of tridymite in a mudstone in Gale Crater is one of the most unexpected discoveries that the Curiosity rover has made in 10 years of exploring Mars,” said Rice’s Kirsten Siebach, co-author of a study published online in Earth and Planetary Science Letters. “Tridymite is typically associated with quartz-forming, explosive, evolved volcanic systems on Earth, but we discovered it at the bottom of an ancient lake on Mars, where most volcanoes are extremely primitive.”
The discovery of tridymite in a mudstone in Gale Crater is one of the most unexpected discoveries that the Curiosity rover has made in 10 years of exploring Mars. Tridymite is typically associated with quartz-forming, explosive, evolved volcanic systems on Earth, but we discovered it at the bottom of an ancient lake on Mars, where most volcanoes are extremely primitive.
Kirsten Siebach
Siebach, an assistant professor in Rice’s Department of Earth, Environmental and Planetary Sciences, is a mission specialist on NASA’s Curiosity team. To suss out the answer to the mystery, she partnered with two postdoctoral researchers in her Rice research group, Valerie Payré and Michael Thorpe, NASA’s Elizabeth Rampe and Caltech’s Paula Antoshechkina. Payré, the study’s lead author, is now at Northern Arizona University and preparing to join the faculty of the University of Iowa in the fall.
Siebach and colleagues began by reevaluating data from every reported tridymite find on the planet. They also reexamined sedimentary evidence from the Gale Crater lake and reviewed volcanic materials from Mars volcanism models. They then devised a new scenario that matched all of the evidence: Martian magma sat in a chamber beneath a volcano for longer than usual, undergoing a partial cooling process known as fractional crystallization, which concentrated silicon.
The volcano erupted in a massive eruption, spewing ash containing extra silicon in the form of tridymite into the Gale Crater lake and surrounding rivers. Water aided in the breakdown of ash through natural chemical weathering processes, and it also aided in the separation of minerals produced by weathering.
The scenario would have concentrated tridymite, yielding minerals similar to the 2016 discovery. It would also explain other geochemical evidence found in the sample by Curiosity, such as opaline silicates and lower concentrations of aluminum oxide.
“It’s actually a simple evolution of other volcanic rocks found in the crater,” Siebach explained. “We contend that because we only saw this mineral once and it was highly concentrated in a single layer, the volcano erupted at the same time as the lake. Although the sample we examined was not entirely volcanic ash, it 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,” she said. “This work suggests that Mars may have a more complex and intriguing volcanic history than we would have imagined before Curiosity.”
“We argue that because we only saw this mineral once and it was highly concentrated in a single layer, the volcano erupted at the same time the lake was present. Although the sample we examined was not entirely volcanic ash, it was ash that had been weathered and sorted by water.”
Because this silicic eruption is an evolved type that differs from the basaltic volcanism for which evidence abounds on Mars, the team’s analysis suggests that the red planet may have had a much more complex volcanic history than we currently know. The Curiosity rover is still operational, and NASA is gearing up to commemorate the 10th anniversary of its landing next month.
