In 2016, the Curiosity rover found something really strange in Gale Crater on Mars.
On the slopes of Mount Sharp, where Curiosity worked, there were large amounts of rare minerals; Rare at least here on Earth. Tridymite, a form of quartz, forms only very rarely, and under high temperatures, such as you might find in magma.
Although Mars shows extensive evidence of past basaltic volcanic activity in some areas, Gale Crater, once filled with water, is not one of those areas, giving scientists a better idea of how the mineral came about.
Now a team led by planetary scientist Valerie Pere of the University of Arizona has figured out the mystery: that tridymite may have come from a single, explosive volcanic eruption about 3.0 to 3.7 billion years ago.
“The discovery of tridymite in a mudstone at Gale Crater is one of the most surprising observations the Curiosity rover has made in 10 years of Mars exploration,” said Rice University Mars geologist Kirsten Seeback.
“Tridymite is usually associated with quartz-forming, explosive, evolved volcanic systems on Earth, but we found it at the bottom of an ancient lake on Mars, where most volcanoes are very primitive.”
Because we can’t actually go to Mars, scientists had two tools to unravel the mystery: tridymite deposits found here on Earth, and mineral samples collected from Gale Crater and Mount Sharp — the crest in the crater’s center. – by Curiosity, which sends data on its discoveries back to Earth.
So that’s where Pere, then at Rice University, and his colleagues changed.
First, Earth. Each documented tridymite deposit and the conditions under which it was formed were carefully examined by the research team.
Then, they are sifted through data collected by Curiosity on the composition of the long-dried sedimentary lake in Gale Crater.
Tridymite forms at temperatures above 870 °C (1,600 °F) and turns into a phase called cristobalite at about 1,470 °C. Both these forms were found in the same layer on the slopes of Mount Sharp.
In addition, Curiosity found feldspar and opaline silica, which can be found in volcanic contexts on Earth.
Putting these pieces together leads to a fascinating landscape involving a magical chamber at the bottom of Gale Crater billions of years ago. The team estimated that this chamber may have been sitting under the lake for some time, longer than usual.
During this time, cooling will result in a process called partial crystallization, in which minerals must be removed and separated to produce an excess of silica.
When the chamber eventually erupted, it did so in a massive explosion that spewed silica-rich ash into the air—now in the form of tridymite—to rain back into the lake at Gale Crater and its surrounding tributaries.
The team said these waters used to sieve and sort the ashes to produce the chemical composition of the layer, as observed by Curiosity.
“It’s actually a direct evolution of the other volcanic rocks found in the crater,” explained Seebach.
“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 as the lake. However the specific sample we analyzed was exclusively volcanic. It was not ashes, it was ashes that had been weathered and sorted out by the water.”
Since this silicic eruption is an evolution type that would have been different from basaltic volcanism, for which evidence is abundant on Mars, the team’s analysis suggests that the Red Planet may have a more complex volcanic history than we know.
Future missions, the team said, should look for evidence of other examples of this evolved volcanism to delineate when and in what contexts it occurred on Mars.
“Mars,” he wrote in his paper, “is not just a basaltic world.”
research has been published in Earth and Planetary Science Papers,