Crater-in-crater on Mars provides rare glimpse into ancient volcanoes and glaciers

March 16, 2021

A crater is showing us some of Mars' history. (NASA)

Taking advantage of an impact crater inside a larger crater on a Martian volcanic dome, planetary scientists identified the composition of a volcano billions of years old, in addition to evidence of ancient water flows and unprecedented glacial formation in the red planet's far past.

The team of Indian and French scientists analyzed satellite images of the unusual geological feature on Mars and published the results Feb. 25 in a paper in Icarus. The findings flesh out trends in the red planet's geologic activity and climate evolution since its formation in the solar system.

Four billion years ago, Mars was the host to frequent volcano eruptions, large bodies of liquid water and continuous orbital bombardment from asteroids and other space debris. But it has since become a dry, freezing planet that only has water at its poles, frozen, and most likely has not experienced a volcanic eruption for about 50 million years

The researchers peered into the planet's more geologically active past by studying a roughly 30-mile-wide impact crater called Degana and another impact crater about 12 miles wide inside it, which they dubbed Degana-A. Both craters were created by meteorites that struck an ancient volcanic dome nearly 4 billion years old, burrowing 1.7 miles deep into the crust through the 0.75-mile-high dome.

Both impacts scattered material from the dome as well as from below it, making it visible to NASA's Mars Reconnaissance Orbiter satellite, which took the images of visible and nonvisible light used in the analysis. Being able to see and study this once-buried ground is a rare occurrence because Martian surface dust usually obscures ancient parts of the planet's crust, according to Harish, the paper's lead author.

"These craters, they are very, very fascinating because they are excavating the inner material," said Harish, a Ph.D. student studying the geological processes of Mars and the moon at India's Physical Research Laboratory. "They are providing the inner material for us to study."

After analyzing the scattered materials, Harish and his colleagues found that the ancient volcano contained high-magnesium olivine and low-calcium pyroxene. These minerals indicate that the volcano's lava likely originated at a relatively shallow depth, in the lower crust or upper mantle, Harish said.

Knowing what old volcanoes are made of "is critical to understand the evolution of ancient volcanic dust on Mars," the researchers wrote. The volcanic dome underlying the Degana crater is one of the first such volcanoes to have its composition known.

The paper is also the first to identify signs of past water flows and glaciers on this volcanic dome. The floors of both Degana and Degana-A are covered with deposits from alluvial fans, which are left behind when a narrow source of water flows into a wide area.

"If we found an alluvial fan on a planetary surface — for example, here on Mars — then it is a very good indicator that water may have flown in the past," Harish said.

The scientists suggested that the water was formed from precipitation of snow or ice and snowmelt, which then breached a 0.6-mile-wide hole in the wall of Degana-A and caused water to flow inside. 

Ridges found on these fans also indicated the former presence of glaciers made from water ice, forming after the alluvial fans were created by water flows. They are an unusual and significant finding because glaciers have not been found so close to the equator in Mars' southern hemisphere, Harish said.

It also depicts an evolution in climate at this location, from hosting liquid water to solid glaciers, not too long before most water disappeared from the Martian surface.

The scientists estimated that the glaciers occurred during the early Amazonian period, a period of Martian geologic history that began roughly 3 billion years ago. The Degana crater — named after an Indian town — formed around 3.7 billion years ago, while the volcanic dome is about 4 billion years old. The researchers could not easily determine Degana-A's age.

Harish said he is pursuing the composition of other ancient volcanoes and looking for more glacial activity at low latitudes as part of ongoing research, searching for whether his study's most notable findings were part of larger trends in Mars' past.

The study, "Evidence for fluvial and glacial activities within impact craters that excavated into a Noachian volcanic dome on Mars," published Feb. 25 in Icarus, was authored by Harish, Physical Research Laboratory and Indian Institute of Technology; Vijayan S., Physical Research Laboratory; and Nicolas Mangold, Laboratory of Planetology and Geodynamics.

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