Mars’ elusive internal structure revealed by ancient zircon minerals from the red planet

Analysis of an ancient meteorite from Mars suggests that the mineral zircon may be abundant on the red planet’s surface.

By determining the age and isotopic composition of the zircon hafnium, researchers at the University of Copenhagen showed that a population of these crystals came from the inner depths of Mars. If the researchers are right, it means that the young zircons contain information about the deep and inaccessible interior of Mars, which provides information about the internal structure of the planet.

The uranium-containing mineral zircon is an abundant constituent of the earth’s continental crust, providing information on the age and origin of the continents and on large geological features such as mountain ranges and giant volcanoes. But unlike Earth, the crust of Mars has not evolved and is compositionally similar to the crust found under Earth’s oceans, where zircon is rare. Therefore, zircon shouldn’t be a common mineral on Mars.

“We were quite surprised and thrilled when we found so many zircons in this Martian meteorite. Zircons are incredibly resistant crystals that can be dated and hold information that tells us about their origins. Having access to so many zircons is like opening a time window into the geological history of the planet, “describes Professor Martin Bizzarro of the GLOBE Institute, who led the study.

The team studied the ancient Martian meteorite NWA 7533 (Figure 1), nicknamed “Black Beauty”, which was discovered in the Moroccan desert in 2011. After shattering 15 grams of this rock, they extracted about 60 zircons. By dating the zircons, they found that most of the crystals date back to about 4.5 billion years ago, which is the beginning of the planet’s life. But they also made an unexpected discovery: some zircons have defined much younger ages, ranging from around 1500 million years up to 300 million years.

Figure 1: The NWA 7533 zircon-rich meteorite containing fragments of the ancient crust of Mars. Credit: © University of Copenhagen / Deng et al.

“These young ages were a big surprise,” says Bizzarro. “The Black Beauty meteorite is believed to come from the southern hemisphere of Mars, which has no young volcanic soils. The only possible source for these young zircons is the volcanic province of Tharsis located in the northern hemisphere of the planet, which contains large recently active volcanoes, “adds Martin Bizzarro.

The Tharsis bulge on Mars is a huge volcanic province that is home to the largest volcanoes in the Solar System, up to 21 km high. Scientists believe that this volcanic province is the expression of a very deep magmatism that explodes on the surface of the planet. The Earth analogy is the Hawaiian volcanic island chain, which is also believed to reflect deep volcanic activity. But due to plate tectonics, the Pacific Plate is constantly moving in such a way that instead of accumulating in one place, a chain of progressively younger volcanic islands has formed. Since Mars does not have plate tectonics, volcanoes accumulate in one spot and consequently reach gigantic sizes.

If Bizzarro’s team is right, it means the young zircons may contain information about the deep and inaccessible interior of Mars. This is the first time that scientists have direct access to the inner depths of the red planet via these samples, which may allow them to discover the internal structure and composition of Mars.

“Having samples of the inner depths of Mars is essential. This means that we can now use these zircons to probe the origin of volatile elements on Mars, including its water, and see how it compares to Earth and other planets in the Solar System, “explains Mafalda Costa, the first other in the new study.

But to understand the nature of the deep Martian interior, the researchers turned to analyzing the isotopic composition of the element hafnium in the same zircons.

“Since hafnium is one of the main building blocks of zircon, it keeps a memory of where zircon was formed,” says Martin Bizzarro. “We found that the isotopic composition of hafnium of the young zircons is unlike any known Martian meteorite, indicating that the young zircons come from a primitive reservoir that we did not know existed within Mars,” he adds.

Figure 2: Unraveling the internal structure of Mars. An ascending plume of primitive material rises from the deep mantle, feeding a volcano on the planet’s surface. Credit: University of Copenhagen

The isotopic composition of hafnium of young zircons is similar to the most primitive objects of the Solar System, that is, chondrite meteorites. These chondrite meteorites are samples of asteroids that have never been modified since their formation. This implies that the deep interior of Mars has not been substantially modified by the formation of the planet (Figure 2). The existence of such a primitive reservoir is former meteorites are samples of asteroids that have never been modified since their formation. This implies that the deep interior of Mars has not been substantially modified by the formation of the planet (Figure 2). The existence of such a primitive reservoir is predicted for a planet devoid of plate tectonics. Unlike Earth, where material formed on the surface is continually recycled into the interior by plate tectonics, the deep interior of Mars has remained unchanged since the planet’s birth and, as such, retains its original composition.

Finally, the discovery that zircon could be abundant on the Martian surface could guide future robotic exploration of the planet, especially in the context of the return of samples to Earth.

“Our study makes it clear that a return mission aimed at acquiring samples containing zircons will be of high scientific value for understanding the geological history of Mars,” concludes Martin Bizzarro.

Reference: “The internal structure and geodynamics of Mars deduced from a 4.2 Gyr zircon record” by Maria M. Costa, Ninna K. Jensen, Laura C. Bouvier, James N. Connelly, Takashi Mikouchi, Matthew SA Horstwood , Jussi-Petteri Suuronen, Frédéric Moynier, Zhengbin Deng, Arnaud Agranier, Laure AJ Martin, Tim E. Johnson, Alexander A. Nemchin and Martin Bizzarro, 16 November 2020, Proceedings of the National Academy of Sciences of the United States of America.
DOI: 10.1073 / pnas. 2016326117

The study was supported by the Carlsberg Foundation, the Danish National Research Foundation and the European Research Council.