Friday, December 8, 2023


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The location of the meteor impact contains a snapshot of the atmosphere from 200 million years ago.

A flaming asteroid entering a planet’s atmosphere

Approximately 200 million years ago, a meteorite collided with the area known as Rochechouart, France. The location of the impact has been drastically worn down by erosion, making the crater difficult to identify. However, the minerals that were left behind from the crash have managed to endure and provide a glimpse into the composition of Earth’s atmosphere during that era, as reported in the study published in Earth and Planetary Science Letters.

“That is an incredibly intelligent proposal.”

During an impact event, hot water may circulate briefly in the crust, creating a hydrothermal system that leaves behind minerals like quartz. In a recent study, researchers discovered that the fluids trapped within these hydrothermal minerals in the Rochechouart crater contain inert gases from the ancient atmosphere. These gases can provide a preserved glimpse of the atmosphere for millions of years, making impact craters a unique source of information about the physical changes that have shaped our planet.

Geochemist Sujoy Mukhopadhyay of the University of California, Davis, who was not part of the study, praised the idea of a potential new repository for ancient atmospheric samples as highly intelligent. He noted that the authors thoroughly demonstrated that the samples were not from modern air.

Stubbornly Stable Gases

Xenon and argon, both noble gases, are considered the elitists of the periodic table. These gases are colorless, tasteless, and odorless, and their stability makes them reluctant to engage in chemical reactions. In fact, they tend to be inactive chemically.

According to study author Guillaume Avice, a geochemist at the Institut de Physique du Globe de Paris, these entities are unaffected by biological activities, biogeochemical processes, and chemical reactions. They are completely impervious to these factors and only track physical processes.

This attribute is the main reason why scientists find noble gases intriguing when trying to comprehend the evolution of Earth over a long period of time. Natural phenomena like the transfer of material from the Earth’s interior to its surface, as well as the creation of land masses, release different gases into the atmosphere, each with their own abundance of isotopes. Due to their lack of reactivity, noble gases have a lasting and dependable record of these physical processes through their isotopic ratios.

Unfortunately, the issue is that there are not numerous trustworthy records of noble gases from the ancient atmosphere. While ice cores are valuable, they can only provide data dating back to about 1 million years.

Avice and his team are attempting to alter this situation. When Avice was a graduate student, he pondered if hydrothermal minerals found in impact craters could serve as the elusive record he had been seeking. It is known that fluid inclusions in hydrothermal minerals, like quartz, can retain ancient atmospheric gases.

“It’s not a completely clean signal,” Avice stated. “The fluid has been moving through the Earth’s crust for a long time, sometimes for millions of years. And naturally, it has been affected by the surrounding rocks.”

The properties of impact hydrothermal minerals are unique, according to Avice’s hopes. This type of mineral formation is caused by short-lived hydrothermal activity triggered by impact events, lasting less than a few million years. Avice refers to this as a small moment in the grand scheme of geologic time. Due to the limited time for circulation and interaction with crustal isotopes, the composition of the fluids should closely resemble that of the atmosphere at the time they were incorporated into the hydrothermal mineral.

Avice and his team examined hydrothermal quartz veins from the Rochechouart crater in search of the initial impact image. They pulverized the samples in a vacuum to collect noble gases and irradiated them with neutrons to generate new noble gas isotopes, useful for dating the material.

The minerals found in Rochechouart contained Argon with isotope ratios that could not have originated from present-day air. This suggests that the fluid inclusions were indeed able to preserve a snapshot of the atmosphere from 200 million years ago.

The samples provided strong evidence that the air was ancient due to the low ratio of heavier argon-40 to lighter argon-36. Argon-40 is produced through the decay of potassium-40, a radioactive element, over billions of years. Unlike modern air, ancient air has lower ratios of argon-40 to argon-36 because argon-40 released from the solid Earth has gradually accumulated in the atmosphere.

Measuring Ancient Atmospheres

According to Mukhopadhyay, the findings demonstrate the feasibility of a novel archive of noble gases in the atmosphere.

“We need to examine additional impact craters.”

Mukhopadhyay stated that the main importance lies in the possibility of utilizing these materials as storage units for preserving historical data of atmospheric noble gases. He also suggested exploring other impact craters for further research.

Avice suggested that the next course of action should involve testing the method on additional samples from a variety of craters. If the method proves to be dependable, it could provide insights into the evolution of other planets. Mukhopadhyay noted that many of the Mars rovers are currently located in impact craters, making them ideal locations for collecting samples.

Avice expressed his anticipation that gathering snapshots of the atmosphere from additional craters will aid in solving a nearby enigma: the longstanding inquiry into how Earth attained its unique nitrogen-based atmosphere.

According to Avice, although nitrogen is not a noble gas, it exhibits similar properties and is highly unreactive. If historical records of the atmosphere accurately reflect the presence of nitrogen, investigating more craters could provide further insight into the past of this prevalent and significant gas.

Currently, nitrogen makes up approximately 80% of the Earth’s atmosphere. This high level of atmospheric pressure has an impact on various aspects such as climate and cloud formation. Avice stated that if this was not the case in the past, there would be significant differences in these factors.

—Elise Cutts (@elisecutts), Science Writer

Reference: Cutts, E. (2023), Discovery of Ancient Atmosphere Preserved at Meteor Impact Site, Eos, 104, Published on October 19th, 2023.

Text © 2023. The authors. CC BY-NC-ND 3.0

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