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    Tiny meteorites shed light on Earth’s early atmosphere

    These tiny meteorites, about half a millimeter across, fell into the ocean and were collected from the deep sea. Like the samples used in the new study, these more recent micrometeorites are made of iron. In the simulation video, the micrometeorite's color reflects its temperature and becomes white-hot when iron melts and reacts with atmospheric gases. The micrometeorite melts for only a few seconds above the Earth’s surface. The micrometeorite is oxidized by oxygen in the modern atmosphere, but can be oxidized by carbon dioxide in the oxygen-free atmosphere of the early Earth. The percentage of iron in the micrometeorite that becomes oxidized is shown as "Iron Oxidation %." (Image credit: Donald Brownlee/U. Washington)

    Very old samples of small meteorites show that they could have reacted with carbon dioxide on their journey to Earth, researchers report.

    Previous work suggested the meteorites ran into oxygen, contradicting theories and evidence that the Earth’s early atmosphere was virtually devoid of oxygen.

    “Our finding that the atmosphere these micrometeorites encountered was high in carbon dioxide is consistent with what the atmosphere was thought to look like on the early Earth,” says first author Owen Lehmer, a doctoral student in Earth and space sciences at the University of Washington.

    At 2.7 billion years old, these are the oldest known micrometeorites. They were collected in limestone in the Pilbara region of Western Australia and fell during the Archean eon, when the sun was weaker than today. A 2016 paper by the team that discovered the samples suggested they showed evidence of atmospheric oxygen at the time they fell to Earth.

    That interpretation would contradict current understandings of our planet’s early days, which is that oxygen rose during the “Great Oxidation Event,” almost half a billion years later.

    Knowing the conditions on the early Earth is important not just for understanding the history of our planet and the conditions when life emerged. It can also help inform the search for life on other planets.

    Life formed more than 3.8 billion years ago, and how life formed is a big, open question. One of the most important aspects is what the atmosphere was made up of—what was available and what the climate was like,” Lehmer says.

    The new study takes a fresh look at interpreting how these micrometeorites interacted with the atmosphere, 2.7 billion years ago. The sand-sized grains hurtled toward Earth at up to 20 kilometers (12.4 miles) per second. For an atmosphere of similar thickness to today, the metal beads would melt at about 80 kilometers (49.7 miles) elevation, and the molten outer layer of iron would then oxidize when exposed to the atmosphere.

    A few seconds later the micrometeorites would harden again for the rest of their fall. The samples would then remain intact, especially when protected under layers of sedimentary limestone rock.

    The previous paper interpreted the oxidization on the surface as a sign that the molten iron had encountered molecular oxygen. The new study uses modeling to ask whether carbon dioxide could have provided the oxygen to produce the same result. A computer simulation finds that an atmosphere made up of from 6% to more than 70% carbon dioxide could have produced the effect seen in the samples.

    “The amount of oxidation in the ancient micrometeorites suggests that the early atmosphere was very rich in carbon dioxide,” says coauthor David Catling, a professor of Earth and space sciences.

    For comparison, carbon dioxide concentrations today are rising and are currently at about 415 parts per million, or 0.0415% of the atmosphere’s composition.

    High levels of carbon dioxide, a heat-trapping greenhouse gas, would counteract the sun’s weaker output during the Archean era. Knowing the exact concentration of carbon dioxide in the atmosphere could help pinpoint air temperature and acidity of the oceans during that time.

    More of the ancient micrometeorite samples could help narrow the range of possible carbon dioxide concentrations, the authors write. Grains that fell at other times could also help trace the history of Earth’s atmosphere through time.

    “Because these iron-rich micrometeorites can oxidize when they are exposed to carbon dioxide or oxygen, and given that these tiny grains presumably are preserved throughout Earth’s history, they could provide a very interesting proxy for the history of atmospheric composition,” Lehmer says.

    The study appears in Science Advances.

    Additional coauthors are from the University of Washington and Rutgers University. NASA, the University of Washington Astrobiology Program, the the University of Washington Virtual Planetary Laboratory, and the Simons Foundation’s Collaboration on the Origins of Life funded the research.

    Source: University of Washington

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