Carbon’s interstellar journey to Earth


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We’re made of stardust, the saying goes, and a couple of studies, including research from the University of Michigan, find that it may be more true than we previously thought.

The first study, led by UM researcher Jie (Jackie) Li and published in Progress of science, finds that most of the carbon on Earth was likely delivered from the interstellar medium, the material that exists in the space between stars in a galaxy. This likely happened long after the protoplanetary disk, the cloud of dust and gas that surrounded our young sun and contained the planets’ building blocks, formed and heated up.

Carbon was also likely sequestered into solids within a million years after the sun was born, meaning that carbon, the backbone of life on earth, survived an interstellar journey to our planet.

Previously, researchers thought that carbon on Earth came from molecules that were initially present in nebular gas, which later accumulated on a rocky planet when the gases were cool enough for the molecules to rush. Li and his team, which includes UM astronomer Edwin Bergin, Geoffrey Blake of the California Institute of Technology, Fred Ciesla of the University of Chicago, and Marc Hirschmann of the University of Minnesota, point out in this study that gas molecules that they carry carbon will not be available to build the Earth because once carbon vaporizes, it does not condense back into a solid.

“The condensation model has been widely used for decades. It is assumed that during the formation of the sun, all the elements of the planet vaporized and, as the disk cooled, some of these gases condensed and supplied chemical ingredients to the solid bodies. But that doesn’t work for carbon, “said Li, a professor in the UM Department of Earth and Environmental Sciences.

Much of the carbon was delivered to the disk in the form of organic molecules. However, when carbon vaporizes, it produces much more volatile species that require very low temperatures to form solids. More importantly, the carbon does not condense back into an organic form. Because of this, Li and his team inferred that most of Earth’s carbon was likely inherited directly from the interstellar medium, avoiding vaporization entirely.

To better understand how the Earth acquired its carbon, Li estimated the maximum amount of carbon that the Earth could contain. To do this, he compared the speed at which a seismic wave travels through the core with the known speeds of sound of the core. This told the researchers that carbon probably makes up less than half a percent of the Earth’s mass. Understanding the upper limits of how much carbon the Earth could hold gives researchers information about when the carbon might have been delivered here.

“We asked a different question: We asked how much carbon could be stored in the Earth’s core and still be consistent with all the constraints,” said Bergin, professor and chair of the UM Department of Astronomy. “There is uncertainty here. Let’s accept the uncertainty to ask ourselves what are the true upper limits of the amount of carbon that is in the depths of the Earth, and that will tell us the true landscape that we are in.”

The carbon on a planet must exist in the right proportion to support life as we know it. Too much carbon and Earth’s atmosphere would be like Venus, trapping the sun’s heat and maintaining a temperature of about 880 degrees Fahrenheit. Too little carbon, and Earth would look like Mars: an inhospitable place unable to support water-based life, with temperatures around -60 degrees.

In a second study by the same group of authors, but led by Hirschmann of the University of Minnesota, researchers looked at how carbon is processed when tiny precursors to planets, known as planetesimals, retain carbon during their initial formation. Examining the metal cores of these bodies, now preserved as iron meteorites, they found that during this key step of planetary origin, much of the carbon must be lost as the planetesimals melt, form nuclei, and lose gas. This changes the previous thinking, says Hirschmann.

“Most of the models have carbon and other materials essential for life, such as water and nitrogen, that go from the nebula to primitive rocky bodies, and then they are sent to growing planets like Earth or Mars,” he said Hirschmann, professor of environmental and earth sciences. . “But this misses a key step, in which planetesimals lose much of their carbon before it accumulates on planets.”

Hirschmann’s study was recently published in procedures of the National Academy of Sciences.

“The planet needs carbon to regulate its climate and allow life to exist, but it is a very delicate thing,” Bergin said. “You don’t want to have too little, but you don’t want to have too much.”

Bergin says the two studies describe two different aspects of carbon loss and suggest that carbon loss appears to be a central aspect in building Earth as a habitable planet.

“Answering whether or not Earth-like planets exist elsewhere can only be achieved by working at the intersection of disciplines such as astronomy and geochemistry,” said Ciesla, professor of geophysical sciences at the U. of C. Approaches and the specific questions researchers work to answer differ across fields, building a coherent story requires identifying topics of mutual interest and finding ways to bridge the intellectual gaps between them. Doing so is challenging, but the effort is both challenging and rewarding. . “

Blake, a co-author of both studies and a Caltech professor of cosmochemistry, planetary science and chemistry, says this kind of interdisciplinary work is critical.

“During the history of our galaxy alone, rocky planets like Earth or slightly larger have assembled hundreds of millions of times around stars like the Sun,” he said. “Can we extend this work to examine carbon loss in planetary systems more broadly? Such research will require a diverse community of scholars.”

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More information:
“Earth’s carbon deficit caused by early loss by irreversible sublimation” Progress of science (2021).… .1126 / sciadv.abd3632

Provided by the University of Michigan

Citation: From Stardust to Pale Blue Dot: Carbon’s Interstellar Journey to Earth (April 2, 2021) Retrieved April 3, 2021 from -blue-dot-carbon.html

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