Eddie Gonzales Jr. – MessageToEagle.com – Most of the carbon on Earth was likely delivered from the interstellar medium, the material that exists in space between stars in a galaxy, according to a new study led by Jie (Jackie) Li, a professor in the U-M Department of Earth and Environmental Sciences.
Carbon is an essential element for life, but its behavior during Earth’s accretion is not well understood. However, it is known that a planet’s carbon must exist in the right proportion to support life as we know it.
Too much carbon and the Earth’s atmosphere would be like Venus, trapping heat from the sun and maintaining a temperature of about 880 degrees Fahrenheit. Too little carbon and Earth would resemble Mars: an inhospitable place unable to support water-based life, with temperatures around minus 60.
Previously, researchers believed carbon in the Earth came from molecules that were initially present in nebular gas, which then accreted into a rocky planet when the gases were cool enough for the molecules to precipitate.
In this study, Li and her team point out that the gas molecules that carry carbon wouldn’t 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 assumes that during the formation of the sun, all of the planet’s elements got vaporized, and as the disk cooled, some of these gases condensed and supplied chemical ingredients to solid bodies. But that doesn’t work for carbon,” Li sain in a press release.
Much of carbon was delivered to the disk in the form of organic molecules. However, when carbon is vaporized, it produces much more volatile species that require very low temperatures to form solids. More importantly, carbon does not condense back again into an organic form. Because of this, Li and her team inferred most of Earth’s carbon was likely inherited directly from the interstellar medium, avoiding vaporization entirely.
To better understand how Earth acquired its carbon, the team compared how quickly a seismic wave travels through the core to the known sound velocities of the core. This told the researchers that carbon likely makes up less than half a percent of Earth’s mass.
“We asked a different question: We asked how much carbon could you stuff in the Earth’s core and still be consistent with all the constraints,” U-M astronomer Edwin Bergin said.
“There’s uncertainty here. Let’s embrace the uncertainty to ask what are the true upper bounds for how much carbon is very deep in the Earth, and that will tell us the true landscape we’re within.”
In a second study, researchers examined the metallic cores of these bodies, now preserved as iron meteorites, and found that during this key step of planetary origin, much of the carbon must be lost as the planetesimals melt, form cores and lose gas.
“Most models have the carbon and other life-essential materials such as water and nitrogen going from the nebula into primitive rocky bodies, and these are then delivered to growing planets such as Earth or Mars,” said Marc Hirschmann of the University of Minnesota., professor of earth and environmental sciences.
“But this skips a key step, in which the planetesimals lose much of their carbon before they accrete to the planets.”
The two studies both describe two different aspects of carbon loss—and suggest that carbon loss appears to be a central aspect in constructing the 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 like astronomy and geochemistry,” said Ciesla, a U. of C. professor of geophysical sciences.
“While approaches and the specific questions that researchers work to answer differ across the 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 stimulating and rewarding.”
Written by Eddie Gonzales Jr. – MessageToEagle.com Staff