From Interstellar Dust to ‘faint blue Dot’ : How did Carbon come to Earth

“Pale Blue Dot,” a famous image of Earth taken by Voyager 1, shows earth hovering against a dark background in the solar system. One theory is that we are all made of interstellar dust. Two recent studies have found that this may be closer to the truth than previously thought.
The first study was led by Jie (Jackie) Li, a professor in the Department of Earth and Environmental Sciences at the University of Michigan. The study found that most of the carbon on Earth probably came from interstellar material, and that this process likely occurred long after the protoplanetary disk formed and heated up. Interstellar material refers to the space material and radiation that exists between star systems in galaxies, while protoplanetary disks are clouds of dust and gas that orbit the early sun and contain the basic building blocks of planets.
Carbon could also have been locked away in a solid for a million years after the sun’s birth, meaning it took a long interstellar journey to finally reach Earth and become a major component of life there.
Previously, researchers thought carbon on Earth came from molecules originally present in nebula gas; When the gas cools enough for the molecules to fall, they condense into a rocky planet. In the study, Li’s team pointed out that the carbon-bearing gas molecules in the nebula would not have been used to build the Earth, because carbon does not condense back into a solid after evaporating.
Professor Li Jie said: “Coagulation models have been widely used for decades. The model postulates that during the sun’s formation, all the elements on Earth evaporated, and as the protoplanetary disk cooled, some of the gas began to condense and provide chemistry for the solid parts of the planet. But that’s not the case with carbon.”
Most of the carbon is transported to the protoplanetary disk as organic molecules. When carbon evaporates, however, it produces more volatile materials that require very low temperatures to form a solid. What’s more, the carbon doesn’t condense back into organic form. As a result, Li’s team reasoned, most of the carbon on Earth probably came directly from interstellar material and did not all undergo evaporation.
To better understand how the earth gets its carbon, Li estimates the maximum amount of carbon the earth can contain. To do this, she compared the speed of seismic waves passing through the core with the known speed of sound in the core. The researchers found that carbon probably makes up less than half a percent of the earth’s mass. Knowing the upper limit on earth’s carbon content helps to understand when carbon is likely to reach the planet.
“We asked a different question: how much carbon can you put in the core of the Earth and still meet all the constraints,” said Edwin Bergin, professor and chair of the Astronomy department at the University of Michigan. “There is uncertainty. We need to embrace that uncertainty and understand what the true upper limit for carbon is deep in the Earth, and that will tell us the truth about our environment.”
A planet must contain the right proportion of carbon to support life as we know it. If the carbon content is too high, the Earth’s atmosphere, like Venus, absorbs heat from the sun and keeps it hot at about 470 degrees Celsius. If the carbon content is too low, Earth, like Mars, would be an inhospitable place, unable to support water-based life, with an average temperature of around -55 ° C.
In another study, led by Mark Hirschman, a professor of Earth and environmental sciences at the University of Minnesota, researchers analyzed how tiny precursors of planets, known as planetesimal, process carbon early in its formation to preserve it. By examining the metal cores of these microplanets, now in the form of iron meteorites, they found that most of the carbon was lost as the microplanets melted, formed their cores and lost their gas during this crucial step in planetary origin. Hirschman points out that this overturns previous ideas.
“Most of the models agree that carbon and other essential substances, such as water and nitrogen, etc., from micro planetary nebula into the original rocky body, and then be transported to a growing planet, such as earth or Mars,” Hector seaman said. “but these models over a key steps, namely before micro planetary accretion to the planet, will lose most of the carbon.”
“The earth needs carbon to regulate the climate and allow life to survive, but it’s a very delicate thing,” Edwin Bergin said. “You can’t have too little carbon, but you can’t have too much.” He added that the two studies describe two different aspects of carbon loss and show that carbon loss appears to play a key role in making Earth a habitable planet.
“The question of whether earth-like planets exist elsewhere in the universe can only be answered through the intersection of astronomy and geochemistry.” “While researchers in different fields vary in the progress they are making and the specific problems they are addressing, building a coherent story requires finding topics of common interest and finding ways to bridge gaps in knowledge,” said Fred Sisla, a professor of geophysical sciences at the University of California. It’s challenging, but the effort is both exciting and rewarding.”
Jeffrey Black is a co-author of both studies and a professor of cosmic chemistry, planetary science and chemistry at Caltech. Such interdisciplinary work is crucial, he says. “In the history of the Milky Way alone, rocky planets like Earth or larger have coalesced hundreds of millions of times around stars like the Sun,” he said. “Can we extend this work to a broader analysis of carbon loss in planetary systems? Such research will require a more diverse group of scholars.”