You ever wondered what Earth’s atmosphere was like 4.5 billion years ago?
It’s a tough one to answer, because back in the old days Earth was more like a swirling collection of rocks and debris barely holding it together.
By the time life was bountiful on Earth, it made oxygen the foremost gas in the atmosphere and changed the way the earth would foster life for billions of years.
And yet, every new exoplanet discovery holds a new urgency of discovering the answer to Earth’s past. A new generation of telescopes will soon bring blurred outlines of Earthlike planets into focus, as they learn about atmospheres on planets bigger and hotter than our own.
Scientists must be able to compare the gaseous silhouettes with the worlds’ skeletons: their rocks and minerals, in order to find out what makes the gaseous worlds up, whether they have water, and perhaps even if they are capable of supporting life.
The weather, life, and plate tectonics of Earth have recycled the elements that first formed our world over billions of years. But the remnants of Venus, Mars, and Earth that escaped their chaotic production were able to survive.
A team of researchers have now experimented with pulverizing, roasting, and examining a bunch of nuggets from these primitive materials to catch a whiff of noxious gases that cannot be collected any other way.
The only way to get an understanding of the rocky planet atmospheres' makeup from the early Earth or other planets in the early solar system is actually to study the samples directly, says astrophysicist Maggie Thompson at the University of California at Santa Cruz.
Building An Atmosphere from Scratch
A 5kg meteorite rock which landed in a former Khmer Rouge zone, in northwest Cambodia. (Image for representation)
Saturn and Jupiter have a continuous atmosphere because of their mass and gravitational pull. They can just grab whatever’s hanging around forever—mostly hydrogen and helium.
Terrestrial planets have rocky interiors and can make their own air.
This planet forms from whirling clouds of mega-asteroids, which range from tens to hundreds of kilometers wide and heat up to thousands of degrees Celsius. The planet's starter atmosphere is formed by boiling away frozen elements in the rock.
Scientists have understood for a long time that asteroids are copies of the stuff which formed the planet, and thus the key to understanding it's formation. We can’t reach these asteroids because they are too far away, so instead they investigate remnants of asteroids that have fallen to Earth: meteorites.
“They really are like leftover Legos of planets in our solar system,” Thompson says. “It’s really lucky that they come to us.”
Meteorite composition data of asteroid material varieties compared to Earth’s crust.
At first, models were used to determine the makeup of the atmosphere of the young Earth. Now a Stanford University planetary scientist, Laura Schaefer led one such effort in 2010.
In previous years she had already picked out the minerals and elements that were thought to be associated with the formation of planets in the main class of meteorites. Based on those findings, she used chemistry to predict what gases these materials would release as the materials were heated.
She concluded that the early atmosphere would have been dominated by water and carbon dioxide, with traces of less anticipated greenhouse gases such as sulfur and carbon. It was a plausible conclusion, but it also relied on ideal conditions and the use of pure materials.
After several years, Thompson and her advisor Myriam Telus got in touch with Schaefer. To make Stanford’s simulations real, they would bake real meteorite fragments to see what gases leaked.
There have been studies where meteorites have been heated prior to, but the questions behind them are typically different, which require different experimental techniques. There had never been a one off experiment involving space rocks to make a primitive atmosphere for Earth, and Schaefer was in.
“It’s every modeler’s dream that somebody will come along and test their models with real data,” she says.
Baking A Meteorite: A Recipe
A Mass spectrometer connected to a furnace and vacuum system was used for the meteorite experiment
One meteorite that had recently fallen to Earth and crashed through the roof of a dog house in Costa Rica a few years ago yielded just a few raisins’ worth of material, thanks to Telus' connections in the meteorite community.
Thompson ground and exposed the powdered meteorites to conditions assumed to have existed 4.5 billion years ago if they had contributed to the formation of the Earth. In a near vacuum, she heated the samples to nearly 1,200 degrees Celsius at pressures a hundred million times lower than at sea level. Then, using a sensitive instrument meant to look for traces of contamination, she sniffed the gas.
Hydrogen and other gases, including sulfide and hydrogen, accompanied by smaller amounts of water vapor and carbon monoxide comprised the protoatmosphere released by the burning meteorite powder.
Schaefer's predictions had largely come true, but he also predicted that sulfuric gases would increase. “That’s something we’re still working out,” says Schaefer, who helped analyze the results. The researchers announced their results last Thursday in Nature Astronomy.
There is about 80 percent of their meteorite material left, so the Santa Cruz group plans to continue baking it. In the future they intend to fine-tune the gas sensor so that it can sniff out much more rare gases (it can only gather more than ten compounds at once). To assess what type of atmosphere meteorites produce, they’d also like to switch up ingredients and cook with different kinds of meteorites.
The Future of Meteorite Based Baking
In the coming decade, the James Webb Space Telescope and the next generation of ground-based telescopes will be coming online, and the team’s home-baked atmosphere will help astronomers better understand what they are seeing.
Researchers will then compare the observations with atmospheric predictions to see which blend fits the best as the telescopes return raw data about the light that makes it to Earth from rocky planets (like those in the TRAPPIST solar system).
Thompson’s atmosphere will be one of them. If it matches these, it will tell us how much water and carbon dioxide exists in the atmosphere, and whether that solar system was formed from the same type of asteroids.
The study also lays the groundwork for interpreting biosignatures, or evidence of life.
Organisms on Earth radically altered our atmosphere. Now that researchers have a better handle on how the atmospheres of rocky planets might start out, they’ll be more prepared to recognize an atmosphere that life has tampered with.
“To understand biosignatures, we have to get a handle on what’s the natural range [of atmospheres that] rocky planets will make without life,” Schaefer says.
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