Fractal Haze Could Solve Weak-Sun Mystery for Early Earth
A thick haze of organic material let the early Earth soak up the sun’s warmth without absorbing harmful ultraviolet rays, according to a new study.
The model offers a new twist on an old puzzle: Although the sun was so dim billions of years ago that the Earth should have been a ball of ice, the young planet had liquid oceans capable of supporting life.
“Given these recent papers, we can probably say the early faint sun problem is not one of the problems anymore in solving the origin of life,” said astrophysicist Christopher Chyba of Princeton University, who was not involved in the new work.
The sun should have been up to 30 percent less bright 3.8 billion to 2.5 billion years ago, according to studies of the lifecycles of sun-like stars. If the Earth’s atmosphere had the same composition then as it does now, it would have frozen over completely, like Jupiter’s moon Europa. But geological records show the Earth was at least as warm and wet then as it is today.
Scientists have struggled with this “faint young sun paradox” since 1972, when astronomers Carl Sagan and George Mullen suggested that an atmosphere containing a small amount of ammonia, a powerful greenhouse gas, could have warmed the Earth enough to keep the oceans liquid. But a later study showed that ultraviolet radiation from the sun would destroy the ammonia in the atmosphere and cancel out its warming effects.
Sagan countered in 1996 that the early atmosphere would have produced a thick cloud of organic haze, much like the orange cloud that enshrouds Saturn’s moon Titan. This haze would have blocked ultraviolet light but let in visible light, letting the Earth tan without getting burnt.
But early models assumed the haze particles were spheres, and that when individual particles collided, they globbed together to make bigger spheres. These spheres blocked visible light as well as ultraviolet light, and left the Earth’s surface even colder.
“It basically led us to a dead end where we couldn’t have a warm early Earth,” said Eric Wolf, a graduate student in atmospheric sciences at the University of Colorado at Boulder and the first author of the new study in Science June 4.
Wolf and coauthor Brian Toon realized that assuming the haze particles were spherical was too simple. Instead of combining to make bigger spheres, tiny haze particles no more than 100 nanometers across could form long chains, like strings of pearls. These chains would link up and branch off each other in a complicated fractal geometry, similar to the structure of clouds.
These strands of haze would form fluffy, airy structures that would let in visible light while blocking ultraviolet light, Wolf said.
“If you take into account the shape factor,” he said, “it turns out that the haze would be quite a strong ultraviolet shield while being relatively transparent in the visible. Visible light can reach through the haze and reach the surface.”
Without the destructive ultraviolet light, ammonia could build up under the haze and warm the Earth efficiently, Wolf said. Only a few parts per million of ammonia would be enough to offset the faint young sun.
But if early organisms could have looked up, they wouldn’t have seen a clear blue sky. The sky would be dim and rust-colored, like Titan’s.
“We’re really dealing with this completely alien world on the early Earth,” Wolf said.
Wolf’s study comes shortly after an April 1 paper in Nature that proposed another solution to the faint young sun paradox: The early Earth was darker, and therefore absorbed more heat. Both explanations could be right, Chyba said.
“It seems likely that the answer is going to be a composite explanation,” he said. “You cobble together a number of factors and you solve the paradox that way.”
The next step should be looking at ancient rocks to determine what the early Earth’s atmosphere was really made of, Chyba added. “That’s going to be really hard, because those rocks are really worked over. But that’s probably where the field is heading now.”