Venus, often called Earth's twin, is in fact a planet of extremes. Beneath its thick carbon dioxide atmosphere lie crushing surface temperatures and dense clouds of sulfuric acid. While the planet's main cloud layer sits between 47 and 70 kilometers above the surface, scientists have long been puzzled by a mysterious layer of particles below 47 kilometers, known as the "lower haze." First detected by spacecraft in the 1970s, the origin of this haze remained unexplained for more than half a century.

Now, a research team led by Hiroki Karyu, Takeshi Kuroda, and Naoki Terada of Tohoku University, in collaboration with the Royal Belgian Institute for Space Aeronomy, has finally solved the mystery. Using a state-of-the-art microphysical model, the team showed that the lower haze is formed from cosmic dust - tiny particles left behind by "shooting stars" that constantly rain down on Venus.

"When we traced the life cycle of these particles in our simulations, everything suddenly fit together," said Karyu. "Cosmic dust, which might seem insignificant, turns out to be the missing ingredient needed to explain Venus's lower haze."

According to the study, incoming cosmic dust burns up high in the atmosphere, producing nanometer-sized mineral particles (Fig. 1). These particles become embedded within Venus's sulfuric acid clouds. As they drift downward into the hotter lower atmosphere, the sulfuric acid evaporates, leaving behind solid mineral cores. These cores then collide and stick together, forming the haze layer observed by past missions such as Venera and Pioneer Venus. The model's results closely match measurements collected decades ago, lending strong support to the team's conclusions.

The researchers also found that these cosmic particles play an important role in Venus's climate. Acting as "seeds" for cloud formation, they increase cloud production by an estimated 20-30% (Fig. 2). In addition, the team suggests that metallic elements within the dust, such as iron, may be responsible for the long-mysterious "unknown UV absorber" in Venus's atmosphere, a substance that strongly absorbs sunlight and affects the planet's energy balance.

"These findings show that material from space is not just a passive visitor," said Terada. "It can actively shape a planet's atmosphere and climate."

The study reshapes how scientists think about planetary atmospheres, suggesting that similar processes may occur on gas giants like Jupiter and Saturn, as well as on distant exoplanets. The team hopes to test their predictions with future missions, including NASA's DAVINCI mission to Venus, scheduled for launch in the late 2020s.