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Venus has long been known for its mysterious yellowish lower haze, which was first seen by the Venera and Pioneer Venus probes in the 1970s. For decades, scientists could not explain this layer. Planetary scientist Hiroki Karyu and his team at Tohoku University in Sendai City, Japan, recently determined that the haze is made of cosmic dust from meteorites. When space rocks enter Venus’s atmosphere, friction causes them to burn up, leaving behind particles. Sulfuric acid then interacts with these particles to form the haze. Karyu stated in a study published in Nature Astronomy that this continuous influx of cosmic dust sustains the observed haze layer.

Previous theories suggested volcanic ash or surface dust were the source, but researchers ruled both out. Even if large amounts of volcanic or surface dust existed, they would not interact with atmospheric sulfur correctly to form the haze. Instead, the process works like cloud formation on Earth, where tiny aerosol particles act as condensation nuclei. On Venus, droplets of sulfuric acid condense around meteorite particles left hovering in the atmosphere. However, more particles do not create more droplets; instead, several particles stick together to form larger clusters before acid condenses around them.

This discovery also solved another mystery regarding what absorbs ultraviolet rays in Venus’s atmosphere. While iron compounds were previously suspected, magnesium and silicon found in meteorites are poor absorbers. Iron sulfate, detected by earlier probes, matched the haze properties perfectly. At certain heights between 40 and 50 kilometers above the surface, a high-energy barrier prevents sulfuric acid from sticking to particles. Hotter air rises and carries these particles to upper cloud layers where they cool enough to form droplets. Karyu noted that observing metal layers in outer planets could help determine deposition rates of metals within their atmospheres and resulting haze abundances.