For about half a century, scientists believed that the thick clouds enveloping Venus were made up mainly of concentrated droplets of sulfuric acid suspended in an atmosphere rich in carbon dioxide.
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But a reanalysis of data collected by NASA’s Pioneer Venus mission in 1978 suggests a more complex picture, indicating that the aerosol in one cloud region may have contained up to 8% water by mass, mostly chemically bound within hydrated iron and magnesium salts, rather than in the form of liquid water.
The result is significant because it may prompt scientists to reconsider the composition of Venus's cloud particles and how they form and interact with ultraviolet light.
As it passed through the middle and lower cloud layers, fine particles entered the sampling systems, causing a temporary blockage at one of their inlets between altitudes of 51 and 48 kilometers. The trapped material then heated up as the probe continued its descent.
What did scientists find in the old data?
In a peer-reviewed study published in 2025, researcher Rakesh Mughal and his colleagues reinterpreted this data as an unintended thermal experiment, as water signals appeared at various temperatures, including around 35, 80, 118, 185, 242, 276, and 414 degrees Celsius.
Simultaneously, signals associated with sulfur dioxide, oxygen, sulfur trioxide, and metallic compounds were detected. According to chemical reconstruction, the estimated composition of the sample is 8% water, 3% ferric sulfate, and 4% sulfuric acid.
However, the first figure requires careful interpretation. Most of the water detected in the measurements was not free-moving water within a solution; the proportion of water in the liquid or mobile phase was estimated at only about 0.1%. The majority, according to the proposed interpretation, was trapped within the structure of hydrated salts, compounds that contain water molecules in their chemical composition. When heated, these molecules are released as vapor, as occurs in some hydrated earth minerals.
This is not the end of the sulfuric acid theory
The study does not imply that the traditional model of Venus' clouds is wrong. Measurements of optical properties, atmospheric chemistry, spectroscopy, and data from various space missions have remained consistent with the presence of sulfuric acid in the clouds. In fact, the reanalysis itself gives sulfuric acid a 4% estimate of the total proposed composition.
What's new is the potential presence of significant quantities of iron sulfate and other hydrated compounds, possibly magnesium sulfate. However, a fundamental problem remains: the probe did not collect and preserve the particles in their original state; instead, the materials were subjected to pressure, heat, vaporization, and chemical reactions.
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