A reanalysis of data collected by NASA’s Pioneer Venus mission in 1978 has revealed that aerosols in one region of Venus’s clouds may have contained water equal to 8% of their mass. Most of this water was neither liquid nor free, but chemically bound within hydrated iron and magnesium salts, a finding that could prompt a reassessment of the composition of cloud particles and how they form and interact with ultraviolet radiation.
The prevailing model of Venus’s cloud composition
The finding adds a more complex picture to the model that has prevailed for about half a century, which holds that the dense clouds surrounding Venus consist mainly of concentrated droplets of sulphuric acid suspended in a carbon dioxide-rich atmosphere.
The study does not rule out the presence of the acid, but raises the possibility that hydrated salts and mineral compounds contribute to the composition of some particles. The multi-probe Pioneer Venus mission was launched in 1978 and comprised one large probe and three smaller probes. The four probes entered Venus’s atmosphere on 9 December of that year.
The mission was not designed to collect and study cloud particles directly, but the large probe was equipped with instruments for analysing gases during its descent. As the probe passed through the middle and lower cloud layers, fine particles entered the sampling systems, temporarily blocking one of the inlets between altitudes of 51 and 48 kilometres.
Revisiting measurements from the Pioneer Venus mission
As the probe continued its descent, material suspended inside the system was heated, later allowing the recorded measurements to be treated as an unplanned thermal experiment. Researcher Rakesh Mugal and colleagues reanalysed the data in a peer-reviewed study published in 2025.
The team detected signals indicating the release of water at multiple temperatures, including about 35, 80, 118, 185, 242, 276 and 414 degrees Celsius. This suggests that the water was not present in a single form within the sample that entered the probe’s instruments. The water signals coincided with indicators associated with sulphur dioxide, oxygen and sulphur trioxide, as well as mineral compounds.
Most of it may instead have been held within the chemical structure of hydrated salts containing water molecules, releasing them as vapour when heated, as happens with some hydrated minerals on Earth. This interpretation does not necessarily undermine the traditional model based on the presence of sulphuric acid in Venus’s clouds.
Measurements of optical properties, atmospheric chemistry and spectroscopic observations, along with data from multiple space missions, remain consistent with its presence. The reanalysis itself also attributed 4% of the proposed sample’s total composition to it. The new element is the possible presence of significant quantities of iron sulphates and other hydrated compounds, which may include magnesium sulphate.
Measurement limitations and questions for future research
But interpreting the results faces a fundamental limitation: the probe did not collect and preserve the particles in their original state. The material passed through heated tubes and was subjected to pressure, heat, evaporation and chemical reactions during measurement. Under these conditions, some hydrated salts may have formed after the particles entered the sampling instruments.
The data therefore do not establish that similar crystals were present in the same form while suspended in the clouds. The study is also based on a single descent path and provides no picture of how the composition changes with altitude or location in Venus’s atmosphere. The findings are not evidence of life on Venus or that its clouds are habitable.
But they raise questions about the source of the iron, magnesium and chemically bound water in the clouds, as well as the nature of the mysterious material that absorbs ultraviolet radiation there. Resolving these questions will require a future probe to collect particles directly and analyse the water, acids, salts and minerals before heat or chemical reactions alter them.
