50-year-old mistake by the Pioneer Venus probe reveals hidden water Earth's twin

Venus, our neighbor, has long teased astronomers by being the brightest planet in our night sky. That curiosity about what hides behind its twinkle prompted NASA scientists to send the Pioneer Venus Large Probe diving through the planet’s atmosphere in 1978.

Apart from snooping on the planet’s composition, the probe also set out to answer a far bigger question: are we alone in our solar system, or could life exist beyond our planet?

En route to the planet’s surface, the probe sampled the atmosphere at different altitudes. It analyzed aerosol and cloud particles, gathering data on temperature, pressure, and cloud chemistry. 

The mission’s data led scientists to our existing belief that Venus’s clouds were dominated by concentrated sulfuric acid and held only traces of water. 

Half a century later, Earth’s twin is revealing new secrets. A second look at the Pioneer Venus data, tucked away on microfilm in the NASA Space Science Data Coordinated Archive, involved Cal Poly Pomona, the University of Wisconsin–Madison, Arizona State University, and NASA itself. 

Reexamining this old data showed that the atmosphere was approximately 62% water trapped in hydrated compounds.

The water bound within these hydrated minerals reveals a hidden reservoir in the planet’s clouds that scientists have overlooked until now.

Chemistry finds water on Venus

Aboard NASA’s Pioneer Venus probe were two crucial scientific instruments: the Large Probe Neutral Mass Spectrometer and Large Probe Gas Chromatograph.

The sole mission of the duo was to measure the composition of Venus’s atmosphere as the probe descended in 1978. 

As the Venus probe plunged deeper into the hot atmosphere, the mass spectrometer detected H₂O⁺, SO₂⁺, O₂⁺, and FeO⁺.

The original scientists interpreted this as direct measurements of atmospheric gas composition at each altitude.

They concluded the aerosols were concentrated sulfuric acid solution (≥75% by mass) with relatively minor water (≤25% by mass). 

Beyond this dominant sulfuric acid-water mixture, they proposed several minor species, including ferric chloride and/or ferric sulfates, condensed sulfur, and tentative traces of bulk iron and phosphorus.

Unfortunately, as the probe plunged through the planet’s thick clouds, tiny aerosol particles clogged the instrument’s inlet. Decades later, researchers turned what seemed like a mishap into a lucky break.

Accidentally finding water on Venus

The trapped aerosols broke down under the extreme heat encountered by the probe as it descended deeper into Venus’s atmosphere.

Temperatures rose from about −30°C at an altitude of 65 km to around 462°C at the surface. During this process, the spectrometer recorded readings not just from around Venus but also from the chemical breakdown of the cloud material itself.

During their reanalysis, the team realized that the Venus probe had unintentionally performed a thermal and evolved gas analysis.

This is the same type of analysis that Mars rovers conduct to analyze soil samples on the Red Planet. 

So, instead of treating the data as direct atmospheric measurements, the new study treated the chemistry data as measurements of aerosols decomposing due to heat.

Not your average water

The water in Venus’s clouds isn’t in a liquid or vapor state familiar to us. It is mostly solid, not the icy kind, but as chemically bound hydrates.

It is mainly locked within the crystalline structure of hydrated iron sulfates, hydrated magnesium sulfates, and other hydrated mineral species. 

Analysis of the Pioneer Venus data showed that the original altitude measurements were actually temperature-dependent breakdown profiles of aerosol material clogging the instrument. 

The researchers plotted the released gases during the descent against the rising temperature to create a thermal decomposition profile. 

The most significant water release peaked at about 414°C, which was unusually high for liquid or vapor. The profile, however, matched the breakdown profile of thermally stable hydrated compounds. 

The study also indicates an aerosol solution phase, suggesting that some water dissolves in sulfuric acid within the aerosols.

Even so, the dominant form remains water-bearing compounds that release water only when heated to decomposition temperatures.

The finding challenges the long-standing belief that Venus is almost completely dry. 

New chemistry explains anomalies

The original mission data contained puzzling anomalies that scientists couldn’t explain: sudden pressure surges, strange chemical spikes, and unusually high levels of water and sulfur dioxide below the cloud deck. The team at the time wrote them off as contamination from Earth. 

The new study showed that these anomalies weren’t errors at all. The evidence showed aerosol particles breaking down inside the instruments, revealing their actual chemical makeup.

This reevaluation led researchers to identify aerosols’ composition as 62% water, 22% sulfuric acid, and 16% ferric sulfate.

The researchers suggest that the iron detected in Venus’s cloud aerosols could originate from cosmic sources.

The lead author, R. Mogul, told CPP NEWS that this newly identified aerosol composition will require scientists to reassess their models of cloud chemistry and rethink whether the planet could support life.

The findings were published in the Journal of Geophysical Research: Planets. 

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