Venus’s clouds are 60% water, according to reanalyzed Pioneer data

Reanalyzing old data with our modern understanding seems to be in vogue lately. However, the implications of that reanalysis are more impactful for some topics than for others.

One of the most hotly debated topics of late in the astrobiological community has been whether or not life can exist on Venus—specifically in its cloud layers, some of which have some of the most Earth-like conditions anywhere in the solar system, at least in terms of pressure and temperature.

A new paper from a team of American researchers has just added fuel to that debate by reanalyzing data from the Pioneer mission to Venus NASA launched in the 1970s—and finding that Venus’s clouds are primarily made out of water.

The work is published in the Journal of Geophysical Research: Planets.

That doesn’t mean that it’s water in the traditional sense of how we think water vapor makes up clouds here on Earth. The dihydrogen monoxide in Venus’s clouds seems to be tied up in hydrated materials rather than stand alone as pure water droplets. But that is still a drastic change from our current understanding that Venus’s clouds are made up primarily of sulfuric acid. There is still some of that floating around—22% of the cloud material according to the paper—but how could the scientists of the ’70s be so far off the mark in terms of the readings of their instruments?

To answer that required some scientific sleuthing from a series of researchers at various institutions, including Cal Poly Pomona, the University of Wisconsin, Arizona State, and even NASA itself, to uncover the old Pioneer data. It had been stored on microfilm in NASA’s Space Science Data Coordinated Archive office—so the first step in reanalyzing the data was to fish it from the archives and digitize it.

Inspiration for the idea came from a conversation between Rakesh Mogul of Cal Tech Pomona and Sanjay Limaye, a Venus expert of the University of Wisconsin, who were talking about the composition of Venus’s clouds and then agreed they should reanalyze the mass spectrometry data Pioneer originally collected, as they thought there might be some new insights to glean there.

There were. The data came from two instruments on board the Pioneer Venus Large Probe—part of the Pioneer mission that descended through Venus’s clouds—the neutral mass spectrometer (LNMS) and the gas chromatograph (LGC).

Drs. Mogul and Limaye realized that as the probe descended through the thicker parts of the atmosphere, the inlets for these instruments, which were designed to measure atmospheric gases, became clogged with aerosolized particles from the clouds. For evidence of this clog, they point to a massive but temporary drop in the CO₂ levels in the atmosphere as the probe descended through the cloud layers.

Instead of chalking this up as an instrument failure, they looked at the data as a way of analyzing the types of aerosols that were trapped in the inlet—and they did so by looking at their burn-off temperatures. As the probe continued to descend through the atmosphere, it melted the various aerosols at different temperatures (and allowed the inlet to flow freely again, which caused the CO₂ reading to spike back up).

Analyzing which gases were released at the temperatures at which those aerosols melted would help them understand what comprised the aerosols, and hence the clouds themselves.

The first thing they noticed were massive spikes in water at 185^o C and 414^o C, which were indicative of hydrates such as hydrated ferric sulfate and hydrated magnesium sulfate. They also noticed that water made up the bulk of the aerosols at 62%, though almost all of it was bound up in these hydrates.

As expected, sulfuric acid was also present in the aerosols. It showed up in a major release as SO₂ around 215^o C, which is the temperature sulfuric acid decomposes. Interestingly, there was also another release of SO₂ around 397^o C, which indicated there was another, more thermally stable sulfate compound in the aerosols as well.

A hint at what that compound might be came from a spike in another, though unexpected, chemical signature—iron. At the same temperature as the second SO₂ spike, the LNMS detected a spike in iron ions. Combined with the release of SO₂ at that temperature, there’s a strong indication that one of the aerosols is ferric sulfate, which decomposes to iron oxide and sulfur oxides around those temperatures. Estimates put the ferric sulfate content of the aerosols as high as 16%, almost matching the 22% estimated for the sulfuric acid that was thought to dominate the cloud banks until this paper.

So where did the iron come from? The authors believe it comes from cosmic dust that is pulled into Venus’s atmosphere and then reacts with the acid cloud bank. But ultimately the biggest finding from this new analysis is the significant presence of water.

It also solves a mystery as to why there was a discrepancy between probes that collected data from the actual clouds compared to those that simply remotely scanned Venus’s cloud layer with spectroscopy equipment in terms of the water content of the clouds. The remote sensing devices wouldn’t be able to detect the water bound up in hydrates—only the amount of atmospheric vapor, making the descent probes much more accurate in their calculation of total water content.

All this new understanding obviously has big implications for the search for life in Venus’s clouds, as one of the main arguments against that possibility was the scarcity of water in that environment. It turns out that water is much more abundant than previously thought—though admittedly it’s rather acidic for the taste of most Earth-bound microbes.

This new understanding shows how useful even old data can be, and how it can effectively contribute to even modern discussions of unanswered scientific questions. The problem might just be finding it buried somewhere in NASA’s archives—which can be a scientific feat in itself.