Venus' atmosphere is rightfully famous for a combination of being stunningly hot and containing sulfuric acid. Those conditions, not surprisingly, have ensured that every bit of hardware we've sent through said atmosphere has had an extremely short lifespan.

But at least one of those pieces of hardware—the Soviet Union's VeGa-2 probe—sent back some data that's hard to explain, a hint of an unstable atmosphere. Now, a pair of scientists is suggesting that the oddity can be explained by an equally odd feature of the atmosphere: it's a supercritical fluid where different chemicals partially separate at different altitudes.

The sulfuric acid and a surface temperature of 464 degrees Celsius, which is hot enough to melt lead, would seem to be more than sufficiently unusual for a planet. Yet Venus' oddities don't stop there. Its atmosphere is so dense and reflective that its surface gets less sunlight than the Earth does, even though Venus is closer to the Sun.

Supercritical

The atmosphere is also incredibly dense. Combined, the heat and density turn its mix of nitrogen and carbon dioxide into something called a supercritical fluid. Under extreme conditions, the neat boundary that separates liquid phases from gas phases goes away, and you end up with something that has a mix of the two properties. It can behave like a gas but also dissolve solids as if it were a liquid.

As a result of this phase change, the density of the atmosphere at the surface is roughly 6.5 percent that of liquid water. It's so dense that it's thought to have brought Venus' rotation to a standstill and then started it moving backwards. The planet is now rotating very slowly in the opposite direction to Earth's rotation—so slowly that, if you were there to see a sunrise, you'd have to wait 243 Earth days to see the next.

So, if you want to understand Venus, you'd have to study the atmosphere. And, according to researchers Sebastien Lebonnois and Gerald Schubert, we haven't really done much of that. "There's a lot of interest in the community in possibly returning to Venus and exploring the atmosphere and the surface environment," Schubert told Ars. "So there's all this activity going on, discussing new science missions to Venus. As part of these study efforts, we've been trying to review and learn as much about the atmosphere and surface of Venus as possible."

That review included a look at the data we have from the lower atmosphere, which turned out to mostly be a legacy of failure. The US' Pioneer Venus mission sent four probes into the atmosphere; every single one saw its temperature-measuring hardware fail at 12km above the surface. Several Soviet probes failed before reaching the surface, too. So, what we're left with is VeGa-2 data. That had two temperature sensors that remained active through the lower atmosphere and all the way to the surface.

And the VeGa-2 data is decidedly odd. "The measurements showed that the temperature near the surface was hot compared to the temperature at an altitude of seven kilometers," Schubert said. "The atmosphere shouldn't have been in that state; it was highly unstable. Whenever you have very hot material underlying a layer of fluid, the fluid tends to turn over." The convection that turns things over happens everywhere on Earth, from the air in its atmosphere to the semi-molten rock in its mantle. Why wasn't it happening on Venus?

The problem is that we don't know if that turnover really isn't happening, since all we have is a single data point. "We have just the one probe at one time in one place," Schubert told Ars. So, the VeGa-2 probe may have been subjected to some sort of temporary oddity in the atmosphere when it landed.

Separation

But Lebonnois and Schubert decided to take the measurement seriously.

"We searched for explanations for how the atmosphere could exist in this state," Schubert said. "How could you stabilize the unstable temperature gradient?" The answer they came up with is chemistry. If the gas at the bottom was heavier, then it could stay there despite being hotter. And there's at least one measurement that suggests gases might separate out under Venusian conditions.

Supercritical fluids are used for a variety of applications; one of them is injecting carbon dioxide into oil wells, where it helps push more crude to the surface. People are studying a similar approach for carbon sequestration, as well. And, according to Schubert, one of the studies of this process looked at a mix of carbon dioxide and nitrogen. Even though it was a small vessel (18cm high), the two molecules stratified. The top ended up about 70-percent nitrogen, and the bottom was 90-percent carbon dioxide.

Schubert was quick to point out that this wasn't done under Venusian conditions, as the materials were kept at room temperature. Still, if anything similar happens in the supercritical fluid atmosphere of Venus, the lower atmosphere could end up being nearly pure carbon dioxide and sufficiently dense to allow the awkward temperature gradient to persist.

To really know whether this does apply to Venus, we've got two choices, according to Schubert: "You either go back to Venus and measure the temperature and composition again using a descent probe, or you go into a laboratory on Earth and reproduce the Venus conditions." Since the former could take years, he and Lebonnois have applied to use a NASA facility that can reproduce Venus-like conditions.

Why would this be important enough to sort out? The paper notes that there would be much less nitrogen in the atmosphere of Venus if it were excluded from the lowest, most dense layer. And that could influence how we understand both the formation and evolution of the planet. It could also inform our understanding of a variety of other planets, where gases exist under similarly extreme conditions. Plus, let's face it—it would be pretty cool to have something so bizarre happening on a planet that could be Earth's twin if it were elsewhere in the Solar System.

Nature Geoscience, 2017. DOI: 10.1038/NGEO2971 (About DOIs).