The ‘anti-weather’ of Venus

Conditions on the surface of Venus remained a mystery for decades. Carl Sagan pointed out that people were quick to jump to conclusions, for example about the presence of dinosaurs, from the little evidence collected on the planet. But just because we have little real data doesn’t mean we can’t draw conclusions, and better yet models, from the data we have.

A new paper by Maxence Lefèvre of the Sorbonne and colleagues takes what little data has been collected from the surface of Venus and uses it to validate a model of what wind and dust conditions there would be, all with the aim of making work easier for the Venusian explorer’s next tour.

The paper, available in pre-print form on arXiv, focuses on two main measurements: temperature variations and dust transport. It is important to note that it models different parts of the planet differently. This is the first time such a study has been carried out, but it is absolutely essential to isolate some of the characteristics that are the driving forces behind these two conditions. But the main force underlying temperature and dust transport is the same on Venus as on Earth: wind.

Measurements taken by Venera, one of the only craft to successfully land on the surface of Venus, reduced wind speeds deep in the atmosphere to a measly 1 m/s. Compared to 20 m/s on Earth or even 40 m/s on Mars, this may not seem like much. But Venus’ atmosphere is thicker than ours or Mars’, so it would take a lot more energy to reach a speed equivalent to that of its sister planets. However, it still has a major impact on both the surface temperature and the amount of dust in the air.

Venus has a “day” that lasts 117 Earth days and a night that is just as long. This causes massive changes in the atmosphere, as the planet is gradually warmed by solar radiation during the day and gradually cooled by its own infrared radiation at night. But these changes are different in different regions of the planet, according to the paper – and particularly different between “highlands” (i.e. mountainous regions) and “lowlands” (i.e. plains), and different again between the tropics and the poles.

In the tropics, there is a very distinct “diurnal shift,” meaning that winds occur in very different patterns depending on whether it is day or night on their part of the planet. At noon, winds blow upward (called “anabatic” in technical jargon) due to the warming of the ground below them pushing it upward. However, at night this process reverses as IR cooling of surfaces causes cooling of the air, causing downslope winds called “katabatic”.

These processes have a direct effect on surface temperature, because katabatic winds cause compression of descending air, thereby warming it and counteracting surface IR cooling in a process called adiabatic heating. Essentially, the winds in the mountains keep the temperature stable, with a variation of less than 1 degree Kelvin between the night cycle and the day cycle. Compare this to an oscillation of around 4 degrees Kelvin for the “lowlands” which does not have the same cooling effect.

Closer to the poles, this dynamic changes, with winds constantly in katabatic flow, which again compensates for the constant IR cooling of this planet at these latitudes. Since future missions, like Envision and Veritas, will have their eyes on the poles, it is good to understand these processes before they arrive.

Another probe, DaVINCI, is expected to land on the Venusian surface for the first time in decades. The planned descent will take place in a region called Alpha Regio, a mountainous plateau near the equator, which would be subject to more moderate temperature variations than some of the surrounding lowlands. But will the DaVINCI probes be destroyed by floating dust? Most likely: according to the researchers’ calculations, 45% of Alpha Regio’s land is subject to wind forces sufficient to lift “fine sand” with a particle size of 75 µm. This would place DaVINCI’s planned landing zone directly in the path of an ongoing fine particle storm, which could vary depending on the time of day it arrives.

All of this work was driven by a new “regional” simulation of the planet that divided these individual areas into computable weather patterns, rather than trying to model the entire surface as a single block. But that doesn’t mean that this work can’t still be improved: the authors mention adding different thermal characteristics to different parts of the surface based on their albedo and thermal inertia or taking into account the thermal absorption value of CO.₂which predominates in the atmosphere of Venus, at different temperatures.

But the paper’s authors and other researchers studying Venus’ atmosphere still have time before the new batch of probes arrive at the second planet. At least when they do, they’ll have a better idea of ​​what might be causing some of the features they found.