Conditions on Venus’ surface have largely remained a mystery for decades. Carl Sagan famously pointed out that people were quick to jump to conclusions, such as that there are dinosaurs living there, from scant little evidence collected from the planet. But just because we have little actual data doesn’t mean we can’t draw conclusions, and better yet models, from the data we do have. A new paper from Maxence Lefèvre of the Sorbonne and his colleagues takes what little data has been collected from Venus’ surface and uses it to valid a model of what the wind and dust conditions down there would be like - all for the sake of making the work of the next round of Venusian explorer easier.
The paper, which is available in pre-print form on arXiv, focuses on two main metrics - temperature swings and dust transport. Importantly, it models different parts of the planet differently, the first time any such study has been done, but one that is absolutely critical to isolate some of the features that are the driving forces behind those two conditions. But the key underlying force for both temperature and dust transport is the same on Venus as it is on Earth - the wind.
Measurements from Venera, one of the only craft to ever successfully land on Venus’ surface, put the wind speed down at the bottom of the atmosphere at a measly 1 m/s. Compared to 20 m/s on Earth or even 40 m/s on Mars, that may not sound like much. But Venus’ atmosphere is thicker than either ours or Mars’, so it would require a lot more energy to get it up to speeds equivalent to that of its sister planets. Even so, it still has a major impact on both the temperature on the surface and the amount of dust in the air.
Fraser discusses the history of the Venera program - one of the only mission to ever successfully land on Venus.Venus has a “day” that is 117 Earth days long, and a night that is equally as extended. This causes massive changes in the atmosphere as the planet is gradually warmed up by solar radiation during the day, and gradually cooled by its own infrared radiation at night. But those changes are different for different regions of the planet, according to the paper - and especially different from the “highlands” (i.e. mountainous regions) and the “lowlands” (i.e. the plains), and different again between the tropics and the poles.
In the tropics, there is a very clear “diurnal shift”, meaning that the winds happen in very different patterns depending on whether its day or night on their part of the planet. During midday, the winds blow upslope (called “anabatic” in technical jargon) due to the heating of the ground underneath them pushing it up. However, at night this process reverses as the IR cooling of the surfaces causes the air to cool, causing downslope winds known as “katabatic”.
These processes have a direct effect on surface temperature, as the katabatic winds cause the air flowing downhill to compress, thereby heating it up, and counteracting the IR cooling from the surface in a process called adiabatic warming. Essentially, the winds in the mountains hold the temperature steady - with a swing of less than 1 degree Kelvin between the night and day cycle. Compare that to a swing of around 4 degrees Kelvin for the “lowlands” that don’t have the same cooling effect going on.
Fraser discusses the future Venus missions that will benefit from this preliminary research into the planet's surface conditions.Nearer the poles, this dynamic shifts, with the winds constantly in katabatic flow, which again offsets the constant IR cooling of that planet at those latitudes. Given future missions, such as Envision and Veritas, will have their eyes on the poles, its good to have an understanding of these processes before they arrive.
Another probe, DaVINCI, is currently scheduled 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 highland plateau near the equator, which would be subject to the more moderate temperature swings than some of the surrounding low-land areas. But will the DaVINCI probes be blasted by dust floating around? Quite possibly - by the researcher’s calculations, 45% of the land in Alpha Regio has wind strengths that are enough to lift “fine sand” of 75 µm particle size. That would put DaVINCI’s planned landing zone straight in the path of an ongoing fine particle storm, which could vary some depending on the time of day it arrives.
All of this work was driven by a new “regional” simulation of the planet that broke up these individual areas into calculable weather models, rather than trying to model the whole surface as one singular block. But that doens’t mean this work can’t still be improved upon - the authors mention adding different thermal characteristics to different parts of the surface based on their albedo and thermal inertia or accounting for the thermal absorption value of CO2, which is predominant in Venus’ atmosphere, at different temperatures. But the paper authors and other researchers looking at Venus’ atmosphere still have some 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 find.
Learn More:
M. Lefèvre et al. - The effect of near-surface winds on surface temperature and dust transport on Venus
UT - Winds on Venus
