The most accurate version of the full three-dimensional physical model of Venus' atmosphere provides insights into its highly unstable and largely unexplored outer layers, according to two co-authored and lead studies by researchers at the Institute of Astrophysics and Space Science (IA).
Air bursts, faster than sound, 130 kilometers above the surface, blowing from the east, but is suddenly stopped at dusk, compressed in shock.
It could be happening in the atmosphere of Venus, but it is in fact a simulation of several days running on supercomputers, in France, where it was first developed, but also in Portugal, the United States and Canada.
The enhanced version of the General Circulation Model of Venus (VGCM), a three-dimensional model fully equipped with physical and dynamic processes, simulates the atmosphere of Venus with the highest resolution ever, up to an altitude of 150 kilometers.
It is now able to faithfully mimic recent scientific observations, while also revealing details that could explain some of the many unknowns on Earth's fake twin planet.
These results are reported in a pair of articles published online in March in the scientific journal Icarus, respectively co-authored and led by Gabriella Gilli, from the Institute of Astrophysics and Space Sciences (IA) and the Faculty of Sciences of the University of Lisbon (Ciências ULisboa).
The two studies focused on altitudes between 80 and 120 kilometres, a highly variable region of Venus' atmosphere sandwiched between the high-powered winds of cloud level and the thermosphere.
“Some of my colleagues call these upper layers the 'ignotosphere', to underscore the lack of knowledge and measurements that we have in this region,” says Gabriella Gilli, who has been trying to interpret it by extending the VGCM with physical processes that occur at higher altitudes, and validating the model with observations from the Venus Express spacecraft, the European Space Agency (ESA), and from telescopes on Earth.
The first of the two studies now published “revealed details never before simulated and exhibits a highly variable nocturnal atmosphere suggested by observations but never described by other numerical simulations,” says co-author Gabriella Gilli.
This researcher led the second study, in which her team shows that the model is able to provide reliable estimates for temperatures and winds in regions where measurements are scarce.
In the absence of clouds at high altitudes, the abundances of molecules such as carbon monoxide or oxygen allow scientists to track and monitor the dynamics of the atmosphere.
In the past, observations on the planet's night side of bright spots of specific infrared luminescence from oxygen molecules and detected at high latitudes have left scientists intrigued. This enhanced version of the VGCM model is the first numerical simulation to reproduce this phenomenon.
Based on unexpected features that emerged from the simulations but not yet observed on Venus, Thomas Navarro (UCLA and McGill University) and his team put forward the first explanation for these patterns in nighttime infrared luminescence: a shock structure produced by a sharp decrease in supersonic wind speed at dusk and dawn, and also a planetary scale type of atmospheric gravity wave4 called the Kelvin wave.
Navarro, the other first author and co-author of these studies, explains: “This luminescence changes in a matter of a few hours because of the wind variability accentuated by the shock. And it reaches high latitudes due to the wind circulation towards the poles intensified by the Kelvin wave.”
The validation of the model with the data, and the complementarity of the two studies, give investigators confidence in interpreting those unexpected characteristics as being responsible for the variability observed in the nocturnal hemisphere of Venus. Day and night on Venus are in fact radically different because the planet rotates so slowly.
“A day on Venus is very long, about 117 Earth days, with implications for the distribution of solar radiation,” explains Gilli. “The night side is so cold that it was called 'cryosphere' above 100 kilometers. A strong temperature and pressure gradient between day and night produces strong winds, faster than the sound waves, characteristic of the day-night circulation of these upper layers, moving from midday to the night side.”
The VGCM was first developed at the Institut Pierre-Simon Laplace (IPSL) in France. This updated version discriminates details at the equator between cells of just 200 by 400 kilometers, and even smaller ones at other latitudes. It is crucial to interpret observations and reveal the physical mechanisms at play on Venus.
It may also provide insights into the past or future of our own parent planet, but Venus is also an analogue for anticipating the study of worlds outside the Solar System – slowly rotating exoplanets with high surface pressure and a dense, cloudy atmosphere , in line with the IA's research in this field.
The results now published in the scientific journal Icarus call for more observations of the outer atmospheric Venusian envelope: the upper mesosphere and the thermosphere. While we still have to wait about two decades for another space mission to our neighbor, telescopes on Earth can monitor the abundances of dynamic markers (chemical compounds) and map winds and temperatures.
IA researchers and MSc students in Sciences ULisboa, Diogo Quirino and Vasco Silva, were also involved in these studies, fine-tuning the model's parameters to better reproduce the temperatures in accordance with data from the Venus Express, and in the study of the dynamic markers extracted from the model in the same regions and times of the day of the observations.
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