A trio of researchers with the University of California and the Sorbonne Universités have found a possible explanation of why Venus probes have found different day lengths for the planet. In their magazine published in the journal Nature Geoscience T. Navarro, G. Schubert and S. Lebonnois describe a theory that they have developed based on observation data.
Measurements of Venus speeds have varied over the years for unknown reasons. It is known that it takes 243 earth days for the planet to spin once, but accurate measurements have varied with an average of seven minutes. Earlier research has also shown that the atmosphere circulates around the planet much faster ̵
The researchers began a long-standing wave that has been observed in Venus cloud formations – a wave about 10,000 kilometers long. They noted that similar waves have been seen on Earth because of the air colliding with mountains, but they usually spread rapidly due to air currents. But the atmosphere of Venus is particularly thicker than on earth, an observation that fascinates the researchers. They created a simulation to recreate the cloud formations seen on Venus and introduced the idea of mountains on the surface as the cause. After adding to all known ingredients in the Venus atmosphere, describing the size and density of the planet, they stopped by laying mountains on the surface. They then ran the simulation.
The researchers report that the simulation showed a wave formed in the cloud peaks, similar to the one seen on the actual planet. But they also found that the braking effect caused by the atmosphere that stretched into the mountains slowed the path of the planet's spinning amount due to the time of day. They found that on average the effect was sufficient to cause up to two minutes variation in the plane's spinning speed – not enough to account for the observed seven minute variation, but enough to suggest that other physical properties could play a similar role.
Picture: Cloudy Venus
T. Navarro et al. Atmospheric rock wave generation on Venus and its influence on the rotational speed of the solid plane, Nature Geoscience (2018). DOI: 10.1038 / s41561-018-0157-x
Akatsuki Spacecraft observed a 10,000 km long meridional structure on top of Venus cloud cover that seemed stationary with respect to the surface and was interpreted as a gravitational wave. Furthermore, several four Venus soldiers observed observations that other such waves occurred in the afternoon over the equatorial highlands. This indicates a direct impact of the solid plane on the entire Venusian atmosphere, despite different rotational speeds of 243 and 4 days respectively. How such gravity waves can be generated on Venus does not understand. Here we use general circulatory modeling simulations of the Venusian atmosphere to show that observations are consistent with stationary gravity waves over topographic high or mountain waves – generated in the afternoon in equatorial regions in the daily cycle with near surface atmospheric stability. We find that these mountain waves contribute substantially to the total atmospheric torque that acts on the surface of the planet. We estimate that mountain waves along with thermal tide and baroque waves can cause a change in the body's rotational speed in about 2 minutes per day of sunshine. This interplay between the solid plane and the atmosphere can explain some of the difference in rotational speeds (corresponds to a change in daytime of about 7 minutes), as measured by spacecraft over the past 40 years.