Venus, often regarded as Earth’s twin, is a source of perpetual intrigue for planetary scientists. Although it may once have harbored water on its surface, Venus is now a hell planet with surface temperatures over 800 degrees Fahrenheit, likely due to a runaway greenhouse effect. Its toxic atmosphere contains mysterious dark spots that some scientists think may indicate the presence of extraterrestrial bacteria.
Yet one of the biggest Venusian mysteries has to do with the planet’s rotational period, which is slowly speeding up. Two measurements taken 16 years apart in the 90s and early 2000s indicated that length of the planet’s day had decreased by nearly seven minutes, which isn’t a ton considering the planet only rotates about once every 243 Earth days. Still, scientists couldn’t explain how Venus’ rotational speeds varied over time. New research published in Nature Geoscience indicates that the planet’s changing rotational velocity may result from Venus’ thick, fast-moving atmosphere pushing on the planet’s mountains.
In 2015, the Japanese Space Agency’s Akatsuki spacecraft observed a 6,200-mile long wave in the Venusian atmosphere above the Aphrodite highland region that rises to an altitude of about three miles. This discovery was remarkable in itself, but even more odd was that this atmospheric wave wasn’t moving. Venus’ atmosphere circles the planet every four days propelled by winds moving at well over 200 miles per hour, but for whatever reason this feature wasn’t getting dragged along for the ride.
This atmospheric wave was interpreted as a gravity wave, which are also found in Earth’s atmosphere. As explained by NASA, gravity waves occur when “buoyancy pushes air up and gravity pulls it back down,” resulting in observable disturbances in the atmosphere. Yet as planetary scientist Thomas Navarro and his colleagues write in their new paper, “how such gravity waves might be generated on Venus is not well understood.”
Based on current scientific knowledge the observed gravity wave shouldn’t have been possible. The atmosphere on Venus has a “neutral,” mostly unchanging low layer that shouldn’t have allowed a gravity wave to propagate to the upper portion of the Venusian atmosphere. Furthermore, this gravity wave only appeared during the daytime on Venus, generally in the afternoon.
The standard physical model for simulating the atmosphere of Venus (known as the Institut Pierre-Simon Laplace general circulation model, or IPSL GCM for short) takes into account all the known physical properties of Venus, but had never produced a gravity wave like the one observed by Akatsuki. According to the researchers, the only time a planet-wide gravity wave has been produced was by using a “rather crude model of the atmosphere” following Akatsuki’s discovery of the huge gravity wave in 2015.
To tackle the mystery of the Venusian gravity wave, Navarro and his colleagues wanted to clean up this crude model by adding more realistic atmospheric physics and the topography of Venus’ surface. When these elements were added to the simulation, the researchers found that the gravity wave could be accounted for by air getting pushed up over mountains. Incredibly, the results also suggested that these gravity waves “substantially contribute to the atmospheric torque that acts on the planet’s surface.”
The researchers estimate that the gravity waves in Venus’ atmosphere likely cause the planet’s rotation rate to change by two minutes every 243 Earth days. This increased speed is offset, however, by the atmospheric rotation at the upper levels in the atmosphere and the effects of the Sun on Venus’ air pressure. The interplay of these factors could explain why Venus’ rotation speed is so variable.
Navarro’s research adds to the mounting argument for more missions to Venus. To better understand our neighbor’s peculiar rotation, he argued that we need long term observations of the gravity wave as well as measurements of Venus’ interior
“We don’t know anything about the interior of Venus,” Navarro told Cosmos Magazine. “This is frustrating because Venus is the closest planet to Earth in terms of size, and yet we don’t know what the interior looks like.”
Sounds to me like it's time to start thinking seriously about building some cloud cities on Venus.