Scientists Discover That Methane Causes Superstorms on Uranus and Neptune.
Uranus and Neptune are the outermost planets in the Solar System, often referred to as ice giants due to their high water content. Scientists know very little about these distant planets. However, after the Voyager 2 spacecraft flew past them in the 1980s, researchers discovered that these planets sometimes experience massive storms that last for a short period. These intense yet fleeting storms appear every few years and are so large that they can be observed (through telescopes) from Earth.
Image of a storm on Uranus in 2018. (Photo: NASA).
Researchers have long wondered why storms on these planets are so unpredictable. A group of astronomers has found that methane may be the “key” to understanding these storms. The research results were published in the arXiv database on September 3.
To generate a storm, heat must rise from the planet’s interior to its surface. There, warm gases begin to cool, which can cause disturbances and trigger storm formation. But the interiors of these planets remain warm, while the outer surfaces are always cold. So why don’t storms occur all the time?
According to the research, methane is the third most abundant molecule, after hydrogen and helium, in the deep atmospheres of both planets. Typically, methane gas does not have much effect other than floating within the atmosphere, but researchers used a 3D cloud-resolving model to simulate the convective processes. Using these simulations, the team concluded that the typical velocity of dry convection in the deep atmosphere is relatively low (around 1 m/s) but sufficient to maintain the upward transport of methane and significantly suppress moist convection at the level where methane condenses.
Methane typically exists in gaseous form, but in the upper regions of the atmospheres of these icy worlds, methane can condense, forming droplets that fall to lower altitudes, the study’s authors suggest. There, they warm up and rise again, completing a cycle similar to the water cycle on Earth. When the atmosphere becomes overly saturated with methane, a stable layer forms. Like a wet blanket, this stable layer prevents heat from reaching the surface, thus inhibiting storm formation.
These layers are commonly found at all latitudes of Neptune and around the equator and mid-latitudes of Uranus. However, the poles of Uranus do not have enough methane to create a stable saturated layer. As a result, heat can easily rise to the surface, generating larger storms, the research states.
On the other hand, Neptune has more methane, and researchers found that sometimes this methane can rise from the stable layer and disperse throughout the atmosphere, allowing heat circulation and storm formation before everything stabilizes again.
On these ice giant planets, dry convection is weak, and moist convection is significantly suppressed. However, when enough methane is transported upward through dry convection and chaotic diffusion, infrequent moist convective storms can form. These storms are likely to occur more frequently on Neptune than on Uranus, due to Neptune’s internal heat flow, the research team concluded.
The authors suggest that further research is needed to understand how the factors in these massive icy atmospheres interact with each other.
