Lightning on Venus — A ‘Charged’ Affair!
Understanding the mechanism of lightning on Venus through a study published by PRL, Ahmedabad.
The sky is filled with dark grey water-laden clouds. Rain is imminent… a few drops fall on the parched Earth raising a wafting petrichor in the air. Suddenly a blinding streak of electricity breaks across the sky, and a few seconds later, a thundering sound hits the ears. Lightning on Earth is a marvellous phenomenon indeed if one has the time to savour it! Now a new study claims that even though Venus’s atmosphere is quite different from Earth’s, its lightning mechanism may be more similar to the Earth’s than thought.
Venus doesn’t have a global magnetic field like we have on Earth. This makes it more susceptible to bombardment by cosmic rays. Earlier studies suggested this as a reason for clouds getting electrified on Venus. The present study, conducted by a group involving scientists from Physical Research Laboratory, Ahmedabad, and published in Icarus, shows that the charges produced by cosmic rays are not enough to produce a discharge of electricity. They found that the number of charges getting accumulated due to cosmic rays was 1000 times lower than that required to produce lightning. They instead suggest that collisions could lead to such a number of charges as is required to produce lightning on Venus. Collisions are just one among several mechanisms that lead to lightning.
Let us first try to understand how lightning is caused on Earth.
Lightning occurs when charges separate, either within a cloud, between two clouds, or between a cloud and the ground. Let us take the simple case of lightning occurring between the cloud and the ground. The cloud, let’s say, has a negative charge, and the ground has a positive charge. A potential difference is created between the cloud and the ground. The charges then try to ‘equalise’ by flowing from the cloud to the ground. But the air between the cloud and the ground is an insulator, meaning it stops the flow of charges. So the charges keep building up and at one point, the charges build up so much that the insulating atmosphere can hold no more, and then the charges even out by creating an electrically conducting channel from the cloud to the ground. This spark of electricity is what we see as lightning. The temperature in this conducting spark is around 27,000 degrees Celsius which is five times higher than the surface of the Sun. Apart from Earth, lightning strikes have been found on Venus, Jupiter, Saturn and Uranus among the planets of our Solar System. “Detection [of lightning] in other planetary bodies has always sparked an interest.” say the authors in the paper.
Even after decades of study, lightning on Venus is still a controversial subject. The Russian Venera mission and NASA’s Pioneer mission in the ’70s and ’80s indicated that lightning could be present on Venus. Then Cassini-Huygens did not find any signature of lightning during its Venus flyby in 1999. Later, Venus Express which began orbiting Venus in 2006 heard radio pulses which could have their origin in lightning on Venus. The Pioneer Venus Multiprobe launched in 1978, a part of the Pioneer program, gave some insights which were used in this study.
The Large Cloud Particle Size Spectrometer on board Pioneer Venus Mission earlier found that particles which undergo collision or friction, causing lightning, are found at altitudes between 45 and 70 km on Venus. These particles are mostly sulphuric acid droplets. It’s interesting to note that clouds on Venus, unlike Earth, are made of sulphuric acid, and not water. (This also means that it rains acid and not water on Venus!) These droplets can get charged through different mechanisms, but a common way for them to get charged is friction generated through collision.
In this study, the researchers estimated the minimum charge density, that is the minimum number of charges that need to get accumulated, and the minimum dimensions of charge regions within the clouds required for lightning to occur on Venus. As per their model, the values match those on Earth. As the charge density and dimensions of charge regions are the same as that required on Earth, and collisions between water droplets and ice particles do lead to lightning initiation on Earth, the scientists suggest that collisions and not cosmic rays could be the main reason for lightning on Venus too.
The calculations done by scientists in the present study could aid in refining other simulations related to Venusian lightning. Even though Venus is our nearest neighbour, it may be surprising that the mere presence of lightning on Venus is still shrouded in mystery. Hence, any research that helps clarify a part of it, takes us closer to unveiling this mystery.
Such studies about lightning on other planets help us better understand the atmospheres of those planets and also their composition. Not just that, such studies also help us better understand the weather and the atmosphere here on Earth.
So the next time you get time to savour a lightning event, spare a thought for something similar that could be occurring on another spinning rock 115 million km away!
Till then, here’s some thundering lightning trivia for you:
1. 6000 lightning bolts occur between clouds and Earth every minute.
2. The fear of lightning is called ‘astraphobia’.
3. The science of lightning is called ‘fulminology’.
4. It’s a common myth that lightning does not strike the same place twice. Lightning favours tall objects and objects that conduct electricity more easily. And hence tall objects get struck more often than shorter ones.
5. Apollo 12 survived a lightning strike shortly after lift-off!
Further reading:
https://ui.adsabs.harvard.edu/abs/2021Icar..36514473K/abstract
https://www.feynmanlectures.caltech.edu/II_09.html#Ch9-S6
https://www.lightningmaps.org/#m=oss;t=3;s=0;o=0;b=;ts=0;y=17.3087;x=30.7617;z=2;d=2;dl=2;dc=0
This article was edited by Vagisha Bhatia and Atotmyr
