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Planet Electric

by Perry Pezzolanella

Somewhere on Earth there are thunderstorms raging with brilliant flashes of lightning and deafening rumbles of thunder unleashing torrents of rain. It is not hard to ponder whether other planets that have clouds and storms may also have lightning. Imagine lightning flashing in blazing hot acid skies above a scorched Venera lander on Venus; lightning flashing between towering red clouds on Jupiter; lightning flashing in white anvil storm clouds with Saturn’s rings arcing above them in the distance; or lightning flashing off a vast sea during a frigid lake effect methane rain squall on Titan. These alien sights may be happening for real!

There are three signatures of lightning or ways that it can be detected. The most obvious is optical emission or simply the brilliant flash from the intense heating of the channel it makes in the atmosphere. Planetary lightning can be photographed with a simple camera much in the same way as on Earth. Voyager 1 managed to do exactly that when it flew past the night side of Jupiter during March 1979 and photographed numerous flashes. Jupiter’s lightning is super powerful in part due to the planet’s enormous size and its powerful magnetic field, and therefore is larger and far brighter than Earth’s lightning, which made it easier for Voyager 1 to image the flashes. More-sensitive equipment is necessary to image lightning on the other planets. A camera would have to be tuned to a certain spectrum depending on which planet was being explored. Earth’s lightning spectrum is strongest in the infrared nitrogen absorption line and the green excitation oxygen line which makes sense since these are the primary gases in our atmosphere. These filters can be used to detect lightning during the day from satellites. The green oxygen excitation line can also be used for Venus with its dense carbon dioxide atmosphere. Jupiter, Saturn, Uranus, and Neptune are predicted to emit in the red spectrum due to their dominant hydrogen atmospheres.

Radio emission is another signature that is familiar when trying to listen to AM radio during a thunderstorm. The ionized channel of lightning acts like an antenna for broadband radio emissions, known as sferics. The strength and duration of each burst of static corresponds to the size and length of the flash, and its proximity to the receiver. Detection is not directly possible in space; therefore, it can only be made within the atmosphere, or from the surface, close to the flash. Whistlers are very low frequency radio waves generated by lightning that are guided along magnetic field lines and are detected by spacecraft either orbiting the planet or flying through its magnetic field.

The third signature is the one that seems to scare us the most. Acoustic signals, fondly known as thunder, are caused by atmospheric pressure waves created by the heated lightning channel. Thunder can only be heard within the atmosphere so it can only be detected by atmospheric probes such as a balloon, drone, surface lander or rover. Sound cannot propagate through the vacuum of space; therefore, a spacecraft in orbit or flying by cannot detect it. The probe must be close to the lightning since sound waves diminish with increasing distance. Thunder may sound different on each planet, but the same physics still applies. The thickness and temperature of the atmosphere will affect the characteristics of thunder along with the landscape and cloudscape.

The techniques used to optically detect planetary lightning flashes include a telescope with a filter looking mainly at the night side of the planet from Earth, a camera with a filter observing from a spacecraft during a flyby or while in orbit, and simply a camera on the surface. Radio telescopes on Earth can detect radio (electromagnetic) emissions, while radio receivers and magnetometers are used on spacecraft and landers/rovers. A microphone or a pressure sensor can be used on a lander or rover to detect thunder.

Lightning is mainly generated in deeply convective clouds or thunderclouds where water can exist in several phases. The turbulence causes charge transfer between graupel (soft hail) and ice crystals, with the ice crystals becoming positively charged and carried to the top of the thundercloud while the graupel falls downward. This separates the charges creating a potential difference. If the potential difference within the cloud, between two clouds, or between the cloud and the ground becomes large enough, lightning will occur. Water content of the cloud affects the characteristics of its lightning, where it concentrates and discharges, and the discharge/flash rate.

The second type of lightning seen on Earth is volcano lightning, which is caused by a convective ash plume. If the concentration of ash is high enough, particles can transfer charge by rubbing against each other. Smaller particles tend to become negatively charged while larger particles become positively charged and fall towards the ground creating charge separation that triggers lightning.

Any world with a reasonably thick atmosphere has the potential to generate lightning if proper conditions exist. These would include Venus, Earth, Jupiter, Saturn, Uranus, Neptune, and Saturn’s moon, Titan. Lightning on Venus is still up for debate. The atmosphere is thick, turbulent, and full of clouds but consists primarily of sulfuric acid and may lack ice. Spacecraft have failed to detect both flashes and whistlers. Venus is suspected to have explosive volcanoes that could generate lightning, but Venus Express and Akatsuki have found no evidence. Venus will finally receive extensive exploration later this decade, which should nail down these basic questions regarding the existence of lightning and active volcanoes once and for all. Jupiter and Saturn have enormous lightning storms where lightning can span a continent. Their atmospheres are deep, turbulent and contain water-rich clouds and ice crystals. Spacecraft have observed flashes and detected whistlers at both worlds. Lightning can be found within the white ovals of both worlds along with Jupiter’s Great Red Spot and Saturn’s Great White Spot. Voyager 2 is the only spacecraft to fly by Uranus and Neptune. It did not observe any flashes at either planet, but radio emissions were detected at Uranus and whistlers at Neptune confirming lightning. Saturn’s largest moon, Titan, has a thick nitrogen-methane atmosphere, but 126 close flybys with Cassini while it was orbiting Saturn from 2004-2017 detected nothing. However, lightning generation is possible within the rare convective methane clouds. The Huygens probe may have detected sferics during its descent to the surface, but it could not be confirmed. Mars’ atmosphere is too thin for convective storms and is no longer volcanically active so the only possibility of lightning would be during a massive dust storm which could potentially generate lightning since it is such a windy, dry, and dusty environment. Large dust devils can also generate lightning and those on Mars often grow as large as tornadoes on Earth. It may not be much longer before lightning is confirmed on Mars given the large fleet of spacecraft exploring it and even more highly sophisticated spacecraft on the way and planned.

Lightning is a vital component for planetary environments in the creation and evolution of life. It can synthesize complex precursor compounds for life as happened on Earth. It may also have been involved in life on Venus if it ever did exist before the suspected shallow seas boiled away long ago and it became a superheated inferno, or perhaps life may still exist there in the clouds. It is not beyond reason that simple life could exist on any planet since lightning has the power to assist it.