Astronomers debate whether or not lightning strikes even once on Venus
Where are the bolts of light? Why are the low-frequency radio waves lightning makes going backwards?
Contrary to a decades-old hypothesis, there may be very little lightning in the atmosphere of Venus – or even none at all – according to researchers analysis of data collected by NASA's Parker Solar Probe.
In 1978, the Pioneer Venus spacecraft orbited the planet and detected what appeared to be electromagnetic "Whistler" Waves hundreds of miles above its surface. Whistlers are low-frequency radio waves that travel along magnetic field lines and ripple through the atmosphere On Earth they are often generated by lightning.
Astronomers proposed that the existence of Whistler Waves on Venus was proof that lightning was generated in its clouds. But a group of researchers led by the University of Colorado Boulder has cast doubt on that interpretation.
"There's been debate about lightning on Venus for close to 40 years," opined Harriet George, lead author of the new study and a postdoctoral researcher at the school's Laboratory for Atmospheric and Space Physics. "Hopefully, with our newly available data, we can help to reconcile that debate."
George and her colleagues analyzed data collected by the Parker Solar Probe, which is currently five years into its trip to study the effects of solar winds. The craft's path to Sol involves zipping past Venus seven times to perform gravity-assist maneuvers, which will accelerate it to 430,000mph (690,000km/h) and make it the fastest human-made object in history.
In 2021, during its fourth flyby, the spacecraft's electric and magnetic field sensors detected dozens of Whistler Waves on Venus. The data revealed an odd property of the waves: they were traveling down towards the planet. On Earth, Whistler Waves generated by lightning typically propagate outwards into space.
"The direction that the Whistler Waves are traveling in tells us about the location that they must have formed in, which gives us clues about the generation mechanism," George explained to The Register.
"If the waves were generated by lightning or any other atmospheric process, they would travel away from Venus. Since the waves are traveling towards Venus, it means that they could not have been generated by lightning but must have been caused by some other process further away in space, likely in a region on the night side of the planet called the magnetotail," she said.
If lightning were as common on Venus as astronomers previously thought, the waves should also be accompanied by flashes of light – but these events are rare, the paper suggests.
"Venus encounters by multiple spacecraft have not detected lightning," the team wrote in their paper published in the journal Geophysical Research Letters.
"These non-detections and rare optical observations are consistent with low Venusian lightning occurrence rates, which is incompatible with the high Whistler-derived rates. We present observations of whistlers during a Parker Solar Probe Venus gravity assist and eliminate lightning as a possible source."
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Instead, they believe the Whistler Waves are from magnetic reconnection – where Venus's magnetic field lines break and recombine, releasing jolts of energy.
The researchers argue that they need more data to figure out whether there really is no lightning or just very little of it occurring on the planet, and will rely on data collected when the Parker Solar Probe makes its final flyby, swooping 250 miles above the planet's surface.
"It's very rare that new scientific instruments make it to Venus," said David Malaspina, co-author of the new study and an assistant professor at the University of Colorado Boulder's Department of Astrophysical and Planetary Sciences. "We don't get a lot of chances to do this kind of interesting research. ®