Recent research conducted by space physicists at CU Boulder has delved into the longstanding debate surrounding lightning on Venus, the second planet from the sun. Contrary to previous assumptions, their findings provide compelling evidence suggesting that lightning is not a common occurrence within Venus’ thick, acidic clouds.
Venus, often described as one of the most enigmatic and inhospitable bodies in our solar system, shares a similar size to Earth but boasts a dense, carbon dioxide-rich atmosphere that has triggered a runaway greenhouse effect. This results in scorching surface temperatures of 900 degrees Fahrenheit and crushing atmospheric pressures, rendering Venus extremely hostile to spacecraft, with none surviving for more than a few hours on its surface.
To explore this extreme environment, the researchers harnessed an unexpected tool: NASA’s Parker Solar Probe. This spacecraft, launched in 2018 on a seven-year mission to investigate the sun’s corona and solar wind, flew within approximately 1,500 miles of Venus in February 2021. During this encounter, the probe’s instruments detected numerous “whistler waves,” which are typically associated with lightning on Earth. However, the team’s analysis suggests that these whistler waves on Venus may arise from disturbances in the planet’s weak magnetic fields rather than lightning strikes.
These findings align with a 2021 study led by Marc Pulupa of the University of California, Berkeley, which also failed to detect radio waves generated by lightning on Venus. David Malaspina, a co-author of the new study, emphasized that these results highlight the limited knowledge humans possess about our neighboring planet and the rarity of scientific instruments being deployed to study Venus in detail.
Dark and stormy nights
The debate over lightning on Venus traces its roots back to 1978 when NASA’s Pioneer Venus spacecraft entered orbit around the planet, Earth’s seething twin. Almost immediately, the spacecraft started detecting signals of whistler waves, hundreds of miles above Venus’s surface.
For many scientists, these signals bore a striking resemblance to a phenomenon well-known on Earth: lightning. Harriet George explained that on our planet, whistler waves are frequently associated with lightning. Lightning strikes can disrupt electrons in Earth’s atmosphere, creating waves that propagate outward into space. These waves generate a distinctive whistling sound that early radio operators could hear through headphones, thus earning them the name “whistlers.”
If Venus’ whistler waves originate in a similar manner, then Venus might be a lightning powerhouse, potentially experiencing about seven times more lightning strikes than Earth. Lightning has also been observed on other planets like Saturn and Jupiter.
Harriet George elaborated, saying, “Some scientists observed these signatures and thought, ‘This could be lightning.’ Others countered with, ‘Actually, it might be something else.’ This back-and-forth has persisted for decades.”
A brush with Venus
The Parker Solar Probe presents a promising opportunity for scientists to potentially settle the long-standing debate about lightning on Venus once and for all.
Harriet George explained that the spacecraft is set to make seven flybys of Venus during its mission, gradually drawing nearer to the sun with each pass. In 2021, during its fourth flyby, the probe ventured exceptionally close to Venus, passing into the shadow created by the planet—an ideal location for investigating whistler waves.
To detect these signals, George, David Malaspina, and their team utilized the FIELDS Experiment on the Parker Solar Probe, which consists of electric and magnetic field sensors extending from the spacecraft. (The Digital Fields Board, responsible for analyzing signals from the FIELDS sensors, was designed and built by a team at CU Boulder and LASP.)
Upon analyzing a set of these whistlers, the researchers stumbled upon an unexpected revelation: Venus’ whistler waves were behaving contrary to expectations. Instead of radiating outward into space, as typically seen in lightning storms, they appeared to be moving downward towards the planet.
David Malaspina noted, “They were heading backward from what everybody had been imagining for the last 40 years.” The cause of these reverse whistlers remains uncertain, but George and Malaspina suspect that they might result from a phenomenon known as magnetic reconnection. This involves the twisting magnetic field lines around Venus coming apart and snapping back together explosively.
To definitively rule out lightning as the source, further analysis of whistlers is required. The next opportunity for this investigation will come in November 2024 when the Parker Solar Probe completes its final pass by Venus, descending to within 250 miles of the planet’s surface, skimming the top of Venus’ dense atmosphere, as described by Malaspina. In his words, “Parker Solar Probe is a very capable spacecraft. Everywhere it goes, it finds something new.”