Natural radio signal buzzes in Venus’ atmosphere
Venus has been in the news a lot since last September, when researchers announced the possible detection of phosphine, a possible life sign, in its atmosphere. On May 3, 2021, NASA’s Parker Solar Probe announced another discovery: a never-before-observed natural low-frequency radio signal in the atmosphere of Venus. The probe, designed primarily to study the sun, came close to Venus to use it as a gravity slingshot, needed to propel the probe sunward. Parker Solar Probe was at its closest to Venus yet – only about 500 miles (800 km) above Venus’ surface on July 11, 2020 – when it found the surprising signal.
I was just so excited to have new data from Venus.
The measurements from Parker Solar Probe are the first new direct measurements of Venus’ atmosphere in nearly 30 years. The results also show that the atmosphere undergoes changes during the sun’s 11-year solar cycle, and that it is now quite different from what it was in the past.
How did the spacecraft detect the radio signal?
It did so using its FIELDS instrument, which measures electrical and magnetic fields in the atmosphere. The signal was detected when the spacecraft was closest to the planet, for a period of only seven minutes. Collinson saw it in the data and recognized it, saying:
Then the next day, I woke up. And I thought, ‘Oh my god, I know what this is!’
He had seen the exact same kind of radio signal before from the Galileo orbiter, which had explored Jupiter and its moons. That mission ended in 2003. In this case, the signal, which looked like a thin “frown” in the data, was found in the ionospheres of some of the moons. The similarity of the signal meant that Parker Solar Probe had unexpectedly skimmed through the upper atmosphere of Venus, or more specifically, its ionosphere.
The ionosphere is a region of charged gases or plasma in the upper atmosphere that emits natural radio waves. Galileo had detected those radio waves at Jupiter’s moons, and now Parker Solar Probe had also found them in Venus’ atmosphere. The last time Venus’ ionosphere was measured directly was in 1992 by the Pioneer Venus Orbiter. The sun also happened to be at the peak of its solar cycle at the time.
For a long time afterward, however, there were no more Venus missions that could take new measurements. Instead, scientists relied on Earth-based telescopes, which showed that the ionosphere was changing while the sun’s solar cycle started to wane again and become calmer. During solar minimum, the ionosphere on Venus was at its thinnest.
Now, the new measurements from Parker Solar Probe support the previous findings. The close flyby occurred six months after solar minimum – the least active period on the sun – and the data showed that again, the ionosphere was significantly thinner than it was during solar maximum, the most active period. This confirms that the density of Venus’ ionosphere is directly correlated to the strength of the solar cycle. According to co-author Robin Ramstad:
When multiple missions are confirming the same result, one after the other, that gives you a lot of confidence that the thinning is real.
As outlined in the paper:
On July 11, 2020, NASA’s Parker Solar Probe made a close flyby of Venus. During the 7 minutes around the closest approach, one of its scientific instruments detected low-frequency radio emission of a type naturally generated by planetary ionospheres. By measuring the frequency of this emission, we can directly calculate the density of the ionosphere around Parker, finding it to be far less dense than previous missions have encountered. This supports the theory that the ionosphere of Venus varies substantially over the 11-year solar cycle.
Why is this important?
Scientists want to better understand how Venus changed from a former habitable world, according to the latest research, to the inhospitable inferno we see today. The fact that the planet’s ionosphere thins at solar minimum can provide valuable clues as to how the sun affects Venus’ atmosphere. It is also known that the planet’s ionosphere can “leak” into space. Changes like that can tell scientists how the atmosphere has evolved over time.
The discovery of the radio signal is a fortuitous development, since Parker Solar Probe’s main mission is to study the sun, not Venus. As Parker Solar Probe project scientist Nour Raouafi commented:
The goal of flying by Venus is to slow down the spacecraft so that Parker Solar Probe can dive closer to the sun. But we would not miss the opportunity to gather science data and provide unique insights into a mysterious planet such as Venus.
Parker Solar Probe took some stunning images of Venus during its flyby, which NASA released on February 24, 2021. It also imaged the entire dust ring that orbits the sun along Venus’ orbit, the first time that has ever been accomplished by any spacecraft.
There are no current U.S. missions at Venus, so these close flybys provide a great opportunity to gather more data. As Collinson noted:
To see Venus now, it’s all about these little glimpses.
Bottom line: NASA’s Parker Solar Probe has detected an unexpected natural radio signal being emitted by Venus’ ionosphere.