During a close flight from the planet Venus in July 2020, NASA’s Parker Solar Probe discovered something strange.
As it dipped just 833 kilometers above the Venusian surface, the probe’s instruments detected a low-frequency radio signal – a sign that Parker had foamed through the ionosphere, a layer of the planet’s upper atmosphere.
This was the first time an instrument had been able to record directly in situ measurements of Venus’ upper atmosphere for almost three decades, and the recorded data give us a new understanding of how Venus changes in response to cyclical changes in the sun.
“I was just so excited to have new data from Venus,”
Venus is a fascinating world for us here on Earth. It looks so much like our own planet in size and composition, but so much different: a toxic, burning hot hell world that is probably completely uneven for life as we know it.
How the two planets could have evolved into such radically different animals is of deep interest to planetary scientists and astrobiologists searching for other habitable worlds out there in the Milky Way.
Yet missions to explore Venus have been relatively few. There is not much to send landers; they cannot survive the planet’s surface at 462 degrees Celsius (864 degrees Fahrenheit).
Dispatch of orbital probes is also considered problematic due to the incredibly thick atmosphere of carbon dioxide and sulfuric acid rain clouds that make it difficult to tell what is happening on the surface.
For these reasons, Venus has not been a popular target for dedicated missions for some time (Japan’s Akatsuki orbiter is the recent exception), and much of our recent data has come from instruments with other primary targets, such as the Parker Solar Probe.
As Parker performs his mission to study the sun in detail, it has used Venus for gravity-assisted maneuvers – slinging around the planet to change speed and orbit. It was on one of these gravity-assisted flybys that the probe’s instruments recorded a radio signal.
Collinson, who has worked on other planetary missions, noted a strange intimacy that he could not quite position himself in terms of the signal.
“So the next day I woke up,” he said. “And I thought, ‘Oh my God, I know what this is!'”
It was the same kind of signal picked up by the Galileo probe as it skimmed through the ionospheres of Jupiter’s moons – a layer of atmosphere, also seen on Earth and Mars, where solar radiation ionizes the atoms, resulting in a charged plasma producing low radio frequency emission.
Once scientists understood what the signal was, they were able to use it to calculate the density of the Venusian ionosphere and compare it with the last direct measurements made back in 1992. Fascinatingly, the ionosphere was an order of magnitude. . thinner in the new measurements than it was in 1992.
The team believes this has something to do with solar bikes. Every 11 years, the sun’s poles change places; south becomes north and north becomes south. It is not clear what drives these cycles, but we know that the poles change when the magnetic field is at its weakest.
Because the sun’s magnetic field controls its activity – such as sunspots (temporary regions with strong magnetic fields), sunbeams and coronal mass emissions (produced by magnetic field lines that snap and reconnect) – this step in the cycle manifests as a period of very minimal activity. It is called solar minimum.
Once the poles are shifted, the magnetic field strengthens and solar activity rises to a solar maximum before being lowered again to the next polar switch.
Measurements of Venus from Earth suggested that Venus’ ionosphere changed synchronously with the solar cycles, becoming thicker at the sun’s maximum and thinner at the sun’s minimum. But without direct measurements, it was difficult to confirm.
Guess what? The 1992 measurement was taken at a time close to the maximum of the sun; The 2020 measurement close to the sun’s minimum. They were both consistent with the ground-based measurements.
“When multiple missions confirm the same result, one after the other, it gives you great confidence that the thinning is real,” said astronomer Robin Ramstad of the University of Colorado, Boulder.
Exactly why the solar cycle has this effect on Venus’ ionosphere is unclear, but there are two leading theories.
The first is that the upper limit of the ionosphere could be compressed to a lower altitude below the minimum of the sun, preventing atoms ionized on the day side from flowing to the night side, resulting in a thinner night side ionosphere. The second is that the ionosphere leaks faster into space below the minimum of the sun.
None of these mechanisms could be ruled out by the Parker data, but the team hopes that future missions and observations may clarify what is going on. In turn, it can help us gain a better understanding of why Venus is as it is compared to Earth.
Maybe it’s time for another Venus mission, right?
The research is published in Geophysical research letters.
Credit for the top image: Venus under Parker’s airport in July 2020. (NASA / Johns Hopkins APL / Naval Research Laboratory / Guillermo Stenborg and Brendan Gallagher)