During a close flyby of Venus in July 2020, NASA’s Parker solar probe discovered something strange.
As it descends only 833 kilometers (517 miles) above the Venusian surface, the probe’s instruments record a low-frequency radio signal – a telltale sign that Parker has passed through the ionosphere, a layer of the planet’s upper atmosphere.
This was the first time an instrument could record directly on the spot measurements of the upper atmosphere of Venus 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 so excited to get new data from Venus,”
Venus is a fascinating world for us here on Earth. It is so similar to our own planet in size and composition, but it is so important: a toxic, burning hot hellish world that is probably completely inhospitable to life as we know it.
How the two planets could have become such radically different animals is of deep interest to planetary scientists and astrobiologists looking for other inhabited worlds there in the Milky Way.
Yet the missions to explore Venus were relatively few. It doesn’t make much sense to send landers; they cannot survive on the planet’s surface at 462 degrees Celsius (864 degrees Fahrenheit).
Sending orbital probes is also considered problematic due to the incredibly dense atmosphere of rain clouds of carbon dioxide and sulfuric acid, which make it difficult to understand what is happening on the surface.
For these reasons, Venus has not been a popular target for special missions for some time (Japanese orbiter Akatsuki is the recent exception) and much of our latest data comes on pieces of instruments with other primary purposes, such as the Parker Solar Probe.
As Parker fulfills his mission to study the Sun closely, he uses Venus for gravitational maneuvers – assistance around the planet to change speed and trajectory. It was on one of these gravity-assisting flies that the probe’s instruments recorded a radio signal.
Collinson, who has worked on other planetary missions, notices a strange knowledge that he can’t put exactly in the form of a signal.
“Then I woke up the next day,” he said. And I thought, Oh my God, I know what this is! “
It was the same type of signal recorded by the Galileo spacecraft as it passed through the ionospheres of Jupiter’s moons, a layer of atmosphere also seen on Earth and Mars where solar radiation ionizes atoms, resulting in charged plasma that produces low-frequency frequencies. .
Once the researchers understood the signal, they were able to use it to calculate the density of the Venus ionosphere and compare it to recent measurements until 1992. Charmingly, the ionosphere was an order of magnitude thinner in the new measurements than in 1992
The team believes this has something to do with solar cycles. Every 11 years the poles of the Sun change places; the south becomes north and the north becomes south. It is not clear what drives these cycles, but we know that the poles switch when the magnetic field is weakest.
As the Sun’s magnetic field controls its activity – such as sunspots (temporary areas of strong magnetic fields), solar flares and coronal mass ejections (produced by clicking and reconnecting magnetic field lines) – this stage of the cycle manifests itself as period of very minimal activity. It’s called the solar minimum.
Once the poles are switched, the magnetic field amplifies and solar activity rises to a solar maximum before weakening again for the next polar switch.
Measurements of Venus from Earth suggest that the ionosphere of Venus changes in sync with the solar cycles, becoming thicker at the solar maximum and thinner at the solar minimum. But without direct measurements, it was difficult to confirm.
Well, guess what? The 1992 measurement was made at a time close to the solar maximum; measurement in 2020 close to the solar minimum. Both were in line with ground measurements.
“When multiple missions confirm the same result, one after the other, it gives you a lot of 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 the ionosphere of Venus is unclear, but there are two leading theories.
The first is that the upper limit of the ionosphere can be compressed at a lower altitude during the solar minimum, which prevents the flow of atoms ionized on the day side to the night side, resulting in a thinner ionosphere on the night side. The second is that the ionosphere leaks into space at a faster rate during the solar minimum.
None of these mechanisms can be ruled out from Parker’s data, but the team hopes that future missions and observations may be able to clarify what is happening. In turn, this can help us better understand why Venus is the way it is compared to Earth.
Maybe it’s time for another mission to Venus, huh?
The study was published in Geophysical research letters.
Best Image Credit: Venus during Parker’s Flight in July 2020 (NASA / John Hopkins APL / Naval Research Laboratory / Guillermo Stenborg and Brendan Gallagher)