As it dived just 833 kilometers (517 miles) above the Venusian surface, the spacecraft’s instruments recorded a low-frequency radio signal – a telltale sign that Parker had flown over the ionosphere, a layer of the upper atmosphere of the planet.
It was the first time that an instrument could record directly in situ measurements of Venus’ upper atmosphere over nearly 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 have new data from Venus,” said astronomer Glyn Collinson of NASA’s Goddard Space Flight Center.
Venus is a fascinating world for us here on Earth. It is so similar to our own planet in size and makeup, yet so fundamentally different: a toxic, scorching hell world that is likely completely inhospitable to life as we know it.
How the two planets could have developed into radically different beasts is of deep interest to planetary scientists and astrobiologists looking for other habitable worlds in the Milky Way.
Yet the exploration missions of Venus have been relatively few. There is no point in sending landers; they cannot survive on the planet’s surface at 462 degrees Celsius (864 degrees Fahrenheit).
Sending probes into orbit is also seen as 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 hasn’t been a popular target for dedicated missions for quite some time (the Akatsuki orbiter from Japan being the recent exception), and much of our recent data has come piecemeal, from instruments with other main objectives, such as Parker Solar. Probe.
As Parker carries out his mission to study the Sun in detail, he uses Venus for gravity-assist maneuvers – slingshot around the planet to alter speed and trajectory. It was on one of these gravity-assisted overflights that the probe’s instruments recorded a radio signal.
Collinson, who worked on other planetary missions, noted a strange familiarity that he couldn’t quite place in the form of the signal.
“Then the next day I woke up,” he says. “And I thought, ‘Oh my god, I know what this is!’ “
This was the same type of signal recorded by the Galileo probe when it flew over the ionospheres of Jupiter’s moons – a layer of the atmosphere, also seen on Earth and Mars, where solar radiation ionizes atoms, resulting in a charged plasma which produces radio frequency emission.
Once the researchers figured out what the signal was, they were able to use it to calculate the density of the Venusian ionosphere and compare it to the last direct measurements taken, in 1992. Fascinatingly, the ionosphere was an order of magnitude. thinner in the new measures than it was in 1992.
The team believe it has something to do with solar cycles. Every 11 years, the Sun’s poles change place; south becomes north and north becomes south. It is not known what drives these cycles, but we do know that the poles switch when the magnetic field is at its weakest.
Because the Sun’s magnetic field controls its activity – such as sunspots (temporary regions of strong magnetic fields), solar flares, and coronal mass ejections (produced by magnetic field lines breaking and reconnecting) – this cycle stage manifests as a period of very minimal activity. This is called the solar minimum.
Once the poles are switched, the magnetic field strengthens and solar activity reaches a solar maximum before descending for the next pole switch.
Measurements of Venus from Earth suggested that Venus’ ionosphere was changing in sync with 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 taken at a time close to the solar maximum; the 2020 measurement close to the solar minimum. They were both consistent with Earth-based measurements.
“When several missions confirm the same result, one after the other, it gives you great confidence in the reality of the thinning,” said astronomer Robin Ramstad of the University of Colorado at Boulder.
It is not clear exactly why the solar cycle has this effect on the ionosphere of Venus, but there are two main theories.
The first is that the upper limit of the ionosphere could be compressed to a lower altitude during solar minimum, preventing ionized atoms on the day side from flowing to the night side, resulting in an ionosphere on the night side more thin. The second is that the ionosphere is leaking into space at a faster rate during solar minimum.
None of these mechanisms could be ruled out by Parker data, but the team hopes that future missions and observations can clarify what is going on. In turn, this could help us better understand why Venus is as it is, relative to Earth.
Maybe it’s time for another Venus mission, eh?
The research was published in Geophysical research letters.
Top image credit: Venus in Parker flyby in July 2020 (NASA / Johns Hopkins APL / Naval Research Laboratory / Guillermo Stenborg and Brendan Gallagher)