During its third flyby of Venus, NASA’s Parker Solar Probe recorded natural radio emissions from within the Venusian atmosphere. The finding is confirmation that Venus’s upper atmosphere undergoes major changes in accordance with the Sun’s 11-year cycle, providing new insights into this enigmatic — and utterly hostile — Earth-like planet.
The Parker Solar Probe was designed to study the Sun, but some of its best work so far has revolved around Venus. Launched in 2018, the spacecraft is using Venus’s gravity to inch increasingly closer to the Sun. These flybys will eventually place Parker to within 6.9 million km of our host star, allowing the probe to study the solar winds and corona.
These gravitational assists have proven fruitful, as Parker’s instruments are smartly being used to study Venus. Data acquired by the probe recently allowed astronomers to capture our first complete view of Venus’s orbital dust ring, and — quite unexpectedly — to peer through the clouds and visualise the planet’s toasty surface.
And now, as new research published in Geophysical Research Letters shows, Parker has managed to discover natural radio emissions from within the Venusian atmosphere. Venus expert Glyn Collinson from the Heliophysics Science Division at NASA’s Goddard Space Flight Centre led the new research.
Parker took the readings on July 11, 2020, when it was performing its third flyby of Venus. That astronomers are using these moments to study the planet makes total sense, as we still have much to learn about this planet, which is very similar to ours in terms of its size, chemical composition, and location within the solar system. But famously, while Earth is teeming with life, Venus is a scorching hellhole.
A possible reason for this big difference is that Earth features a protective magnetic field, and Venus does not. Our magnetic field could be a contributing factor to habitability, as it prevents our atmosphere from bleeding out into space. At least, that’s the theory. By this logic, Venus, without a magnetic field, should feature an atmosphere that leaks out into space during periods of intense solar activity. Trouble is, observations made by ground-based telescopes have shown the opposite to be the case, revealing thinner ionospheres — the very top of the atmosphere — during periods in which the Sun is least active. It presented a major puzzle.
That’s why the third flyby of Venus, in which Parker came to within 517 miles (833 km) of the Venusian surface, was so important, as scientists weren’t entirely sure about the trustability of the remote-sensing data. For a period of seven minutes, the probe took measurements of Venus’s top atmosphere, which it did using its onboard FIELDS instrument (this tool will later be used to study the Sun’s electric and magnetic fields). Unlike the radio in your car, this instrument simultaneously scans frequencies within the entire radio spectrum.
At first, Collinson didn’t know what to make of the new Parker data, but then he remembered seeing something similar from NASA’s Galileo orbiter, which explored Jupiter and its moons. The frown-like signal detected by Parker was just like the signal picked up by Galileo when the probe zipped through the ionospheres of the Jovian moons.
The Parker Solar Probe, Collison realised, had actually travelled through Venus’s atmosphere, providing the first direct measurement of the Venusian atmosphere in nearly 30 years. Parker had detected natural low-frequency radio emissions, which are associated with planetary ionospheres — an atmospheric region packed with plasma, or charged gases.
These radio emissions allowed Collinson to calculate the density of Venus’s ionosphere, or at least the part of the ionosphere explored by Parker. His team compared these findings to data recorded by NASA’s Pioneer Venus Orbiter.
When the Pioneer probe visited Venus in 1992, the Sun was near the maximum point of activity in its 11-year solar cycle. The “cool thing about Parker is that its flyby happened six months after the solar minimum,” Collinson explained during a phone interview, allowing him to “nail down the ionosphere” during this period, he said.
“We were able to mathematically prove significant differences between this atmosphere and the one Pioneer saw many years ago,” added Collinson.
As the data showed, Venus’s atmosphere appeared to be considerably thinner compared to the previous measurements taken during the solar maximum, which actually confirms observations made by ground-based telescopes.
“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,” wrote the scientists in their paper. “This supports the theory that the ionosphere of Venus varies substantially over the 11 year solar cycle.”
Indeed, the team was “able to confirm what we previously guessed from the remote-sensing measurements,” explained Collinson.
So a very good result, as this “variance was expected,” said Collinson, but planetary scientists now have a big mystery on their hands: Why is this happening? Venus, it would appear, is prone to atmospheric leakage, resulting in the escape of plasma into space — but not during periods in which the Sun is most active.
“This tells us that we really don’t have a good understanding of how Earth’s closest sister planet works,” said Collinson. “This is an indication that there’s an Earth-like planet which experiences huge changes in its upper atmosphere, revealing mechanisms we don’t fully understand.”
Collinson said the new data provides a “tantalising clue about how Venus works,” and we should now compare this to how things work here on Earth. By doing so, we might be able to figure out “what makes a planet habitable and why we’re here” and not on Venus, he said.