NASA’s Parker Solar Probe has traveled closer to the Sun than any other mission in history, and is already unraveling some of the star’s many secrets. When it's not Sun-surfing like some robotic daredevil, Parker swings out into the solar system to pass by Venus so that it can use the planet’s gravity to slow down.
Scientists are now elated to discover that Parker managed to record a radio signal in the atmosphere of Venus during one of these close passes, on July 11, 2020. The radio emission is natural in origin and was captured at an altitude of just 517 miles (833 km) above Venus’ surface and marks the first time the planet’s upper atmosphere has been directly measured since 1992, according to a study published on Monday in Geophysical Research Letters.
“I was just so excited to have new data from Venus,” said Glyn Collison, a Venus expert at NASA Goddard Space Flight Center who led the study, in a statement.
Though Venus has been visited by spacecraft and orbiters in recent decades, Parker’s new observations occurred at just the right time to shed new light on a persistent mystery about the cloudy planet’s reactions to the Sun’s moods over its 11-year solar cycle.
Planets and moons with atmospheres are usually sheathed by an upper layer called the ionosphere, so named because the Sun’s radiation ionizes particles inside of it. Earth, Venus, and Mars have ionospheres, as do all the giant gas planets in our solar system, and even some moons, like Jupiter’s Europa or Saturn’s Titan. Planetary ionospheres can produce natural radio emission created by the interactions of charged particles within them.
Because ionospheres are influenced by the Sun, scientists have suspected that the upper atmosphere of Venus might thicken and thin as our star cycles through its maximums, a period of peak solar activity, and its minimums, when solar flares and sunspots reach a lull.
NASA’s Pioneer Venus Orbiter was the last spacecraft to measure Venus’ ionosphere nearly 30 years ago, at a time when the Sun was approaching its maximum. Given that Parker made its July approach just a few months after the solar minimum, the mission had the fortuitous opportunity to directly test whether Venus’ ionosphere really does thin during the Sun’s quiet phase.
Captured with Parker’s FIELDS instrument, the radio signal reminded Collison of similar signatures detected by NASA’s Galileo orbiter in the ionospheres of Jupiter’s moons. His team was able to estimate the density of Venus’ atmosphere from the natural emission, revealing that the ionosphere was significantly thinner during the recent solar minimum.
“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,” Collison and his colleagues wrote in the study. “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.”
The discovery not only resolved a decades-long puzzle, it also reveals details about the weird dynamics of Venus’ skies, a tantalizing aerial environment that some scientists think may be habitable. In 2020, scientists announced a tentative detection of phosphine in Venus’ atmosphere, a chemical sometimes associated with life, though the results of that study are still heavily contested.
While it’s still very unclear whether the Venusian skies could host life, Parker has placed one more piece into the puzzle of the planet’s complex atmosphere, which will have implications for future research. The probe may be focused on the Sun, but that doesn’t mean it can’t occasionally moonlight as a Venus explorer.