During a short swing past Venus, NASA̵
Born of similar processes, Earth and Venus are twins: both stony and of the same size and structure. But their paths deviated from birth. Venus lacks a magnetic field, and the surface broiler at temperatures hot enough to melt lead. The room has at most only ever survived a few hours there. Studying Venus, harmless as it is, helps scientists understand how these twins evolved and what makes Earth-like planets habitable or not.
On July 11, 2020, Parker Solar Probe swung off Venus in its third flying city. Each flyby is designed to utilize the planet’s gravity to fly the spacecraft closer and closer to the sun. The mission – managed by the Johns Hopkins Applied Physics Laboratory in Laurel, Maryland – made its nearest flight of Venus yet and passed only 833 km above the surface.
The data sonication in the video translates data from the Parker Solar Probes FIELDS instrument into audio. FIELDS discovered a natural, low-frequency radio emission as it moved through Venus’ atmosphere, helping scientists calculate the density of the planet’s electrically charged upper atmosphere, called the ionosphere. Credit: NASA’s Scientific Visualization Studio / Mark SubbaRao / Glyn Collinson
“I was just so excited to have new data from Venus,” said Glyn Collison of NASA’s Goddard Space Flight Center in Greenbelt, Maryland, the lead researcher in the study published today (May 3, 2021) in Geophysical research letters. A Venus expert, Collinson, has reviewed all available Venus data – from previous missions such as NASA’s Pioneer Venus Orbiter and ESA’s (European Space Agency) Venus Express – several times.
One of Parker Solar Probe’s instruments is FIELDS, named after the electric and magnetic fields it measures in the sun’s atmosphere. In just seven minutes – when the Parker Solar Probe was closest to Venus – FIELDS detected a naturally low-frequency radio signal. The thin wrinkle in the data caught Collinson’s attention. The shape and strength of the signal seemed familiar, but he could not place it. “So the next day I woke up,” he said. “And I thought, ‘Oh my god, I know what this is!'”
Collinson recognized the signal from his previous work with NASA’s Galileo orbiter, which explored Jupiter and its moons before the mission ended in 2003. A similar wrinkle appeared each time the spacecraft passed through the ionospheres of Jupiter’s moons.
Like Earth, Venus has an electrically charged gas layer in the upper edge of its atmosphere, called the ionosphere. This sea of charged gases, or plasmanaturally emits radio waves that can be detected by instruments such as FIELDS. When Collinson and his team identified this signal, they realized that the Parker Solar Probe had foamed Venus’ upper atmosphere – a pleasant surprise, though one they might have expected based on previous data, he said.
The researchers used this radio emission to calculate the density of the ionosphere through which the Parker Solar Probe flew. Scientists last obtained direct measurements of Venus’ ionosphere from the Pioneer Venus Orbiter in 1992. Thereafter, the sun was near the maximum of the sun, the stormy peak of the solar cycle.
In the years that followed, data from terrestrial telescopes suggested that major changes were taking place as the sun settled in its quiet phase, the solar minimum. While most of the atmosphere remained the same, the ionosphere – which is at the top where gases can escape into space – was much thinner below the sun’s minimum.
Without direct measurements, it was impossible to confirm.
Observations from Parker Solar Probe’s latest flyby, which took place six months after the last solar minimum, confirm the puzzle in Venus’ ionosphere. In fact, Venus’ ionosphere is much thinner compared to previous measurements taken below the sun’s maximum.
“When several missions confirm the same result, one after the other, it gives you great confidence that the thinning is real,” said Robin Ramstad, a study author and postdoctoral researcher at the Laboratory of Atmospheric and Space. Physics at the University of Colorado, Boulder.
Understanding why Venus’ ionosphere thins near the Sun’s minimum is part of revealing how Venus responds to the Sun – which will help scientists determine how Venus, once so similar to Earth, became a world of burning, toxic air as it is today. For example, Venus’ ionosphere tends to leak, which means that energy from gases enters space. Collecting data on this and other changes in the ionosphere is the key to understanding how Venus’ atmosphere has evolved over time.
This study was about 30 years in the making. It took a mission to Venus, and decades later, a state-of-the-art mission to the sun. “The goal of flying with Venus is to slow down the spacecraft so that the Parker Solar Probe can dive closer to the Sun,” said Nour E. Raouafi, Parker Solar Probe project researcher at the Applied Physics Laboratory. “But we will not miss the opportunity to collect science data and provide unique insights into a mysterious planet like Venus.”
Collinson compared the research to lifts. Venus scientists were eager to return from Parker Solar Probe’s flyby for new data and views of the Earth’s twin planet. “To see Venus now is all about those little glimpses,” he said.
Reference: “Depleted Plasma Densities in the Ionosphere of Venus Near Solar Minimum From Parker Solar Probe Observations of Upper Hybrid Resonance Emission” by Glyn A. Collinson, Robin Ramstad, Alex Glocer, Lynn Wilson III and Alexandra Brosius, May 3, 2021, Geophysical research letters.
DOI: 10.1029 / 2020GL092243