April 15, 2021
NASA’s Parker Solar Probe captured the first complete view of Venus’ dust ring, a band of particles that stretches for the entirety of the planet’s path around the Sun. The new images, published April 6 in The Astrophysical Journal, cover nearly the entire 360-degree view of the ring, completing a picture that scientists had seen only hints of before, with images from the Helios probes in the 1970s and multiple observations from NASA’s twin Solar Terrestrial Relations Observatory (STEREO) probes from 2007 to 2014.
“This is the first time that a circumsolar dust ring in the inner solar system could be revealed in its full glory in ‘white light’ images,” said study lead author Guillermo Stenborg from the U.S. Naval Research Laboratory in Washington, D.C. “I find that pretty special.”
While Parker Solar Probe’s chief objective is to study the Sun’s corona and solar wind, the team planned from the mission’s outset to try and capture images of Venus’ dust ring using the spacecraft’s Wide-field Imager for Parker Solar Probe, or WISPR, instrument.
With two telescopes that together provide a field of view of more than 95 degrees, WISPR was built to capture wide-angle images of the solar wind in white light. Initially, the dust ring was revealed using images from the spacecraft’s third orbit around the Sun in August and September 2019, when it performed a series of rolling maneuvers to help manage its momentum. Those rolls incidentally made seeing the ring possible because it allowed for customized image processing to reveal faint, stationary features.
“That process didn’t erase the dust ring from the images,” as seems to have happened with images from the first two orbits, when the spacecraft didn’t perform those rolling maneuvers, explained Parker Solar Probe Project Scientist Nour Raouafi from the Johns Hopkins Applied Physics Laboratory in Laurel, Maryland. With that realization, the team went back and reprocessed the earlier images, and there, indeed, was the ring.
“It’s funny that spacecraft operations can sometimes lead to the discovery of new things,” Raouafi remarked, smiling. “It’s kind of amazing.”
The team at first thought the ring — which when resolved appears as a bright band stretching across the blackness of space — was a streamer, a coronal structure that glows thanks to electrons tied up within it. But that bright band continued to the outermost edge of WISPR’s field of view, very unlike a streamer, whose brightness would fall off the farther it stretches from the Sun.
Eliminating the possibility that it was an artifact from the image processing, the researchers plotted the orbits of several planetary objects known to have circumsolar dust rings (Earth, Venus and an odd group of asteroids called the Karin family) and found the bright band lined up perfectly with Venus’ orbit.
The Venus dust ring, like Earth’s ring and Mercury’s ostensible ring, is made up of microscopic particles thought to be from the cloud that formed our solar system, as well as the continuous crumble of comets and collision of asteroids.
But how the dust rings formed is still uncertain. Scientists have proposed two hypotheses, explained Russell Howard, a now-retired astrophysicist from the U.S. Naval Research Laboratory and study co-author. “One idea is that the dust rings naturally formed from the primordial cloud, but several researchers contend that each planet’s gravity has gradually trapped the particles, perhaps even asteroid or cometary particles within its orbit.”
In the second scenario, the dust particles might move in waves, staying in orbit until some are ejected, usually by bumping into one another, while others move in to take their place.
Imaging dust rings, such as the Venus circumsolar ring, opens a new window into how dust is captured and redistributed throughout the solar system, Stenborg said. “We’re learning things about the dynamics, the exchanges, of dust particles throughout the heliosphere that before Parker Solar Probe we didn’t know.”
Still, general questions remain about Venus’ dust ring, including its density and radial extent — details that could also help shed light on its origin. The team hopes to use the European Space Agency and NASA’s Solar Orbiter mission, whose orbit around the Sun takes it beyond Venus’ path and high above the ecliptic plane, to provide a new vantage that could provide answers.
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