While staring at the sun is its main mission, NASA’s Parker Solar Probe will use every opportunity to return data to Earth.
The planet Venus represents just such an opportunity, or rather seven of them. Seven times during its mission, the probe will swing around Venus for a gravity assist, using the planet’s gravity as a slingshot for course and speed corrections as it draws closer to the sun.
The solar probe made the third of these maneuvers on July 11, 2020, and as it got closer, it took a glamorous photo of the planet’s night side using the Wide-field Imager for Parker Solar Probe (WISPR) instrument.
Parker isn’t the only probe taking pictures of Venus as it makes its way through the inner solar system. BepiColombo, a joint European and Japanese space agency Mercury probe, captured a video of Venus performing a gravity maneuver last year.
BepiColombo’s flight to Venus. (ESA / BepiColombo / MTM)
Those images show the planet as relatively smooth and characterless. That’s not surprising at all – Venus is shrouded in a thick, toxic atmosphere with clouds of mostly sulfuric acid that reflect about 70 percent of the light that falls on it. Hence, Venus is one of the brightest objects in the night sky.
The Parker team expected to see a similar bland sphere, but that’s not what they saw when they processed the WISPR data.
If you look at the image, you can see a bright glow around the edge of the planet. That, the team thinks, is nightglow.
This is produced by atoms in the upper atmosphere. On the day side of the planet, solar radiation splits carbon dioxide in the upper atmosphere into oxygen and carbon monoxide. When night falls, the atoms recombine into carbon dioxide, creating a glow.
This is something that also occurs on Earth and Mars, and it has been seen before on Venus; its presence in the Parker image is not surprising.
Neither do the white stripes – while the Parker team isn’t sure what they are, there are a number of candidates, including dust, cosmic rays, material thrown from the spacecraft after being hit by dust, or some combination of all of these .
(NASA / Johns Hopkins APL / Naval Research Laboratory / Guillermo Stenborg and Brendan Gallagher)
What’s surprising is that dark spot on the face of the planet. That is an area called the Aphrodite Terra, the largest highland area on the planet’s surface.
Designed to image the solar corona and coronal ejections, WISPR is optimized for visible light observations – but somehow it peered through the clouds of Venus.
However, the scientists think they know what happened. Venus currently has one active mission, the Japanese space agency’s Akatsuki probe. It sends back similar images, taken with the infrared camera, which are sensitive to temperature changes.
The Aphrodite Terra, with its greater elevation, is a lot cooler than the surrounding terrain, so it would be visible in infrared or near infrared images of the planet.
“WISPR has effectively captured the thermal emission from the Venusian surface,” said astrophysicist and WISPR team member Brian Wood of the US Naval Research Laboratory. “It is very similar to images taken by the Akatsuki spacecraft at near infrared wavelengths.”
This means that WISPR may be more sensitive to infrared light than it was designed for – which in turn opens up new possibilities for Parker’s main mission to study the sun. The Parker team is currently reviewing the instrument’s specifications to find out exactly what it did.
“Regardless,” said WISPR project scientist Angelos Vourlidas of the Johns Hopkins Applied Physics Laboratory, “we have some exciting scientific opportunities ahead.”