Last July, NASA's Parker Solar Probe swung around the planet Venus during a gravity-assist maneuver designed to bring the spacecraft ever closer to the sun. During its seven-year mission the probe will whip around Venus seven times. Each time it does, the planet's gravity bends the spacecraft's orbit closer and closer to the sun.
NASA is putting Parker in the hot seat so it can better study the solar wind — the high-speed stream of particles our star pew pews — close to its source rather than 93 million miles away. By mission's end in late 2025, the spacecraft will skim just 4 million miles (6 million km) from the sun's searing surface.
As during any journey there are often surprises along the way. During the mission’s third Venus gravity assist on July 11, 2020, Parker's wide-field camera called WISPR (Wide-field Imager for Parker Solar Probe) captured a striking and unexpected image of the planet’s nightside from 7,693 miles (12,381 km) away.
WISPR is designed to take photos in visible light just like your camera does. Its subjects are the solar corona, the pearly bonnet of light surrounding the blackened moon during a total solar eclipse, as well as the solar wind and its structures as they approach and fly by the spacecraft.
At Venus, the camera detected a bright rim around the edge of the planet from airglow. Space station astronauts have photographed a similar phenomenon that shows up as a green or yellowish "rind" around the circumference of the Earth from orbit. It's also visible as streaks of faint light to ground observers under dark skies. On Venus, ultraviolet (UV) light in sunlight breaks up carbon dioxide (CO2) molecules, liberating oxygen (O) atoms. The atoms travel around to the nightside of the planet and recombine into oxygen molecules (O2), emitting infrared and other colors of light in the process.
Venusian airglow is a fascinating topic of its own, but here's the really bizarre thing. No one expected to see that prominent, dark feature just above the center of the image. Named Aphrodite Terra, it's the largest highland region on Venus, comparable in size to Africa. The feature appears dark because it's about 85° F (30° C) cooler than its surroundings.
Remember, WISPR is a visual light camera, so naturally, scientists expected to see nothing but clouds, which Venus possesses in abundance. On the dayside alone, the cloud layer is some 12.4 miles (20 km) thick. So by what magic did Parker's camera see all the way down to the surface?
“WISPR effectively captured the thermal (heat) emission of the Venusian surface,” said Brian Wood, an astrophysicist and WISPR team member. Heat emission is another word for light emitted in the infrared part of the spectrum. If you ever wondered what color shines just beyond the red edge of the rainbow arc, it's infrared. Pity, we can't see it with the human eye.
This pleasant surprise sent the WISPR team back to the lab to test the camera's sensitivity to heat emissions. If WISPR can somehow see infrared light, scientists could use it to photograph dust around the sun and in the inner solar system. Dust glows in infrared when it absorbs sunlight. How serendipitous would that be?
But WISPR's apparent superpowers may have a more intriguing explanation. If the team discovers the camera is blind to infrared, it may have revealed a previously unknown “window” through the Venusian atmosphere instead. And that would be an exciting new discovery. Good news either way.
Similar nightside observations were planned on Feb. 20, 2021 during the fourth flyby. Data and images from that encounter are expected to arrive by the end of April, so we should know what's up soon. Sometimes in science, it's what you don't anticipate that leads to the most interesting discoveries. Already, a mission sent to the sun has unintentionally enlightened us about Venus. And who knows what else may be in the pipeline. Stay tuned.
"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at duluthnewstribune.com/astrobob.