During two gravity-assist flybys on July 11, 2020 and February 20, 2021, the Wide-Field Imager for Parker Solar Probe (WISPR) imager on board NASA’s Parker Solar Probe observed the night side of Venus and unexpectedly penetrated its thick atmosphere, detecting thermal emission from the planet’s surface, making this the first detection of the Venusian surface by an optical telescope. The thermal emission is detectable due to the high temperature of the Venusian surface, which even on the night side reaches an estimated 462 degrees Celsius (864 degrees Fahrenheit). The new WISPR images also show a bright rim of emission at the limb, associated with nightglow emission from molecular oxygen, somewhat analogous to auroral emissions observed at Earth.
The first WISPR images of Venus were taken on July 11, 2020 as Parker Solar Probe embarked on its third flyby, which the spacecraft uses to bend its orbit closer to the Sun.
WISPR was designed to see faint features in the solar atmosphere and wind, and some scientists thought they might be able to use the camera to image the cloud tops veiling Venus as Parker passed the planet.
“The objective was to measure the speed of the clouds,” said WISPR project scientist Dr. Angelos Vourlidas, a researcher at Johns Hopkins University Applied Physics Laboratory.
But instead of just seeing clouds, WISPR also saw through to the surface of the planet.
The images were so striking that the researchers turned on the cameras again during the fourth pass on February 20, 2021.
During the 2021 flyby, Parker’s orbit lined up perfectly for WISPR to image Venus’ night side in entirety.
“We’re thrilled with the science insights Parker Solar Probe has provided thus far,” said Dr. Nicola Fox, division director for the Heliophysics Division at NASA Headquarters.
“Parker continues to outperform our expectations, and we are excited that these novel observations taken during our gravity assist maneuver can help advance Venus research in unexpected ways.”
When flying past Venus on July 11, 2020, the WISPR instrument onboard Parker Solar Probe detected a bright rim around the edge of the planet that may be nightglow. The prominent dark feature in the center of the image is Aphrodite Terra, the largest highland region on the Venusian surface. Bright streaks in WISPR, such as the ones seen here, are typically caused by a combination of cosmic rays, sunlight reflected by grains of space dust, and particles of material expelled from the spacecraft’s structures after impact with those dust grains. The number of streaks varies along the orbit or when the spacecraft is traveling at different speeds, and scientists are still in discussion about the specific origins of the streaks here. The dark spot appearing on the lower portion of Venus is an artifact from the WISPR instrument. Image credit: NASA / Johns Hopkins Applied Physics Laboratory / Naval Research Laboratory / Guillermo Stenborg / Brendan Gallagher.
“Venus is the third brightest thing in the sky, but until recently we have not had much information on what the surface looked like because our view of it is blocked by a thick atmosphere,” added Dr. Brian Wood, a physicist at the Naval Research Laboratory and lead author of a paper published in the journal Geophysical Research Letters.
“Now, we finally are seeing the surface in visible wavelengths for the first time from space.”
Clouds obstruct most of the visible light coming from Venus’ surface, but the very longest visible wavelengths, which border the near-infrared wavelengths, make it through.
On the dayside, this red light gets lost amid the bright sunshine reflected off Venus’ cloud tops, but in the darkness of night, the WISPR cameras were able to pick up this faint glow caused by the incredible heat emanating from the surface.
The WISPR images show features on the Venusian surface, such as the continental region Aphrodite Terra, the Tellus Regio plateau, and the Aino Planitia plains.
Since higher altitude regions are cooler than lower areas, they show up as dark patches amidst the brighter lowlands.
These features can also be seen in previous radar images, such as those taken by NASA’s Magellan mission.
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Brian E. Wood et al. Parker Solar Probe Imaging of the Night Side of Venus. Geophysical Research Letters, published online February 9, 2022; doi: 10.1029/2021GL096302
