Parker Solar Probe pulls back the veil on Venus’s night side

When flying past Venus in July 2020, Parker Solar Probe’s WISPR instrument, short for Wide-field Imager for Parker Solar Probe, detected a bright rim around the edge of the planet that may be nightglow — light emitted by oxygen atoms high in the atmosphere that recombine into molecules in the nightside. 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 charged particles — called 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. Credits: NASA/Johns Hopkins APL/Naval Research Laboratory/Guillermo Stenborg and Brendan Gallagher

An ultra-sensitive camera on NASA’s Parker Solar Probe caught an ethereal glimpse of the night side of Venus during a flyby last year, unexpectedly seeing through thick atmospheric haze to reveal part of the planet’s hellishly hot landscape.

Parker’s wide-field imaging instrument, known as WISPR, is designed to observe the large-scale structure of the sun’s atmosphere, or corona, while other instruments on the probe measure electric and magnetic fields, and solar wind particles. The mission launched from Cape Canaveral in August 2018 and has traveled closer to the sun than any previous spacecraft.

The mission is designed to study the origin of the solar wind, a fast-moving flow of particles streaming away from the sun in every direction. The solar wind drives space weather and influences the entire solar system, and can bring impacts such as communications and electricity blackouts, damage to satellites, and colorful auroral displays to Earth.

Named for Eugene Parker, who correctly predicted the existence of the solar wind in 1958, Parker Solar Probe will also investigate why the solar corona is many times hotter than the sun’s surface. The spacecraft is armored to withstand the extreme temperatures of the sun’s outer atmosphere, where temperatures reach 3.6 million degrees Fahrenheit (2 million degrees Celsius).

Parker Solar Probe will continue breaking its own record in the coming years. The spacecraft uses gravitational slingshot maneuvers around Venus to alter its trajectory, moving the closest point of its orbit — or perihelion — ever-closer to the sun.

The flybys with Venus also offer opportunities for bonus science. An image released by NASA last week shows the night side of Venus in stark contrast to a starry sky.

Parker’s wide-field imager captured the image of Venus during a flyby July 11, 2020, from a distance of 7,693 miles (12,380 kilometers) away, according to NASA. Developed at the Naval Research Laboratory, the WISPR instrument has two overlapping cameras that take pictures in visible light.

Scientists expected Parker Solar Probe’s images of Venus to show the planet’s sulfuric acid clouds, which typically hide Venus’s landscape. Instead, Parker’s cameras saw hints of the surface of Venus.

“WISPR is tailored and tested for visible light observations. We expected to see clouds, but the camera peered right through to the surface,” said Angelos Vourlidas, the WISPR project scientist from the Johns Hopkins Applied Physics Laboratory.

The picture released by NASA shows a section of Venus named Aphrodite Terra, the planet’s largest highland region. NASA says Aphrodite Terra appears darker than its surroundings because it is about 85 degrees Fahrenheit (30 degrees Celsius) cooler than neighboring regions.

The bright rim around Venus’s atmosphere may show a phenomenon known as nightglow created as oxygen atoms high above the surface recombine into molecules over the night side of the planet, NASA said. The streaks visible in the image may be the signature of cosmic rays, interplanetary dust grains, or materials from the spacecraft itself, officials said.

“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,” NASA said in a statement.

U.S. scientists coordinated Parker’s observations with the Japanese team managing the Akatsuki spacecraft, the only probe currently in orbit around Venus.

“WISPR effectively captured the thermal emission of the Venusian surface,” said Brian Wood, an astrophysicist and WISPR team member from the Naval Research Laboratory in Washington. “It’s very similar to images acquired by the Akatsuki spacecraft at near-infrared wavelengths.”

Artist’s concept of Parker Solar Probe flying near Venus. Credit: NASA/Johns Hopkins APL/Steve Gribben

NASA said scientists are studying whether the WISPR instrument might be sensitive to infrared light, which would be an “unforeseen capability” for the cameras. If they can see in infrared light, the cameras might be able to observe dust around the sun and in the inner solar system in a new way.

If it turns out WISPR is not sensitive to infrared light, the images of Venus’s surface “may have revealed a previously unknown ‘window’ through the Venusian atmosphere,” NASA said.

Parker Solar Probe’s flight plan includes seven flybys with Venus during its seven-year mission. The spacecraft completed its fourth Venus flyby Feb. 20, bending Parker’s orbit to set up for close approaches to the sun April 29 and Aug. 9. During those solar encounters, Parker will set a new record when it flies approximately 6.5 million miles (10.4 million kilometers) from the sun’s surface, about 1.9 million miles closer than on the mission’s previous perihelion Jan. 17.

Ground teams sent up commands for the WISPR instrument to take more pictures of Venus during the Feb. 20 flyby. That data will be downlinked to Earth by the end of April, according to NASA.

“We are really looking forward to these new images,” said Javier Peralta, a planetary scientist from the Akatsuki team, who first suggested a Parker Solar Probe campaign with Akatsuki. “If WISPR can sense the thermal emission from the surface of Venus and nightglow — most likely from oxygen — at the limb of the planet, it can make valuable contributions to studies of the Venusian surface.”

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