Almost all of the photographs of Pluto's unusually complex surface taken by NASA's New Horizons mission in 2015 were of the side illuminated by the sun. The dwarf planet's other hemisphere was cloaked in darkness. Some of it had not seen the sun in decades, such as the region near the south pole.

However, Pluto's dark side has come into dim view, thanks to the light of the dwarf planet's moon.

Seeing Pluto's Dark Side

Scientists from the expedition have now revealed a blurry image of the dwarf planet's dark side. The researchers detail the photo-taking procedure and what it shows about Pluto's nitrogen cycle and its atmosphere in the Planetary Science Journal on October 20.

The scientists thought that the dwarf planet's biggest moon, Charon, may reflect enough light to reveal the distant world's surface before New Horizons flew past Pluto. As a result, the researchers had the spacecraft return to the sun for a final look at Pluto.

Initially, the photographs showed a ring of sunlight passing across Pluto's thick atmosphere (SN: 7/24/15). "It's like trying to read a street sign while driving toward the setting sun with a dirty windshield," says planetary scientist John Spencer of the Southwest Research Institute in Boulder, Colo. "It's tough to see anything in that glare."

Spencer and his colleagues took a few steps to allow features of Pluto's dark side to emerge from the brightness. To begin, the researchers had the spacecraft take 360 brief photos of the dwarf planet's illuminated surface. To avoid overexposing the images, each was just 0.4 seconds long. The crew also took photographs of the sun without Pluto in the frame so that the sun could be removed later.

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Initial Response

When Tod Lauer of the National Optical Astronomy Observatory in Tucson, Ariz., received the data in 2016, he analyzed the photos. Unfortunately, he didn't have the time to handle such a complex undertaking because the rest of the material from New Horizons was still fresh and took up most of his focus.

But, as Lauer describes it, "it was something that just sat there and ate away at me." So, in 2019, he attempted once again. The photographs were somewhat smeared or blurred since the spacecraft was moving while taking them. To eliminate the fuzz from each individual frame, Lauer used a computer program. The reflected Charon light in each of the hundreds of photos was combined to create a single image.

"We finally saw something emerge in the black there when Tod conducted that arduous study," Spencer says, "offering us a little bit of a view of what the dark pole of Pluto looks like."

Carly Howett, a planetary scientist at the Southwest Research Institute who is also on the New Horizons team but was not engaged in this investigation, believes that the team's achievement is outstanding. "It's, challenging to deal with this dataset," she says. "Congratulations to this group. This is not something I would have chosen to undertake."

Understanding Pluto

According to Howett, the image will aid scientists in understanding how Pluto's freezing nitrogen atmosphere changes throughout its decades-long seasons. The amount of nitrogen in the gaseous air and how much is frozen on Pluto's surface control the atmosphere. The atmosphere grows thicker as more nitrogen ice evaporates. On the other hand, the atmosphere might completely collapse if too much nitrogen freezes the earth.

Pluto's south pole seemed darker than its north pole when New Horizons arrived. Even though it was approaching winter, this indicates that there was not a lot of new nitrogen frost freezing out of the sky there. "The last summer ended decades ago," Spencer explains, "but Pluto cools off slowly." "Perhaps it's still too warm for frost to form there, preventing the atmosphere from collapsing."

In the center of the photograph, a bright patch appeared that might be a fresh ice deposit. That's also not surprising, according to Howett. As Pluto advances deeper into its winter, the ice may still be flowing from the north pole to the south pole.

"This has been on our minds for a long time. But, she adds, "It makes sense." "However, it's encouraging to see it happen."

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