A group of astronomers have photographed the dark side of Pluto. This is special, because in that part of the dwarf planet the night has been shadowed for more than 20 years. It is now illuminated only indirectly by Pluto’s moon Charon. Picture Researchers introduced last month in Planetary Science Journal.
“They’ve really done a feat in terms of what you can glean from these observations. He’s so clever that they did it,” says Bert Vermeersen, professor of planetary sciences at TU Delft. He was not involved in the investigation.
Pluto is a dwarf planet at the edge of the solar system that revolves around the sun in 248 Earth years. This extreme tropical period also results in long seasons. The different seasons appear in a similar way to what happens on Earth: because the axis of rotation, around which the planet revolves every day, is skewed with respect to the plane in which the planet revolves around the Sun. This angle to the Earth is 23 degrees. On Pluto, this angle is much larger, about 120 degrees. This ensures that a large part of the planet lies in the arctic circles and receives alternate decades of continuous sunlight, or no light at all. There the days and nights can be very long depending on the season. By comparison, if the Earth were skewed like Pluto, then all of North America and Europe would see long months of midnight sun every year and long nights in winter, According to NASA. At the south pole of Pluto, it has been night for twenty years now, And it will remain that way for another eighty years.
The decades-long darkness makes this part of Pluto difficult to see. In order to be able to see something, the researchers cleverly used the reflection of sunlight from the moon Charon. It is half the size of Pluto, making it the largest known natural moon relative to its parent planet. That is why the pair Charon and Pluto are called the only double planet in the solar system. The centers of mass of the Moon and Earth lie within the Earth, but the centers of the two dwarfs lie outside of Pluto. So Charon is not really orbiting around Pluto, but around a point between the two dwarfs.
But whatever I qualify as Charon, his role in this search is to reflect sunlight off part of Pluto’s shadow side. Because of the large distance from the Sun, the moonlight of Charon from Pluto is about Five times weaker From a full moon seen from Earth. NASA’s New Horizons spacecraft captured this bright side of Pluto as it became the first spacecraft to cross the dwarf planet in 2015. New Horizons captured a series of 360-degree images. All of them are overexposed, due to the backlight of the sun. To filter this, the researchers used a powerful example of image processing. For example, New Horizons also took a 360-degree image with the spacecraft’s same position relative to the Sun, but without Pluto in between. Based on this, the researchers made a model of the backlight in order to filter it. This is how they “fix” the images. Then they stacked those 360 photos into one.
Grainy and understated details
The image is grainy and has few details, but this is not surprising given the exceptionally limited lighting. The image shows that Pluto’s southern hemisphere is generally darker than the northern hemisphere. The lighter the surface, the more methane and nitrogen ice. So not much ice formed in the Southern Hemisphere, according to the researchers. This is not surprising, since summer ended only thirty years before New Horizons passed. That’s in a nutshell by Pluto standards. The beating is a distinctly lighter spot in the same area. These are likely glacial deposits, where nitrogen and methane have been sublimated from the atmosphere into the ice due to the cold.
Although the image-editing technology is very clever, the researchers offer little explanation for the image, Vermiersen says. ‘Everything is reasonable’ sincere. It confirms what we already expect about changes in ice cover with seasons.” According to Vermeersen, it would be great if another research group could continue with these findings. “I think there is still some work to do in interpreting these observations.”
A version of this article also appeared on NRC on the morning of November 10, 2021
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