A GIF posted to Twitter last week captured an intriguing sight: a black-and-white scene of a "snowstorm" on the surface of a far-flung comet.
The scene is a clip Twitter user landru79 recently created from 25 minutes’ worth of images from the European Space Agency (ESA) archives. Captured on June 1, 2016 by the Rosetta spacecraft, the images feature towering cliffs of the rubber-ducky shaped comet 67P/Churyumov–Gerasimenko surrounded in what appears to be falling snow.
The series of images have recently caused quite a stir and sparked rampant speculation as to the cause. While the view is real, the “snowstorm” is largely an illusion—a crazy combination of apparent star motion in the background and dust and cosmic rays in the foreground. As Mark McCaughrean, senior advisor science and exploration at the ESA, writes in an email to Smithsonian.com: “Things are not quite as they seem.”
In 2014, ESA’s Rosetta entered its orbit around Comet 67P, becoming the first to circle a frosty space rock. For two years, it circled around the comet, gathering some impressive data in the process, which it beamed back to Earth. ESA recently released some of these breathtaking images to the public, resulting in the mind-bending new GIF.
The driving force behind the snowy illusion is the dense backdrop of stars that appear to form a curtain of falling snow in the distance. “But of course, they’re not ‘falling,’” McCaughrean writes. The motion of the stars is the result of a combination of Rosetta’s changing position as it snaps each image and the comet’s rotating motion.
On the right side of the image are stars from the constellation Canis Major. And in the upper-left hand corner of the image, star cluster NGC 2362 makes an appearance. McCaughrean identified this vibrant bundle of stars, which lie around 4,500 light years from Earth, using the website astrometry.net.
To create this sped-up GIF, landru79 would have rotated the original images by 90 degrees, McCaughrean notes. Without this rotation, “the ‘snow stars’ would move sideways,” he writes. “Of course, as there's no up or down in space, it's perfectly fine that landru79 chose to rotate the sequence, but it does suggest that it was a deliberate aesthetic choice to help create an illusion.”
Most of the flecks in the foreground of the GIF are actually particles floating far away from Comet 67P—and not on the surface of the icy world. Rosetta captured the images while circling some 13 kilometers (8 miles) away. At this distance, the craft's OSIRIS camera doesn’t have the sensitivity and resolution to pick up dust particles flying around directly above the comet's surface, says McCaughrean.
This foreground "snow" is likely part of the hazy envelope of dust, known as the coma, that commonly forms around the comet’s central icy body or nucleus. As comets pass close to the sun, the emanating warmth causes some of the ice to turn to gas, which generates a poof of dust around the icy nucleus.
And comet 67P certainly doesn’t lack dust. By mass, the comet is around 80 percent dust and just 20 percent ice, notes McCaughrean. This dusty abundance also suggests that most of the streaks in the foreground are dust particles rather than water or carbon dioxide ice.
The apparent “fluttering” motion of these particles is largely thanks to the movement of the spacecraft through 67P’s coma. As McCaughrean writes: “There's a preponderance of movement from bottom right to top left, I think, which suggests the motions aren't semi-random, as you'd expect in the cloud of slow-moving dust surrounding the comet.”
Since the images are compressed into a short GIF, the action appears much, much faster than how it occurred in real time. Here’s a (slightly) slower version Twitter user Avi Solomon posted:
Some of these streaks may also be the result of high-energy particles striking the camera, writes Ryan F. Mandelbaum for Gizmodo. But McCaughrean believes that the contribution of this effect, known as cosmic ray events, is minor.
One final telling detail that demonstrates the effect is more illusion than snowstorm is the relative amount of “snow” in the foreground and background. The stars create a densely speckled background, while only a few glimpses of white appear to zip across the comet’s surface. If it was truly a “snowstorm” the two should appear more equally populated.
“And yet it's all real: nothing fake about it,” writes McCaughrean. “Thus a remarkably powerful optical illusion in which people are imprinting something familiar from the Earth … onto a cosmic scene.”