Dust Whirls, Swirls and Twirls at Rosetta’s Comet

Tell me this montage shouldn’t be hanging in the Lourve Museum. Every time I think I’ve seen the “best image” of Rosetta’s comet, another one takes its place. Or in this case four! When you and I look at a comet in our telescopes or binoculars, we’re seeing mostly the coma, the bright, fluffy head of the comet composed of dust and gas ejected by the tiny, completely invisible, icy nucleus.

As we examine this beautiful set of photos, we’re  privileged to see  the individual fountains of gas and dust that leave the comet to create the coma. Much of the outgassing comes from the narrow neck region between the two lobes. 

This photo taken on Feb. 27 shows the comet with peacock-like display of dusty jets. Below center is a streak that may be a dust particle that traveled during the exposure. Other small white spots are also likely dust or bits of comet that have broken off. Credits: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0

All were taken between February 25-27 at distances around 50-62 miles (80 to 100 km) from the center of Comet 67P/Churyumov-Gerasimenko. Looking more closely, the comet nucleus appears to be “glowing” with a thin layer of dust and gas suspended above the surface. In the lower left Feb. 27 image, a prominent streak is visible. While this might be a cosmic ray zap, its texture hints that it could also be a dust particle captured during the time exposure. Because it moved a significant distance across the frame, the possible comet chunk may be relatively close to the spacecraft. Just a hunch.

Another close-up individual image from Rosetta’s NAVCAM. Credit: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0

While most of Rosetta’s NAVCAM images are taken for navigation purposes, these images were obtained to provide context in support of observations performed at the same time with the Alice ultraviolet (UV) imaging spectrograph on Rosetta. Observing in ultraviolet light, Alice determines the composition of material in coma, the nucleus and where they interface. Alice will also monitor the production rates of familiar molecules like H2O, CO (carbon monoxide) and CO2 as they leave the nucleus and enter 67P’s coma and tail.

Alice makes its observations in UV light through a long, narrow slit seen here superimposed on a graphic of comet 67P/ C-G. Credit: ESA/NASA

From data collected so far, the Alice team has discovered that the comet is unusually dark in the ultraviolet, and that its surface shows no large water-ice patches. Water however has been detected as vapor leaving the comet as it’s warmed by the Sun. The amount varies as the nucleus rotates, but the last published measurements put the average loss rate at 1 liter (34 ounces) per second with a maximum of 5 liters per second. Vapors from sublimating carbon monoxide and carbon dioxide ice have also been detected. Sometimes one or another will dominate over water, but overall, water remains the key volatile material outgassed in the greatest quantity.

Particularly striking and collimated jets emerge from the comet’s shadowed Hathor region between the two lobes. Credit: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0
Look at those spirals! In this separate image, taken Feb. 28, ESA suggests the curved shape of the outflowing material likely results from a combination of several factors, including the rotation of the comet, differential flows of near-surface gas, and gravitational effects arising due to the uneven shape of the comet. The viewing perspective of the image might also distort the true shape of the outflowing material. Credit: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0

That and dust. In fact, 67P is giving off about twice as much dust as gas. We see the comet’s dual emissions by reflected sunlight, but because there’s so much less material in the jets than what makes up the nucleus, they’re fainter and require longer exposures and special processing to bring out without seriously overexposing the comet’s core.

67P’s coma will only grow thicker and more intense as it approaches perihelion on August 13.

Bob King

I'm a long-time amateur astronomer and member of the American Association of Variable Star Observers (AAVSO). My observing passions include everything from auroras to Z Cam stars. I also write a daily astronomy blog called Astro Bob. My new book, "Wonders of the Night Sky You Must See Before You Die", a bucket list of essential sky sights, will publish in April. It's currently available for pre-order at Amazon and BN.

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