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Rosetta detects exposed water ice on comet’s surface

Rosetta_OSIRIS_NAC_comet_67P_20140803_1 European Space Agency image posted on SpaceFlight Insider

The European Space Agency's Rosetta spacecraft has detected the presence of water ice on the surface of Comet 67P/Churyumov-Gerasimenko. Photo Credit: ESA

Scientists using the high-resolution science camera on board the European Space Agency’s (ESA ) Rosetta spacecraft have identified over a hundred patches of water ice a few meters in size on the surface of comet 67P/Churyumov-Gerssimenko. A new study which focused on an analysis of the bright patches of exposed ice was recently published in the journal Astronomy and Astrophysics.

The Rosetta spacecraft arrived at the comet in August 2014, orbiting at a distance of approximately 62 miles (100 kilometers) and eventually descended to a distance of 6 miles (10 kilometers) or less in order to acquire high-resolution images of the surface.

Observations of the gas that  emerges from comets indicate that they are rich in ices. As comets move closer to the Sun, their surfaces warm up and the ices sublimate into gas, which streams away from the nucleus and drags along particles of dust embedded in the ice to form the coma and tail of the comet. Some of the dust remains on the surface of the comet or fall back onto the nucleus, coating it with a thin layer of dusty material and leaving very little exposed ice on the surface.

Examples of six different bright patches identified on the surface of Comet 67P/Churyumov-Gerasimenko in OSIRIS narrow-angle camera images acquired in September 2014. Image Credit: ESA

Examples of six different bright patches identified on the surface of Comet 67P/Churyumov-Gerasimenko in OSIRIS narrow-angle camera images acquired in September 2014. Image Credit: ESA

Rosetta’s science instruments have detected a variety of gasses around the comet, including water vapor, carbon dioxide, and carbon monoxide. These gasses are thought to originate from frozen reservoirs beneath the surface of the comet. Scientists using NASA’s Microwave Instrument for Rosetta Orbiter (MIRO) have generated maps of water vapor in the comet’s coma.

Scientists using images taken by Rosetta’s OSIRIS narrow-angle camera have identified some 120 areas on the surface of the small frozen body that are as much as ten times brighter than the average surface brightness.

The bright patches were all found in areas that receive little sunlight, such as in the shadow of a cliff, and no significant changes were noted between images that were taken about a month apart.

This image, by the Rosetta navigation camera, was taken from a distance of about 53 miles (86 kilometers) from the center of Comet 67P/Churyumov-Gerasimenko on March 14, 2015. Image Credit: ESA

This image, by the Rosetta navigation camera, was taken from a distance of about 53 miles (86 kilometers) from the center of Comet 67P/Churyumov-Gerasimenko on March 14, 2015. Image Credit: ESA

“Water ice is the most plausible explanation for the occurrence and properties of these features,” says Antoine Pommerol of the University of Bern and lead author of the study. “At the time of our observations, the comet was far enough from the Sun such that the rate at which water ice would sublimate would have been less than 1 mm per hour of incident solar energy. By contrast, if carbon dioxide or carbon monoxide ice had been exposed, it would have rapidly sublimated when illuminated by the same amount of sunlight. Thus, we would not expect to see that type of ice stable on the surface at this time.”

The researchers conducted laboratory experiments that tested the behavior of water ice mixed with different minerals under simulated solar illumination in order to better understand the process. The scientists found that after a few hours of sublimation, a dark mantle of dust a few millimeters thick had formed. While in some places this layer completely concealed any traces of the ice below, occasionally large dust grains or chunks would lift from the surface and move elsewhere, exposing bright patches of water ice.

“A 1 mm thick layer of dark dust is sufficient to hide the layers below from optical instruments,” confirms Holger Sierks, OSIRIS principal investigator at the Max Planck Institute for Solar System Research in Göttingen. “The relatively homogeneous dark surface of the nucleus of Comet 67P/Churyumov-Gerasimenko, only punctuated by some metre-scale bright dots, can be explained by the presence of a thin dust mantle composed of refractory mineral and organic matter, with the bright spots corresponding to areas from which the dust mantle was removed, revealing a water-ice-rich subsurface below.”

On Tuesday, June 23, ESA announced that its Rosetta mission will be extended until the end of September 2016. The mission had originally been scheduled to end in December of this year (2015).

“This is fantastic news for science,” says Matt Taylor, ESA’s Rosetta Project Scientist. “We’ll be able to monitor the decline in the comet’s activity as we move away from the Sun again, and we’ll have the opportunity to fly closer to the comet to continue collecting more unique data. By comparing detailed ‘before and after’ data, we’ll have a much better understanding of how comets evolve during their lifetimes.”

Artist impression of ESA's Rosetta approaching comet 67P/Churyumov-Gerasimenko. Image Credit: ESA

Artist’s impression of ESA’s Rosetta approaching comet 67P/Churyumov-Gerasimenko. Image Credit: ESA

 

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Jim Sharkey is a lab assistant, writer and general science enthusiast who grew up in Enid, Oklahoma, the hometown of Skylab and Shuttle astronaut Owen K. Garriott. As a young Star Trek fan he participated in the letter-writing campaign which resulted in the space shuttle prototype being named Enterprise.

While his academic studies have ranged from psychology and archaeology to biology, he has never lost his passion for space exploration. Jim began blogging about science, science fiction and futurism in 2004.

Jim resides in the San Francisco Bay area and has attended NASA Socials for the Mars Science Laboratory Curiosity rover landing and the NASA LADEE lunar orbiter launch.

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