A new color image of Comet 67P snapped by the Rosetta space probe doesn't show much -- color, that is -- as the comet is for the most part even more dark and gray than expected, scientists say.
The image, created by sequential snapshots of the comet taken with filters acquiring red, blue and green wavelengths, was posted by the European Space Agency on its Rosetta mission blog.
During the sequence of photos, as each filter was brought into play, both the comet and the Rosetta space craft were moving, so the three images were slight shifted in their perspective and had to be painstakingly worked on to create an accurate superimposition.
"As it turns out, 67P/C-G looks dark grey, in reality almost as black as coal," said mission scientist Holger Sierks of the Max Planck Institute for Solar System Research.
Some previous images of the comet, in which it appeared to have a significantly reddish tint, were posted online and promoted as the first true color pictures of the comet.
Not so fast, ESA says.
"This [red tint] is a well-known phenomenon observed at many other small bodies in the solar system and is due to the small size of the surface grains," the agency wrote in the blog. "That does not, however, mean that the comet would look red to the human eye.
"Natural sunlight has its peak intensity in the yellow part of the spectrum and the response of the human eye is similarly matched" it said. "Thus, overall, the comet would look rather grey to the human eye, as seen here."
Ground-based telescopes also observed that gray color long before the Rosetta spacecraft intercepted the comet, although those observations were unable to resolve any details of the comet's surface. The extremely homogeneous color captured by the spacecraft at a detailed scale suggests there's little or no variation in the composition of the comet's surface, scientists say.
Most of its surface seems to be covered in a uniformly dark dust, they say, explaining that more images using other combinations of the 25 filters the spacecraft's instrument contain may yield better information about the composition of that dust. The researchers say they will examine the dust more closely for various minerals such as pyroxene, commonly found in the crust of the Earth, or other minerals that might contain water.
There is no evidence in the current image of any ice patches on the surface, which would have shown up as blue patches in the filtered image, the researchers say.
