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In this delightful portrayalof Comet 67P/Churyumov-Gerasimenko, we see the 2.5-mile-wide object close to true colorwith downtown Los Angeles, Calif. for size reference.Compare to the same image (below) as viewed from space. Despite appearances in photos, comets are coal-black objects. Credit: ESA and anosmicovni

All the pictures weve seen of Rosettas target comet 67P/C-G show it reflecting brightly against the background of outer space. And well they should. Space is black as night. But if we were to see the comet against a more familiar earthly backdrop, wed be shocked by its appearance. Instead of icy white, Rosettas would appear the color of a fresh asphalt parking lot. Most comets, including Rosettas, are no brighter than the charcoal briquettes you use to grill hamburgers.

Astronomers rank an objects reflectivityby its albedo (al-BEE-do). A body that reflects 100% of the lightis said to have an albedo of 1.0.Venus albedo is .75 and reflects 75% of the light it receives from the sun, while the darker Earths average is 30%. Trees and thedarker-toned continents reflect much less light compared to Venus pervasive cloud cover. In contrast, the coal-dark moon reflects only 12% of the sunlight falling on it and fresh asphalt just 4% smack in the middle ofthe 2-6% range of most known comets.

Photo of Comet 67P/C-G taken by Rosetta on August 6, 2014. Against the blackness of space, it appears whitish-grey. Credit: ESA

The brightest object in the solar system is Saturns icy moon Enceladus with areflectivity of 99%.So why are comets so dark? Its funny because before we sent the Giotto spacecraft to snap close-up pictures of Halleys Comet in 1986, astronomers thought comets, being made of reflective ice, were naturally white.Not Halley and not every comet seen up close since then.

Comets are as dark as charcoal but appear light only because the sun illuminates them against the blackness of outer space. I shone a flashlight on a charcoal briquette (left) to simulate comet lighting.The same charcoal, when viewed in normal light, appears black. Credit; Bob King

Astronomers hypothesizethat a comet grows a dark skin both from accumulated dust andirradiation of its pristine ices by cosmic rays. Cosmic rays loosen oxygen atoms from water ice, freeing them to combine with simple carbon molecules present on comets to form larger, more complex and darker compounds resembling tars and crude oil.

Comet colored parking lots have been the rage for years. Both comets and fresh asphalt reflect about the same amount of light. Credit: Bob King

Excerpt from:

What Comets, Parking Lots and Charcoal Have in Common