During the darkest days of December, it makes me feel better to think about all the other, more profound darknesses out there in the universe. A little dose of the old perspective, you know. And boy, there are a lot of them--not just a lot of dark places, but a lot of different forms of darkness out there. In fact, there's a lot more darkness than most of us realize, for an obvious if easily overlooked reason: Space images are calibrated to highlight faint or even invisible detail, making the universe seem like a much brighter place than it really is. Consider, for instance, Comet Churyumov–Gerasimenko, the now-famous comet being explored by the Rosetta spacecraft (and home to the intrepid, hibernating Philae lander). In all the images you see online, it looks brightly lit. Even the allegedly "true color" image, which is supposed to show what the Comet C-G would look like to the human eye up close, is a bright green-tinged gray. Here's the truth: The comet is blacker than coal.
The range of darkness, starting with the whitest moon in the solar system (Saturn's Enceladus, which is essentially covered with fresh snow). Earth's moon is actually pretty dark--but that's nothing compared to the blackness of Rosetta's comet, at bottom right. (Full info and credits here.) Take a look at the blackness of the comet, compared to three other solar system objects. This is how they would look if seen together, equally illuminated. See how dark the comet is? Look closely. It's there, at bottom right; it's just really really dark. You may notice that the moon is not so bright either. If you look at properly exposed images from Apollo you can see a sharp contrast between the whiteness of the space suits and the dull gray of the lunar surface. Another kind of darkness is found in the shadows of space, blacker than almost anything we experience. The Cassini spacecraft, flying behind Saturn, looked back at the planet's night hemisphere and saw the dark side of the rings. In this case, the rings are seen not by reflected sunlight but by scattered light that filters through. All else is shadow. This view--never possible from Earth--shows structures in the rings that do not show up in conventional illumination.
Backlit Saturn, with the sun behind the planet, shows the nighttime half of the planet and the dark parts of the rings made bright. The dot just below the rings on the left is Enceladus--the same moon seen in the previous image. (Credit: NASA/JPL-Caltech) The farther you go from the sun, the deeper the darkness. That will be a key challenge for the New Horizons spacecraft when it reaches Pluto and its moons this coming July. Sunlight at Pluto is just 1/1000th as strong as it is on Earth, creating tough shooting conditions for mission scientists as they try to snap off as many images as possible in the solar system's twilight zone. But really, that challenge pales in comparison to that of getting a look at things that give off essentially no light at all. Sometimes we get lucky and are able to observe them in silhouette against a bright background, as seen in this Spitzer Space Telescope view of the ultra-black clouds where new stars are born.
The deepest shadows ever observed are caused by dense, dusty interstellar clouds in the constellation Aquila. Eventually these clumps will condense into massive, brilliant stars--light from the dark. (Credit: NASA/JPL/M. Butler) A silhouette is still a play of light, though. What about a more profound kind of darkness--the universe seen without light at all? It sounds like a paradox (or perhaps a Zen koan), but the IceCube observatory in Antarctica is doing just that. It is observing the sky not in radiation but in neutrinos. The resulting map is a staggering expansion of human vision. Neutrinos are particles so inert that they fly right through you, and right through the whole Earth, almost completely unhindered. IceCube's detectors manage to pick up the very veryvery few particles that interact with ordinary atoms to show a side of the cosmos never seen before, revealed in the "dark light" of neutrinos.
First-ever map of the neutrino sky, compiled over four years using detectors buried in an Antarctic glacier. (Credit: IceCube) And then there is the ultimate darkness: a world that cannot interact with light at all. I'm talking about dark matter, the invisible component of the universe whose presence is known only from its gravitational pull. This material cannot be observed directly because it neither absorbs or emits radiation. It is utterly invisible to us, and we are utterly invisible to it. But gravity produces its own signal, bending and distorting the radiation that we can see. By measuring that distortion against the cosmic microwave background, scientists with the Planck space telescope managed to produce the first-ever map of all the matter in the universe. The vast majority of the signal in this map--about five-sixths--is produced by dark matter.
All the matter in the universe in one map, detected by its gravitational pull on radiation left over from the Big Bang. About 85 percent of this structure is dark matter, invisible by any other means. (Credit: ESA/NASA/JPL-Caltech) How's that for a holiday present? A whole new universe revealed to you, the first generation in history with the ability and opportunity to see it. Truly, it is a candle in the darkness. For more cosmic updates, follow me on Twitter: @coreyspowell
