Just how dim is the sunlight on Pluto, some three billion miles away? This artist's concept of the frosty surface of Pluto with Charon and our Sun as backdrops illustrates that while sunlight is much weaker than it is here on Earth, it isn't as dark as you might expect. In fact, you could read a book on the surface of Pluto.
Image Credit: NASA/Southwest Research Institute/Alex Parker
Explanation from: http://photojournal.jpl.nasa.gov/catalog/PIA19682