Where they walked, it was dark most of the time. That was the price of being sheltered from cosmic rays and coronal mass ejections. Looking up, however, they could see sunlight gilding the edges of the crevasse above them. They talked about how to set up mirrors that would bounce sunlight downward onto the arklets, which could grow food and scrub air in their translucent outer hulls. Doob spoke of Endomement, the idea that, in time, a ceiling could be thrown over the top of the crevasse and walls built to keep in the air, whereupon a whole section of the valley could be given an atmosphere and turned into a place where children could go "outside" without the need for space suits.