Close to Critical planet's air is a little less than the density of liquid water will the raindrops descend slowly; otherwise, they would come down like meteors and make life on Tenebra impossible. But life is possible on Tenebra. Although Clement doesn't say so explicitly, Tenebra's plants do not use photosynthesis. The optical depth of the atmosphere is too great for enough of Altair's light to reach the ground. Instead, Tenebra's plants use chemosynthesis based on the transformation of sulphur oxides. The process is much like that used by plants growing around the hydrothermal vents known as black smokers on the bottom of
Photosynthesis pigments present. For example, in green plants, the action spectrum resembles the absorption spectrum for chlorophylls and carotenoids with absorption peaks in violet-blue and red light. In red algae, the action spectrum is blue-green light, which allows these algae to use the blue end of the spectrum to grow in the deeper waters that filter out the longer wavelengths (red light) used by above ground green plants. The non-absorbed part of the light spectrum is what gives photosynthetic organisms their color (e.g., green plants, red algae, purple bacteria) and is the least effective for photosynthesis in the respective organisms. In