the surface of Titan at night, except for eclipses. From outer space, Cassini images from near-infrared to UV wavelengths have shown that the twilight periods (phase angles > 150°) are brighter than the daytime on Titan. This observation has not been observed on any other planetary body with a thick atmosphere. The Titanean twilight outshining the dayside is due to a combination of Titan's atmosphere extending hundreds of kilometers above the surface and intense forward Mie scattering from the haze. Radiative transfer models have not reproduced this effect. Atmospheric evolution The persistence of a dense atmosphere on Titan has been enigmatic as the atmospheres of the structurally similar satellites of Jupiter, Ganymede and Callisto, are negligible. Although the disparity is still poorly understood, data from recent missions have provided basic constraints on the evolution of Titan's atmosphere. Roughly speaking, at the distance of Saturn, solar insolation and solar wind flux are sufficiently low that elements and compounds that are volatile on the terrestrial planets tend to accumulate in all three phases. Titan's surface temperature is also quite low, about 94 K (-179 C/-290 F). Consequently, the mass fractions of substances that can become atmospheric constituents are much larger on Titan than on Earth. In fact, current interpretations suggest that only about 50% of Titan's mass is silicates, with the rest consisting primarily of various H2O (water) ices and NH3·H2O (ammonia hydrates). NH3, which may be the original source of Titan's atmospheric N2 (dinitrogen), may constitute as much as 8% of the NH3·H2O mass. Titan is most likely differentiated into layers, where the liquid water layer beneath ice Ih may be rich in NH3. Tentative constraints are available, with the current loss mostly due to low gravity and solar wind aided by photolysis. The loss of Titan's early atmosphere can be estimated with the 14N–15N isotopic ratio, because the lighter 14N is preferentially lost from the upper atmosphere under photolysis and heating. Because Titan's original 14N–15N ratio is poorly constrained, the early atmosphere may have had more N2 by factors ranging from 1.5 to 100 with certainty only in the lower factor. Because N2 is the primary component (98%) of Titan's atmosphere, the isotopic ratio suggests that much of the atmosphere has been lost over geologic time. Nevertheless, atmospheric pressure on its surface remains nearly 1.5 times that of Earth as it began with a proportionally greater volatile budget than Earth