Photochemical Smog Hides an Icy World

摘要

Lifting Titan's Veil is an account of the exploration of Titan, Saturn's largest moon, and the joint effort by the European Space Agency (ESA) and NASA to reveal its secrets with the Cassini-Huygens mission. Why Titan? It is the solar system's second largest moon, with a radius about 60 km smaller than that of Jupiter's Ganymede. Whereas Ganymede's atmosphere is measured in picobars (10~(-12) times the surface pressure of Earth's atmo- sphere), the nitrogen atmosphere of Titan is about 1.5 bar. What really distinguishes Titan's atmosphere is its mildly reducing character, conditions similar to those many believe characterized Earth's prebiotic atmosphere, and its large suite of hydrocarbons, organic molecules, and nitriles. Among the nitriles, hydrogen cyanide is a known precursor of α-amino acids and nucleic-acid bases; thus, Titan's environment is of great importance for understanding chemical evolution in Earth's early atmosphere. Present-day Titan could be a natural laboratory for chemical synthesis analogous to the pioneering laboratory experiments on the origin of life that Urey and Miller carried out in the 1950s.