Formose Reaction

Abstract

A facula is a bright area on the icy ▶ satellites of ▶ Jupiter, ▶ Ganymede, ▶ Callisto, and Amalthea, and on ▶ Saturn's satellite ▶ Titan. Faculae on Ganymede and Callisto are circular or elliptical and up to several hundred kilometers in diameter. Faculae are thought to have been created by impacts into the icy ▶ crusts of these two Jovian satellites, possibly with plastic or liquid material present in the subsurface. Titan shows two globally abundant surface units characterized by either bright or dark ▶ albedo. Faculae on this satellite are irregularly shaped, represent slivers or islands of bright terrain, are located within extensive areas of dark terrain, and are possibly of non-impact origin. See also ▶ Albedo Feature ▶ Callisto ▶ Crater, Impact ▶ Crust ▶ Ganymede ▶ Impact Basin ▶ Jupiter ▶ Macula, Maculae ▶ Satellite or Moon ▶ Saturn ▶ Titan Synonyms Faint young sun problem; Weak sun paradox; Weak young sun paradox Keywords Climate, greenhouse gases, solar mass loss, young Sun Definition The apparent contradiction between model calculations that indicate a dimmer young Sun and consequently a colder climate on Earth on the one hand, and the geological evidence that the early climates on Earth and Mars were mild and liquid water was abundantly present on the other hand. Overview There is clear evidence for a mild climate on the young Earth and Mars, allowing liquid water-a prerequisite for the formation of life as we know it-to exist on the surface of both planets at ages of a few 100 Myr to 1 Gyr. Liquid water seems to have subsequently disappeared from the surface of Mars. However, detailed stellar evolutionary theory applied to the Sun during its main-sequence life indicates that it was fainter by about 30% when it arrived on the main sequence 4.6 billion years ago, and the radi-ative output increased only slowly in the next billion years. Both planetary surfaces would thus have been completely frozen. To solve this apparent paradox, various hypotheses have been proposed although a conclusive answer is still outstanding. The most popular theory assumes higher admixtures of greenhouse gases in the atmospheres of Earth and Mars; such gases include carbon monoxide, ammonia, and methane (CO 2 , NH 3 , and CH 4 , respectively). A radically different hypothesis posits that the young Sun was not faint-it may rather have been brighter because it was more massive by a few percent. Direct evidence for the implied strong mass loss in the younger epochs of solar evolution is still outstanding. Further hypotheses assume a lower albedo due to less efficient cloud formation, for example, as a consequence of more efficient suppression of cosmic rays in the young solar system, or a smaller fractional area of land, or the lack of biologically induced cloud condensation nuclei. Basic Methodology The average, effective equilibrium temperature of a planetary surface in the absence of an atmosphere follows from the energy balance between optical/near-infrared emission Muriel Gargaud (ed.), Encyclopedia of Astrobiology,