Preface

Abstract

Saturn's largest Moon, Titan, is among the most fascinating worlds in our Solar System, hosting a substantial atmosphere rich in complex chemistry, a diverse and weathered surface and a highly variable outer space environment driven primarily by Saturn's dynamic magnetosphere. While measurements by the Voyager spacecraft during their flybys in the early 1980s provided the first opportunity to study Titan in more detail, a true revolution in our knowledge of this enigmatic body had to await the arrival of the Cassini/Huygens spacecraft into orbit around Saturn in 2004. This international mission has given and continues to provide the most detailed measurements to date of the Titan system. A particular highlight was the successful landing of the Huygens probe on Titan's surface on 14 January 2005, which carried out ground-breaking in situ measurements of both the atmosphere and surface. The Cassini orbiter continues to measure Titan's surface, atmosphere and space environment both in situ and by remote sensing during periodic flybys of Titan, and our hope is to gain a first-level understanding of how this complex and closely coupled system works before the end of the mission. The exploration of a body as diverse as Titan, its history, geology, meteorology, chemistry, upper atmosphere, exosphere as well as plasma and magnetic environment requires expertise from many different scientific communities. What the case of Titan shows particularly well is that all of these processes and regions are intimately coupled and need the different communities to work closely together. To foster this communication and obtain an overview of the status of our knowledge of Titan's atmosphere and its space environment towards the end of the Cassini/Huygens primary mission (2004)(2005)(2006)(2007)(2008), we organized a scientific Discussion Meeting at the Royal Society in London on 3-4 December 2007.The 14 invited talks at this event were given by scientists from Europe and the USA and form the basis for this issue. One overarching question the Cassini/Huygens mission is trying to answer is that of the origin of Titan and its atmosphere, a topic explored in the papers by Owen & Niemann (2009) and Tobie et al. (2009). With a surface pressure 1.6 times that on the Earth and the main constituent being molecular nitrogen, many parallels have been drawn between the atmospheres of the Earth and Titan, and the comparison of their atmospheres is featured in several papers of this issue. For instance, the study by Tokano (2009) highlights important differences between Titan and the Earth as far as wind direction and seasonal variation in the lowest atmospheric layer, the troposphere, are concerned. Flasar & Achterberg's (2009) paper extends discussions of winds into Titan's stratosphere and points out the strong coupling between temperatures, winds and composition there. This strong coupling is also reproduced by the latest general circulation models of Titan's stratosphere, as discussed by

Ground and low-lying excited states of propadienylidene (H2C=C=C:) obtained by negative ion photoelectron spectroscopy

“…The C 3 H 2 isomers constitute a family of reactive organic species that play a role in the chemistry of harsh environments, including combustion, 1, 2 planetary atmospheres, [3][4][5] and interstellar space. [6][7][8] These extremely hydrogen-deficient species possess unusual bonding arrangements and high reactivity.…”