Composition and chemistry of Titan's thermosphere and ionosphere

Abstract

Titan has long been known to harbour the richest atmospheric chemistry in the Solar System. Until recently, it had been believed that complex hydrocarbons and nitriles were produced through neutral chemistry that would eventually lead to the formation of micrometre sized organic aerosols. However, recent measurements by the Cassini spacecraft are drastically changing our understanding of Titan's chemistry. The Ion and Neutral Mass Spectrometer (INMS) and the Cassini Plasma Spectrometer (CAPS) revealed an extraordinary complex ionospheric composition. INMS detected roughly 50 positive ions with m/z!100 and a density higher than 0.1 cm K3 . CAPS provided evidence for heavy (up to 350 amu) positively and negatively charged (up to 4000 amu) ions. These observations all indicate that Titan's ionospheric chemistry is incredibly complex and that molecular growth starts in the upper atmosphere rather than at lower altitude. Here, we review the recent progress made on ionospheric chemistry. The presence of heavy neutrals in the upper atmosphere has been inferred as a direct consequence of the presence of complex positive ions. Benzene (C 6 H 6 ) is created by ion chemistry at high altitudes and its main photolysis product, the phenyl radical (C 6 H 5 ), is at the origin of the formation of aromatic species at lower altitude.

Heavy ion formation in Titan's ionosphere: Magnetospheric introduction of free oxygen and a source of Titan's aerosols?

“…This does not mean to say that PAHs cannot also form in Titan's atmosphere since the aromatic benzene C6H6 and its radical phenyl C6H5 are present in Titan's atmosphere (Vuitton et al, 2009a) and similar polymerization reaction paths may occur to make PAHs (Waite et al, 2007;Vuitton et al, 2007). As we have stated previously both processes could very well be occurring at the same time and heavier aerosols could be mixtures or conglomerations of fullerenes and PAHs (see comments by Thaddeus, 1994Thaddeus, , 1995 on large aerosol formation).…”

“…At Titan the Cassini INMS instrument detected both acetylene (Waite et al, 2005) and benzene (Waite et al, 2007;Vuitton et al, 2009a) and their radicals C2H "Le., indirectly detected by Vuitton et al, 2009b) and C6H or phenyl by Vuitton et al (2009a). Vuitton et al (2007) also reported the detection of numerous hydrocarbons such as polyynes (C4H 2 , C611 2, C91-1 2 ), possible detection of methylpolyynes (CH 3C4H,CH3C6H) and benzene (C 61-I6).…”

Composition and chemistry of Titan's stratosphere

“…Note that data from the Cassini/INMS (Ion and Neutral Mass Spectrometer) indicate an additional source of benzene in the ionosphere, at approximately 900 km (Waite et al 2007;Vuitton et al 2008). A general discussion of the ionospheric chemistry is presented by Vuitton et al (2009).…”

Preface

“…One of the surprise discoveries of Cassini/Huygens was the unexpected complexity of chemical processes that appear to be active in the outer layers of the atmosphere, the thermosphere and ionosphere, something not found in any other atmosphere in the Solar System. The paper by Vuitton et al (2009) reviews the current understanding of upper atmosphere chemistry, which is still in its infancy, and highlights the discovery of heavy neutral molecules above 1000 km altitude from in situ mass spectrometer measurements. Titan's exosphere is discussed by Dandouras et al (2009), who describe the population of energetic neutral atoms there which result from bombardment by energetic ions populating Saturn's magnetosphere.…”