Between heaven and Earth: the exploration of Titan

Abstract

The atmosphere of Titan represents a bridge between the early solar nebula and atmospheres like ours. The low abundances of primordial noble gases in Titan's atmosphere relative to N 2 suggest that the icy planetesimals that formed the satellite must have originated at temperatures higher than 75-100 K. Under these conditions, N 2 would also be very poorly trapped and thus Titan's nitrogen, like ours, must have arrived as nitrogen compounds, of which ammonia was likely the major component. This temperature constraint also argues against the trapping of methane. Production of this gas on the satellite after formation appears reasonable based on terrestrial examples of serpentinization, disproportionation and reduction of carbon. These processes require rocks, water, suitable catalysts and the variety of primordial carbon compounds that were plausibly trapped in Titan's ices. Application of this same general scenario to Ganymede, Callisto, KBOs and conditions on the very early Earth seems promising.

Interaction of CH4 and H2O in ice mixtures

“…It has been detected in comets and in interstellar grain mantles, [5][6][7][8] and the recent results of the Cassini-Huygens mission to Titan suggest the existence of methane clathrates under the surface of the satellite. [9][10][11] There is growing interest in a possible methane sequestration into, or release from, CH 4 /H 2 O mixtures, and, in general, in all possible kinds of association between these species, including clathrates, aerosols and in the solid state. [12][13][14] Infrared spectroscopy is well suited to study these systems.…”

The fate of aerosols on the surface of Titan

“…According to Owen et al 31 the interpretation of the low levels of primordial noble gases in Titan's atmosphere as a manifestation of a warm environment (T $ 75 K) for the formation of constituent planetesimals leads to a requirement for nitrogen to be delivered as compounds, mainly ammonia (NH 3 ), while methane (CH 4 ) needed to be stored within the interior of the satellite. It is reasonable to assume that the icy planetesimals that formed Titan could easily supplied the amount of nitrogen that is now found in the atmosphere.…”

TandEM: Titan and Enceladus mission

“…In this case, only silicates and moderately refractory volatiles could have been incorporated in the planetesimals that formed Titan and the other indigenous regular satellites. It is then difficult to trap the noble gases and CH 4 in icy grains but the trapping of H 2 O, NH 3 , CO 2 and non-volatile compounds would be facilitated [1,5,30,31]. The small amount of 36 Ar detected by the Huygens GCMS ( 36 Ar/N < 1/1000 × Earth's ratio, [27]) implies an ambient formation temperature of ∼100 K in the sub-nebula [30,31], under the assumption that Argon was trapped in amorphous ice.…”

“…It is then difficult to trap the noble gases and CH 4 in icy grains but the trapping of H 2 O, NH 3 , CO 2 and non-volatile compounds would be facilitated [1,5,30,31]. The small amount of 36 Ar detected by the Huygens GCMS ( 36 Ar/N < 1/1000 × Earth's ratio, [27]) implies an ambient formation temperature of ∼100 K in the sub-nebula [30,31], under the assumption that Argon was trapped in amorphous ice. It has been proposed that methane on Titan was not initially incorporated, but was produced within the satellite from chemical (catalysed) reactions between carbon compounds such as CO 2 and H produced from H 2 O by water-rock reactions [5].…”