Rain, winds and haze during the Huygens probe's descent to Titan's surface

Abstract

The irreversible conversion of methane into higher hydrocarbons in Titan's stratosphere implies a surface or subsurface methane reservoir. Recent measurements from the cameras aboard the Cassini orbiter fail to see a global reservoir, but the methane and smog in Titan's atmosphere impedes the search for hydrocarbons on the surface. Here we report spectra and high-resolution images obtained by the Huygens Probe Descent Imager/Spectral Radiometer instrument in Titan's atmosphere. Although these images do not show liquid hydrocarbon pools on the surface, they do reveal the traces of once flowing liquid. Surprisingly like Earth, the brighter highland regions show complex systems draining into flat, dark lowlands. Images taken after landing are of a dry riverbed. The infrared reflectance spectrum measured for the surface is unlike any other in the Solar System; there is a red slope in the optical range that is consistent with an organic material such as tholins, and absorption from water ice is seen. However, a blue slope in the near-infrared suggests another, unknown constituent. The number density of haze particles increases by a factor of just a few from an altitude of 150 km to the surface, with no clear space below the tropopause. The methane relative humidity near the surface is 50 per cent.

Direct detection of pyridine formation by the reaction of CH (CD) with pyrrole: a ring expansion reaction

“…While the recent identification of benzene 53 on Titan strongly suggests that much more complex PAH synthesis 54 may proceed in the cold photoactive atmosphere, the presence of both pyrrole and pyridine, the simplest 5-and 6-membered N-heterocyclic aromatic molecules, has been indirectly inferred from several laboratory experiments 14,15 simulating Titan tholins. 55 In comparison to a PAH homologue, calculations suggest that the inclusion of a nitrogen atom could facilitate ring formation by lowering ring closure barriers. 19 Because it is thought that PAHs are probably the most abundant sources of organic carbon in the Interstellar Medium (ISM), 56 it could also be argued that the presence of N atoms in such polycyclic aromatics could eventually favor molecular weight growth of much larger N-substituted PAHs.…”

AVIATR—Aerial Vehicle for In-situ and Airborne Titan Reconnaissance

“…We wish to better understand how the circulation controls the locations where clouds and precipitation most often occur, and how the moist convection in turn modifies the circulation. Where, how often, and how intensely precipitation occurs bears on such questions as to whether the polar lakes are fed primarily by rain or underground 'aquifers', whether heavy rains occur at low latitudes in the present climate and contribute significantly to the formation of the channels seen in Cassini DISR images [155], and what processes return methane to the atmosphere to balance the loss due to precipitation. We do not yet have a complete picture of where and how often clouds form on Titan, nor how often it rains.…”

“…Rain erodes mountains [8]. Ephemeral streams carve channels [155]. At the equator the streams carry sediments into low-lying plains like the Huygens landing site [57].…”

Numerical modelling of sedimentary structures in rivers on Earth and Titan

“…On Ti tan, there is not liq uid wa ter on the sur face, but liq uid meth ane and other hy dro car bons. Wa ter ice is the main compo nent of the sur face Titanian rocks and of its regolith (Tomasko et al, 2005), i.e. its role is sim i lar to that of the sil icates and other rocks on Earth.…”

“…There are sev eral in ter est ing pub li ca tions re gard ing Titanian rivers (e.g., Tomasko et al, 2005;Burr et al, 2006;Lorenz et al, 2008;Lunine and Lorenz, 2009;Langhans et al, 2012); they are dis cussed in the text. How ever, ac cord ing to our best knowl edge, our re search is the first model of flu vial pro cesses on Ti tan us ing a dy nam i cal model based on phys i cal equa tions of mo tion, con ti nu ity, and sed i men ta tion.…”