Laboratory investigations of Titan haze formation: In situ measurement of gas and particle composition

Abstract

Prior to the arrival of the Cassini-Huygens spacecraft, aerosol production in Titan's atmosphere was believed to begin in the stratosphere where chemical processes are predominantly initiated by far ultraviolet (FUV) radiation. However, measurements taken by the Cassini Ultraviolet Imaging Spectrograph (UVIS) and Cassini Plasma Spectrometer (CAPS) indicate that haze formation initiates in the thermosphere where there is a greater flux of extreme ultraviolet (EUV) photons and energetic particles available to initiate chemical reactions, including the destruction of N 2 . The discovery of previously unpredicted nitrogen species in measurements of Titan's atmosphere by the Cassini Ion and Neutral Mass Spectrometer (INMS) indicates that nitrogen participates in the chemistry to a much greater extent than was appreciated before Cassini. The degree of nitrogen incorporation in the haze particles is important for understanding the diversity of molecules that may be present in Titan's atmosphere and on its surface. We have conducted a series of Titan atmosphere simulation experiments using either spark discharge (Tesla coil) or FUV photons (deuterium lamp) to initiate chemistry in CH 4 /N 2 gas mixtures ranging from 0.01% CH 4 /99.99% N 2 to 10% CH 4 /90% * sarah. N 2 . We obtained in situ real-time measurements using a high-resolution timeof-flight aerosol mass spectrometer (HR-ToF-AMS) to measure the particle composition as a function of particle size and a proton-transfer ion-trap mass spectrometer (PIT-MS) to measure the composition of gas phase products. These two techniques allow us to investigate the effect of energy source and initial CH 4 concentration on the degree of nitrogen incorporation in both the gas and solid phase products. The results presented here confirm that FUV photons produce not only solid phase nitrogen bearing products but also gas phase nitrogen species. We find that in both the gas and solid phase, nitrogen is found in nitriles rather than amines and that both the gas phase and solid phase products are composed primarily of molecules with a low degree of aromaticity. The UV experiments reproduce the absolute abundances measured in Titan's stratosphere for a number of gas phase species including C 4 H 2 , C 6 H 6 , HCN, CH 3 CN, HC 3 N, and C 2 H 5 CN.

Atmospheric Circulation, Chemistry, and Infrared Spectra of Titan-like Exoplanets around Different Stellar Types

“…For example, both models (e.g., Lavvas et al 2013) and observations (Coates et al 2007;Crary et al 2009;Liang et al 2007;Wahlund et al 2009;Waite et al 2007) of Titan's atmosphere indicate that ion chemistry plays an important role in haze formation; our photochemical model lacks ion chemistry, and therefore our haze formation picture is incomplete. Laboratory experiments investigating the chemical precursors, pathways, and energy sources that lead to Titan-like hazes and their resulting composition (Cable et al 2012;Hörst & Tolbert 2013;Hörst et al 2017;Imanaka & Smith 2010;Trainer et al 2013;Sciamma-O'Brien et al 2014) are critical to improving our understanding of the formation of exoplanetary hazes, including its dependence on external factors like stellar forcing. Thus, the combination of such ongoing laboratory work and modeling like ours-along with various observations, including short wavelength studies that might probe the existence of haze (Checlair et al 2016)-is required to make further progress in characterizing the diversity of hazy, cool, terrestrial exoplanets.…”

Exploring the Atmosphere of Neoproterozoic Earth: The Effect of O2 on Haze Formation and Composition

“…The instruments used to characterize the aerosol products of these experiments and data analysis techniques have been previously described by Hörst & Tolbert (2013) and Hörst et al (2018a) (and references therein). Briefly, we use an HR-ToF-AMS (referred to hereafter as AMS; Aerodyne Research) to measure particle composition.…”

“…The additional flow was accounted for during data analysis. For all gas mixtures (see Table 1), identical experiments were performed using the spark discharge from a tesla coil as the energy source instead of the deuterium lamp (see e.g., Hörst et al (2018a)). However, a detectable quantity of particles was not generated.…”

“…The total volume of particles produced for all of the experiments listed in Table 1 measured by the SMPS are shown in Panel A. The shaded lines indicate the production rate from our standard Titan experiment using 0.1% CH4 (Hörst & Tolbert 2013;Hörst et al 2018a) (orange), laboratory air during one of the experimental runs (gray), and standard background measurement (the experiment running as usual, but without the lamp on) (green). Panel B shows the particle diameter and Panel C shows the number density of particles both from SMPS measurements.…”

In situ investigation of neutrals involved in the formation of Titan tholins

“…Lastly, experimental techniques have also been used to simulate Titan's atmospheric conditions (Khare et al, 1984;Sagan and Thompson, 1984;Pintassilgo et al, 1999;Cable et al, 2012;Carrasco et al, 2013;Sciamma-O'Brien et al, 2015, Tigrine et al, 2016Hörst et al, 2018) and determine chemical mechanisms that are otherwise out of Cassini's reach. A wide range of laboratory simulations have been used to complementarily investigate Titan's atmospheric chemistry and different energy inputs (see Cable et al, 2012 for a thorough review of these experiments).…”