Aerosol growth in Titan’s ionosphere

Abstract

Photochemically produced aerosols are common among the atmospheres of our solar system and beyond. Observations and models have shown that photochemical aerosols have direct consequences on atmospheric properties as well as important astrobiological ramifications, but the mechanisms involved in their formation remain unclear. Here we show that the formation of aerosols in Titan's upper atmosphere is directly related to ion processes, and we provide a complete interpretation of observed mass spectra by the Cassini instruments from small to large masses. Because all planetary atmospheres possess ionospheres, we anticipate that the mechanisms identified here will be efficient in other environments as well, modulated by the chemical complexity of each atmosphere.planetary sciences | heterogeneous chemistry | particles charging | plasma P hotochemical aerosols are observed in many atmospheres of our solar system (1-3), and their presence is also detected in exoplanet atmospheres (4). Apart from their direct influence on the atmospheric properties through their interaction with the radiation field, aerosols have further astrobiological implications; their presence in the early Earth's atmosphere could have protected the surface and any life evolving there from UV radiation (5), and laboratory studies of Titan aerosol analogs have identified an in vitro formation of amino acids during aerosol production (6). However, the general mechanisms involved in the production and growth of photochemical aerosols from atmospheric gases have remained elusive. Titan, as the most extreme example of an aerosol-dominated atmosphere, provides a unique opportunity to investigate these mechanisms.Cassini observations revealed the presence of aerosols in multiple regions of the atmosphere, from the troposphere (7), stratosphere (8), and mesosphere (9, 10) up to the thermosphere where the detection of large mass positive and particularly negative ions has been suggested to be the signature of aerosol formation (11)(12)(13)(14). During a recent flyby (T70), the Cassini spacecraft penetrated to deeper regions of Titan's thermosphere than usual, reaching altitudes close to 880 km that had not previously been sampled with in situ measurements. Measurements from the Langmuir probe (LP) during this unique flyby reveal a high abundance of negative ions at closest approach, comparable to the positive ion density, and a corresponding decrease in the electron density (15). This conclusion is supported by the analysis of multiple Cassini observations, which reveals that the observed electron density is smaller than the density anticipated from photochemical equilibrium, implying that the photochemical models are missing a significant electron loss mechanism (16, 17). Thus, current observations demonstrate a significant decrease of the electron density relative to the positive ion density in the lower ionosphere with a concurrent increase in the density of the negative ions.Aerosols, or dust particles in general, are known to interact with free electr...

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

“…However, it should be noted that we rely on a parameterization of haze production using haze precursors to arrive at our estimated production rates, and that the actual chemical pathways leading to haze formation are as yet poorly understood (see Hörst 2017, for a review). 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.…”

The effect of like-charge attraction on aerosol growth in the atmosphere of Titan

“…Remarkably, Titan is the only satellite in the Solar System known to display a dense atmosphere [1], harbouring several organic compounds and This irradiation triggers the dissociation and ionization of the simple primordial molecules and subsequently leads, through a series of chemical and physical 20 processes, to the formation of charged aerosol particles with an average mass of 500 Da [8] at altitudes between 950 and 1150 km (upper atmosphere) [6,7,9,10,11]. With decreasing altitude, these particles grow spherically until they reach the detached haze layer at 520 km in Titan's mesosphere, from where they start to form fractal aggregates [7,12,13,14].…”

Ionization balance in Titan’s nightside ionosphere

“…The Cassini mission has revealed a chemically complex ionosphere around Titan. N 2 and CH 4 are ionized and/or dissociated by solar photons or particle irradiation marking the onset of a chain of chemical reactions, which produce hydrocarbon and nitrile ions, heavy positive and negative ions, and eventually aerosols (e.g., Vuitton et al, 2007;Waite et al, 2007;Wahlund et al, 2009;Crary et al, 2009;Ågren et al, 2012;Shebanits et al, 2013;Lavvas et al, 2013;Wellbrock et al, 2013). However, Titan dayside ionospheric models have shown difficulties in reproducing observed electron number densities (e.g., Vigren et al, 2013), as well as the observed number densities of HCNH + , the dominant ion in the main ionosphere (e.g., Vuitton et al, 2009;Westlake et al, 2012).…”