Chemical and optical evolution of TITAn's aerosols under high-energy RADiation – TITARAD

TITARAD is a multidisciplinary project gathering a broad array of scientists with complementary expertise on energetic particle irradiation, analytical techniques, space-based infrared observations and photochemical/microphysical modeling. They will unite their strengths to tackle the impact of high energy ion irradiation on Titan's aerosol formation and its feedback on the gas phase chemistry. We plan to achieve these objectives by following three distinct but complementary paths. TITARAD will first determine the composition of photochemical haze particles throughout the atmosphere thanks to the modeling of their formation processes and laboratory simulations of the O+ irradiation of nitrogen-rich organic compounds (WP1). TITARAD will then investigate the galactic cosmic rays (GCR) irradiation of organic ices (WP2) and use these experimental constraints to analyze the Cassini observations of stratospheric ice clouds and model their formation and impact on the gas phase (WP3).
- WP1: We will deposit films of organic materials simulating Titan aerosols (Task 1.1) and irradiate them with oxygen ions of energy between 1 and 100 keV (Task 1.2). We will study the impact of the irradiation on the molecular complexity of the samples (Task 1.3). We will model the haze chemical composition throughout the atmosphere, with special emphasis on the oxygen incorporation (Task 1.4). We will address objectives 1 and 4.
- WP2 : We will irradiate with particles ranging from 10 keV to 1 MeV/u pure organic ices entering in Titan’s clouds composition (Task 2.1) and mixtures of these same ices (Task 2.2) and determine their optical constants. We will analyze ex situ the molecular composition of the organic residue formed during irradiation (Task 2.3). We will address objectives 2, 3 and 4.
- WP3 : We will analyze the Cassini CIRS and VIMS observations of Titan’s polar stratospheric clouds (Task 3.1). We will simulate the cloud formation and role of GCR, first at Titan’s equatorial conditions to benchmark the model (Task 3.2) and then at polar conditions (Task 3.3). We will address objectives 2, 3 and 4.

The 13-year (2004-2017) exploration of the Saturnian system by the Cassini-Huygens mission has revealed that Titan's atmospheric chemistry is by far the most complex of any observed in the solar system. In the upper atmosphere, Cassini's mass spectrometers (INMS and CAPS) detected the presence of positive and negative ions reaching up to thousands of atomic mass units that have been interpreted as polycyclic aromatic (nitrogen bearing) hydrocarbons (PANHs). It was subsequently shown that these macromolecules originate from the photochemistry initiated by the impact of solar radiation on N2 and CH4, the two major atmospheric constituents, and that they are the precursors of the organic haze observed at lower altitudes. In the lower atmosphere, ice clouds were observed by Cassini's remote sensing instruments (CIRS and VIMS). They result from the condensation on haze particles of the numerous organic compounds (hydrocarbons and nitriles) formed in the upper atmosphere, producing a suite of pure and co-condensed ices, typically observed at high winter polar latitudes. Ultimately, both photochemical haze and condensation clouds fall as snow towards the surface, where they are expected to either contribute to Titan's regolith or soak into the hundreds of hydrocarbon-filled polar lakes and seas.
Although Cassini-Huygens has brought amazing discoveries, it also left us with numerous mysteries about Titan's atmospheric chemistry. The origin of the oxygen-bearing gas phase species (CO, CO2 and H2O) detected in this reducing atmosphere is under debate and the extent to which oxygen is being incorporated in complex molecules is unknown. The optical properties (and therefore the chemical composition) of Titan's photochemical haze are not fully understood. The interaction of nitriles with aerosols and the photochemical processes in the solid phase are still to be explored. Several cloud features detected in the lower stratosphere are still uninterpreted and the clouds formation process, their chemical evolution and the impact on the gas phase remains unclear.
TITARAD aims at building a pioneering image of aerosol formation on Titan, experimentally, theoretically and observationally consistent. It will shed light on how magnetospheric ions and GCR modify the chemical composition and optical properties of the photochemical haze and condensation clouds, and whether the sputtering of aerosol surface material can have some impact on the gas phase, topics that have not been investigated so far. We will fill this gap in knowledge thanks to (i) new experimental setups and large infrastructures, (ii) state-of-the-art photochemical-microphysical models and (iii) a 3D radiative transfer code developed in the framework of the RaD3-net ANR project (funded from 2021 to 2025).
We will provide key elements that will address the following objectives:
• Objective 1: What is the required dose for O+ ion irradiation to have an impact on the molecular complexity and the surface properties of nitrogen-rich organics? What is the elemental composition of aerosol embryos in Titan’s upper atmosphere?
• Objective 2: Do new spectroscopic bands appear during particle irradiation of pure and co-condensed organic ices? What are the composition, physical and optical properties of the Titan stratospheric clouds? Do GCR play a role in their formation process?
• Objective 3: Are gas-phase species released during particle irradiation of pure and co-condensed organic ices? Does sputtering from the Titan stratospheric clouds surface have an impact on the gas phase?
• Objective 4: What are the mass fluxes of chemical compounds with prebiotic relevance deposited on the surface?
Answering these questions could lead to fundamental reinterpretations of Titan's atmospheric system as a whole with consequences for future in situ exploration, including NASA-led mission Dragonfly.