Study of gamma-ray activated aqueous chemistry on Titan: the impact of phosphate salts on the organic transformations at Selk Crater towards prebiotic chemistry.

Introduction:

During its years of activity, the Cassini spacecraft alongside the Huygens probe unveiled some of the mystery surrounding Titan, depicting it as an active world where molecules of great complexity have been detected^1,2. Building upon the results of the Cassini-Huygens Mission, the investigation of Titan’s prebiotic chemistry has been set as one of the main scientific objectives of the NASA Dragonfly New Frontiers Mission⁴. The organic aerosols produced by Titan’s atmospheric chemistry serve as hydrocarbon-based starting material from which complex oxygenated prebiotic molecules can be synthesized at the surface, via hydrolysis during local melt of the icy crust or putative cryovolcanic events^5–8. Dragonfly will land on the dune area south of the Selk crater, a site that might have held liquid water for up to 10³ to 10⁴ years after impact^9,10 and thus may have been favorable for the production of prebiotic molecules. In order to sustain its interior liquid water ocean, the ices of Titan are thought to be mixed with ammonia¹¹ which would allow for faster and more extensive chemistry at the location of the Selk liquid pool¹². Modelling work done by Sittler et al. (2020)¹³ estimates that 0.4% of the incoming Galactic Cosmic rays (GCR) energy is deposited to the surface of Titan. Over extended periods of time, the GCR interactions produce a non-negligeable quantity of radiation (~30 krad / 10⁸ year) that could also help to activate the chemistry of a reactive medium like the Selk-crater liquid pool, allowing for even more complex molecules to be synthesized and potentially preserved and detectable by the Dragonfly instrumental suite. When compared to other planetary bodies of the solar system such as Mars or Europa, the relatively low energy GCR dose deposited at or near  the surface of Titan could be sufficient to trigger some chemistry without extensively degrading key prebiotic molecules⁸. Inorganic salts are also expected to have been present in the Selk crater liquid medium, either endogenously present or brought by the impactor. These components offer a broader array of reactivities for aerosols organic material, potentially catalyzed by the mineral phases of the remains of the meteorite impactor. Indeed, meteorite mineral phases mixed with formamide ices under GCR-like proton irradiations have been shown to enable the synthesis of complex organics such as nucleosides¹⁴. The effects of sodium carbonate⁶ and magnesium sulfate⁸ on the alkaline hydrolysis of tholins – artificial chemical analogs of the Titan aerosols – have already been proven to broaden and accelerate the transformation of organic molecules in Titan like conditions and allow the productions of biomolecules such as nucleobases and amino acids. So far, the effect of phosphate on the transformation of organic molecules in similar conditions has never been investigated, and it could be one of the salts present in the Selk liquid pool brought by the impactor. Phosphates are ubiquitously used by living organisms on Earth as constituting parts of the nucleotides that make up DNA and RNA and are therefore salts of prime interest to study for astrobiology purposes.

Experimental plan:

This on-going study is centered around understanding the effect of phosphate coupled to GCR-like irradiation on the transformation of organic matter at a Titan impact crater after impact. Different types of organic samples have been subjected to room temperature hydrolysis in various aqueous media coupled to gamma irradiation to understand which prebiotic molecules could be expected to be found in the ices that will be analyzed by Dragonfly. Various reaction parameters have been explored: presence of 5% ammonia, presence of 0.01% of sodium phosphate (Na₂HPO₄), presence of a suspension of 1% of mineral phases analogs of CI chondrite asteroids from Space Resource Technologies, and five gamma-ray irradiation doses ranging from 10^-3 to 300 krad meant to be representative of GCR surface irradiation doses for the duration of the Selk liquid pool lifetime. The doses are based on Sittler et al. (2020)’s model¹³ and data from the MIMI instrument of Cassini¹⁵ that were previously used to simulate Titan’s surface radiative environment⁸. The samples were prepared in flame-sealable ampules before irradiation using the ⁶⁰Co gamma-ray source at NASA Goddard Space Flight Center. The samples have then been analyzed using analytical techniques that will be present on Dragonfly : Laser Desorption – Mass Spectrometry (LDMS) and Gas Chromatography – Mass Spectrometry (GCMS) with wet chemistry pre-treatment, as well as complementary techniques: Matrix Assisted Laser Desorption Ionization – Ion Trap (MALDI-IT) and High Performance Liquid Chromatography – Mass Spectrometry (HPLC-MS) to confirm the presence of key organic products.

A primary organic component that has been irradiated in liquid media is tholin synthesized from the PHAZER Titan simulation chamber (95:5 N₂/CH₄ gas mixture energized by a cold plasma source), meant to be representative of the endogenous organic material that Dragonfly will analyze. These irradiated samples give us information on what sort of molecules can realistically have been synthesized at the location of Selk crater. A particular attention is given to the detection of nucleotide products, one of the most interesting molecules potentially synthesized via the presence of phosphate. To understand whether the abiotic synthesis and polymerization of nucleotide is indeed possible in our Selk-like reactive conditions, standards of either (D)-ribose and a set of nucleobases (adenine, guanine, cytosine, uracil, thymine, hypoxanthine) or a set of ribonuleotides (ATP, GTP, CTP, UTP, ITP) have also been subjected to the same radiative hydrolysis. Finally, as tholin mixtures contain organic condensing agents capable of favoring the synthesis of nucleosides¹⁶, tholins spiked with ribose and nucleobases have also been hydrolyzed under gamma-irradiation to see if the synthesis of nucleoside and nucleotides is helped by the presence of tholins.

 

 

 

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