VIS-IR Spectroscopy of Mixtures of Water Ice, Organic Matter, and Opaque Mineral in Support of Small Body Remote Sensing Observations

Visual-to-infrared (VIS-IR) remote sensing observations of different classes of outer solar system objects indicate the presence of water ice and organics. Here, we present laboratory reflectance spectra in the 0.5–4.2 μm spectral range of binary particulate mixtures of water ice, organics analogue (kerite), and an opaque iron sulphide phase (pyrrhotite) to investigate the spectral effects of varying mixing ratios, endmember grain size, and mixing modality. The laboratory spectra are also compared to different implementations of the Hapke reflectance model (Hapke, 2012). We find that minor amounts (≲1 wt%) of kerite (investigated grain sizes of 45–63 μm and <25 μm) can remain undetected when mixed in coarse-grained (67 ± 31 μm) water ice, suggesting that organics similar to meteoritic insoluble organic matter (IOM) might be characterized by larger detectability thresholds. Additionally, our measurements indicate that the VIS absolute reflectance of water ice-containing mixtures is not necessarily monotonically linked to water ice abundance. The latter is better constrained by spectral indicators such as the band depths of water ice VIS-IR diagnostic absorptions and spectral slopes. Simulation of laboratory spectra of intimate mixtures with a semi-empirical formulation of the Hapke model suggests that simplistic assumptions on the endmember grain size distribution and shape may lead to estimated mixing ratios considerably offset from the nominal values. Finally, laboratory spectra of water ice grains with fine-grained pyrrhotite inclusions (intraparticle mixture) have been positively compared with a modified version of the Hapke model from Lucey and Riner (2011).

The cometary matter between volatiles and macromolecules

&lt;p&gt;Small and volatile molecules are the most abundant constituents of a comet&amp;#8217;s neutral coma. Thanks to ESA&amp;#8217;s Rosetta mission, the neutral coma of comet 67P/Churyumov-Gerasimenko (67P hereafter) has been analyzed in great spatial and temporal detail, e.g., by Rubin et al. (2019) or by L&amp;#228;uter et al. (2020). However, the Double Focusing Mass Spectrometer (DFMS) &amp;#8211; part of the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA; Balsiger et al. 2007) &amp;#8211; delivered data which contains information about the transition region between volatiles and macromolecular matter. Manual fitting of individual spectra allows to resolve pure hydrocarbon from heteroatom-bearing species also in the higher mass-range of the instrument, up to mass-to-charge (m/z) ratios of 140.&lt;/p&gt; &lt;p&gt;While Altwegg et al. (2019) have reported tentative detections of some heavier species like benzoic acid or naphthalene, spectra of m/z&gt;70 have not been investigated systematically. Here, we will present preliminary results from the first comprehensive analysis of a full data set (from m/z=12 to m/z=140) collected on August 3, 2015. On this day, the comet was close to its perihelion and the dust activity, as seen by the OSIRIS camera (Vincent et al. 2016), was high. Probably due to sublimation of molecules from ejected and heated-up dust grains, ROSINA/DFMS registered many signals above m/z=70. Due to the problem of isomerism and the lack of reference data, we chose to follow a statistical approach for our analysis. Larger species tend to expose a lower degree of saturation and the H/C ratio seems to approach that of highly unsaturated insoluble organic matter (IOM), cf., e.g., Sandford 2008. Although we cannot identify individual molecules in the complex gas mixture that makes up for the cometary coma, we are able to characterize for the first time the larger organic species that bridge the small volatiles and the macromolecular matter observed in 67P&amp;#8217;s dust by the Rosetta secondary ion mass spectrometer COSIMA (Fray et al. 2016).&lt;/p&gt; &lt;p&gt;&amp;#160;&lt;/p&gt; &lt;p&gt;&amp;#160;&lt;/p&gt; &lt;p&gt;&amp;#160;&lt;/p&gt; &lt;p&gt;Altwegg et al., 2019, Annu. Rev. Astron. Astrophys., 57, 113-55.&lt;/p&gt; &lt;p&gt;Balsiger H. et al., 2007, Space Sci. Rev., 128, 745-801.&lt;/p&gt; &lt;p&gt;Fray et al., 2016, Nature, 538, 72-74.&lt;/p&gt; &lt;p&gt;L&amp;#228;uter et al., 2020, MNRAS, 498, 3, 3995-4004.&lt;/p&gt; &lt;p&gt;Rubin et al., 2019, MNRAS, 489, 594-607.&lt;/p&gt; &lt;p&gt;Sandford, 2008, Annu. Rev. Anal. Chem. 1, 549&amp;#8211;78.&lt;/p&gt; &lt;p&gt;Vincent et al., 2016, MNRAS, 462 (Suppl_1), 184-194.&lt;/p&gt;

Carbonaceous Chondrite Outgassing Experiments: Implications for Methane Replenishment on Titan

&lt;p&gt;Titan is the only known moon in the Solar System with a substantial atmosphere of N2 and CH4. However, its origin and evolution are not well understood. Titan&amp;#8217;s present amount of atmospheric CH4 was predicted to be destroyed photochemically on very short timescales (~ 10 Myrs, Yung et al. 1984). This suggests that a methane resupply mechanism is necessary. The Huygens probe GCMS measurements of noble gases suggest that Titan&amp;#8217;s atmosphere is likely linked to its interior instead of being incorporated during formation (Nieman et al., 2005). Recent theoretical modeling works of Titan&amp;#8217;s atmosphere and interior suggest that its atmosphere could have originated partly by outgassing primordial organics in its interior (Neri et al. 2019; Miller et al. 2019). If this theory holds, volatiles like methane could be outgassing from Titan&amp;#8217;s interior to sustain its current observed abundances. Insoluble organic matter (IOM) found in carbonaceous chondrites may serve as an analog for the organic material in Titan&amp;#8217;s interior and provide experimental constraints on the outgassed component of its atmosphere (Thompson et al. 2021). By heating carbonaceous chondrite samples and measuring the abundances of their released volatiles, specifically methane, we may be able to connect what we see in the lab to species in Titan&amp;#8217;s atmosphere today.&lt;/p&gt; &lt;p&gt;We performed outgassing experiments using three primordial CM carbonaceous chondrites: Murchison, Aguas Zarcas, and Jbilet Winselwan. The first two are &quot;fall&quot; meteorite (1969 and 2019), and Jbilet Winselwan is a desert &quot;find'' meteorite (2013). We used two sizes of samples for each CM chondrite for the measurements: a small grain sample with diameters &lt; 20 &amp;#181;m and a normal grain sample with diameters of 20-100&amp;#181;m. Each sample underwent a step heating scheme where they are heated and held at every 100&amp;#176;C from room temperature to 1200&amp;#176; C. The whole heating scheme takes 12 hours. We continuously monitored the partial pressures of 10 outgassed mass peaks using a residual gas analyzer (RGA).&lt;/p&gt; &lt;p&gt;We can estimate how much methane can be outgassed from the insoluble organics in the CM chondrites with the RGA data. We found that chondrite outgassing can resupply methane that can last for ~0.5-2 Gyrs. If organics indeed makes a significant fraction of Titan's interior, outgassing through thermal instability of Titan's interior can potentially resupply Titan's atmospheric methane for a period of time.&lt;/p&gt;

Speciation of organosulfur compounds in carbonaceous chondrites

Despite broad application of different analytical techniques for studies on organic matter of chondrite meteorites, information about composition and structure of individual compounds is still very limited due to extreme molecular diversity of extraterrestrial organic matter. Here we present the first application of isotopic exchange assisted Fourier transform ion cyclotron resonance mass spectrometry (FTICR MS) for analysis of alkali extractable fraction of insoluble organic matter (IOM) of the Murchison and Allende meteorites. This allowed us to determine the individual S-containing ions with different types of sulfur atoms in IOM. Thiols, thiophenes, sulfoxides, sulfonyls and sulfonates were identified in both samples but with different proportions, which contribution corroborated with the hydrothermal and thermal history of the meteorites. The results were supported by XPS and thermogravimetric analysis coupled to FTICR MS. The latter was applied for the first time for analysis of chondritic IOM. To emphasize the peculiar extraterrestrial origin of IOM we have compared it with coal kerogen, which is characterized by the comparable complexity of molecular composition but its aromatic nature and low oxygen content can be ascribed almost exclusively to degradation of biomacromolecules.

Synthesis of 13C-enriched amino acids with 13C-depleted insoluble organic matter in a formose-type reaction in the early solar system

Solvent-soluble organic matter (SOM) in meteorites, which includes life’s building molecules, is suspected to originate from the cold region of the early solar system, on the basis of 13C enrichment in the molecules. Here, we demonstrate that the isotopic characteristics are reproducible in amino acid synthesis associated with a formose-type reaction in a heated aqueous solution. Both thermochemically driven formose-type reaction and photochemically driven formose-type reaction likely occurred in asteroids and ice-dust grains in the early solar system. Thus, the present results suggest that the formation of 13C-enriched SOM was not specific to the cold outer protosolar disk or the molecular cloud but occurred more widely in the early solar system.

Calcium Bridging Drives Polysaccharide Co-Adsorption to a Proxy Sea Surface Microlayer

Saccharides comprise a significant mass fraction of organic carbon in sea spray aerosol (SSA), but the mechanisms through which saccharides are transferred from seawater to the ocean surface and eventually into SSA are unclear. It is hypothesized that saccharides cooperatively adsorb to other insoluble organic matter at the air/sea interface, known as the sea surface microlayer (SSML). Using a combination of surface-sensitive infrared reflection-absorption spectroscopy and all-atom molecular dynamics simulations, we demonstrate that the marine-relevant, anionic polysaccharide alginate co-adsorbs to an insoluble palmitic acid monolayer via divalent cationic bridging interactions. Ca²⁺ induces the greatest extent of alginate co-adsorption to the monolayer, evidenced by the ~30% increase in surface coverage, whereas Mg²⁺ only facilitates one-third the extent of co-adsorption at seawater-relevant cation concentrations due to its strong hydration propensity. Na⁺ cations alone do not facilitate alginate co-adsorption, and palmitic acid protonation hinders the formation of divalent cationic bridges between the palmitate and alginate carboxylate moieties. Alginate co-adsorption is largely confined to the interfacial region beneath the monolayer headgroups, so surface pressure, and thus monolayer surface coverage, only changes the amount of alginate co-adsorption by less than 5%. Our results provide physical and molecular characterization of a potentially significant polysaccharide enrichment mechanism within the SSML.

Calcium Bridging Drives Polysaccharide Co-Adsorption to a Proxy Sea Surface Microlayer

Saccharides comprise a significant mass fraction of organic carbon in sea spray aerosol (SSA), but the mechanisms through which saccharides are transferred from seawater to the ocean surface and eventually into SSA are unclear. It is hypothesized that saccharides cooperatively adsorb to other insoluble organic matter at the air/sea interface, known as the sea surface microlayer (SSML). Using a combination of surface-sensitive infrared reflection-absorption spectroscopy and all-atom molecular dynamics simulations, we demonstrate that the marine-relevant, anionic polysaccharide alginate co-adsorbs to an insoluble palmitic acid monolayer via divalent cationic bridging interactions. Ca²⁺ induces the greatest extent of alginate co-adsorption to the monolayer, evidenced by the ~30% increase in surface coverage, whereas Mg²⁺ only facilitates one-third the extent of co-adsorption at seawater-relevant cation concentrations due to its strong hydration propensity. Na⁺ cations alone do not facilitate alginate co-adsorption, and palmitic acid protonation hinders the formation of divalent cationic bridges between the palmitate and alginate carboxylate moieties. Alginate co-adsorption is largely confined to the interfacial region beneath the monolayer headgroups, so surface pressure, and thus monolayer surface coverage, only changes the amount of alginate co-adsorption by less than 5%. Our results provide physical and molecular characterization of a potentially significant polysaccharide enrichment mechanism within the SSML.