Carbonaceous Chondrite Outgassing Experiments: Implications for Methane Replenishment on Titan

<p>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’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’s atmosphere is likely linked to its interior instead of being incorporated during formation (Nieman et al., 2005). Recent theoretical modeling works of Titan’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’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’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’s atmosphere today.</p> <p>We performed outgassing experiments using three primordial CM carbonaceous chondrites: Murchison, Aguas Zarcas, and Jbilet Winselwan. The first two are "fall" meteorite (1969 and 2019), and Jbilet Winselwan is a desert "find'' meteorite (2013). We used two sizes of samples for each CM chondrite for the measurements: a small grain sample with diameters < 20 µm and a normal grain sample with diameters of 20-100µm. Each sample underwent a step heating scheme where they are heated and held at every 100°C from room temperature to 1200° 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).</p> <p>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.</p>

A primordial 15N-depleted organic component detected within the carbonaceous chondrite Maribo

We report on the detection of primordial organic matter within the carbonaceous chondrite Maribo that is distinct from the majority of organics found in extraterrestrial samples. We have applied high-spatial resolution techniques to obtain C-N isotopic compositions, chemical, and structural information of this material. The organic matter is depleted in 15N relative to the terrestrial value at around δ15N ~ -200‰, close to compositions in the local interstellar medium. Morphological investigations by electron microscopy revealed that the material consists of µm- to sub-µm-sized diffuse particles dispersed within the meteorite matrix. Electron energy loss and synchrotron X-ray absorption near-edge structure spectroscopies show that the carbon functional chemistry is dominated by aromatic and C=O bonding environments similar to primordial organics from other carbonaceous chondrites. The nitrogen functional chemistry is characterized by C-N double and triple bonding environments distinct from what is usually found in 15N-enriched organics from aqueously altered carbonaceous chondrites. Our investigations demonstrate that Maribo represents one of the least altered CM chondrite breccias found to date and contains primordial organic matter, probably originating in the interstellar medium.