PAMPRE: A dusty plasma experiment for Titan's tholins production and study

Organic aerosols play a significant role in the properties and evolution of Titan's atmosphere. But our knowledge of them and their physico-chemical mechanisms of formation and evolution are currently limited to a few data obtained by Titan observations from the Earth or from space probes. For this reason, laboratory experiments are developed to simulate the atmospheric chemistry and produce analogues of these aerosols in order to understand better their properties and how they are formed. The plasma discharges are the most efficient devices for the production of such analogues. However, the existing plasmas simulations introduce experimental biases compared with the conditions of aerosols production in Titan's atmosphere: chemistry is induced by electrons instead of photons; the solid analogues are produced and deposited on solid surfaces; direct analysis of the particles inside the reactive chamber is not easy. In order to avoid some of these experimental problems, we have developed another method of production of Titan's aerosols analogues. It is based on a capacitively coupled radio-frequency (RF) cold plasma system at low pressure in a N₂-CH₄ gaseous mixture. In this plasma, solid particles produced from the gas phase are in levitation, thus preventing any wall effect on their production, and allowing the study of the formation and growth of the particles directly in the plasma. Moreover, the electron energy distribution of this plasma can be compared with the solar spectrum. This article describes the RF plasma experiment and presents the first results obtained with an initial N₂-CH₄ (90:10) gaseous mixture which produced our first studied analogues of Titan's aerosols.     

L‐chondrite asteroid breakup tied to Ordovician meteorite shower by multiple isochron 40Ar‐39 Ar dating

Abstract— Radiochronometry of L chondritic meteorites yields a rough age estimate for a major collision in the asteroid belt about 500 Myr ago. Fossil meteorites from Sweden indicate a highly increased influx of extraterrestrial matter in the Middle Ordovician ～480 Myr ago. An association with the L‐chondrite parent body event was suggested, but a definite link is precluded by the lack of more precise radiometric ages. Suggested ages range between 450 ± 30 Myr and 520 ± 60 Myr, and can neither convincingly prove a single breakup event, nor constrain the delivery times of meteorites from the asteroid belt to Earth. Here we report the discovery of multiple ⁴⁰Ar‐³⁹Ar isochrons in shocked L chondrites, particularly the regolith breccia Ghubara, that allow the separation of radiogenic argon from multiple excess argon components. This approach, applied to several L chondrites, yields an improved age value that indicates a single asteroid breakup event at 470 ± 6 Myr, fully consistent with a refined age estimate of the Middle Ordovician meteorite shower at 467.3 ± 1.6 Myr (according to A Geologic Time Scale 2004). Our results link these fossil meteorites directly to the L‐chondrite asteroid destruction, rapidly transferred from the asteroid belt. The increased terrestrial meteorite influx most likely involved larger projectiles that contributed to an increase in the terrestrial cratering rate, which implies severe environmental stress.     

Influence of methane concentration on the optical indices of Titan’s aerosols analogues

This work deals with the optical constant characterization of Titan aerosol analogues or “tholins” produced with the PAMPRE experimental setup and deposited as thin films onto a silicon substrate. Tholins were produced in different N₂–CH₄ gaseous mixtures to study the effect of the initial methane concentration on their optical constants. The real (n) and imaginary (k) parts of the complex refractive index were determined using the spectroscopic ellipsometry technique in the 370–1000 nm wavelength range. We found that optical constants depend strongly on the methane concentrations of the gas phase in which tholins are produced: imaginary optical index (k) decreases with initial CH₄ concentration from 2.3 × 10^?2 down to 2.7 × 10^?3 at 1000 nm wavelength, while the real optical index (n) increases from 1.48 up to 1.58 at 1000 nm wavelength. The larger absorption in the visible range of tholins produced at lower methane percentage is explained by an increase of the secondary and primary amines signature in the mid-IR absorption. Comparison with results of other tholins and data from Titan observations are presented. We found an agreement between our values obtained with 10% methane concentration, and Imanaka et al. (Imanaka, H., Khare, B.N., Elsila, J.E., Bakes, E.L.O., McKay, C.P., Cruikshank, D.P., Sugita, S., Matsui, T., Zare, R.N. [2004]. Icarus, 168, 344–366) values, in spite of the difference in the analytical method. This confirms a reliability of the optical properties of tholins prepared with various setups but with similar plasma conditions. Our comparison with Titan’s observations also raises a possible inconsistency between the mid-IR aerosol signature by VIMS and CIRS Cassini instruments and the visible Huygens-DISR derived data. The mid-IR VIMS and CIRS signatures are in agreement with an aerosol dominated by an aliphatic carbon content, whereas the important visible absorption derived from the DISR measurement seems to be incompatible with such an important aliphatic content, but more compatible with an amine-rich aerosol.     

Thermal and irradiation history of lunar meteorite Dhofar 280

Dhofar 280 recorded a complex history on the Moon revealed by high‐resolution ⁴⁰Ar‐³⁹Ar dating. Thermal resetting occurred less than 1 Ga ago, and the rock was exposed to several impact events before and afterwards. The cosmic ray exposure (CRE) age spectrum indicates a 400 ± 40 Ma CRE on the lunar surface. A unique feature of this lunar sample is a partial loss of cosmogenic ³⁸Ar, resulting in a (low‐temperature) CRE age plateau of about 1 Ma. This was likely caused by the same recent impact event that reset the (low‐temperature) ⁴⁰Ar‐³⁹Ar age spectrum and preceded the short transit phase to Earth of ≤1 Ma. Dhofar 280 may be derived from KREEP‐rich lunar frontside terrains, possibly associated with the Copernicus crater or with a recent impact event on the deposits of the South Pole–Aitken basin. Although Dhofar 280 is paired with Dhofar 081, their irradiation and thermal histories on the Moon were different. An important trapped Ar component in Dhofar 280 is “orphan” Ar with a low ⁴⁰Ar/³⁶Ar ratio. It is apparently a mixture of two components, one endmember with ⁴⁰Ar/³⁶Ar = 17.5 ± 0.2 and a second less well‐constrained endmember with ⁴⁰Ar/³⁶Ar ≤10. The presence of two endmembers of trapped Ar, their compositions, and the breccia ages seem to be incompatible with a previously suggested correlation between age or antiquity and the (⁴⁰Ar/³⁶Ar)_trapped ratio (Eugster et al. 2001; Joy et al. 2011a). Alternatively, “orphan” Ar of this impact melt breccia may have an impact origin.     

Optical constants from 370 nm to 900 nm of Titan tholins produced in a low pressure RF plasma discharge

Determining the optical constants of Titan aerosol analogues, or tholins, has been a major concern for the last three decades because they are essential to constrain the numerical models used to analyze Titan’s observational data (albedo, radiative transfer, haze vertical profile, surface contribution, etc.). Here we present the optical constant characterization of tholins produced with an RF plasma discharge in a (95%N₂–5%CH₄) gas mixture simulating Titan’s main atmospheric composition, and deposited as a thin film on an Al–SiO₂ substrate. The real and imaginary parts, n and k, of the tholin complex refractive index have been determined from 370 nm to 900 nm wavelength using spectroscopic ellipsometry. The values of n decrease from n = 1.64 (at 370 nm) to n = 1.57 (at 900 nm) as well as the values of k which feature two behaviors: an exponential decay from 370 nm to 500 nm, with k = 12.4 × e^?0.018λ (where λ is expressed in nm), followed by a plateau, with k = (1.8 ± 0.2) × 10^?3. The trends observed for the PAMPRE tholins optical constants are compared to those determined for other Titan tholins, as well as to the optical constants of Titan’s aerosols retrieved from observational data.     

A new extraction technique for in situ analyses of amino and carboxylic acids on Mars by gas chromatography mass spectrometry

In order to target key organic compounds in the Martian regolith using gas chromatography mass spectrometry (GC-MS), we have developed a new extraction procedure coupled with chemical derivatization. This new technique was tested on a Mars analog soil sample collected from the Atacama Desert in Chile. We found that amino and carboxylic acids can be extracted from the Atacama soil in a 1:1 mixture of isopropanol and water after ultrasonic treatment for 30 min. The extracted organic compounds were then derivatized in a single-step reaction using N-methyl-N-(tert-butyldimethylsilyl)-trifluoroacetamide (MTBSTFA) as the silylating agent in order to transform these compounds into volatile species that can then be detected by GC-MS. We are currently developing a miniaturized reaction cell suited for spaceflight, where both organic extraction and chemical derivatization processes can take place in a single step.     

Dhofar 225 and Dhofar 735: Relationship to CM2 chondrites and metamorphosed carbonaceous chondrites,Belgica‐7904 and Yamato‐86720

Abstract– Dhofar (Dho) 225 and Dho 735 are carbonaceous chondrites found in a hot desert and having affinities to Belgica‐like Antarctic chondrites (Belgica [B‐] 7904 and Yamato [Y‐] 86720). Texturally they resemble CM2 chondrites, but differ in mineralogy, bulk chemistry and oxygen isotopic compositions. The texture and main mineralogy of Dho 225 and Dho 735 are similar to the CM2 chondrites, but unlike CM2 chondrites they do not contain any (P, Cr)‐sulfides, nor tochilinite 6Fe_0.9S*5(Fe,Mg)(OH)₂. H₂O‐contents of Dho 225 and Dho 735 (1.76 and 1.06 wt%) are lower than those of CM2 chondrites (2–18 wt%), but similar to those in the metamorphosed carbonaceous chondrites of the Belgica‐like group. Bulk compositions of Dho 225 and Dho 735, as well as their matrices, have low Fe and S and low Fe/Si ratios relative to CM2 chondrites. X‐ray powder diffraction patterns of the Dho 225 and Dho 735 matrices showed similarities to laboratory‐heated Murchison CM2 chondrite and the transformation of serpentine to olivine. Dho 225 and 735’s oxygen isotopic compositions are in the high ¹⁸O range on the oxygen diagram, close to the Belgica‐like meteorites. This differs from the oxygen isotopic compositions of typical CM2 chondrites. Experimental results showed that the oxygen isotopic compositions of Dho 225 and Dhofar 725, could not be derived from those of typical CM2 chondrites via dehydration caused by thermal metamorphism. Dho 225 and Dho 735 may represent a group of chondrites whose primary material was different from typical CM2 chondrites and the Belgica‐like meteorites, but they formed in an oxygen reservoir similar to that of the Belgica‐like meteorites.     

Laboratory light-scattering measurements with Titan's aerosols analogues produced by a dusty plasma

The chemistry leading to the formation of solid aerosols (tholins) in Titan's atmosphere is simulated by a capacitively coupled plasma in a N₂-CH₄ gas mixture. The solid grains are produced in volume directly in the gas phase and studied ex-situ by SEM imaging and by light scattering on clouds of particles. The scattered light properties depend on the physical properties of the particles (morphologies, size distribution), as well as on the phase angle and the wavelength of the light. The particles may be aggregated or agglomerated grains. The grains size distribution is studied as a function of plasma parameters such as initial methane concentration introduced into the discharge, gas flow, absorbed RF power and plasma duration. The average grain size increases when the amount of CH₄ increases, when the gas flow decreases, and when the plasma duration increases up to a limit for each production condition.For all the samples, the absorption decreases with increasing wavelength in the visible domain. As usually found for irregular particles, the polarization phase curves have a bell-shaped positive branch and a shallow negative branch. The maximum of polarization (P_max) increases when the average grain size decreases (sub-μm-sized grains). To obtain P_max values within the range of those measured in Titan's atmosphere; the average grains diameter has to be smaller than 100 nm, in agreement with the space observations results. In the light-scattering experiment, the size of the agglomerates in the clouds is in the 40-80 μm range in equivalent diameter. As a consequence P_max increases with decreasing wavelength due to the increasing absorption, in agreement with observations of Titan from outside the atmosphere.