0000-0002-4414-4917The attached files are the published version of the article from the 49th Lunar and Planetary Science Conference 2018 (LPI Contrib. No. 2083), and the open access abstract (Geophysical Research Abstracts, Vol. 20, EGU2018-16223, 2018, EGU General Assembly 2018, © Author(s) 2018. CC Attribution 4.0 license.

Garbout, A

Greshake, A

Hamann, C

Hecht, L

Heunoske, D

Osterholz, J

Salge, T

Schäffer, S

0000-0002-4414-4917The attached files are the published version of the article from the 49th Lunar and Planetary Science Conference 2018 (LPI Contrib. No. 2083), and the open access abstract (Geophysical Research Abstracts, Vol. 20, EGU2018-16223, 2018, EGU General Assembly 2018, © Author(s) 2018. CC Attribution 4.0 license.)NHM Repositor

Natural History Museum Repository

  Title Impact vaporization and Condensation: LaserIrradiation Experiments with Natural PlanetaryMaterialsAuthors Hamann, C; Hecht, L; Schäffer, S; Heunoske, D;Salge, T; Garbout, A; Osterholz, J; Greshake, ADescription The attached files are the published version of thearticle from the 49th Lunar and Planetary ScienceConference 2018 (LPI Contrib. No. 2083), and theopen access abstract (Geophysical ResearchAbstracts, Vol. 20, EGU2018-16223, 2018, EGUGeneral Assembly 2018, © Author(s) 2018. CCAttribution 4.0 license.)Date Submitted 2018-05-09IMPACT VAPORIZATION AND CONDENSATION: LASER IRRADIATION EXPERIMENTS WITH NATURAL PLANETARY MATERIALS.  C. Hamann1,2, L. Hecht1,2, S. Schäffer3, D. Heunoske3, T. Salge4, A. Garbout4, J. Osterholz3, and A. Greshake1, 1Museum für Naturkunde, 10115 Berlin, Germany (chris-topher.hamann@mfn–berlin.de), 2Institut für Geologische Wissenschaften, Freie Universität Berlin, 12249 Berlin, Germany, 3Fraunhofer-Institut für Kurzzeitdynamik, Ernst-Mach-Institut, 79104 Freiburg, Germany, 4Imaging and Analysis Centre, Natural History Museum, London SW7 5BD, United Kingdom.   Introduction: Hypervelocity impacts of asteroids or comets onto planetary surfaces typically induce melting and partial to complete vaporization of the involved materials close to the point of impact [e.g., 1], and may produce a vapor plume above the impact crater that subsequently condenses after it reaches the liquid–vapor coexistence curve [2,3]. Compared to impact melts and owing to their rarity, natural prod-ucts of impact vaporization and condensation have scarcely been studied [e.g., 4–7]. Moreover, previous experiments that focused on impact vaporization and condensation have either used numerical modeling to predict physical (e.g., spherule size) and thermody-namic (e.g., temperature, oxidation state) characteris-tics of vaporized and condensing materials [e.g., 2,3], or directly investigated physicochemical properties of experimentally generated vapor plumes using spec-troscopic techniques [e.g., 8]. The products of impact vaporization and subsequent condensation have, however, rarely been studied [e.g., 9].  Here, we present a petrographic-geochemical characterization of vapor condensates produced by laser irradiation of planetary materials [cf. 10] of chondritic, basaltic, dioritic, and granitic composi-tions to constrain mineralogy and composition of impact condensates formed from similar materials.  Methods: We used a continuous-wave (CW) fi-ber laser with a wavelength of 1.07 µm at Fraunhofer Institut für Kurzzeitdynamik, Freiburg, Germany [cf. 10], to irradiate pieces of the H5 chondrite Ham-madah al Hamra 077 (HaH 077) as well as terrestrial rocks of basaltic, dioritic, and granitic composition in ambient air at 1 bar and room temperature (Fig. 1).   Figure 1: Laser irradiation of andesite. (Exemplary im-age to illustrate experimental setup; witness plates were not used here to fully observe the vapor plume.) The CW fiber laser emitted a power of 8 kW over a specified time (5–10 s) during which the target was moved either vertically or horizontally. The profile of the laser beam was close to a Gaussian distribution with a 1/e2 diameter of 2 mm on the target. The aver-age laser intensity within the 1/e2 diameter was 2.2 × 105 W/cm2 and the laser–matter interaction zone was observed using a similar setup as described in [10].  The vapor plumes were sampled by witness plates of aluminum metal or ceramics that had a horizontal, 5 mm wide slit machined into the material to allow passage of the laser beam. The witness plates were positioned opposite and parallel to the target surface at distances between 1 and 3 cm, and sampled a mix-ture of condensate precipitates and melt droplets that were excavated from the irradiation zones. Textural and compositional characterization of condensates employed field-emission scanning electron microsco-py (SEM), low-voltage energy dispersive spectrosco-py (EDX), and 3D X-ray microscopy (CT) at the Imaging and Analysis Centre, Natural History Muse-um, London, UK. Results: Direct CW laser irradiation of the sam-ples in the parameter region considered here resulted in formation of self-luminous vapor plumes (Fig. 1) that precipitated onto the witness plates macroscopi-cally white (silicate starting materials) or tan to dark brown (HaH 077 starting material), fine-grained, fluffy coatings or layers. In addition, the laser exca-vated melt droplets of sub-micrometer to millimeter diameters that were ballistically transported to the witness plates; if deposited early, these melt droplets were also coated by thin condensate layers.  SEM analysis obtained from the surfaces of the condensate films reveals that the condensate coatings consist of fluffy, dust-like, globular, ~100-nm-diameter nanoparticles (Fig. 2a,b) that accreted to continuous, chemically alternating sequences or lay-ers (Fig. 2c–e). Individual condensate sub-layers seem to evolve from more refractory to more volatile compositions (e.g., from Si-rich to Na-rich composi-tion) with decreasing depth (i.e., from bottom to top; Fig. 2e). Moreover, platy to acicular, idiomorphic crystallites (presumably sodium sulfate) of 2–5 µm size were found interbedded into rod-shaped aggrega-tions of nanoparticles in some experiments (e.g., 2144.pdf49th Lunar and Planetary Science Conference 2018 (LPI Contrib. No. 2083)gabbroic starting composition; Fig. 2b), indicating that the vapor plume precipitated a mixture of pre-sumably amorphous nanoparticles and crystallites. Discussion: Following methods outlined in [8] and [10], we estimate that the irradiation intensities used in our study result in entropy increases that correspond to impact velocities in the range of 10 to some 25 km/s and, thus, are capable of simulating impact vaporization of planetary materials (i.e., typi-cal silicate rocks and chondrites).  The experimentally generated condensate deposits share similarities to condensates formed in previous experiments that used pulsed laser irradiation [e.g., 9,11]. The chemical zoning of individual condensate layers observed here likely reflects slight changes in temperature and, thus, composition [12] of the vapor plume of a given experiment. As irradiation was continuous over several seconds, continuously “re-plenishing” plumes were formed that precipitated several sequences of alternating composition, whereat each sequence seems to reflect a specific (and rhyth-mically recurring) temperature interval.  In future work we plan to extend our characteriza-tion of our experimental vapor condensates to the nanometer scale using transmission electron micros-copy, and to perform similar experiments in vacuum to simulate impact vaporization on atmosphere-less bodies.  Acknowledgments: This work was supported by a Eu-roplanet 2020 RI TA Grant to CH and LH and a MfN Inno-vation Funds Grant to CH. Europlanet 2020 RI has received funding from the European Union's Horizon 2020 research and innovation program under grant agreement No 654208. References: [1] Pierazzo E. et al. (1997) Icarus, 127, 408–423. [2] Johnson B. C. and Melosh H. J. (2012) Ica-rus, 217, 416–430. [3] Svetsov V. V. and Shuvalov V. (2016) Geochim. Cosmochim. Acta, 173, 50–63. [4] Keller L. P. and McKay D. S. (1992) Proc., 22nd Lunar Planet. Sci. Conf., pp. 137–141. [5] Keller L. P. and McKay D. S. (1993) Science, 261, 1305–1307. [6] Warren P. H. (2008) Geochim. Cosmochim. Acta, 72, 3562–3585. [7] Belza J. et al. (2017) Geochim. Cosmochim. Acta, 202, 231–263. [8] Kadono T. et al. (2002) Geophys. Res. Lett., 29, 40-1–40-4. [9] Gerasimov M. B. V. et al. (2002) Deep Sea Res. Part II, 49, 995–1009. [10] Hamann C. et al. (2016) Geophys. Res. Lett., 43, 10602–10610. [11] Dikov. Y. et al. (1996) LPS XXVII, Abstract #311. [12] Lodders K. (2003) Astrophys. J., 591, 1220–1247. Figure 2: a Condensate nanoparticles on the surface of a condensed film (HaH 077 starting material); secondary electron (SE) image. b Idiomorphic crystallites, presumably sodium sulfate, interbedded into aggregates of condensate nanoparticles (gabbroic starting material); SE image. c Representative CT slice of condensate layers and silicate melt droplets (granitic starting material). The dashed white line indicates the position of an aluminum witness plate (removed before CT imaging). d Back-scattered electron image of two condensate layers featuring various sub-layers adhering to a silicate melt droplet. e Elemental distribution of Na, Si and Al of alternating condensate layers deposited onto an aluminum witness plate (removed before section-ing; position indicated by dashed white line) and silicate melt droplets. Note that each layer evolves from Si-rich, more refractory to Na-rich, more volatile compositions.  2144.pdf49th Lunar and Planetary Science Conference 2018 (LPI Contrib. No. 2083)

Impact vaporization and Condensation: Laser Irradiation Experiments with Natural Planetary Materials

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Impact vaporization and Condensation: Laser Irradiation Experiments with Natural Planetary Materials

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