Research ArticlePLANETARY SCIENCE

Hypervelocity impacts as a source of deceiving surface signatures on iron-rich asteroids

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Science Advances  28 Aug 2019:
Vol. 5, no. 8, eaav3971
DOI: 10.1126/sciadv.aav3971
  • Fig. 1 Features and characteristics of craters generated by our hypervelocity impact experiments on steel metallic targets.

    (A) Secondary electron microscopy (SEM) images of the target taken orthogonal to the section of the n0 crater (6.89 km/s; 1899 J). This representative example (27) shows that craters have the typical regular bowl shape surrounded by an almost continuous frozen metallic lip that rises at the edge of the crater. Note that the bottom of the crater backscatters electrons less efficiently than the unprocessed metallic surface (and thus appears darker in the image). This is explained by the low average atomic number of the basaltic-like impact melt coating in the processed area of the target. (B) Vertical SEM image of the inside edge of the n4 crater (6.88 km/s; 710 J) depicting the impact melt coating with glassy and degassing features. (C) Composite x-ray element map (Al, green; Fe, blue; Si, red) of the same area of the n4 crater (6.88 km/s; 710 J). Note that the coating spreads efficiently over the entire surface of the bowl shape crater [modified from previous work (27)]. (D) Secondary electron–SEM image of a cross section of the n0 crater (6.89 km/s; 1899 J) showing a regular, almost symmetrical concave section. Concentric fractures are apparent from the top down to the bottom of the crater. There is a strong concentration of fractures and voids at the bottom of the crater, which produces a substantial increase of porosity at a depth well below the crater’s bottom. Notice shear bands and the occurrence of crystal-oriented texture in the steel. (E) Electron backscattered SEM image showing the basalt-like impact melt coating of the crater surface on its sides, with a thickness of a few tens of micrometers [see (D) for location]. The creation of fracture areas by the injection of the melt is observable. Note that the basaltic melt is spotted with minute metallic blebs and gas bubbles. Target metal is highly deformed. (F) Electron backscattered SEM image of the crater bottom [see (D) for location]. In addition to the minute metallic blebs and bubbles shown in (E), large metal chips that are extracted from the metallic target during the impact are shown.

  • Fig. 2 Electron backscattered (BSE) characteristic images of craters generated by hypervelocity impact experiments on the Gibeon iron meteorite target chunks using both dunite and basalt projectiles.

    (A) #101 dunite at 3.25 km/s at room temperature. The crater’s morphology is a typical bowl-like shape with a clear darkening of its interior. Notice that the crater’s metallic rims are irregular and largely protruding. (B) #102 dunite at 3.28 km/s at 151 K. This bowl-like shape crater is obtained in the same experimental conditions than run #101, but the dwell target temperature was set at 151 K. Similar features are observable, except the tearing off part of the crater wall on its left part (see section S6). (C) #103 dunite at 6.97 km/s at room temperature. This is the higher-velocity impact crater obtained with a dunite projectile. Its interior consists of a frozen emulsion of metal-silicate immiscible liquids. (D) #107 basalt at 5.08 km/s at 131 K. The darkening of the inside of the crater is more pronounced than in the other impact experiments using dunite as a projectile. Concentric cracks are also present. (E) Electron backscattered SEM image showing the dunite-like glassy coating (pink) of the run #103 iron crater surface (green) on its sides. The glassy coating contains numerous bubbles. (F) Electron backscattered SEM image showing the basalt-like glassy coating (pink) of the #107 crater. Notice here the heterogeneous thickness of the glassy coating and its foamy texture (iron-rich materials are in green).

  • Fig. 3 SEM images of glassy coating in impact craters.

    (A) Thin glass threads connecting the two rims of the same concentric fracture in the n4 crater, suggesting that the formation and the opening of the fractures are necessarily before the quenching of the impact melt. (B) Secondary electron images of the frothy and foamy bottom of the crater NW2. (C) BSE view of the bottom of the n5 crater dunite resulting from the impact of a dunite projectile at 4.91 km/s and showing a vesicular impact coating constituted by a mixture of dunite (dark) and iron (light) materials. (D) Detail of the #103 crater interior resulting from the impact of a dunite projectile at 6.97 km/s and showing a frozen emulsion of metal (light color)–silicate (dark color) immiscible liquids.

  • Fig. 4 VIS-NIR spectra of the produced craters.

    (A) Impact experiments performed on a steel target with basaltic (colored solid lines) and dunite projectiles (dashed-dotted line). The spectra of the target outside of the crater are displayed in black. (B) Impact experiments performed on the iron meteorite Gibeon with basaltic (blue line) and dunite projectiles (dashed-dotted lines). Note the appearance of broad absorption bands around 1 and 1.9 μm for the craters produced with the basaltic projectile. The spectra of the target outside of the crater are in black. All spectra in (A) and (B) were measured at an incidence of 0° and an emergence of 20°. See table S1 for run conditions.

  • Fig. 5 VIS-NIR spectra of the produced craters compared to observation of probable metal-rich asteroids.

    (A) VIS-NIR spectra of the craters produced with a dunite projectile, run n5 at 4.91 km/s. The black spectra show the spectral signature of the metal outside of the crater and its dependency on observation geometry (same incidence angle, 0°; three different emergence angles, 15°, 20°, and 30°). The light blue spectra were measured inside the crater (incidence, 0°; emergence, 20°). The dark blue spectra were measured after treating the sample with HF-nital, removing the material coating the crater floor (i.e., glassy dunite). The removal of this material induces an increase of the overall reflectance level. (B) Normalized reflectance spectra of the craters produced in this study compared to observation of probable metal-rich asteroid (14) as well as metal-related spectral end-members (36). The obtained craters show variations of spectral slopes, including slope values similar to ground-based observation of probable metal-rich asteroids.

  • Fig. 6 VIS-NIR reflectance spectra of craters resulting from impact experiments (5.4 km/s) using a dry (ND1) or a wet (NW2) basalt.

    The clear occurrence of a 3-μm absorption band only in the NW2 crater indicates the entrapment of water, very likely in the form of hydroxyl, within impact glasses. (16) Psyche NIR (37) and obsidian, smectite, and condensed water spectra are given for comparison.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/8/eaav3971/DC1

    Section S1. Experimental and effect of surface roughness on the reflectance spectra of metallic meteorites

    Section S2. Hypervelocity impact experiments on steel targets (more images and observations)

    Section S3. Characteristics of impact melt coating inside hypervelocity craters

    Section S4. Hypervelocity impact of a dunite projectile on metallic target

    Section S5. Hypervelocity impact experiments on (Gibeon) iron meteorite target

    Section S6. Hypervelocity impact experiments using dry and hydrated basalt projectile on steel target

    Fig. S1. Impact experiments.

    Fig. S2. Various surface roughness of the Gibeon iron meteorite.

    Fig. S3. VIS-NIR spectra of the Gibeon meteorite after progressive roughening of the sample.

    Fig. S4. Morphology of hypervelocity impact craters on steel target.

    Fig. S5. Crater glassy coating characterization.

    Fig. S6. Cross section of the impact melt glassy coating at the bottom of hypervelocity crater.

    Fig. S7. Secondary electron microscopy (SEM) images of the impact melt coating inside craters caused by basalt-like projectiles.

    Fig. S8. Secondary electron microscopy (SEM) image of impact crater caused by a dunite projectile on steel target (n5; 4.91 km/s; 892 J).

    Fig. S9. Characterization of the crater coating of the molten dunite.

    Fig. S10. Morphology of hypervelocity impact craters on Gibeon iron meteorite target.

    Fig. S11. Morphology of hypervelocity impact craters using dry and hydrated basalt projectile on steel target.

    Fig. S12. VIS-NIR spectra of impact crater on Gibeon iron meteorite targets using dry dunite and basalt projectiles.

    Table S1. Experimental conditions.

  • Supplementary Materials

    This PDF file includes:

    • Section S1. Experimental and effect of surface roughness on the reflectance spectra of metallic meteorites
    • Section S2. Hypervelocity impact experiments on steel targets (more images and observations)
    • Section S3. Characteristics of impact melt coating inside hypervelocity craters
    • Section S4. Hypervelocity impact of a dunite projectile on metallic target
    • Section S5. Hypervelocity impact experiments on (Gibeon) iron meteorite target
    • Section S6. Hypervelocity impact experiments using dry and hydrated basalt projectile on steel target
    • Fig. S1. Impact experiments.
    • Fig. S2. Various surface roughness of the Gibeon iron meteorite.
    • Fig. S3. VIS-NIR spectra of the Gibeon meteorite after progressive roughening of the sample.
    • Fig. S4. Morphology of hypervelocity impact craters on steel target.
    • Fig. S5. Crater glassy coating characterization.
    • Fig. S6. Cross section of the impact melt glassy coating at the bottom of hypervelocity crater.
    • Fig. S7. Secondary electron microscopy (SEM) images of the impact melt coating inside craters caused by basalt-like projectiles.
    • Fig. S8. Secondary electron microscopy (SEM) image of impact crater caused by a dunite projectile on steel target (n5; 4.91 km/s; 892 J).
    • Fig. S9. Characterization of the crater coating of the molten dunite.
    • Fig. S10. Morphology of hypervelocity impact craters on Gibeon iron meteorite target.
    • Fig. S11. Morphology of hypervelocity impact craters using dry and hydrated basalt projectile on steel target.
    • Fig. S12. VIS-NIR spectra of impact crater on Gibeon iron meteorite targets using dry dunite and basalt projectiles.
    • Table S1. Experimental conditions.

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