All Figures

$\begin{figure} \par\includegraphics[width=7.5cm,clip]{5606fig1.eps} \end{figure}$ Figure 1: A sketch of the shot-peening treatment used to simulate the bombardment of micro-meteoroids on asteroid surfaces. One hundred micron diameter quartz spheres were pneumatically propelled to impact onto 5 mm thick sections of ordinary chondrites. The air pressure controls the mean impact velocity on the target between 12.5 and 25.0 m/s.
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In the text

$\begin{figure} \par\includegraphics[width=7.5cm,clip]{5606fig2CMJN.eps} \end{figure}$ Figure 2: Images of a shot-peened H6 S2 W1 ordinary chondrite (visible light source at 30 degrees from the vertical). The sample was first polished with a 1 $\mu \rm m$ diamond paste (untreated region) before shot-peening with 100 $\mu \rm m$ quartz spheres at 15 m/s impact velocity for 180 s and 360 s. In the bottom frame, a close up image showing the difference between the untreated and the 360 s shot-peening regions. Metallic zones have not been fragmented by the collisions, while craters formed in the stony, more brittle regions. Some of the metallic zones are marked as a, b, c and d: they appear dark in the polished and untreated region since the light reflected by their surface is not along the vertical. The metallic zones can instead appear brighter in the bombarded region, depending on the deformation of their surface.
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In the text

$\begin{figure} \par\includegraphics[width=7.7cm,clip]{5606fig3.eps} \end{figure}$ Figure 3: Top: a photomicrograph from the thick section of the ordinary chondrite imaged in Fig. 1 (in reflected light optical microscopy), before the shot-peening treatment. The thin section was polished with a 1 $\mu$ m diamond paste. The horizontal field of view is approximately 1.5 mm. Light regions are the metallic zones. Bottom: the same region after shot-peening with 100 $\mu \rm m$ quartz spheres for 180 s at 15 m/s impact velocity. The metals are mainly preserved in their dimension and shape, while in the silicate regions, a pronounced cratering occurred.
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In the text

$\begin{figure} \par\includegraphics[width=7.7cm,clip]{5606fig4.eps} \end{figure}$ Figure 4: Top: the same thick section shown in Fig. 1, where the craters obtained with the shot-peening treatment were filled with a 30 $\mu$ m grain size powder obtained from the same OC (see text). The white frame indicates the region imaged below. Bottom: a composition of photomicrographs in reflected light of the polished section, before the shot-peening treatment. Metallic zones appear white. The dashed rectangles represent the $2 \times 8$ mm² areas whose spectra are shown in Fig. 5.
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In the text

$\begin{figure} \par\includegraphics[width=7cm,clip]{5606fig5.eps} \end{figure}$ Figure 5: Reflected spectrum of the powder alone (approx. 30 $\mu$ m grain size, dashed line) obtained from the ordinary chondrite H6 S2 W1 shown in Figs. 2-4 and spectra from $2 \times 8$ mm² regions of the 360 s bombarded surface whose craters were filled with an powder (continuous lines, see also Fig. 4). Spectra were obtained with a Perkin-Elmer spectrometer with an integrating sphere and normalized to 0.56 $\mu \rm m$ .
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In the text

$\begin{figure} \par\includegraphics[width=7.5cm,clip]{5606fig1.eps} \end{figure}$	Figure 1: A sketch of the shot-peening treatment used to simulate the bombardment of micro-meteoroids on asteroid surfaces. One hundred micron diameter quartz spheres were pneumatically propelled to impact onto 5 mm thick sections of ordinary chondrites. The air pressure controls the mean impact velocity on the target between 12.5 and 25.0 m/s.
Open with DEXTER