Scarabeo Egizio, materiale: Cometa

Il primo frammento di cometa rinvenuto sulla terra.

10 ottobre 2013

Redazione Tiscali

E' stato prodotto dall'esplosione di una cometa avvenuta 28 milioni di anni fa sull'Egitto, il vetro giallo incastonato in uno dei gioielli della mummia di Tutankhamon. E' la prima testimonianza in assoluto dell'impatto di una cometa nell'atmosfera terrestre.

Il frammento è stato lucidato e scolpito dagli antichi Egizi, che lo hanno modellato come uno scarabeo, ma la sua origine è sicuramente cosmica: lo dimostra la ricerca in via di pubblicazione sulla rivista Earth and Planetary Science Letters (EPSL) e condotta dal gruppo internazionale di geologi, fisici ed astronomi coordinato da Jan Kramers, dell'Università sudafricana di Johannesburg.

Tra i ricercatori c'è anche l'italiano Marco Andreoli, della South African Nuclear Energy Corporation. Secondo la ricostruzione dei ricercatori l'esplosione della cometa, avvenuta sul deserto del Sahara, riscaldò la sabbia portandola alla temperatura di circa 2.000 gradi. Si formò in questo modo un'enorme quantità di vetro di silice giallo, sparso su una su una superficie di oltre 6.000 chilometri quadrati. Uno di questi frammenti è stato modellato e incastonato nel gioiello del faraone.

Ad attirare inizialmente l'attenzione degli studiosi è stata una pietra scura trovata anni fa da un geologo egiziano: analizzata nuovamente adesso risulta essere tutt'altro che un comune meteorite: è quello che resta del nucleo della cometa esplosa sulla Terra milioni di anni fa.

L'esplosione ha anche prodotto microscopici diamanti. "I diamanti normalmente si formano nelle viscere della terra dove la pressione è alta - osserva Kramers - ma è anche possibile generare una pressione molto elevata attraverso una forte esplosione''. I ricercatori sono convinti che la scoperta del materiale prodotto dalla cometa permetterà di aggiungere tasselli preziosi per ricostruire la storia del Sistema Solare.

ARTICOLO ORIGINALE (da EPSL):

Unique chemistry of a diamond-bearing pebble from the Libyan Desert Glass strewnfield, SW Egypt: Evidence for a shocked comet fragment

• Jan D. Kramers^{a, ,} ,
• Marco A.G. Andreoli^{b, c},
• Maria Atanasova^d,
• Georgy A. Belyanin^a,
• David L. Block^e,
• Chris Franklyn^b,
• Chris Harris^f,
• Mpho Lekgoathi^b,
• Charles S. Montross^g,
• Tshepo Ntsoane^b,
• Vittoria Pischedda^h,
• Patience Segonyane^b,
• K.S. (Fanus) Viljoen^a,
• Johan E. Westraadt^g

• ^a Department of Geology, University of Johannesburg, Auckland Park 2006, South Africa
• ^b NECSA, PO Box 582, Pretoria 0001, South Africa
• ^c School of Geosciences, University of the Witwatersrand, PO Box 3, Wits 2050, South Africa
• ^d Council for Geoscience, PO Box 112, Pretoria 0001, South Africa
• ^e AECI and AVENG Cosmic Dust Laboratory, School of Computational and Applied Mathematics, University of the Witwatersrand, PO Box 60, Wits 2050, South Africa
• ^f Department of Geological Sciences, University of Cape Town, Rondebosch 7701, South Africa
• ^g Element Six (Pty) Ltd, Springs 1559, South Africa
• ^h LPMCN, Université Lyon 1 and CNRS, UMR 5586, F-69622 Villeurbanne, France

Received 27 November 2012

Revised 27 July 2013

Accepted 3 September 2013

Available online 25 September 2013

Editor: T. Elliot

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Highlights

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A carbonaceous stone found in the Libyan Desert Glass strewnfield was studied.

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It contains sub-micrometer diamonds in an amorphous, carbon-dominated matrix.

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Carbon isotope data (δ¹³C∼0${\delta }^{13}\mathrm{C}\sim 0$) do not fit terrestrial coal or carbonaceous chondrites.

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Argon, Kr and Xe isotope data show extraterrestrial origin different from chondrites.

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We propose that the stone is a fragment of a comet nucleus, shocked on impact.

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Abstract

We have studied a small, very unusual stone, here named “Hypatia”, found in the area of southwest Egypt where an extreme surface heating event produced the Libyan Desert Glass 28.5 million years ago. It is angular, black, shiny, extremely hard and intensely fractured. We report on exploratory work including X-ray diffraction, Raman spectroscopy, transmission electron microscopy, scanning electron microscopy with EDS analysis, deuteron nuclear reaction analysis, C-isotope and noble gas analyses. Carbon is the dominant element in Hypatia, with heterogeneous O/C and N/C ratios ranging from 0.3 to 0.5 and from 0.007 to 0.02, respectively. The major cations of silicates add up to less than 5%. The stone consists chiefly of apparently amorphous, but very hard carbonaceous matter, in which patches of sub-μm$\text{μ}\mathrm{m}$ diamonds occur. δ¹³C${\delta }^{13}\mathrm{C}$ values (ca. 0‰) exclude an origin from shocked terrestrial coal or any variety of terrestrial diamond. They are also higher than the values for carbonaceous chondrites but fall within the wide range for interplanetary dust particles and comet 81P/Wild2 dust. In step heating, ⁴⁰Ar/³⁶Ar ratios vary from 40 to the air value (298), interpreted as a variable mixture of extraterrestrial and atmospheric Ar. Isotope data of Ne, Kr and Xe reveal the exotic noble gas components G and P3 that are normally hosted in presolar SiC and nanodiamonds, while the most common trapped noble gas component of chondritic meteorites, Q, appears to be absent. An origin remote from the asteroid belt can account for these features.

We propose that the Hypatia stone is a remnant of a cometary nucleus fragment that impacted after incorporating gases from the atmosphere. Its co-occurrence with Libyan Desert Glass suggests that this fragment could have been part of a bolide that broke up and exploded in the airburst that formed the Glass. Its extraordinary preservation would be due to its shock-transformation into a weathering-resistant assemblage.

Keywords

• Libyan Desert Glass;
• shock diamonds;
• extraterrestrial carbonaceous matter;
• carbon isotopes;
• noble gas isotopes;
• comet nucleus

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Figures and tables from this article:

Fig. 1. A: Macrophotograph of sample Hyp-1, the largest among a group of subsamples split from the primary Hypatia stone. Note secondary desert varnish coating and a thin open fracture on the left side (white arrows). B, C: scanning electron microscope backscattered electron images of a fracture surface on a second subsample (Hyp-2). In B, dark material is richer in oxygen than light material. In C, light material is Fe–Cr–Ni alloy. Working distance: B, 15.1 mm; C, 14.4 mm. Detectors: B: Everhardt–Thornley; C: Dual backscatter. High voltage: B, 30 kV; C, 20 kV. D–G: element maps for C, Al, Fe, S (element symbols in left top corners, lighter shades indicate higher concentration) produced on a polished section of Hypatia (Hyp-3) using the TeScan instrument (20 kV). Carbon dominates the matrix (D). Aluminum marks secondary clay minerals formed on cracks (E). In F and G, note the close coincidence of Fe with S. Equal abundances signal pyrrhotite.

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Fig. 2. A: optical image of an area of the polished section of Hyp-3. B: Raman spectral map of the same area produced by the WiTek Raman microscope. Light and dark grey patches correspond to the top and bottom spectra shown in C, respectively (variable baseline removed). Bottom spectrum in C shows both D and G bands indicating a carbon structure with both sp² and sp³ bonds, whereas in the top spectrum the G band is almost absent and the D band is very sharp, indicating diamond. Thus light grey areas in B are dominated by diamond. D: bright-field transmission electron microscope image of a grain from Hyp-3. The nm-scale cracks and voids seen are not an artifact of the sample preparation as they are not observed in similar imagery on terrestrial diamond samples. “Dark” specs are cubic diamond domains with orientations causing electron diffraction. A selected area diffraction pattern of this region (Supplementary Fig. 2) shows several rings, indicating a fine polycrystalline aggregate. A line calibrated profile of the selected area diffraction intensity is shown in Supplementary Fig. 3. Supplementary Table 3 shows the d-spacings corresponding to the first 5 rings, and the corresponding diffraction planes in cubic diamond.

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Fig. 3. O/C ratios, N/C ratios (determined both by SEM energy dispersive analysis and nuclear reaction analysis) and δ¹³${\delta }^{13}$C_(PDB) values of Hypatia matrix matter (see Table 1, Supplementary Table 2), compared to terrestrial and extraterrestrial sample sets: coal (,  and ), bulk carbonaceous chondrites (Kerridge, 1985), chondritic IOM (Alexander et al., 2007), carbonados (,  and ), impact-produced diamonds (Koeberl et al., 1997), Comet Halley (Greenberg and Li, 1999), Comet 81P-Wild 2 dust (, , ,  and ), interplanetary dust particles ( and ), Earthʼs mantle (Sharp, 2007). All bars include the uncertainty limits. For O/C and N/C ratios of Hypatia, “dark” and “pale” spots refer to Fig. 1B. For δ¹³C${\delta }^{13}\mathrm{C}$ values, “A” and “B” imply an ultrasonic treatment in ethanol only, whereas “HCl” denotes a cleaning in hot, dilute HCl. Stippled line for carbonados δ¹³C${\delta }^{13}\mathrm{C}$ represents rare occurrences of heavier values (Shelkov et al., 1997).

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Fig. 4. Noble gas abundances (cc stp/g) for heating steps from Hypatia fragments Hyp-7, -9, -10 and -11, normalized to Earth concentrations (total atmospheric inventory divided by the Earthʼs mass). Heating step temperatures are shown in °C. For comparison, the atmosphere—normalized relative solar abundance pattern (summarized by Wieler, 2002; arbitrarily positioned to fit in the frame) is shown in (C).

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Fig. 5. ⁴⁰Ar/³⁶Ar ratios versus amount of ³⁶Ar released at individual incremental heating steps (temperatures shown in °C). Dashed lines “Atm”: Earthʼs atmosphere. f³⁶Ar_ET, the extraterrestrial fraction of ³⁶Ar, is scaled on the right hand side of each panel; the bulk value of f³⁶Ar_ET listed in the panels is the weighted average of the step values.

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Fig. 6. Conventional three-isotope plot for neon extracted from a large (3.1 mg) fragment (Hyp-12) in broad temperature steps (not corrected for blank (b) which is of atmospheric composition; data in Supplementary Table 6), compared with atmospheric (A, Pepin and Porcelli, 2002) and the meteoritic trapped components Q, P3, HL and G (summarized by Ott, 2002, and discussed in the text). No sample gas from the 1900 °C step could be measured, as a huge eruption of gas occurred (larger than in Hyp-11) which had to be pumped out immediately to prevent damage. A remnant of Ar from this gas, trapped on a cold finger, proved to be of atmospheric composition.

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Fig. 7. Three-isotope plots for Kr and Xe from the heating steps of four Hypatia fragments, shown with 2σ error bars, compared with atmospheric and various extraterrestrial isotope compositions. Narrow-dashed arrows with double heads show direction of mass-dependent fractionation enriching heavy (f+) or light (f−) isotopes. Labels on data points deviating strongly from the main populations give the Hyp-number and the step temperature in °C (referring to Supplementary Tables 7 and 8). A: Krypton data. “Common” comprises isotopic compositions of atmospheric, solar wind, Q and P3 components discussed in the text, as summarized by Ott (2002), Wieler (2002) and Pepin and Porcelli (2002), which are not resolved at this scale. Only the HL and G components are clearly different. Broad-dashed arrow points to G composition outside the plot. Grey area is 95% confidence envelope of a York fit through the Hypatia data only (not forced through “common”). B: ¹²⁸Xe/¹³²Xe vs. ¹³⁶Xe/¹³²Xe, Atm: atmosphere (Pepin and Porcelli, 2002), Sol: solar wind (Wieler, 2002). Isotopic compositions of chondritic noble gas components as for A. Broad-dashed arrows with coordinates point to the components outside the plot. Grey area as in A. C: ¹²⁹Xe/¹³²Xe vs. ¹³⁶Xe/¹³²Xe. The dashed triangle (apices Atm, Q and arrows pointing to distant G component) contains most of the data, suggesting 3-component mixing as discussed in the text.