Impact craters up to a few hundreds of meters in diameter are common structures of solid surfaces of planetary bodies in the solar system. Statistics predict that impacts producing small craters on Earth occur on decadal to secular time scales (1, 2). However, small craters are rare on Earth because they are rapidly eroded, and the few identified so far [15 <300 m in diameter out of 176 craters up to 300 km in diameter (3)] have lost most of their primary features. We report the detection in southern Egypt of a rayed impact crater 45min diameter (Fig. 1A) on a Cretaceous sandstone target. The ejecta rays highlight the exceptional freshness of the structure. The crater was identified by V. De Michele during a Google Earth survey and named Kamil Crater after nearby Gebel Kamil. A geophysical expedition undertaken [supporting online material (SOM)] in February 2010 revealed that the crater is bowl shaped and has an upraised rim (~3 m above preimpact surface) (figs. S1 and S2) typical of simple craters (4). The true crater floor depth is 16 m and is overlain by ~6-m-thick crater-fill material (fig. S2). Morphometric parameters agreewith those predicted by models (5) for a transient crater generated by an iron meteorite 1.3 min diameter (equivalent to 9.1 × 103 kg) impacting at a velocity of 3.5 kms−1, assuming an average meteoroid entry velocity and entry angle of 18 kms−1 and 45°, respectively. Centimeterscale masses of scoriaceous impact melt glass (fig. S3) occur in and close to the crater and indicate local shock pressures >60 GPa (4). We identified 5178 iron meteorite specimens totaling ~1.71 tons in the crater and surrounding area during systematic searches (SOM). They consist of <34-kg shrapnel produced by the explosion of the impactor upon hypervelocity collision with the target (Fig. 1B), except one individual fragment of 83 kg (fig. S4). This evidence indicates that the Kamil Crater was generated by an impactor that landed nearly intact without substantial fragmentation in the atmosphere. The meteorite is classified as an ungrouped Ni-rich ataxite [Ni = 19.8 weight % (wt %), Co = 0.75 wt %, Ga = 49.5 mg g−1, Ge = 121 mg g−1, Ir = 0.39 mg g−1; data following (6); fig. S5]. Magnetic anomaly data show no evidence of buried meteorites larger than some tens of centimeters (fig. S1). On the basis of systematic meteorite searches, the estimated total mass of the impactor is of the order of 5 ×103 to 10 × 103 kg, corresponding to a preatmospheric mass of ~20 ×103 to 40 × 103 kg (2). According to geophysical models (2, 7), iron masses <3 × 106 kg normally fragment upon impact with Earth’s atmosphere, thereby reducing the energy of the impact at Earth’s surface. However, the present statistics, which include the recently discovered Whitecourt Crater (8) and the Kamil Crater, suggest that ~35% of the iron meteorites in the above mass range are not disrupted in the atmosphere.
THE KAMIL CRATER IN EGYPT
FOLCO, LUIGI;D'ORAZIO, MASSIMO;
2010-01-01
Abstract
Impact craters up to a few hundreds of meters in diameter are common structures of solid surfaces of planetary bodies in the solar system. Statistics predict that impacts producing small craters on Earth occur on decadal to secular time scales (1, 2). However, small craters are rare on Earth because they are rapidly eroded, and the few identified so far [15 <300 m in diameter out of 176 craters up to 300 km in diameter (3)] have lost most of their primary features. We report the detection in southern Egypt of a rayed impact crater 45min diameter (Fig. 1A) on a Cretaceous sandstone target. The ejecta rays highlight the exceptional freshness of the structure. The crater was identified by V. De Michele during a Google Earth survey and named Kamil Crater after nearby Gebel Kamil. A geophysical expedition undertaken [supporting online material (SOM)] in February 2010 revealed that the crater is bowl shaped and has an upraised rim (~3 m above preimpact surface) (figs. S1 and S2) typical of simple craters (4). The true crater floor depth is 16 m and is overlain by ~6-m-thick crater-fill material (fig. S2). Morphometric parameters agreewith those predicted by models (5) for a transient crater generated by an iron meteorite 1.3 min diameter (equivalent to 9.1 × 103 kg) impacting at a velocity of 3.5 kms−1, assuming an average meteoroid entry velocity and entry angle of 18 kms−1 and 45°, respectively. Centimeterscale masses of scoriaceous impact melt glass (fig. S3) occur in and close to the crater and indicate local shock pressures >60 GPa (4). We identified 5178 iron meteorite specimens totaling ~1.71 tons in the crater and surrounding area during systematic searches (SOM). They consist of <34-kg shrapnel produced by the explosion of the impactor upon hypervelocity collision with the target (Fig. 1B), except one individual fragment of 83 kg (fig. S4). This evidence indicates that the Kamil Crater was generated by an impactor that landed nearly intact without substantial fragmentation in the atmosphere. The meteorite is classified as an ungrouped Ni-rich ataxite [Ni = 19.8 weight % (wt %), Co = 0.75 wt %, Ga = 49.5 mg g−1, Ge = 121 mg g−1, Ir = 0.39 mg g−1; data following (6); fig. S5]. Magnetic anomaly data show no evidence of buried meteorites larger than some tens of centimeters (fig. S1). On the basis of systematic meteorite searches, the estimated total mass of the impactor is of the order of 5 ×103 to 10 × 103 kg, corresponding to a preatmospheric mass of ~20 ×103 to 40 × 103 kg (2). According to geophysical models (2, 7), iron masses <3 × 106 kg normally fragment upon impact with Earth’s atmosphere, thereby reducing the energy of the impact at Earth’s surface. However, the present statistics, which include the recently discovered Whitecourt Crater (8) and the Kamil Crater, suggest that ~35% of the iron meteorites in the above mass range are not disrupted in the atmosphere.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
