PermeateFree said:
>>A giant crater that was formed when a meteorite smashed into Earth, has been uncovered deep below Greenland’s ice sheets.
The 31-kilometre-wide cavity was discovered by an international team of scientists who believe it was caused by a “rare” meteorite that struck Earth as recently as 12,000 years ago.
Evidence suggests the crater was formed when a kilometre-wide iron meteorite penetrated seven kilometres into the Earth’s crust.<<
https://www.independent.co.uk/news/science/meteorite-crater-meteor-greenland-ice-sheet-hiawatha-glacier-scientists-a8633826.html
31 km is big but not all that big. We may have as many as eleven craters that big or bigger in Australia, for starters.
The recent date, 3 million years or less, makes it exceptional at that size. Still awaiting a more accurate dating. In Australia, nothing that young has a diamater more than 1.2 km.
“The association of shocked quartz grains mantled by carbonaceous material, microbreccias with amorphous carbonaceous matrix, and glasses with a range of mineral-like compositions”
The carbonaceous material strongly suggests a carbonaceous chondrite.
“every tested subsample of the same sample that contained shocked quartz (HW21-2016) also contains elevated concentrations of Ni, Co, Cr, PGE, and Au, indicative of a relatively rare iron meteorite.”
Carbonaceos and iron in the same meteorite. Seems impossible.
“Highly fractionated iron asteroid”.
In your dreams. A highly fractionated iron is better known as a stony iron, but even that has no carbonaceous material. Perhaps more likely is that a carbonaceous meteorite hit a buried ore body.
“The formation of a 31-km-wide impact crater in crystalline target rock requires ~3 × 1021 J of energy (17). Assuming that the Hiawatha impactor was iron with a density of 8000 kg m−3 and its impact velocity was 20 km s−1, the required impactor diameter was ~1.5 km (17). The impact would initially produce a bowl-shaped cavity ~20 km in diameter and ~7 km deep, which would quickly collapse (within ~1 min) to form a complex crater more than 31 km in diameter and ~800 m deep with a central uplift (17). This impact scenario would have melted and vaporized up to ~20 km3 of target rock, approximately half of which would have remained within the crater, forming a melt sheet up to ~50 m deep.”
The assumption of 20 km s-1 is too low. That assumes the impactor was a near-Earth asteroid. If the impactor was a comet then a representative speed would be 60 km s-1 with an energy per unit mass nine times as big. The calculation relies a lot on Reference 17.
“No ejecta layer that might be associated with the Hiawatha impact crater has yet been identified in either Greenland’s rock or ice records.”
That argues that it may not even be an impact crater, or that it’s much older. Needs more study to sort out the three conflicts: uncertain age, impossible composition, and lack of ejecta. But certainly looks interesting.
http://advances.sciencemag.org/content/4/11/eaar8173