Saturday, 30 June 2018

The Nakhla Dog

When The Blob was launched in January 2013, it was to record the process of transition from being a gentlemanly part-time, cash-poor, time-rich researcher attached to Ireland's premier University to becoming fire-hose active as a full time teacher in a Technology Institute. Since then, like Topsy, the Blob just growed: extending its curiosity tentacles into places a long way from The Institute; which continues to pay my salary even though I have no classes to teach until September. From the sofa of my mountain fastness, I range across space and time to find things that deserve a better press. There was, for example, Slijper's goat, the little caprid that could; and Mary Anning's dog, killed in the line of duty in 1833.

Anning's dog Tray was an important part of the team, being told to Stay beside an interesting fossil while Anning trudged back along the beach to get the right tools for bagging it. The Nakhla dog, otoh, was mere collateral damage when a meteorite came explosively to earth at 0900hrs on 28th June 1911 at El Nakhla El Bahariya, a village in Egypt. There were lots of people out and about at 9am and, although none of them had dashcams or cell-phones like at Челябинск Chelyabinsk on 15 Feb 2013, there were dozens of eye-witness and 40 fragments were recovered. Many of the bits were dug out of the earth at the bottom of an impact crater and up to 1 meter below grade.

The event gave rise to the eponym Nakhlite for a specific and peculiar class of mineral. They are the N in SNC meteorites which I wrote about last year. SNC meteorites are interesting because the share the same geology, especially wrt gaseous inclusions, as Mars, our nearest neighbour beyond the Moon. For one of these fragments to be found on Earth requires an improbable chain of rare events:
  • A meteor has to strike Mars
  • It has to be sufficiently large to whack off a lump of the red planet
  • That lump has to come within gravitational range of Earth
  • and eventually collide
  • it has to be sufficiently large itself that it doesn't vaporise in the atmosphere
  • it has to fall during the working day 
  • in a place where people are about (ie not in the sea which cover 70% of the Earth's surface)
  • the people have to be idle enough or curious enough to look for the bits, which are unlikely to be lying on the surface.
  • and it's not much use to science if the SNC fragment finishes up on the mantlepiece of a hut in Siberia or Rwanda.
There are nevertheless 132 of these Martian rocks known to science.

Yes yes, but what about the dog?? One of these hot, hard and fast Nakhla rocks was observed by a farmer, Mohammed Ali Effendi Hakim, to hit and vaporize a neighbour's dog. Which sounds a little unlikely, not the hitting, which happens - ask Ann Hodges of Sylacauga AL -  but the vaporisation. You might think that the question "can a rock vaporise a medium sized mammal?" is amenable to testing or modelling by Mythbusters or The Slow Mo Guys [whom bloboprev indeed blobobleve]. The chunks of the Nakhla meteorite were from 20g to 2000g, the larger chunks having more kinetic energy and more vaporisation potential. If the original meteorite was comparable to the Chelyabinsk event, it was travelling at 18,000m/s, which is about 10x faster than the fastest speeding bullet. But 18,000m/s is the impact velocity rather than the terminal velocity having travelled several km through the viscous atmosphere, which is only about 200m/s. I doubt if the [British] Slow Mo Guys are heartless enough to practice their photographic arts on a dog, alive or dead. But I'm sure their ingenuity and contacts will find them the artillery necessary to fire a 500g projectile at 200m/s at a dead goose and see if they can find any solid bits.

Emily Graslie [prev] talking about meteorites and meteowrongs.
Jason Kottke says, The Case Is Altered when you get a really big Cretaceous extinction meteorite incommming.

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