New Moon

OK, so it’s not a picture of a new moon, I just thought it looked quite nice! (image: wikipedia)

In January of this year I wrote an entry on my research blog about the latest versions of Moon-forming giant impact event that had just been published in the journal Science. This followed hot on the heels of a previous entry that discussed how the classic version of the giant impact model was starting to look decidedly ropey.

The formation and earlier evolution of the Earth-Moon system remains a hot topic in planetary science. Very hot in fact, possibly in excess of 6000 K!

To discuss the matter in more detail the Royal Society will be holding a two day meeting next week (23rd and 24th September) entitled, well, you guessed it: Origin of the moon. The speakers at the meeting include many of the star players in lunar research. While it certainly won’t be the last word on the subject, the meeting should help to clarify some of the outstanding problems in current lunar formation theories. It looks like it will be a very lively meeting and if it lives up to expectations should inspire another blog entry. We shall see.

And here is that new blog entry: Giant Steps.

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The Mystery of the Giant Disappearing Meteorite

Camel in the Adrar region of Mauritania (image: wikipedia/Ji-Elle)

Everyone likes a good mystery story. Something that remains unexplained and perhaps also inexplicable. Even the most sceptical of us surely must admit that tales about the Mary Celeste, Roswell incident, or good old Nessie hold a deep fascination.  In fact we often don’t want to know the truth behind such stories. A little bit of us is happy with the thought that there are things that remain beyond reason. Perhaps that’s what explains the enduring fascination of the lost “Fer de Dieu” meteorite.

It all started back in 1916 when a French colonial officer, Captain Gaston Ripert, claimed to have discovered a huge iron meteorite, 40 metres high and 100 metres long, in the desert of North Africa.  He said that the iron mass was located in sand dunes about 45km to the south-east of the town of Chinguetti in Mauritania.

Captain Ripert said he was taken there blindfolded by a local chieftain. He collected a 4 kg fragment lying close to the giant metal mass, which a number of years later ended up in Paris, where it was identified as a relatively rare type of stony-iron meteorite, known as a mesosiderite. The fact that so much more of the meteorite remained in the desert was hailed as a great discovery. But a series of expeditions failed to trace the meteorite. In 1934 the search for the great “Fer de Dieu” meteorite was taken up by Professor Théodore Monod of the Muséum  National d’Histoire Naturelle in Paris. In the following 45 years or so he undertook a series of expeditions, again without success. In 1989 he finally wrote: “The existence of a giant meteorite in the Adrar of Mauritania, largely accepted since 1924, must now be abandoned. There was a mistake on the nature of the rock of a butte that is entirely sedimentary with no trace of metal.”

Professor Monod concluded that the feature thought to be a giant iron meteorite by Ripert was in fact a sandstone and quartzite butte known as guelb Aouinet. However, Dr Brigitte Zanda, who accompanied of Professor Monod to the area in 1991, remains unconvinced. In Ursula Marvin’s 2007 account, Dr Zanda is quoted as saying that Ripert could only have mistaken guelb Aouinet as an iron meteorite if he had come across it “at night, at a distance, in a fog”. At the Meteoritical Society Meeting in London in 2011 I was able to talk briefly to Dr Zanda about the “Fer de Dieu”. Dr Zanda had some insights into Captain Ripert’s character based on his notebooks for other specimens he had sent back to Paris. Dr Zanda had the impression he was an honest, and relatively knowledgeable, observer of all things geological. She remained convinced that guelb Aouinet is not the locality that Ripert first encountered back in 1916.

But what about the meteorite that Ripert claimed to have brought back as a sample of the larger mass. A study published in 2001 led by Dr Kees Welten measured the concentrations of cosmogenic radionuclides in the Chinguetti mesosiderite. These unstable elements are produced in space when the material is bombarded by galactic cosmic rays. Once on Earth the meteorite is shielded by the Earth’s atmosphere and these radionuclides start to decay. Such measurements can define how long a meteorite has been on Earth and more importantly for the” Fer de Dieu” story, how big it was. Dr Welten and his team interpret their data to indicate that the Chinguetti mesosiderite came from an object no larger than 80 cm in radius. So the sample that Ripert collected, and which eventually ended up in Paris, came from a very small meteorite. It seems it could never have been a piece from an object as large as the “Fer de Dieu” as suggested by Ripert.

An additional problem is that a mass of meteoritic iron the size of the “Fer de Dieu” object (40m x 40m x 100m) should not survive intact on impact with the Earth. Thus, the iron meteorite which formed the 1.2 km diameter Meteor Crater in Arizona is estimated to have been about 50 metres in diameter prior to impact, in other words a bit smaller than “Fer de Dieu”. Based on the reported dimensions of the Fer de Dieu it would have had a mass of about 1.25 million tonnes. According to a paper by Dr Philip Bland and Dr Natalya Artemieva published in 2006, an impacting object of this size ought to form a crater of at least 2 km diameter. So where is it?

So the mystery of the Fer de Dieu meteorite remains. It seems very unlikely that such a large object could have survived impact intact. In addition, the sample collected by Captain Ripert appears to have been part of much smaller object. And yet what did the Captain actually see that convinced him he had found a huge iron meteorite? The only way to completely solve the case would be to locate further fragments of the real Chinguetti meteorite. By finding such fragments in the desert we might get a clue about the feature that Ripert mistook for the giant meteorite.  The truth is out there…perhaps!

 

Posted in General, Meteorites | Leave a comment