Lunar Ping Pong

Posted on Saturday 31 July 2004 to Quintessence

While we're on the subject of Martian meteorites, it occurred to me the other day that given the pock-marked surface of the Moon, it's surprising that you don't hear more about lunar rocks arriving here the same way. Well, it turns out of course that they do but, perhaps surprisingly, lunar meteorites are exceedingly rare.

Only 30 have been found mainly in the icy wastes of Antarctica or the stony deserts of Oman. Apparently, it is these extreme climates that provide the best places to search for meteorites because the stones suffer the least from weathering and erosion. These lunar rocks can tell interesting stories about the Moon's evolution but one of the meteorites recently discovered in Oman tells what I reckon is a fair rip-snorter of one (despite not even having a caninocidal angle to it).

Beda Hofmann (left) of the Natural History Museum of Berne and Ali Al-Kathiri of the University of Berne, Switzerland, kneel behind lunar meteorite SaU 169 in the Oman desert Jan. 16, 2002. Photo: Edwin Gnos, University of Berne.

After ejection from the Moon, the rock orbited the Earth or the sun for a period up to 300,000 years. Due to the Earth's gravitation it finally fell as a bright fireball in Oman up to 10,000 years ago. During its travel through the Earth's atmosphere the rock was heated and eroded by friction with the atmosphere which significantly reduced its size and formed its final oval meteorite shape.

The rock is made up of two components: the lighter coloured rock is made up of volcanic material, the darker is lunar soil.

When the researchers got hold of the meteorite SaU 169 in the lab they discovered that it was made of an easily identifiable lunar material known as KREEP (that K for Potassium, REE for rare-earth elements and P for Phosphorus). This type of rock was first encountered during the Apollo missions and subsequent surveying by NASA's Lunar Prospector Orbiter in 1998-9 established that this mildly radioactive material was essentially localised to one place on the Moon, the Mare Imbrium (for Northern Hemisphere readers that the right eye of the Man in the Moon or alternatively the belly of the Jade Rabbit who busily pounds the Elixir of Immorality with his mortar and pestle).


This rock formed as a result of a melting caused by a massive impact nearly 4 billion years ago. This impact, which created an impression the size of the 1160 kilometre-wide Mare Imbrium, was one of the last of its kind and marked the end of the epoch of devastating planetoid-sized collisions that had characterised the early solar system (it was catcylsmic events such as this one but on a much larger scale that lead to the creation of the Moon itself). This cessation of massive impacts, incidentally, was also the time when life really started to get cracking on Earth - surely more than just a coincidence.

Now this is where the story starts to get quite convoluted. As can be seen in the cross-section picture of the meteorite to the left, the rock is made up of two components, the older and lighter coloured KREEP material which makes up 87% of the rock and the layer of regolith or lunar soil which has adhered to it. This was a serendipitous occurrence because it permitted researchers to pinpoint the times and places in this rock's history to an astonishing degree of accuracy.

It is known that the KREEP material formed at the interface of the Moon's crust and mantle and would have laid deep under the surface for a long time. The regolith layer, on the other hand, indicates that it must have later been brought up close to the surface. Furthermore, the KREEP layer shows evidence of undergoing a shock some 2.8 billion years ago and an examination of the regolith indicates that the rock had been exposed to solar winds for a long time implying that it had been near the surface for aeons.

This is evidence that the rock had been involved in a second meteorite impact about 2.8 billion years ago which caused it to be dug out of the ground and thrown somewhere onto the surface. Researchers scoured the NASA lunar survey database in search of regions of surface regolith that matched the meteorite's chemical signature and were in the vicinity of Mare Imbrium. Then they looked for craters that were of the correct age.

A crater named for the French astronomer Lalande (1732-1807), which is about 25 kilometers in diameter and not far south of Mare Imbrium, fitted these requirements to a tee (see map). This feature is believed to have been formed about 2.8 billion years ago. Lalande is known as a "thorium hotspot", with high concentrations of this element providing a strong indication of its connection with the Imbrium impact. This is something that is observable even from Earth.

It is within the lighter coloured ground surrounding the Lalande impact site (left) that the rock is thought to have stayed for another 2.6 billion years before being involved in yet another impact (possibly at sites marked either A or B) that excavated the rock and dumped it into soil only 50 centimetres deep (possibly at the site marked as C).

To get from there into orbit is not all that difficult and requires only a modest impact capable of creating a 3 kilometre crater (such as the one at C). Analysis of isotopic data which gauges how long the rock was in space and directly exposed to solar winds indicates that this impact (its last before striking Earth) that launched the rock happened up to 340,000 years ago.

The rock then went into orbit around the Earth before finally descending as a ball of fire into the atmosphere and landing in Omani desert about 10,000 years ago.

Several research firsts have come out of this study of the meteorite. As already mentioned, it was the lucky combination of two different rock types that enabled the accurate pinpointing of its place origin to a remarkable degree. The rock is also the largest specimen found from the Imbrium impact (the Apollo astronauts were only able to bring back minute quantities) and this has enabled much more detailed analysis and the accurate dating of the end this important epoch in the solar system's history. It also provides a dating of when massive impacts stopped happening to Earth and this must have had an important positive effect on the evolution of life on this planet.

This impact even has a tie in to one of my favourite images from the Apollo missions, the descent of the Apollo 12 lunar module.


Incidentally, the foreground crater to the right is Herschel and the impression to the bottom and left is the more ancient Ptolemaeus.

See Also:
Meteorite from Oman Records Its Lunar Launch Site and Detailed History
Sayh al Uhaymir (SaU) 169 and especially this bit.