planetary meteorites

by space rocks UK

In the early 1980s, some strange achondrites (meteorites containing no spherical chondrules) were found in Antarctica by Japanese researchers. When these were analyzed, they were found to have very similar chemistries and lithologies to the 382 kg of rocks retrieved from the Moon by the twelve Apollo astronauts.

planetary meteoritesAround fifty lunaites have been now collected from Africa, Australia, Antarctica and the Middle East, and, although most are in museum and University laboratories, small samples are freely available at a reasonable price. The Apollo samples, of course, remain largely locked away in the USA: surprisingly only a small fraction has been analyzed since the last mission in 1972.
Interestingly, the majority of lunar meteorites originated in highland regions on the far side of the Moon, which was not visited by the Apollo astronauts: they therefore add considerably to our understanding of the Moon’s formation and turbulent history!
A similar number of achondrites are known to have originated on the planet Mars. This group is referred to as the SNC meteorites, from the ‘type’ specimens from Shergotty in India, Nakhla in Egypt and Chassigny in France. (The Nakhla meteorite is most famous for allegedly killing a dog!) These stones have chemistries similar to samples analyzed on Mars by the many robot explorers that have visited the planet and, more importantly, have young crystallization ages between 1.35 and 0.15 billion years: this compared to the average of 4.5 billion years for chondritic meteorites.

The conclusive evidence of a Martian origin for the SNCs comes from the relative concentrations of two Oxygen isotopes 16O and 18O, each planet in the Solar System having a unique value for this ratio. You may well be wondering how lumps of rock from the lunar and martian surfaces could possibly have reached the Earth!
In both cases, it is most likely that the massive asteroidal impacts that have cratered the surfaces of both bodies since their formation were energetic enough to hurl debris into space: this eventually being captured by the Earth’s gravity.
Additionally, Mars has some enormous shield volcanoes such as Olympus Mons that are quite capable of ejecting igneous rocks into space, given Mars’ low escape velocity of just 5 km/s.

A third group of planetary meteorites are the HED achondrites, which are known to have originated on a number of different asteroids.
These small, rocky minor planets mostly inhabit the region of space between the orbits of Mars and Jupiter, although others can (and do!) visit our part of the Solar System, resulting in cataclysmic collisions in the past. The initials stand for Howardites, Eucrites and Diogenites, the group being classified according to where on the parent body the meteorite originated. The vast majority of HEDs reach us from the asteroid 4-Vesta.

It has recently become apparent that the strange little world Mercury is a potential source of meteorites: its heavily-cratered surface and low escape velocity make this a distinct possibility.
Over the past few years, meteoricists have been re-examining atypical meteorites in a search for candidates, and it seems that some of the Angrite group have the predicted lithologies.

A final, perhaps surprising, possibility is that we may eventually identify meteorites that were flung into space by impacts upon our own planet, the Earth! About 180 terrestrial craters have been discovered to date, many of which are large enough to have been caused by impacts with enough energy to have sent ejecta into space: what goes up must come down!

In conclusion: although planetary meteorites offered for sale are generally fairly small, it is possible to put together a full set from the Moon, Mars, Mercury and several asteroids and comets for a surprisingly reasonable price. In my opinion, such a collection is about as intriguing as it gets!

space rocks uk

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