Saturday, June 21, 2008

Murcheson Meteorite

The Murcheson meteorite has been in the news again, as a result of a slick piece of organic geochemisty described by the Bad Astronomer. Indeed, from the Wikipedia entry you’d think that this is the meteorite’s only claim to fame. But there is far more to this rock than mere molecules.
Murcheson is the first member of the cosmochemical trifecta of 1969 (The others being Allende and the Apollo moon rocks). A primitive chondrite, it is one of the main sources of presolar grains- interstellar dust particles that predate the solar system.
Mind you, the meteorite itself is not pre-solar. The vast majority of it consists of material that condnesed from the solar nebula. But this particular meteorite has suffered no remelting since it formed, and as a result, tiny bits of pre-existing dust can be found in it.
The most well studied and characterized of these grains are silicon carbide. These grains are rare, and isolating them is a lot like finding a needle in a haystack. Indeed the methodology used to recover them is similar. Carbides are chemically resistant material, so they are isolated by simply dissolving the rest of the meteorite in acids and other solvents. This is the metaphorical equivalent of burning down the haystack and pulling the needle from the ashes with a magnet.
These silicon carbide grains have been useful for all sorts of purposes, such as experimental confirmation of stellar nucleosynthesis, chiefly the S process.
But these carbides also present a significant sampling bias. Then can only form in carbon stars- start which have more carbon than oxygen in their atmospheres. If there is more oxygen than carbon, then the carbon will be consumed in the production of carbon monoxide, and the leftover oxygen will form metal oxides instead of carbides. So oxygen stars like the sun cannot form carbides unless the C/O ratio changes as a result of helium burning in the star’s dying days.
Oxygen stars should be able to form refactory oxides which are tough enough to survive the collapse of the solar nebula. And indeed a few presolar corundums are known. But because rocks are made chiefly of metal oxides, finding presolar oxides is more like finding a pine needle in a haystack. Instead of a match, you need a gaggle of patient grad students. However, such people do exist, and are currently trying to invent efficient methods of finding anomalous grains. Hopefully they will find and characterize extrasolar silicates before too long.
I don’t think we’ve heard the last from this rock.

1 comment:

Chris said...

Ivuna's been in the news recently too, in that they're generating a few more samples from a museum sample: