(From: ACOS Bulletin, No 21, October 1979)
All of the 117 known chemical elements exist as any of several different isotopes. The various isotopes of a given element behave nearly identically chemically to each other, but have differing nuclear masses. These masses can be measured in a special instrument : a Mass Spectrometer. The isotopes are usually referred to by a letter symbolising the element followed by a number symbolising the mass eg H-2 means Hydrogen with a nuclear mass of 2 atomic units.
Many isotopes are radioactive, and can be detected this way, but over a long period of time these decay. The stable isotopes have some value for verifying, or otherwise, the extraterrestrial source of an object.
Of the 117 elements, 82 have at least one stable isotope, and 62 have more than one. The element with the largest number of stable isotopes is Tin with 10. A tiny fragment (0.001 gram) of an object analysed by a Mass Spectometer can give the ratios of the amounts of the various isotopes of each of these 62 elements. The three common elements which compose the bulk of living matter are: Hydrogen, Oxygen, and Carbon. These each have several stable isotopes. Their terrestrial relative abundances are:
Just as these abundances vary somewhat according to their terrestrial source, so they vary even more for extraterrestrial sources. Such information is available from:
Analysis of the rocks brought back by Apollo astronauts (Epstein, 1970) gives us some direct information. The rocks are naturally from the surface of the moon, and thus have suffered from bombardment by the solar wind for some millions of years. This wind is composed mainly of H-1, so it affects the values somewhat. Thus a H-2 / H-1 ratio in the range of 18 to 60 was observed in these rocks (cf. 160 in SMOW). The O-18 / O-16 differed from SMOW in the range +0.38% to +1.91%. The C-13 /C-12 ratio was found to be 1.1% to 1.5% higher than PDB.
These also provide a direct source of extraterrestrial material for analysis. There are two main types of meteorite: rocky and metallic. The traces of Hydrogen in these give vales for H-2 / H-1 of 190 and 145 ppm respectively (cf. 160 in SMOW). Traces of Carbon in the two types of meteorite give values differing from PDB by -0.25% and +6% respectively.
This can detect the spectra of molecules in the dust clouds in space. The spectra of the molecules containing different isotopes are distinct. The evaluation of the abundances from these spectra has however some uncertainty due to the unknown thickness of the clouds of interstellar material.
Measurements have been made of the O-18 / O-17 ratio (Gardner, 1976) by examining the spectra of the Hydroxyl radical (OH) at 1627.17 MHz (O-17 H-1) and 1624.43 Mhz (O-18 H-1), giving a value of 4.2 (cf 5.5 in SMOW). These isotopes have also been measured in the spectrum of Carbon Monoxide (CO) giving a O-18 / O-17 ratio of 3.6 in the Orion Nebula, and 4.0 in Rho Ophiucus (Encrenay, 1973).
The heaviest isotope of Carbon, C-13 has been detected and measured in the radio spectrum of Cyanoacetylene (HCCN) in the galactic centre (Gardner, 1975). This gives a ratio of C-12 / C-13 of 36 (cf 89 in PDB). Formaldehyde (HCOH) in the galactic centre gives a value of 13.
The optical spectra of stars provide another source of information, and again the results have some uncertainty due to uncertain absolute intensities of the electronic transitions in the atoms. The C-12 / C-13 ratio has been listed for 53 stars (Dearborn, 1976). The values cluster around 20, but range from 3 to 50 (cf. 89 in PDB).
The ranges of values of just these three elements shows that verification that an object is extraterrestrial is possible by analysis of a very small sample from the object.
(updated 15 August 2007)