Although the press made a great fuss in 1999 about the supposed discovery of signs of life in a meteorite reckoned to have been blasted off Mars by a giant impact, meteorites in general are the only direct means of developing ideas about how the Earth and the rest of the planets formed. The market in meteorites is beginning to resemble the London Metal Exchange in its frenzied bullishness, but being collectibles it is rare types that command the highest prices, rather than their significance. An excellent review of current ideas among meteorite specialists appeared in the 6 July issue of Science (Alexander, C.M.O’D., Boss, A.P. and Carlson, R.W. 2001. The early evolution of the Solar Syetem: a meteoritic perspective. Science, v. 293, p. 64-68).
Since development of theoretical ideas about the generation of the elements in stellar processes, it has become almost a cliché to ponder about the ultimate dependence of every aspect of the natural world on supernovae and their “seeding” of the galaxy with the chemical mix that is so familiar. Even the nuclear processes involved are easily grasped. Not so the means whereby star stuff assembled into planetary systems and laid the potential for life, plate tectonics and virtually everything else. Alexander and colleagues from the Carnegie Institute of Washington span the interactions between physical conditions around young and rapidly evolving stars, derived theoretically, and the kinds of compounds that they can generate. Meteorite chemistry and mineralogy, which are very diverse, put flesh on the bones of these ideas. The tangible properties of different meteorite classes, together with their radiometric ages, are analogous to fossils in piecing together both planetary evolution and the various kinds of environments in the early Solar System.
One conclusion in the review that surprised me concerns the oldest materials known to us – calcium-aluminium-rich inclusions found in some chondrites, such as the famous Allende meteorite that fell in Mexico. The pale inclusions contain evidence for the former presence of short-lived isotopes, such as 26Al. So short are their half-lives that the delay between their nucleosynthesis and the assembly of the pale inclusions can have been a few hundred thousand years at most. There are two possibilities: either such isotopes were generated by energetic particles emitted by the growing early Sun, or they had their source in supernova events. Theoretical work on local genesis has so far failed to match the relative abundance of all such short-lived isotopes, derived from the amounts of their decay products found in pale inclusions. It seems highly likely that collapse of a pre-solar cloud of matter to form the nebula out of which Sun, planets and the parent bodies of meteorites emerged was set in motion by shock waves from a nearby supernova. They would have taken the form of a high-speed interstellar “wind” of gas. Observed differences in oxygen-isotope proportions in meteorites were once ascribed to heterogeneous mixing of this explosive introduction of exotic matter. However, the oxygen heterogeneities do not show up in the isotopes of other elements. That mismatch has led to ideas of chemical fractionation during Solar System evolution, akin to that so familiar from the different behaviours of “light” and “heavy” oxygen during evaporation of water and its uptake in skeletons of living things exposed to different climates. Differences in oxygen isotopes now form a strand in assigning different meteorites to sources at different distances from the evolving Sun, and in deducing that some rare meteorites did indeed come from Mars.
Clearly behind the hype surrounding promotion of staffed and unstaffed missions to Mars and the increasingly shady world of the meteorite trade, exciting research is being done.
Ice and prebiotic chemistry
The problem with ice on Earth is that it will not support living chemistry. The process of crystallization excludes impurities from its structure, so that reactions between organic compounds cannot go on. Comets are mainly ice, and frozen water is a common occurrence in the infrared spectra of interstellar clouds, along with a host of complex CHON compounds (over 100 discovered to date). How organic molecules form in cold molecular clouds is a difficult problem, or at least it was believed to be until recently.
Researchers at the NASA Ames Research Center in California have probed the structure of solid water under all manner of physical conditions. Below a temperature of 200 K (about that of liquid nitrogen) the hexagonal symmetry of ice, familiar from snowflakes, changes to the simpler cubic form. At yet cooler temperatures 10 to 125 K), ice has no crystalline structure. Like flint, it is cryptocrystalline or amorphous. Curiously, even only a few degrees above absolute zero it can flow like a viscous medium, in the manner of glass, when irradiated with ultraviolet radiation. The breaking and reforming of hydrogen bonds, as in liquid water, but slower, creates the conditions for retaining impurities and their chemical combination. This odd behaviour at precisely the temperatures of molecular clouds explains their richness in organic molecules. Quite probably comets form by accretion of such interstellar icy material. The experiments revealed that warming of amorphous ice above 125 K does not result in a complete transition to cubic ice, that would exclude impurities. Instead, around two thirds retains its odd properties. The discovery strongly hints that much of the basic work of producing precursors to life’s chemistry is not only feasible in interstellar space, but that they can be delivered to planets as they collide with comets giving a kick start to the origin of life.
Source: Blake, D.F. and Jenniskens, P. 2001. The ice of life. Scientific American, August 2001, p. 36-41.