What type of meteorite contains organic molecules

For a molecule to be chiral its mirror images must be non-superimposable; hands, ears and feet all have this property. For an organic molecule to achieve chirality a carbon atom must be surrounded by four different structures.

Chiral molecules are said to be optically active because the two molecular forms affect plane-polarized light in different ways. One form rotates the polarized light to the left and is termed laevorotationary — L — while the other form rotates the light to the right and is called dextrorotationary — D.

Chirality is important for theories that link extraterrestrial organic matter with the origin of life, because terrestrial biology preferentially generates L amino acids whereas abiotic reactions generally have no preference and produce racemic mixtures equal amounts of L and D forms. The discussion on whether amino acids in meteorites were racemic with equal amounts of L and D forms or non-racemic with a preference began in the s and continued through the s e.

Mueller , Nagy , but again results suggested the dominance of L forms was caused by terrestrial contamination and analytical artefacts. Following the fall of Murchison in , four of its protein and seven non-protein amino acids were found to be racemic Kvenvolden , and, presumably, abiotic.

In , however, the debate reopened when a slight L-excess in protein amino acids was reported Engel and Nagy and, most recently, L-excess has been reported in non-protein amino acids suggesting that the small but significant preference for left-handed forms is a real pre-terrestrial and abiotic feature Pizzarello and Cronin Proposed mechanisms for producing this L-excess in meteoritic amino acids are themselves chiral in nature and rely on selective destruction of the right-handed form of the amino acids figure 3 , reaction a.

Currently, popular hypotheses involve ultraviolet circularly polarized light spinning away from a neutron star or reflection nebula Bonner and Rubenstein , Bailey It is a necessary consequence of this theory that the amino acids were already formed in interstellar space so that preferential destruction of D-amino acids could take place.

The recent detection of the simplest amino acid, glycine, in interstellar spectra supports this assumption Kuan However, as will be explained below, there are other theories for the formation of amino acids in carbonaceous chondrites and structurally related compounds that attribute their formation to aqueous chemistry on asteroids.

Organic matter in meteorites contains many molecular probes of extraterrestrial environments. The slight L-excess in amino acids may be driven by starlight in interstellar space reaction a. On the parent asteroid low pH and ammonia concentration could lead to the production of hydroxy acids such as lactic acid from aldehydes and hydrogen cyanide reaction b. Alternatively, if pH and ammonia concentration are higher, amino acids such as alanine may dominate reaction c.

Reactions b and c indicate that the ratio between hydroxy acids and amino acids allow the potential for reconstructing the chemical environment on the meteorite parent body. If amino acids such as glutamic acid are heated they can lose carbon dioxide to produce an amine such as propylamine reaction d or cyclize to generate a cyclic amide such as pyroglutamic acid reaction e.

Reactions d and e imply that the type and abundance of amino acid degradation products can help reveal the thermal history of the parent asteroid. Carboxylic acids figure 2 are another type of biologically useful molecule found in meteorites. In life, they are a convenient and compact store of energy and are useful in cell membranes. Carboxylic acids in Murchison contain between two and five carbon atoms and display complete structural diversity, a general decrease in abundance with increasing carbon number, and an equal concentration of branched and straight-chain isomers Yuen and Kvenvolden , Lawless and Yuen Interestingly, the calcium salt of a simple two-carbon dicarboxylic acid has been detected in Murchison, revealing something about the meteoritic form in which these compounds exist.

The hydroxyacids correspond in structure to the more abundant amino acids, a correlation that casts doubt on the idea that all amino acids were produced in interstellar environments. Structural similarities between the hydroxyacids and amino acids suggest that the hydroxy and some amino acids may be molecular siblings born on the parent asteroid and related by a type of chemical reaction known as the Strecker-cyanohydrin synthesis. Hydrogen cyanide and aldehydes combine in aqueous solution to produce hydroxyacids such as lactic acid figure 3 , reaction b but if ammonia is also present then the products become amino acids such as alanine figure 3 , reaction c.

It is the ammonia concentration that determines the ratio of hydroxy low ammonia to amino acids high ammonia produced in the reaction. Therefore, the ratio of hydroxy to amino acids in meteorites can provide some indication as to the volatile content of the asteroid parent body on which these compounds were synthesized Peltzer Another environmental condition, pH, is also important because in the Strecker-cyanohydrin synthesis large amounts of hydroxyacids are produced under acidic conditions.

They are compounds containing a number of hydroxyl OH groups attached to their carbon skeleton figure 2. Sugars are important biologically because they provide the carbon skeletons for many other molecules. They are also a source of energy for organisms and, when combined to form larger molecules, they can act as food stores and give structural support.

Experiments in the s detected polyols in carbonaceous chondrites, but terrestrial contamination was never conclusively discounted Degens and Bajor Recent experiments on Murchison have discovered several sugar-related compounds sugars, sugar alcohols and sugar acids in similar abundances to the amino acids Cooper The Murchison polyols display many of the features of extraterrestrial compounds.

Progressively larger polyols become less abundant, display almost complete structural diversity and contain some terrestrially rare compounds. Short aliphatic molecules figure 2 in Murchison are present in amounts of 1. The short-chain aliphatic hydrocarbons may result from processes in extraterrestrial environments, but their longer counterparts appear to have a very different origin and will be discussed below in the terrestrial contamination section.

Compound sizes range from one to seven aromatic rings with the smaller compounds generally being most abundant Sephton There is evidence of significant heterogeneity in the concentration and compound distribution of the free aromatic hydrocarbons in carbonaceous chondrites.

Some trends in the variations in compound distribution seem explicable. Firstly, low petrographic type meteorites that have experienced alteration by water contain a greater relative abundance of volatile aromatic molecules, while the converse is true in meteorites of higher petrographic types that have experienced thermal metamorphism Sephton a.

Such observations may be attributed to the aqueous degradation and graphitization of organic matter respectively. Secondly, one theory that attempts to account for inter-meteorite variations invokes the selective transportation of aromatic compounds on the meteorite parent body. The Ivuna meteorite contains a predominance of three-ring aromatics and it has been suggested that the separation of mixtures during fluid flow on the meteorite parent body geochromatography has concentrated this particular type of molecule Wing and Bada Thirdly, at least some of the aromatic compound variation is due to terrestrial contamination.

The Allende meteorite was analysed shortly after its fall and revealed small amounts of aromatic contamination at its surface while similar fractions obtained from the interior of the meteorite contained no aromatic material, indicating a terrestrial source for the compound Han et al.

Certain aromatic hydrocarbons act as a record of the thermal history of the parent asteroid. Aromatic hydrocarbons such as methylnapthalene figure 2 are common in the carbonaceous chondrites and the positions of side chains attached to the ringed core of the molecule are sensitive to heat. As temperature increases, the side chain is repositioned to a more stable position; the ratio of stable to less stable molecular configurations is a symptom of the maximum temperatures achieved Sephton Amines are derivatives of ammonia NH 3 where hydrogen atoms are replaced by other structures figure 2.

Where one hydrogen has been replaced they are termed primary amines; two hydrogen atom replacements make a secondary amine. Two possible sources of the amines in Murchison have been proposed: they may be directly inherited from the pre-solar molecular cloud, which may contain methylamine, as detected in the interstellar medium or, as with the hydroxyl acids, the amines may share a common origin with the amino acids. Sixteen of the 20 amines can be made by simply removing carboxylic acid group of amino acids figure 3 , reaction d , a reaction that would take place if the molecules were heated on their parent asteroid.

The amides display the following: structural diversity up to and including molecules with eight carbon atoms, a decline in abundance with carbon number, and the presence of many compounds with no terrestrial source. The chemical structures of cyclic amides permit hydrogen-bonded pair formation and may have acted as parts of a primitive genetic coding apparatus, possible forerunners of the present day nucleic acids. The presence of cyclic amides such as pyroglutamic acid figure 3 , reaction e as near neighbours of free amino acids implies that the amino acids may have been transformed into cyclic forms by a thermal polymerization event, presumably occurring on the meteorite parent body.

Cyclic amides, therefore, may act as thermometers of ancient asteroid processes. Another compound class which is essential to terrestrial life is that of the nitrogen heterocycles figure 2. Some of these compounds are fundamental building blocks for the nucleic acids that store genetic information. All of the purines and pyrimidines found are biologically common and together account for 1.

Prior to there was a reluctance to investigate sulphur compounds figure 2 in meteorites because of the possibility that they were the result of secondary reactions between elemental sulphur and hydrocarbons during extraction. During the thermal extraction-mass spectrometry of sulphur compounds in the Murray meteorite, however, the lack of evolved hydrogen sulphide during extraction indicated that this problem was overstated Hayes and Biemann Murchison contains small amounts of thiophenes, benzothiophenes, dibenzothiophenes and benzonaphthothiophene.

The abundance pattern of thiophenes in Murchison is markedly different from those in terrestrial sediments, indicating an abiotic origin for these molecules Shimoyama and Katsumata Phosphorus compounds have many biological roles and play a part in cell membranes, energy transactions during metabolism and the storage and transfer of genetic information.

Sulphur compounds are also common in living systems and take part in key biochemical reactions. In Murchison a series of alkyl phosphonic acids and alkyl sulphonic acids with up to four carbon atoms have been detected Cooper As observed for many of the other compound classes in Murchison, the sulphonic and phosphonic acids display an exponential decline in amount with increasing carbon number and exhibit complete structural diversity. The solvent-soluble substances are only trace components, yet they are often the focus of studies on meteorite organic matter because they are especially amenable to analysis.

This is a regrettable situation, once compared to trying to understand the internal structure of a house by studying a few bricks intensively Nagy Essentially, the macromolecular materials can be considered as a cross-linked agglomeration of some of the compounds in figure 2 but with an overwhelming dominance of aromatic hydrocarbons.

The macromolecular materials are generally assumed to be completely indigenous to their meteorite host due to their high molecular weight and immobility. As the major organic component the macromolecular material is key to theories of the origin of meteoritic organic matter as a whole.

Hence it is interesting to note that meteoritic organic matter is often compared with that observed in the interstellar medium. Some scientists believe that the molecular cloud that collapsed to form the solar system bequeathed a significant amount of interstellar organic matter, aliquots of which are preserved in primitive asteroids and the meteorites derived from them.

For many years, only one- to four-ring polyaromatic hydrocarbons PAH were commonly observed in Murchison macromolecular material break-down products e. Sephton , which contrasted sharply with the greater than ring PAH proposed for the interstellar medium Pendleton and Allamandola Recently, modern analytical techniques used on the meteoritic macromolecular material have yielded more information.

Up to seven-ring PAH units have been liberated from the macromolecular material in Murchison and it appears that even larger entities are present in the experimental residue Sephton These discoveries partly reconcile the apparent disharmony between the meteoritic and interstellar organic inventories and when combined with stable isotopic data discussed below point to a partly presolar origin for this abundant organic component.

Some of the best evidence that carbonaceous chondrites contain organic matter that predates the formation of the solar system comes from studies involving stable isotopes Sephton and Gilmour The stable isotope data, therefore, is consistent with the previously mentioned L-excesses in chiral amino acids, which may have resulted from ultraviolet circularly polarized light in interstellar space.

Methyl sulphonic acid from Murchison exhibits a relative enrichment in 33 S that can only be explained if the precursor of sulphonic acids was in the gas phase and irradiated by UV in interstellar space Cooper Association with interstellar grains also implies a presolar origin for much of the organic matter Alexander Much meteoritic organic matter, it seems, may predate our solar system altogether.

Although some compound classes in meteorites may contain a record that extends back to interstellar environments before the solar system formed, others can be much more recent in the form of terrestrial contamination. Understanding the record in carbonaceous meteorites requires that terrestrial contamination is effectively identified and excluded. An example of how unusual and subtle contamination sources may be is provided by recent data. The primary specimen of the Orgueil meteorite has a long curation history since its fall in and, recently, a suite of terpene-related compounds figure 2 have been identified in this meteorite Watson Some terpenes have been detected in meteorites previously and camphor, for example, was found in Orgueil with the same authors detecting camphene in Murray Studier These compounds are structurally specific and are unlikely to be produced by abiotic reactions.

Terpenes are common in essential plant oils and are used extensively in cleaning products. It seems that years of diligence by museum cleaning staff may have flooded the atmosphere with sweet-smelling terpenes that percolated their way into the porous Orgueil stones.

A more widespread type of terrestrial contaminant is provided by the long-chain aliphatic hydrocarbons normal alkanes in meteorites figure 2. A detailed historical review of the detection and interpretation of these compounds can be found in Sephton b.

Unlike the short-chain aliphatic hydrocarbons mentioned earlier, the normal alkanes are structurally specific with all carbon atoms arranged almost invariably in a straight chain.

Normal alkanes in meteorites were first detected in Orgueil and were attributed to extraterrestrial biology based on similar distributions for molecules in the meteorite and those found in butter and marine sediments Nagy Terrestrial contamination appeared to be the most likely explanation but, in , there was a revival in the belief that these compounds were indigenous.

Normal alkanes were interpreted as products of an abiotic catalytic reaction called the Fischer-Tropsch synthesis Studier The Fischer-Tropsch synthesis is used on a large scale in industry to produce hydrocarbons from simple hydrogen and carbon monoxide precursors. In the context of meteorite organics, the Fischer-Tropsch reaction was assumed to have occurred on mineral grains in the solar nebula.

In later years, however, it became clear that the necessary catalysts were created on the meteorite parent body and were not present in the solar nebula Kerridge Also, the carbon isotopic compositions of meteoritic organic compounds were not consistent with a Fischer-Tropsch scenario; carbon monoxide, the proposed starting material for the Fischer-Tropsch reaction, contained more 12 C than the hydrocarbons in Murchison, not more 13 C as would be expected Yuen Moreover, the carbon isotopic measurements of individual normal alkanes reveal terrestrial values that are distinct from the majority of meteoritic organic matter Sephton b.

Materials provided by Hokkaido University. Note: Content may be edited for style and length. Science News. Story Source: Materials provided by Hokkaido University. Glavin, Jason P. Dworkin, Shogo Tachibana. Extraterrestrial hexamethylenetetramine in meteorites—a precursor of prebiotic chemistry in the inner solar system. Nature Communications , ; 11 1 DOI: ScienceDaily, 7 December Hokkaido University.

Key building block for organic molecules discovered in meteorites. Retrieved November 12, from www. One way to ponder this question is by observing the distribution and abundance of complex organic molecules in interstellar gas clouds. It may be that small organic molecules first appeared on earth and were later combined into larger molecules, such as proteins and carbohydrates.

This liquid-like ice may enhance the formation of organic compounds ScienceDaily shares links with sites in the TrendMD network and earns revenue from third-party advertisers, where indicated.

Print Email Share. Living Well. View all the latest top news in the environmental sciences, or browse the topics below:. Keyword: Search.