C. X. Mendoza-Gomez

The chemistry of life's origins

Preface. Interstellar dust evolution: a reservoir of prebiotic molecules J.M. Greenberg, C.X. Mendoza-Gomez. Laboratory simulations of grain icy mantles processing by cosmic rays V. Pirronello. Physics and chemistry of protoplanetary accretion disks W.J. Duschl. Chemistry of the solar nebula B. Fegley, Jr. Early evolution of the atmosphere and ocean J.F. Kasting. Origin and evolution of Martian atmosphere and climate and possible exobiological experiments L.M. Mukhin. The possible pathways of the synthesis of precursors on the early earth L.M. Mukhin, M.V. Gerasimov. Physical and chemical composition of comets - from interstellar space to the earth J.M. Greenberg. Organic matter in meteorites: molecular and isotopic analyses of the Murchison meteorite J.R. Cronin, S. Chang. Prebiotic synthesis in planetary environments S. Chang. Prebiotic synthesis on minerals: RNA oligomer formation J.P. Ferris. Biology and theory: RNA and the origin of life A.W. Schwartz. Chirality and the origins of life A. Brack. Early proteins A. Brack. The beginnings of life on earth: evidence from the geological record M. Schidlowski. Index.

Comet Halley as an aggregate of interstellar dust and further evidence for the photochemical formation of organics in the interstellar medium

Photolysis of mixtures of CO:NH3:H2O at 12 K results in the formation of an organic residue which is not volatile in high vacuum at room temperature. Analysis of this fraction by GC-MS resulted in the detection of C2–C3 hydroxy acids and hydroxy amides, glycerol, urea, glycine, hexamethylene tetramine, formamidine and ethanolamine. Use of isotopically labeled gases made it possible to establish that the observed products were not contaminants. The reaction pathways for the formation of these products were determined from the position of the isotopic labels in the mass spectral fragments. The significance of these findings to the composition of comets and the origins of life is discussed.

Approaching the Interstellar Grain Organic Refractory Component

Infrared spectra have been obtained for laboratory residues of photoprocessed low-temperature ices which have been exposed to long-term solar ultraviolet radiation on the EURECA satellite. This is an analog to the ultraviolet processing of interstellar dust mantles in diffuse clouds after leaving molecular clouds. The 3.4 μm absorption features of these organic materials match those of the diffuse cloud interstellar dust better than any other previously suggested analog to the interstellar organics.

Ultraviolet Photoprocessing of Interstellar Dust Mantles as a Source of Polycyclic Aromatic Hydrocarbons and Other Conjugated Molecules

By co-depositing a gas mixture of simple carbon- and nitrogen-containing molecules with water on a 10 K surface and exposing it to ultraviolet radiation, we were able to form a residue. This residue was then placed aboard the EURECA satellite behind a magnesium fluoride window and exposed to solar radiation for 4 months before it was returned and analyzed. The resulting residue is believed to simulate the photoprocessing of organic dust mantles in the interstellar medium. Mass spectrometry indicated that the photoprocessing created a rich mixture of polycyclic aromatic hydrocarbons (PAHs) and other conjugated organic molecules, which may explain how PAHs are replenished in space.

The seeding of life by comets

The evidence that living organisms were already extant on the earth almost 4 Gyr ago and that early bombardment by comets and asteroids created a hostile environment up to about this time has revived the question of how it was possible for prebiotic chemical evolution to have provided the necessary ingredients for life to have developed in the short intervening time. The actual bracketed available temporal space is no more than 0.5 Gyr and probably much less. Was this sufficient time for an earth-based source of the first simple organic precursor molecules to have led to the level of the prokaryotic cell? If not, then the difficulty would be resolved if the ancient earth was impregnated by organic molecular seed from outer space. Curiously, it seems that the most likely source of such seeds was the same a one of the sources of the hostile enviroment, namely the comets which bombarded the earth. With the knowledge of comets gained by the space missions it has become clear that a very large fraction of the chemical composition of comet nuclei consists of quite complex organic molecules. Furthermore it has been demonstrated that comets consist of very fluffy aggregates of interstellar dust whose chemistry derives from photoprocessing of simple ice mixtures in space. Thus, the ultimate source of organics in comets comes from the chemical evolution of interstellar dust. An important and critical justification for assuming that interstellar dust is the ultimate source of prebiotic molecular insertion on the earth is the proof that comets are extremely fluffy aggregates, which have the possibility of breaking up into finely divided fragments when the comet impacts the earths atmosphere. In the following we will summarize the properties of interstellar dust and the chemical and morphological structure of comets indicated by the most recent interpretations of comet observations. It will be shown that the suitable condition for comets having provided abundant prebiotic molecules as well as the water in which they could have further evolved are consistent with theories of the early earth environment.

Interstellar Dust Evolution: A Reservoir of Prebiotic Molecules

The physical and chemical evolution of interstellar dust is followed using a combination of observations, theory and laboratory analog studies. Laboratory analog experimental results on photochemical processes in interstellar dust mantles induced by ultraviolet radiation in space are analyzed. The organic molecular products consist of a wide variety of species, many of which are of obvious prebiotic significance.

Laboratory simulation of organic grain mantles

The most relevant conditions in interstellar space for grain mantle evolution are being simulated at Laboratory Astrophysics. In particular we have photoproduced in the laboratory a material resembling the organic refractory mantle on interstellar grains. These organic refractory samples are being analyzed by several methods, and they have been found to consist of a very complex mixture of long chains, cross-linked and probably aromatic carbon molecules.

Photochemical reactions on interstellar grains

Interstellar grains and gas comprise about 10% of the mass of the stars in our galaxy. A typical grain consists of a silicate core and a mantle. If the grain is present in a diffuse cloud of interstellar gas, the mantle consists of relatively nonvolatile higher molecular weight organic compounds. Grains in dense clouds of interstellar dust are believed to be coated with volatile inorganic and organic compounds such as H20, CH4, CO2, CO, H2S,

Laboratory Dust Studies and Gas-Grain Chemistry

2, NH3, ions such as OCNand higher molecular weight organics.