Ion composition and dynamics at comet Halley

Coloration and darkening of methane clathrate and other ices by charged particle irradiation: applications to the outer solar system

Methane clathrate is expected to be an important carbon-containing ice in the outer solar system. We investigate the effect of electron irradiation by coronal discharge on several simple hydrocarbons enclathrated in or mixed with H2O or H2O+NH3 in simulation of the effects of the solar wind, planetary magnetospheric particles, and cosmic rays on surfaces containing these ices in the outer solar system and interstellar space. H2O+CH4 clathrate, H2O+C2H6, H2O+CH4+NH3, H2O+C2H6+NH3, and H2O+C2H2 are all initially white ices, and all produce yellowish to brownish organic products upon charged particles irradiation. Significant coloration occurs with doses of 10(9) erg cm-2, corresponding to short interplanetary irradiation times. Uranian magnetospheric electrons penetrate to approximately 1 mm depth and deposit this dose in 8, 30, 65, 200, and 500 years into the surfaces of Miranda, Ariel, Umbriel, Titania, and Oberon, respectively. Further irradiation of the laboratory ice surface results in a progressive darkening and a more subdued color. For a conversion efficiency to solids G approximately equal to 1 molecule keV-1, the upper limit for the time for total destruction of CH4 and other simple hydrocarbons in the upper 1 mm is 5 x 10(4) years (Miranda) to 3 x 10(6) years (Oberon). Remote detection of CH4 is possible only when its replenishment rate exceeds the destruction rate at the depth probed by spectroscopy. Reflection spectroscopy or irradiated H2O+CH4 frost is compared with the spectra of several outer solar system objects and to other relevant organic and inorganic materials. Ultraviolet-visible and infrared transmission spectroscopy of the postirradiation residues is presented. Persistence of color and of CH4 ice bands on Triton and Pluto suggests ongoing surface activity and/or atmospheric haze. Over 4 x 10(9) year time scales, > or = 10 m of satellite and cometary surface material is processed by cosmic rays to a radiation-hardened ice-tholin mixture devoid of CH4. Preaccretional chemistry, exogenous materials, and endogenous organic chemistry all contribute to the spectral properties of icy satellites which accreted simple CH(O) molecules. Radiation darkening traces the deposition of mobilized or impact-exposed carbon-bearing volatiles on these satellites. More exhaustive experiments are necessary to work out the detailed relationships between initial composition, exposure age, and color/albedo.

Evidence for methane and ammonia in the coma of comet P/Halley

Methane and ammonia abundances in the coma of Halley are derived from Giotto IMS data using an Eulerian model of chemical and physical processes inside the contact surface to simulate Giotto HIS ion mass spectral data for mass-to-charge ratios (m/q) from 15 to 19. The ratio m/q = 19/18 as a function of distance from the nucleus is not reproduced by a model for a pure water coma. It is necessary to include the presence of NH3, and uniquely NH3, in coma gases in order to explain the data. A ratio of production rates Q(NH3)/Q(H2O) = 0.01-0.02 results in model values approximating the Giotto data. Methane is identified as the most probable source of the distinct peak at m/q = 15. The observations are fit best with Q(CH4)/Q(H2O) = 0.02. The chemical composition of the comet nucleus implied by these production rate ratios is unlike that of the outer planets. On the other hand, there are also significant differences from observations of gas phase interstellar material.

The formation of amino acid precursors in the reaction of atomic carbon with water and ammonia at 77 K

When atomic carbon is condensed on a surface at 77 K containing ammonia and water, glycine, N-methylglycine, alanine, beta-alanine, aspartic acid and serine are generated. It is postulated that these reactions may mimic those which occur when an extraterrestrial carbon atom condenses on a frozen surface coated with water and ammonia and may provide a route to extraterrestrial amino acids. Experiments designed to elucidate the mechanisms of amino acid formation under these conditions have been carried out.

Evidence for Chain Molecules Enriched in Carbon, Hydrogen, and Oxygen in Comet Halley

In situ measurements of the composition and spatial distribution of heavy thermal positive ions in the coma of comet Halley were made with the heavy-ion analyzer RPA2-PICCA aboard the Giotto spacecraft. Above 50 atomic mass units an ordered series of mass peaks centered at 61, 75, 91, and 105 atomic mass units were observed. Each peak appears to be composed of three or more closely spaced masses. The abundances decrease and the dissociation rates increase smoothly with increasing mass. These observations suggest the presence of chain molecules that are enriched in carbon, oxygen, and hydrogen, such as polyoxymethylene (polymerized formaldehyde), in comet Halley.

Comets and life

Some of the chemical species which have been detected in comets include H2O, HCN, CH3CN, CO, CO2, NH3, CS, C2 and C3. All of these have also been detected in the interstellar medium, indicating a probable relationship between interstellar dust and gas clouds and comets. Laboratory experiments carried out with different mixtures of these molecules give rise to the formation of the biochemical compounds which are necessary for life, such as amino acids, purines, pyrimidines, monosaccharides, etc. However, in spite of suggestions to the contrary, the presence of life in comets is unlikely. On the other hand, the capture of cometary matter by the primitive Earth is considered essential for the development of life on this planet. The amount of cometary carbon-containing matter captured by the Earth, as calculated by different authors, is several times larger than the total amount of organic matter present in the biosphere (10(18)g). The major classes of reactions which were probably involved in the formation of key biochemical compounds are discussed. Our tentative conclusions are that: 1) comets played a predominant role in the emergence of life on our planet, and 2) they are the cosmic connection with extraterrestrial life.

Search for organic molecules in the outer solar system

Recent developments of millimeter astronomy have led to the discovery of more and more complex molecules in the interstellar medium. In a similar way, attempts have been made to detect complex molecules in the atmospheres of the most primitive bodies of the Solar System, i.e. outer planets and comets, as well as in Titan's atmosphere. An important progress has been achieved thanks to the continuous development of infrared astronomy, from the ground and from space vehicles. In particular, an important contribution has come from the IRIS-Voyager infrared spectrometer with the detection of prebiotic molecules on Titan, and some complex organic molecules on Jupiter and Saturn. Another important result has been the observation of carbonaceous material in the immediate surroundings of Comet Halley's nucleus. In the near future, the search for organic molecules in the outer Solar System should benefit from the developments of large millimeter antennae, and in the next decade, from the operation of infrared Earth-orbiting spacecrafts (ISO, SIRTF).