Gas and dust jets in the inner coma of comet Halley

Cometary Activity, Discrete Outgassing Areas, and Dust-Jet Formation

Major progress has recently been achieved in the understanding of the coma morphology of dust comets. The conceptual models for the various types of feature observed in the coma (jets, spirals, halos, fans, etc.), their computer simulation, and the hydrodynamic models for jet formation are critically reviewed, and evidence for anisotropic, strongly collimated flows of ejecta emanating from discrete active regions (vents) on the rotating cometary nuclei is presented. The techniques employed to generate synthetic comet images that simulate the observed features are described and their relevance to the primary objectives of coma-morphology studies is exemplified. Modelling of temporal variations in the water emission from discrete active regions suggests that production curves asymmetric with respect to perihelion should be commonplace. Critical comparisons are offered with the activity profiles of Encke’s comet and with light curves of disappearing comets and comets that undergo outbursts. Discussed next is the potential evolutionary significance of nuclear shape, the fractional area of the surface that is active, the vents’ erosion rate, and available information on their activation, aging, dormancy, migration, rejuvenation, and extinction. Recent developments in the understanding of the processes that cause the nongravitational perturbations of cometary motions are reviewed and the observed discontinuities are identified with birth of new sources and/or deactivation of old vents.

Comets and Life on the Primitive Earth

Comets may have contributed substantial amounts of water, volatiles and organic precursors such as HCN for the synthesis of biochemical compounds on the primitive Earth. This suggestion followed closely the prebiotic synthesis of adenine, purines and amino acids from HCN. Recent studies on the terrestrial heavy noble gases provide evidence that comets are the principal external source of Earth’s volatiles. During the encounter of comet Halley strong jets of CN, C2, C3and NH2were measured from Earth observatories, and by spacecraft mass spectrometry HCN, formaldehyde, adenine and many other organic compounds were detected, except amino acids. Obviously the latter require liquid water for their formation. Therefore upon capture of comets by the Earth, and melting of the frozen water, the synthesis of most biochemical compounds could take place readily. The detection of water, HCN and other organics of cometary origin after the impact of Comet SL-9 with Jupiter demonstrated the capability of survival of these molecules even after catastrophic events. Thus on the Earth HCN could be converted into purines, cyanacetylene, after hydration and condensation with urea, into pyrimidines, and formaldehyde into monosaccharides. In the presence of phosphates, which have been detected in cometary IDPs, nucleotides could also be synthesized. In conclusion, comets probably provided the necessary molecular precursors for the generation of life on the Earth.

Structure of the Coma: Chemistry and Solar Wind Interaction

Much has been learned about the physics and chemistry of comets from the successful spacecraft encounters and intensive remote observing programs of Comets Halley and Giacobini-Zinner. Instead of being the panacea for our comet curiosity, these tantalizing “snapshots” have raised new questions, and many fundamental problems remain unsolved. To reap fuller benefits, extensive modeling is necessary to characterize the physical structure and chemical properties of the coma and to infer the composition and structure of the nucleus.

Dust in the Universe: implications for terrestrial prebiotic chemistry

In the present review we analyze the available literature on the distribution of dust in the Universe, methods of its observation and determination of the chemical composition, and the roles for terrestrial prebiotic chemistry. The most plausible natural sources of dust on the Earth in the prebiotic era are sedimentation of interplanetary dust, meteoritic and cometary impacts, volcanic eruptions, and soil microparticulates; the interplanetary medium being among the most powerful supplier of the dust matter. Two fundamental roles of dust particles for the origins of life are considered: (1) catalytic formation of prebiotic compounds; and (2) delivery of organic matter to the Earth by space dust particles. Due to the fact that there is only approximate information on the chemical composition and properties of interstellar, circumstellar, and major part of interplanetary dust, even the simulating experiments are difficult to perform. Until these gaps are filled, it seems reasonable to focus efforts of the scientists dealing with dust-driven catalytic formation of prebiotically important compounds on the volcanic and meteoritic/cometary impact environments.