Nature and origin of comets

Abstract

After an introduction to the basic history and morphology of comets we turn in 0 2 to a description of their motion which is disturbed by the planets, non-gravitational forces due to out-gassing of the cometary nucleus and, for the near-parabolic comets, by the gravitation of nearby passing stars. An overview of the dynamical evolution of long-and short-period comets is presented, paying special attention to the resulting distribution of orbital elements. Section 3 is mainly concerned with the physical properties of the neutral gaseous matter. After a discussion of the excitation mechanisms for radiation of coma molecules we describe the sublimation (vaporisation) of gases and their outflow from the cometary nucleus. The chemistry of the coma gases is then addressed and the individual molecules, radicals and atoms are studied in the framework of their presumed parent species. Section 4 is devoted to the as yet unobserved comet nucleus. We present what is known about nuclear sizes and masses. The nucleus is likely to be a homogeneous mixture of ices (e.g. H 2 0 , CO2, CO, NH3) and dust particles but which may temporarily be covered by an insulating dust crust. Splittings and outbursts of cometary nuclei give information on their reduced internal strength. Certain periodic features observed in the cometary comae allow one to

Nanoscale infrared spectroscopy as a non-destructive probe of extraterrestrial samples

“…Demonstrably, nanoFTIR presents a novel means of optical identification and potential compositional characterization 58,59 for bona fide amorphous silicate materials in returned microscale cometary dust grains, as well as in similarly sized interplanetary dust particles or aerosols. The composition of Iris shown here is decidedly inconsistent with the long-held view that comets formed in the outer solar system from agglomerated circum-or interstellar grains 60 , comprising-for example, dust, ice and silicates amorphized by radiation exposure. Instead, the direct IR spectroscopic evidence presented here for igneous crystalline and amorphous materials coexisting at the micron-scale within a cometary dust grain adds to mounting evidence for a common history of high-temperature formation shared by cometary and chondritic materials deep within the solar nebula 33 …”

SAPT: ligação de hidrogênio ou interação de van der Waals?

“…Por outro lado, a articulação e locomoção de répteis arboríferos, como também a constituição de corpos celestes, podem ser explicadas através de forças típicas das interações de van der Waals, conforme destacado por Autumn et al, 52 Fernandez e Jockers 53 e Whipple e Huebner. 53 As interações intituladas de van der Waals foram assim concebidas em reconhecimento ao físico neerlândes Johannes Diederik van der Waals pelos seus estudos com substâncias gasosas e líquidas, em uma proposta que objetivava a otimização dos modelos de Boyle e Charles 54 através da descrição do tamanho molecular e a força intermolecular atuante, a qual é composta pelas contribuições de Keesom (interação entre dipolo permanente--dipolo permanente), 55 Debye (interação entre dipolo permanente--dipolo induzido) 56 e London (interação entre dipolo induzido-dipolo induzido). 57 As interações de van der Waals têm sido foco de muitos pesquisas, principalmente no aprimoramento de alguns métodos que falham na captura dos efeitos provenientes das forças de dispersão.…”

Nucleus and tail studies of comet P/Swift-tuttle

“…clarified, but may be related to sudden changes in the solar wind flow direction (for a more detailed discussion of the various explanation scenarios see Ip andAxford, 1982, andFernandez andJockers, 1983).…”

“…Contrary to the tail rays, we were not able to verify a connection between the diffuse head streamers in the outer coma of P/Swift-Tuttle and the BEA substructures in the inner coma, although some similarities (for instance the pair appearance and the PAS) are recognized. Both tail rays and head streamers are frequently discussed as ion phenomena which are caused by the same physical process (see Femandez and Jockers, 1983;Ip and Axford, 1982). According to the favoured models, both features may be related to the passage of solar wind discontinuities through the cometary plasma environment.…”