Engineering topics

Delsemme, A. H.

Publications and source records attributed to Delsemme, A. H..

At least 37 records · Page 2

NASA NTRS1981

Nature and origin of organic molecules in comets

The organic molecules sublimating from the cometary nucleus either represent the 'frost' of interstellar molecules that condensed onto those interstellar grains that have later accreted into comets, or they represent the 'snows' that condensed onto silicate grains during the cooling phase of the presolar nebula. Even if one does not accept the speculations of Hoyle and Wickramashinghe (1977), the fact that prebiotic chemistry could have been brought from the interstellar space or from the solar nebula to the Earth by comets is an intriguing possibility that cannot be ruled out at this stage.

Delsemme, A. H.↗

NASA NTRS1980

Pristine nature of comets as revealed by their UV spectrum

Satellite and rocket VUV observations of comets are discussed with reference to observations of OH and NH; the Lyman-alpha halo of hydrogen; and C, O, S atoms, ions, and molecules. Attention is also given to the study of production rates, and elemental and molecular abundances. It is shown that the observation of the VUV spectrum of comets has revealed new clues about comets, which could serve as a link between interstellar molecules and planetary atmospheres as well as between stellar and planetary astrophysics.

Delsemme, A. H.↗

NASA NTRS1980

Brightness profiles of CO/+/ in the ionosphere of Comet West /1976 VI/

The observed sunward and tailward brightness profiles of the (2-0) band of the (A2Pi-X2Sigma) system of CO(+) (4260 A) in Comet West (1976 VI) are established over a range of heliocentric distance from 0.440 to 0.842 AU. From these data the variation with heliocentric distance of the CO(+) production rate is investigated along with the size of the contact surface (the diameter of the onset of the tail). A photoionization model is proposed for the production of CO(+) in Comet West in which CO photoionization is considered to be the principle source of CO(+); the model is consistent with the observational data and the MHD theories.

Combi, M. R.↗

NASA NTRS1980

Neutral cometary atmospheres. I - An average random walk model for photodissociation in comets

A method for constructing physically realistic photochemical models, taking into account the isotropic ejection of dissociated molecular fragments as well as radiation pressure acceleration, has been developed using Monte Carlo techniques. The effect of the isotropic ejection, as opposed to the radial motion arbitrarily assumed in Haser's model, is adequately described by a simple average random walk model. It is shown that measured radial (Haser) scale lengths are in fact only lower limits to a range of possible true scale lengths for a given brightness profile, which explains the current discrepancies between observed scale lengths and those predicted by photochemistry.

Combi, M. R.↗

NASA NTRS1980

Neutral cometary atmospheres. II - The production of CN in comets

Brightness profiles of the CN (0-0) band at 3883 A have been constructed from spectrograms of comets Bennett (1979 II) and West (1976 VI). The subsequent analysis of these profiles shows that the parent molecule of CN had a radial scale length of (2.19 plus or minus 0.07) x 10 to the 4th r (H-squared), in kilometers. This, combined with current theories as well as photochemistry, is consistent with production by simple photodissociation of HCN. The rather random variation of the radial scale length for the apparent decay of CN is indicative not of a true decay scale length, but of the fact that no steady state in the CN parent production rate is achieved for the time scale necessary to build up the observed profile. A revision of the average CN production law with heliocentric distance from published photometry indicates an r(H to the -2nd) law for comet West out to at least 2.555 AU. This implies that the vaporization of this comet was controlled by some species more volatile than water. Based on this and other evidence, CO2 is suggested, but even more volatile molecules like CO cannot be ruled out.

Combi, M. R.↗

NASA NTRS1980

Photodissociation of CO2 into CO plus O/1D/

It is shown that the photodissociation of CO2(1Sigma/g/plus) into CO(X1Sigma/plus/) plus O(1D) with a quantum yield of unity is consistent with all known data on the CO2 ultraviolet absorption band between 1675 and 1400 A. Astrophysical data from the atmospheres of Mars and Venus are not inconsistent with this interpretation and cometary data seem to support it.

Delsemme, A. H.↗

NASA NTRS1980

Origin and chemistry of comets

Long-period and short-period comets are all derived from a steady flux of 'new' comets coming straight from the margin of a sphere whose radius is 50,000 AU (the Oort's cloud). No empirical evidence requires that the Oort's cloud be older than a few million years, but all satisfactory theories have so far linked its formation to the origin of the solar system. 'New' comets have a size distribution consistent with the accretion of planetesimals, in contrast with older comets that are consistent with a fragmentation distribution. The H, C, N, O elemental ratios to Si suggest the comets are more primitive than the most primitive meteorites, namely the C I chondrites. Their chemistry is poorly known, but observational data suggest a protosolar ratio of C/O at least as large as 0.66, and a drastic depletion of hydrogen in the solar nebula. This could for instance imply that the T Tauri phase of the sun happened prior to the condensation of comets. An alternate explanation is possible: the interstellar grains were relatively unprocessed during their contraction in the solar nebula, and their mantles, barely modified, would have followed suit in the accretion of comets.

Delsemme, A. H.↗

NASA NTRS1979

The ultraviolet spectrum of Comet Seargent 1978m

UV spectrograms of Comet Seargent (1978m) have been obtained with the IUE satellite in the low- (1.8-nm) and high- (0.1-nm) dispersion modes. In the long-wavelength region (189-340 nm) emission bands of OH, CS, CO2(+), NH, and CO(+) can be identified, and the rotational structure of OH could clearly be resolved. In the short-wavelength region (119-192 nm) emission lines of H(Lyman-alpha), O I, C I, and S I appear. These observations demonstrate that the IUE observatory can effectively be used to obtain high-quality spectra of comets as faint as ninth magnitude.

Jackson, M. W.↗

NASA NTRS1979

O/1D/ and H2O/+/ in comet Bennett 1970. II

The spatial distribution of O(1D) in comet Bennett on April 18 implies the existence of an average scale length of about 10,000 km for the source of one or several parents, and an average scale length also of about 10,000 km for their destruction. The spatial distribution of H2O(+) has also been studied sunward and tailward for the same comet and the same date. Two spectra taken 20 minutes apart show recognizable wave patterns moving tailward with a velocity of 17 km/sec. The interaction of the H2O(+) ions with the solar wind gives clues to the onset of the plasma tail. A characteristic time of 800 sec seems to be associated with the instabilities seen at the onset of the tail.

Delsemme, A. H.↗

NASA NTRS1979

Empirical data on the origin of 'new' comets

The mass distribution of a group of 62 comets from the sample of Marsden et al (1978) was determined by assuming that the size of a comet is in proportion to the surface area of its vaporizing nucleus. A bimodal distribution was found, with a gap between magnitudes 8.1 and 9. The 50 brighter comets (magnitudes 1 to 8) have been tentatively identified as true 'new' comets; their size distribution cannot be fit to a constant slope on a log-log graph. These comets were apparently formed by a mechanism which differs from the fragmentation process. The 12 fainter may be fragments of comets which split or dimmed during their first passages. Results indicate that new comets could be identified with pristine planetesimals; they decay rapidly, probably by splitting when they enter the inner solar system.

Delsemme, A. H.↗

NASA NTRS1979

Scientific returns from a program of space missions to comets

A program of cometary missions is proposed. The nature and size of interstellar dust, its origin and evolution; identification of new interstellar molecules; clarification of interstellar chemistry; accretion of grains into protosolar cometesimals; role of a T Tauri wind in the dissipation of the protosolar nebula; record of isotopic anomalies, better preserved in comets than in meteorites; cosmogenic and radiogenic dating of comets; cosmochronology and mineralogy of meteorites, as compared with that of cometary samples; origin of the earth's biosphere, and the origin of life are topics discussed in relation to comet exploration.

Delsemme, A. H.↗

NASA NTRS1979

The elemental and isotopic abundances of H, C, N, O in comets

The elemental abundances of light elements in comets is discussed by considering gas and dust separately. It is noted that the icy conglomerate model of the nucleus (Whipple, 1957) has been confirmed, and that the dust-to-gas mass ratio has been estimated within a factor of two for two recent comets. It is found that the total amount of oxygen available is also nominally in solar abundance; this implies that the oxygen of the solar nebula has probably been quantitatively bound in compounds that have been completely condensed and accreted in cometary nuclei. In addition, the amount of hydrogen is clearly depleted by a factor of 2,000, suggesting that the only hydrogen present is that linked in molecular compounds of H, C, N and O.

Delsemme, A. H.↗

NASA NTRS1977

The pristine nature of comets

Abundance considerations suggest that comets are likely to be the most pristine minor bodies in the solar system. In proportion to solar abundances, the present scanty data suggest that cometary oxygen is not depleted, whereas carbon is by a factor of 4 and hydrogen, by a factor of 2000. This implies that comets are less depleted in H, C, N, O than CI chondrites, namely 10:1 in hydrogen, 4:1 in carbon and 3:1 in oxygen. These results have been obtained by using dust-to-gas ratios in comets to measure the relative abundance of silicon and metals to volatile material, and the spectra of atomic lines, mainly from the vacuum ultraviolet, to determine the H/O and C/O ratios of the mixture of volatile molecules.

Delsemme, A. H.↗

NASA NTRS1977

The origin of comets

Empirical data related to questions regarding the origin of comets are examined, taking into account the primitive temperature and its evolution, the cometary C-12/C-13 isotope ratio, the chemistry of comets, and the solar-system related orbits of the comets. The theories of cometary origin considered are related to a condensation of comets from the solar nebula and a later ejection into Oort's cloud, a recent condensation of comets in situ, a capture of interstellar comets, and a reversal of the arrow of time in the traditional evolution of comets. The only hypothesis which seems to survive an evaluation of the theories on the basis of the existing data is that the comets were condensed some five billion years ago in the vicinity of the giant planets or beyond, at the outer edge of the solar nebula.

Delsemme, A. H.↗

NASA NTRS1977

Low-temperature condensates in comets

Recent observational data on the volatile fraction of comets are confronted with a model based on the fractional condensation, in the 80-100 K range, of a higher-temperature equilibrium obtained from a solar mixture, more or less depleted in oxygen and in hydrogen. It is possible to almost duplicate the observational data, only by assuming that the solar ratio of C/O is at least as large as 0.66 and that the hydrogen was drastically depleted by an unknown process in the primitive solar nebula. Although none of these two assumptions is at variance with present knowledge, the latter is sufficiently exotic to propose a simpler explanation, namely that comets could be made of interstellar grains relatively unprocessed by heat.

Delsemme, A. H.↗

NASA NTRS1976

The neutral coma of comets: A review

The hypothesis that water snow controls the vaporization of the nucleus of some comets seems verified from the general order of magnitude of the size of their nucleus and of their nuclear albedo; the largest observed production rates are H and OH which both seem to originate from the photodissociation of H2O, as also confirmed by the scale length of the invisible parent molecule producing OH. However, comet Encke is not uniformly covered by water snow, as it produces only one tenth of the expected vaporization. Early results on comet Kohoutek suggest that the conclusions could be slightly different for some of the new comets in Oort's sense. If the far ultraviolet observations confirm the early assessments of the production rates of C, O and H, then at least another major constituent competing with water has not yet been detected. Such a major constituent is suggested by the ratios C/O = 0.24 and H/O = 2.5.

Delsemme, A. H.↗

NASA NTRS1976

Simulation of a cometary conglomerate of frozen gases and dust in a zero-gravity environment: A critical evaluation of possible space experiments

It is proposed to use the Shuttle (OFT experiments) or eventually Spacelab to study, in the absence of gravity the low velocity accretion and the bulk properties of icy conglomerates simulating cometary material and their sedimentation and bulk properties in very small acceleration fields. Their behavior when exposed to the direct solar flux is also under consideration.

Delsemme, A. H.↗

NASA NTRS1972

Nature and Origin of Cometary Heads

In cometary spectra the three major constituents seem to be OH, H, and O. Their common precursor is likely to be water, whose rates of evaporation and dissociation have so far not led to a major discrepancy with the observations. This type of explanation, however, has not led to a positive identification of other parent molecules. With the known solar intensities, all the molecules that have been suggested, as NH3, CH4, etc, would lead to lifetimes one-to-two orders of magnitude larger than needed by the coma observations. An alternate hypothesis is that the precursors are not molecules, but icy particles stripped from the nucleus by the evaporating gases. By evaporating within an icy halo, the grains would liberate either parent molecules with very short lifetimes or some of the radicals themselves. Recent laboratory work suggests that the grains could be made of clathrate hydrates of gases or of radicals. The observations appear to indicate the presence of icy grains within the inner coma.

Delsemme, A. H.↗