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At least 181 records · Page 10
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Materials Data on Li3Mn2(SiO4)2 by Materials Project

Li3Mn2(SiO4)2 is Clathrate-derived structured and crystallizes in the monoclinic C2 space group. The structure is three-dimensional. there are two inequivalent Li1+ sites. In the first Li1+ site, Li1+ is bonded to four O2- atoms to form LiO4 tetrahedra that share corners with two equivalent LiO4 tetrahedra, corners with two equivalent MnO4 tetrahedra, corners with four SiO4 tetrahedra, and an edgeedge with one MnO4 tetrahedra. There are a spread of Li–O bond distances ranging from 1.91–2.07 Å. In the second Li1+ site, Li1+ is bonded to four O2- atoms to form distorted LiO4 tetrahedra that share corners with two equivalent SiO4 tetrahedra, corners with four equivalent LiO4 tetrahedra, corners with four equivalent MnO4 tetrahedra, and an edgeedge with one SiO4 tetrahedra. There are two shorter (2.07 Å) and two longer (2.08 Å) Li–O bond lengths. Mn+2.50+ is bonded to four O2- atoms to form MnO4 tetrahedra that share corners with four LiO4 tetrahedra, corners with four SiO4 tetrahedra, and an edgeedge with one LiO4 tetrahedra. There are a spread of Mn–O bond distances ranging from 1.96–2.08 Å. There are two inequivalent Si4+ sites. In the first Si4+ site, Si4+ is bonded to four O2- atoms to form SiO4 tetrahedra that share corners with four equivalent LiO4 tetrahedra, corners with four equivalent MnO4 tetrahedra, and an edgeedge with one LiO4 tetrahedra. There is two shorter (1.64 Å) and two longer (1.66 Å) Si–O bond length. In the second Si4+ site, Si4+ is bonded to four O2- atoms to form SiO4 tetrahedra that share corners with four equivalent MnO4 tetrahedra and corners with six LiO4 tetrahedra. There is two shorter (1.64 Å) and two longer (1.67 Å) Si–O bond length. There are four inequivalent O2- sites. In the first O2- site, O2- is bonded in a distorted trigonal planar geometry to one Li1+, one Mn+2.50+, and one Si4+ atom. In the second O2- site, O2- is bonded to two Li1+, one Mn+2.50+, and one Si4+ atom to form edge-sharing OLi2MnSi tetrahedra. In the third O2- site, O2- is bonded in a trigonal planar geometry to one Li1+, one Mn+2.50+, and one Si4+ atom. In the fourth O2- site, O2- is bonded in a distorted rectangular see-saw-like geometry to two Li1+, one Mn+2.50+, and one Si4+ atom.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on CdFe4Cu10(SnSe4)5 by Materials Project

Fe4Cu10Cd(SnSe4)5 is Clathrate-derived structured and crystallizes in the monoclinic C2 space group. The structure is three-dimensional. there are two inequivalent Fe3+ sites. In the first Fe3+ site, Fe3+ is bonded to four Se2- atoms to form FeSe4 tetrahedra that share corners with four SnSe4 tetrahedra and corners with eight CuSe4 tetrahedra. There are one shorter (2.41 Å) and three longer (2.42 Å) Fe–Se bond lengths. In the second Fe3+ site, Fe3+ is bonded to four Se2- atoms to form FeSe4 tetrahedra that share corners with four SnSe4 tetrahedra and corners with eight CuSe4 tetrahedra. There are three shorter (2.42 Å) and one longer (2.43 Å) Fe–Se bond lengths. There are five inequivalent Cu1+ sites. In the first Cu1+ site, Cu1+ is bonded to four Se2- atoms to form CuSe4 tetrahedra that share a cornercorner with one FeSe4 tetrahedra, corners with three equivalent CdSe4 tetrahedra, corners with four CuSe4 tetrahedra, and corners with four equivalent SnSe4 tetrahedra. There are a spread of Cu–Se bond distances ranging from 2.39–2.41 Å. In the second Cu1+ site, Cu1+ is bonded to four Se2- atoms to form CuSe4 tetrahedra that share corners with four equivalent FeSe4 tetrahedra, corners with four CuSe4 tetrahedra, and corners with four SnSe4 tetrahedra. There are three shorter (2.44 Å) and one longer (2.45 Å) Cu–Se bond lengths. In the third Cu1+ site, Cu1+ is bonded to four Se2- atoms to form CuSe4 tetrahedra that share corners with four FeSe4 tetrahedra, corners with four CuSe4 tetrahedra, and corners with four SnSe4 tetrahedra. There are two shorter (2.44 Å) and two longer (2.45 Å) Cu–Se bond lengths. In the fourth Cu1+ site, Cu1+ is bonded to four Se2- atoms to form CuSe4 tetrahedra that share a cornercorner with one CdSe4 tetrahedra, corners with three equivalent FeSe4 tetrahedra, corners with four CuSe4 tetrahedra, and corners with four SnSe4 tetrahedra. There are a spread of Cu–Se bond distances ranging from 2.42–2.45 Å. In the fifth Cu1+ site, Cu1+ is bonded to four Se2- atoms to form CuSe4 tetrahedra that share corners with four FeSe4 tetrahedra, corners with four CuSe4 tetrahedra, and corners with four SnSe4 tetrahedra. There are two shorter (2.44 Å) and two longer (2.45 Å) Cu–Se bond lengths. Cd2+ is bonded to four Se2- atoms to form CdSe4 tetrahedra that share corners with four equivalent SnSe4 tetrahedra and corners with eight CuSe4 tetrahedra. All Cd–Se bond lengths are 2.66 Å. There are three inequivalent Sn+3.20+ sites. In the first Sn+3.20+ site, Sn+3.20+ is bonded to four Se2- atoms to form SnSe4 tetrahedra that share corners with two equivalent FeSe4 tetrahedra, corners with two equivalent CdSe4 tetrahedra, and corners with eight CuSe4 tetrahedra. There are a spread of Sn–Se bond distances ranging from 2.62–2.65 Å. In the second Sn+3.20+ site, Sn+3.20+ is bonded to four Se2- atoms to form SnSe4 tetrahedra that share corners with four FeSe4 tetrahedra and corners with eight CuSe4 tetrahedra. There are one shorter (2.68 Å) and three longer (2.69 Å) Sn–Se bond lengths. In the third Sn+3.20+ site, Sn+3.20+ is bonded to four Se2- atoms to form SnSe4 tetrahedra that share corners with four equivalent FeSe4 tetrahedra and corners with eight CuSe4 tetrahedra. There are two shorter (2.69 Å) and two longer (2.70 Å) Sn–Se bond lengths. There are ten inequivalent Se2- sites. In the first Se2- site, Se2- is bonded to one Fe3+, two Cu1+, and one Sn+3.20+ atom to form corner-sharing SeFeCu2Sn tetrahedra. In the second Se2- site, Se2- is bonded to two Cu1+, one Cd2+, and one Sn+3.20+ atom to form corner-sharing SeCdCu2Sn tetrahedra. In the third Se2- site, Se2- is bonded to one Fe3+, two Cu1+, and one Sn+3.20+ atom to form corner-sharing SeFeCu2Sn tetrahedra. In the fourth Se2- site, Se2- is bonded to one Fe3+, two equivalent Cu1+, and one Sn+3.20+ atom to form corner-sharing SeFeCu2Sn tetrahedra. In the fifth Se2- site, Se2- is bonded to one Fe3+, two Cu1+, and one Sn+3.20+ atom to form corner-sharing SeFeCu2Sn tetrahedra. In the sixth Se2- site, Se2- is bonded to two equivalent Cu1+, one Cd2+, and one Sn+3.20+ atom to form corner-sharing SeCdCu2Sn tetrahedra. In the seventh Se2- site, Se2- is bonded to one Fe3+, two Cu1+, and one Sn+3.20+ atom to form corner-sharing SeFeCu2Sn tetrahedra. In the eighth Se2- site, Se2- is bonded to one Fe3+, two Cu1+, and one Sn+3.20+ atom to form corner-sharing SeFeCu2Sn tetrahedra. In the ninth Se2- site, Se2- is bonded to one Fe3+, two Cu1+, and one Sn+3.20+ atom to form corner-sharing SeFeCu2Sn tetrahedra. In the tenth Se2- site, Se2- is bonded to one Fe3+, two Cu1+, and one Sn+3.20+ atom to form corner-sharing SeFeCu2Sn tetrahedra.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Zn3Fe2Cu10(SnSe4)5 by Materials Project

Fe2Cu10Zn3(SnSe4)5 is Clathrate-derived structured and crystallizes in the monoclinic C2 space group. The structure is three-dimensional. Fe3+ is bonded to four Se2- atoms to form FeSe4 tetrahedra that share corners with four SnSe4 tetrahedra and corners with eight CuSe4 tetrahedra. There are two shorter (2.43 Å) and two longer (2.44 Å) Fe–Se bond lengths. There are five inequivalent Cu1+ sites. In the first Cu1+ site, Cu1+ is bonded to four Se2- atoms to form CuSe4 tetrahedra that share corners with four CuSe4 tetrahedra, corners with four ZnSe4 tetrahedra, and corners with four SnSe4 tetrahedra. All Cu–Se bond lengths are 2.43 Å. In the second Cu1+ site, Cu1+ is bonded to four Se2- atoms to form CuSe4 tetrahedra that share a cornercorner with one FeSe4 tetrahedra, corners with three equivalent ZnSe4 tetrahedra, corners with four CuSe4 tetrahedra, and corners with four SnSe4 tetrahedra. There are a spread of Cu–Se bond distances ranging from 2.42–2.44 Å. In the third Cu1+ site, Cu1+ is bonded to four Se2- atoms to form CuSe4 tetrahedra that share corners with four CuSe4 tetrahedra, corners with four ZnSe4 tetrahedra, and corners with four equivalent SnSe4 tetrahedra. All Cu–Se bond lengths are 2.43 Å. In the fourth Cu1+ site, Cu1+ is bonded to four Se2- atoms to form CuSe4 tetrahedra that share a cornercorner with one ZnSe4 tetrahedra, corners with three equivalent FeSe4 tetrahedra, corners with four CuSe4 tetrahedra, and corners with four SnSe4 tetrahedra. There are a spread of Cu–Se bond distances ranging from 2.43–2.45 Å. In the fifth Cu1+ site, Cu1+ is bonded to four Se2- atoms to form CuSe4 tetrahedra that share corners with four equivalent FeSe4 tetrahedra, corners with four CuSe4 tetrahedra, and corners with four SnSe4 tetrahedra. There are three shorter (2.44 Å) and one longer (2.45 Å) Cu–Se bond lengths. There are two inequivalent Zn2+ sites. In the first Zn2+ site, Zn2+ is bonded to four Se2- atoms to form ZnSe4 tetrahedra that share corners with four SnSe4 tetrahedra and corners with eight CuSe4 tetrahedra. All Zn–Se bond lengths are 2.50 Å. In the second Zn2+ site, Zn2+ is bonded to four Se2- atoms to form ZnSe4 tetrahedra that share corners with four equivalent SnSe4 tetrahedra and corners with eight CuSe4 tetrahedra. All Zn–Se bond lengths are 2.50 Å. There are three inequivalent Sn+3.60+ sites. In the first Sn+3.60+ site, Sn+3.60+ is bonded to four Se2- atoms to form SnSe4 tetrahedra that share corners with two equivalent FeSe4 tetrahedra, corners with two equivalent ZnSe4 tetrahedra, and corners with eight CuSe4 tetrahedra. There are one shorter (2.65 Å) and three longer (2.66 Å) Sn–Se bond lengths. In the second Sn+3.60+ site, Sn+3.60+ is bonded to four Se2- atoms to form SnSe4 tetrahedra that share corners with four ZnSe4 tetrahedra and corners with eight CuSe4 tetrahedra. There are three shorter (2.63 Å) and one longer (2.64 Å) Sn–Se bond lengths. In the third Sn+3.60+ site, Sn+3.60+ is bonded to four Se2- atoms to form SnSe4 tetrahedra that share corners with four equivalent FeSe4 tetrahedra and corners with eight CuSe4 tetrahedra. There are two shorter (2.67 Å) and two longer (2.68 Å) Sn–Se bond lengths. There are ten inequivalent Se2- sites. In the first Se2- site, Se2- is bonded to one Fe3+, two Cu1+, and one Sn+3.60+ atom to form corner-sharing SeFeCu2Sn tetrahedra. In the second Se2- site, Se2- is bonded to two Cu1+, one Zn2+, and one Sn+3.60+ atom to form corner-sharing SeZnCu2Sn tetrahedra. In the third Se2- site, Se2- is bonded to two Cu1+, one Zn2+, and one Sn+3.60+ atom to form corner-sharing SeZnCu2Sn tetrahedra. In the fourth Se2- site, Se2- is bonded to two Cu1+, one Zn2+, and one Sn+3.60+ atom to form corner-sharing SeZnCu2Sn tetrahedra. In the fifth Se2- site, Se2- is bonded to one Fe3+, two equivalent Cu1+, and one Sn+3.60+ atom to form corner-sharing SeFeCu2Sn tetrahedra. In the sixth Se2- site, Se2- is bonded to two Cu1+, one Zn2+, and one Sn+3.60+ atom to form corner-sharing SeZnCu2Sn tetrahedra. In the seventh Se2- site, Se2- is bonded to two equivalent Cu1+, one Zn2+, and one Sn+3.60+ atom to form corner-sharing SeZnCu2Sn tetrahedra. In the eighth Se2- site, Se2- is bonded to two Cu1+, one Zn2+, and one Sn+3.60+ atom to form corner-sharing SeZnCu2Sn tetrahedra. In the ninth Se2- site, Se2- is bonded to one Fe3+, two Cu1+, and one Sn+3.60+ atom to form corner-sharing SeFeCu2Sn tetrahedra. In the tenth Se2- site, Se2- is bonded to one Fe3+, two Cu1+, and one Sn+3.60+ atom to form corner-sharing SeFeCu2Sn tetrahedra.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Si by Materials Project

Si is Clathrate-like structured and crystallizes in the orthorhombic Cmcm space group. The structure is three-dimensional. there are nine inequivalent Si sites. In the first Si site, Si is bonded to four Si atoms to form corner-sharing SiSi4 tetrahedra. There are a spread of Si–Si bond distances ranging from 2.35–2.40 Å. In the second Si site, Si is bonded to four Si atoms to form a mixture of edge and corner-sharing SiSi4 tetrahedra. There are a spread of Si–Si bond distances ranging from 2.33–2.39 Å. In the third Si site, Si is bonded to four Si atoms to form corner-sharing SiSi4 tetrahedra. There are one shorter (2.33 Å) and three longer (2.35 Å) Si–Si bond lengths. In the fourth Si site, Si is bonded to four Si atoms to form corner-sharing SiSi4 tetrahedra. There are a spread of Si–Si bond distances ranging from 2.38–2.48 Å. In the fifth Si site, Si is bonded to four Si atoms to form corner-sharing SiSi4 tetrahedra. The Si–Si bond length is 2.38 Å. In the sixth Si site, Si is bonded to four Si atoms to form corner-sharing SiSi4 tetrahedra. Both Si–Si bond lengths are 2.32 Å. In the seventh Si site, Si is bonded to four Si atoms to form corner-sharing SiSi4 tetrahedra. The Si–Si bond length is 2.41 Å. In the eighth Si site, Si is bonded to four Si atoms to form a mixture of edge and corner-sharing SiSi4 tetrahedra. In the ninth Si site, Si is bonded to four Si atoms to form corner-sharing SiSi4 tetrahedra.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Gas Hydrate Film Growth in Microfluidic Channels for Carbon Dioxide Capture and Sequestration Applications

Gas or clathrate hydrates are a solid, crystalline compound composed of water and guest molecules that typically form at high pressure and low temperature conditions. Carbon dioxide (CO2) hydrates may be involved in several carbon dioxide capture and sequestration (CCS) applications, including CO2 pipeline transportation and CO2 offshore sequestration. Within these applications, the formation mechanism and kinetics must be well understood to manage the CCS processes, either by preventing or promoting hydrate formation. In this work, a high-pressure glass microfluidic reactor is used in tandem with visual microscopy and in-situ Raman spectroscopy to study both the morphological and kinetic behavior of gas hydrate crystals. Subcooling, pressure, and CO2 flow rate are investigated for their impact on the thickening behavior of pure CO2 hydrates, with flow rate being the only parameter to have a significant effect. Visual and Raman spectroscopy evidence show that both a dense hydrate layer and a porous hydrate layer form, and the latter may provide a path for mass transfer to continue hydrate crystallization. A first principles mass transfer model is developed to describe CO2 hydrate crystal thickening at the interface between gas and water. The impacts of gas impurities and channel wettability are also studied. This method is further applied to investigate the conversion of methane hydrate to CO2 hydrate for combined energy recovery and methane hydrate formation. The authors acknowledge the US Department of Energy Basic Energy Science award # DE-SC0022162.

Wadsworth, Lindsey [Colorado School of Mines, Gold↗
DOE OSTI2020

A Moderator Test Station at ORNL

Oak Ridge National Laboratory (ORNL) hosts two world-leading slow neutron sources, the Spallation Neutron Source (SNS) and the High Flux Isotope Reactor (HFIR), and is currently developing the technical design for a Second Target Station (STS) for the SNS. Upon completion of the STS project, ORNL will be uniquely positioned to optimize each of its three neutron sources, the SNS First Target Station (FTS), the STS, and HFIR, in a complementary way. Among the essential aspects of a reimagined FTS and the current STS concept are high-brightness parahydrogen moderators—moderators which are optimized for high per-unit-area neutron brightness rather than integrated-across-large-area neutron intensity. The high-brightness moderators proposed for the STS will significantly outperform the coupled moderators currently on the FTS for appropriately optimized neutron beamlines, provided the moderating hydrogen is converted to near-equilibrium levels of parahydrogen (approximately 99.8% at 20 K). The original FTS moderators, by contrast, were conservatively designed to be relatively insensitive to the exact ortho:para ratio, with a consequent loss in performance. As a result, a redesign of the FTS moderators assuming fully converted parahydrogen could result in significant performance improvements on the FTS coupled moderators, and more consistent performance over time for all hydrogen moderators. This “parahydrogen problem” is a long-standing challenge for the effective implementation of hydrogen cold moderators at high-power neutron sources. In addition, the development of new moderator concepts, whether based on previously unused materials, structured heterogeneous arrays, or even simply on changes in overall shape and size is significantly restricted at a large-scale production facility intended to use the neutron beams so produced. Accordingly, moderators for production neutron sources are often designed in a very conservative, low-risk fashion, even though this compromises the absolute neutronic performance. Advanced moderator concepts worthy of study include features that could not be tested without redesigning and redeploying the entire existing reflector, shielding, and neutron beamline installation, making such development efforts far more expensive than building a stand-alone test facility. We propose a Moderator Test Station (MTS) at the SNS with which we will verify such performance gains and test new moderator concepts. These concepts include both high-brightness and large-volume parahydrogen moderators, as well as moderators with tailored ortho:para hydrogen levels, heterogeneous moderator concepts such as the convoluted moderator or pelletized moderators, and moderators made of materials like ammonia, ethane, and oxygen clathrates, which have not been widely tested let alone deployed at neutron source facilities.

42 ENGINEERING↗
DOE OSTI2026

Towards libraries of different ultrasmall amorphous silica cage topologies

Cage topology controlled at the nanometer length scale is expected to enable original functionalities, e.g., in catalysis and therapeutics. Cages are fundamental structural motifs of clathrates and their topological duals, Frank-Kasper phases, and are constituents of mesoporous silica frameworks. However, with one exception, they have not been synthesized as discrete particles. By varying reactant ratios of surfactant, oil, and silane, we report the discovery of 5(12)6(2) and 5(12)6(8) amorphous silica polyhedra, each coexisting with the previously identified 5(12) cage, using combined cryo-transmission electron microscopy (cryo-TEM) and single-particle reconstruction (SPR). Notably, the 5(12)6(8) polyhedron represents a topology not previously observed in mesoporous silica frameworks. Control over structural features is demonstrated, and insights into cage formation mechanisms are provided. Structural outcomes are summarized in a ternary morphology diagram alongside prior results, bridging serendipitous discovery and intentional design of silica cages displaying a level of control over silica polymerization rivaling nature.

Jang, Dong June [Department of Materials Science a↗
NASA NTRS1972

Cometary Nuclei: Models

Arguments for the existence of some kind of icy-conglomerate cometary nucleus are put forward, and the role of clathrates in the condensation of comets from the solar nebula is discussed. The division of cosmic materials into three types is described, and it is suggested that whereas Jupiter, and to a large extent Saturn, condensed directly from gas and the terrestrial planets and asteroids collected from planetesimals of earthy material, the comets were formed as snow balls in the vicinity of Uranus and Neptune, with these two planets themselves representing accumulations of comets. The evidence for the existence of a comet belt beyond Neptune is considered, and it is concluded that at 50 AU from the sun the mass of such a belt cannot be more than that of the Earth. Finally, some of the problems associated with the mission to periodic Comet d'Arrest are discussed, in particular those concerned with the detectability of the nucleus and the hazards of a close encounter.

Whipple, F. L.↗
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.↗
NASA NTRS1972

Further Comments on Photochemistry

The formation processes for C3 or C2 in comets are discussed along with the possibility of trapping the radical inside the clathrates.

Jackson, W. M.↗
NASA NTRS1973

Liquid carbon dioxide and the Martian polar laminae

One of the most important findings of the Mariner 9 mission was the existence of a set of apparent stacked layers in both the north and south polar regions of Mars. The total thickness of the laminated terrain is approximately several kilometers. Because the Martian atmosphere is composed largely of CO2 and because the frost point of CO2 is known to be reached in both polar regions, it has been assumed that the volatiles in the bright laminae are in appreciable part condensed CO2, although condensed H2O, and a CO2-H2O clathrate are also possible. As long as the ratio of CO2 to H2O is large, the physical properties will be dominated by the CO2.

Sagan, C.↗
NASA NTRS1973

Solubilities of noble gases in magnetite - Implications for planetary gases in meteorites.

Solubilities of noble gases in magnetite were determined by growing magnetite in a noble-gas atmosphere between 450 and 700 K. Henry's law is obeyed at pressures up to .01 atm for He, Ne, Ar and up to .00001 atm for Kr, Xe, with the following distribution coefficients at 500 K: He 0.042, Ne 0.016, Ar 3.6, Kr 1.3, Xe 0.88, some 100 to 100,000 times higher than previous determinations on silicate and fluoride melts. Apparent heats of solution are in sharp contrast with earlier determinations on melts which were small and positive, but are comparable to the values for clathrates. Presumably the gases are held in anion vacancies.

Lancet, M. S.↗
NASA NTRS1974

Mars - The case against permanent CO2 frost caps

Leighton and Murray (1966) have argued that there is a polar reservoir of solid CO2 on Mars that lasts throughout the year and whose vapor pressure determines the mean partial pressure of CO2 in the atmosphere. This model is discussed in the light of recent data, and several difficulties emerge. First, such a system might be unstable, owing to the tendency of poleward heat transport to increase with atmospheric pressure. Second, the annual retreat of the CO2 frost cover would be slower according to the model than that observed. Moreover, the observations seem to indicate that the residual polar cap that lasts throughout the year is composed of water ice rather than CO2. Finally, observations of water vapor in the atmosphere appear to be inconsistent with a permanent CO2 cold trap in continuous existence for many years. These difficulties hold also for a CO2 reservoir buried by water ice and for a hydrated CO2 clathrate.

Ingersoll, A. P.↗
NASA NTRS1976

Ice in space: An experimental and theoretical investigation

The thermodynamics of water ice formation was experimentally investigated under a wide variety of conditions, including those of outer space. This information, and in particular, the lifetime of ice particles as a function of solar distance is an absolute requirement for proper interpretation of photometric profiles of comets. The sublimation of ice particles in a nonequilibrium situation was studied. An oscillating fiber microbalance was used to measure the sublimation rate of water droplets (which were suspended on a long quartz fiber which was oscillating in a vacuum chamber). The influence of particle size, surface temperature, and the index refraction from simulated solar radiation were studied in relation to ice formation. Also examined was the influence of impurities (clathrates) on ice formation. Windows in the vacuum chamber allowed the ice particles to be exposed to a 1 kilowatt xenon arc lamp which was used to simulate solar radiation. Ice is proposed as a possible energy source for comets, as amorphous water ice and ammonia in low temperature and pressure environments demonstrated a clear energy release upon warming. Motion pictures of ice formation were taken and photographs are shown.

Patashnick, H.↗
NASA NTRS1976

Physical processes in Comet Kohoutek

Observational results are brought together to establish the mass loss of Comet Kohoutek (1973 XII) in H2O, solid particles, and other molecules. The production rate of molecules is seen to have flattened prior to peaking near perihelion passage, and then fell below the rate for the incoming branch of the orbit. The total mass loss was about 10 to the 14th power gm. All abundances are consistent with a clathrate-dominant form of water. Comparison of the sizes of the nucleus as calculated from photometry and the rate of water loss requires that either an unrealistically high albedo be assumed or that only a portion of the nucleus was covered by water, in contrast with previous expectations for a new and probable first-passage comet. Moreover, if the preperihelion estimates of the nuclear brightness are rejected as being contaminated, the resulting values indicate an ice-covered nucleus with a radius of 2.1 km.

Odell, C. R.↗
NASA NTRS1977

Saturn's rings - A new survey

Observations of radar returns from Saturn's rings, together with radio interferometry of their absorption of radiation from the disk, combine to require an effective radius of ring particles of about 6 cm or larger. It is suggested that the ring particles may also include, in addition to the known ice constituent, a mixture of the clathrated hydrate of methane and ammonia hydrate. A two-density model for ring particles is possible in which a matrix of low density contains many nodules of higher-density ice particles; in this case, radii nearly as large as the observed ring thickness would be possible. Improved resolution in radio observations at 21 cm or, if necessary, at longer wavelengths for narrow ring openings is perhaps the most useful method for determining upper limits on the particle size.

Cook, A. F.↗
NASA NTRS1978

On the monochromatic brightness variations of comets

The nonsteady development of the multispecies atmosphere of a long-period comet approaching the sun is modeled, and the associated monochromatic brightness variations are discussed in terms of the varying abundances of the atmospheric species. A simple homogeneous clathrate structure, 85% of which is H2O, provides the representation for the cometary nucleus. Monochromatic brightness variations of the emission bands due to OH, CN and C2 are found to follow the law proposed by Levin (1943) for small heliocentric distance where a quasi-steady approximation is valid. However, the large time-scales of residence for emitting species leads to a breakdown of Levin's law at large heliocentric distances.

Mendis, D. A.↗
NASA NTRS1979

Seasonal recession of Mars' south polar cap as seen by Viking

The spring-summer retreat of the south polar cap of Mars is portrayed in photomosaics obtained by Viking Orbiter 2 during 1977. Comparisons of these data to Mariner 9 photos and to the record of telescopic observations attest that the polar retreat viewed by Viking was significantly slower than those previously reported. A global dust storm which occurred at an unusually early season may have effected this retarded recession by introducing dust into the atmosphere of Mars which modified the polar energy balance through scattering of incident radiation. The composition of the south residual cap cannot be unambiguously determined at this time; however, some data suggest that CO2 or clathrate survived the entire summer viewed by Viking.

James, P. B.↗