Engineering topics

Haff, P. K.

Publications and source records attributed to Haff, P. K..

At least 19 records

NASA NTRS1987

Grain dynamics in zero gravity

The dynamics of granular materials has proved difficult to model, primarily because of the complications arising from inelastic losses, friction, packing, and the effect of many grains being in contact simultaneously. One interesting limit for which it was recently possible to construct a theory is that where the grain-grain interactions are dominated by binary collisions. The kinetic model of granular systems if similar to the kinetic theory of gases, except that collisional energy losses are always present in the former and must be treated explicitly. Few granular materials on Earth are describable by this limiting model, since gravity tends to collapse the grains into a high-density state where Coulombic friction effects are dominant. The planned Space Station offers an unusual opportunity to test the kinetic grain model and to explore its predictions. Without gravity, the regime of low interparticle velocities, where an elastic description of the collision is still valid, is investigated. This will allow direct interpretation by dynamical computer simulations as well as by kinetic theory.

Werner, B. T.↗

NASA NTRS1986

Grain dynamics in zero gravity

The dynamics of granular materials has proved difficult to model, primarily because of the complications arising from inelastic losses, friction, packing, and the effect of many grains being in contact simultaneously. The kinetic model of granular systems is similar to the kinetic theory of gases, except that collisional energy losses are always present in the former and must be treated explicity. Few granular materials on Earth are describable by this limiting model, since gravity tends to collapse the grains into a high density state where Coulombic friction effects are dominant. The planned Space Station offers an unusual opportunity to test the kinetic grain model and to explore its predictions. Without gravity, the regime of low interparticle velocities (where an elastic description of the collision is still valid) can be investigated. This will allow for direct interpretation by dynamical computer simulations as well as by the kinetic theory. The dynamics of spherical grains inside a clear box would be examined. Results would be compared with the predictions of the kinetic theory and computer simulations.

Werner, B. T.↗

NASA NTRS1986

Micrometeoroid impact on planetary satellites as a magnetospheric mass source

Proceeding from the observation that planetary satellites are important sources of mass for planetary magnetospheres, it is noted that meteoroid impact vaporization may compete with charged particle sputtering as a supply mechanism. After considering meteoroid-driven vapor sources in the Jovian and Kronian systems, it is concluded that while the larger impact flux values obtained for the outer solar system suggest a role for impact vaporization in the cases of both Jupiter and Saturn, this process will not predominate over sputtering; at the lower end of the impact flux range, however, sputtering everywhere dominates magnetospheric mass loading.

Haff, P. K.↗

NASA NTRS1986

Interactions of planetary magnetospheres with icy satellite surfaces

When natural satellites and ring particles are embedded within magnetospheric plasmas, the charged particles interact with the surfaces of these solid bodies. These interactions have important implications for the surface, the atmosphere of the parent body, and the magnetosphere as a whole. Significant erosion of the surface by sputtering, as well as redeposition of sputter ejecta, can occur over geologic time. The surface can also be chemically modified. Sputter ejecta can make important contributions to the atmosphere; sputtering provides a lower limit to the atmospheric column density even for arbitrarily cold satellite surfaces. Sputter ejecta escaping from the parent body can form extensive neutral clouds within the magnetosphere. Ionization and dissociation within these neutral clouds can be dominant sources of low-energy plasma. The importance of these processes is discussed for the satellites and magnetospheres of Jupiter, Saturn and Uranus.

Cheng, A. F.↗

NASA NTRS1985

Plasma Production by Meteoroid Impact

Material ejected from the surfaces of satellites in the outer solar system plays an important role in the magnetospheres of the outer planets, and may dominate the mass loading, as in the vicinity of the Jovian satellite Io. At least four potential ejection mechanisms can be identified - intrinsic geologic activity, thermal sublimation, sputtering, and micrometeoroid impact vaporization. On all the icy satellites, except possibly Enceladus, sputtering and impact vaporization are the only two potentially important sources of magnetospheric plasma. Sputtering was shown to be an important mass source at both Jupiter and Saturn. The impact mechanism as a plasma source is assessed.

Haff, P. K.↗

NASA NTRS1983

Ring and plasma - The enigmae of Enceladus

The E ring associated with the Kronian moon Enceladus has a lifetime of only a few thousand years against sputtering by slow corotating O ions. The existence of the ring implies the necessity for a continuous supply of matter. Possible particle source mechanisms on Enceladus include meteoroidal impact ejection and geysering. Estimates of ejection rates of particulate debris following small meteoroid impact are on the order of 3 x 10 to the -18th g/(sq cm sec), more than an order of magnitude too small to sustain the ring. A geyser source would need to generate a droplet supply at a rate of approximately 10 to the -16th g/(sq cm sec) in order to account for a stable ring. Enceladus and the ring particles also directly supply both plasma and vapor to space via sputtering. The absence of a 60 eV plasma at the Voyager 2 Enceladus L-shell crossing, such as might have been expected from sputtering, cannot be explained by absorption and moderation of plasma ions by ring particles, because the ring is too diffuse. Evidently, the effective sputtering yield in the vicinity of Enceladus is on the order of, or smaller than, 0.4, about an order of magnitude less than te calculated value. Small scale surface roughness may account for some of this discrepancy.

Haff, P. K.↗

NASA NTRS1983

A two-stage mechanism for escape of Na and K from Io

A two-stage process is presented to identify Io as the source of Na and K ions in the Io plasma torus. The Voyager I IRIS experiment recorded an SO2 abundance of 0.2 cm atm in the Io subsolar region, and further calculations determined that the S(+) and O(+) would have sufficient energy to penetrate the Io atmosphere and produce sputtering of surface atoms at a rate of 800 million/sq cm per sec. If K, Na, and S are present on the surface in cosmic proportions, then an evenly applied sputter distribution would produce the same abundances of sputtered Na and K as observed in the Na and K clouds around Io. Sputtered into the atmosphere, the ions undergo thermal Jean's escape and attain velocities of at least 2.6 km/sec. The sputtering ions are a factor of two greater on the Jupiter side than on the solar side, in agreement with asymmetries measured by the Voyager.

Summers, M. E.↗

NASA NTRS1981

Possible isotopic fractionation effects in material sputtered from minerals

A model is presented which predicts isotope fractionation in material sputtered from mineral surfaces. According to the model, the fractionation is a function of the fractional abundances of each isotope, the low-energy collision cross sections and the atomic masses, and the fractionation pattern can be nonlinear. Calculations are illustrated for all sets of isotopes in the minerals perovskite, anorthite, akermanite, enstatite and troilite, and it is found that while O is always positively fractionated, with heavier isotopes sputtered preferentially, heavier elements are generally negatively fractionated. It is noted that the model may be tested by experiments on a system such as CaF2, CaI2, in which the Ca sputtered from CaF2 is predicted to be strongly fractionated in a negative sense and that sputtered from CaI2 is predicted to be strongly fractionated in a positive sense.

Haff, P. K.↗

NASA NTRS1981

Sputter ejection of matter from Io

Direct collisional interaction of magnetospheric particles, particularly 520-eV S ions, with Io, cause sputter removal of matter. It is estimated that direct sputtering of a full-disk S-containing atmosphere with an exobase at a few hundred km, can provide up to 5 x 10 to the tenth S atoms per sq cm-s. Supplies of S and O required to stabilize the torus are estimated to be from 10 to the 10th to 10 to the 12th per sq cm-s. Sputtering rates are calculated for an atmosphere containing a one percent concentration of Na and K, and are found to be large enough to supply the fluxes required to maintain the Na and K clouds. Sputtering is found to remove heavy molecules from the atmosphere, and the rate of direct sputtering of unprotected surfaces is calculated for ejections of S and Na. Atomic species on the surface are ejected at a rate proportional to the surface abundance; and plume sputtering, avalanche cascading, and ionic saltation which lead to spatial and temporal variations in the number of ejected particles are observed.

Haff, P. K.↗

NASA NTRS1980

Possible isotopic fractionation effects in sputtered minerals

A model which makes definite predictions for the fractionation of isotopes in sputtered material is discussed. The fractionation patterns are nonlinear, and the pattern for a particular set of isotopes depends on the chemical matrix within which those isotopes are contained. Calculations are presented for all nonmonoisotopic elements contained in the minerals perovskite, anorthite, ackermanite, enstatite, and troilite. All isotopes are fractionated at the level of approximately 4-6 deg/o per atomic mass unit. Oxygen is always positively fractionated (heavier isotopes sputtered preferentially), and heavier elements are generally negatively fractioned (light isotopes sputtered preferentially). The value of Delta (O-18:O-16) is always less by about 1.8 deg/o than a linear extrapolation based upon the calculated delta (O-17:O-16) value would suggest. The phenomenon of both negative and positive fractionation patterns from a single target mineral are used to make an experimental test of the proposed model.

Haff, P. K.↗

NASA NTRS1980

Solar wind sputtering effects in the atmospheres of Mars and Venus

It is found through an investigation, combining Monte Carlo simulations and analytical techniques, of the direct collisional interaction of an energetic particle flux with the neutral components of a planetary atmosphere, that solar wind sputtering could provide an important exospheric mass sink on both Mars and Venus under appropriate conditions. The computed rates of helium loss in the Venusian atmosphere and of carbon, nitrogen and oxygen in the Martian atmosphere imply that sputtering would have a significant effect on the noble gas budget of Venus and dominate the chemical and photochemical loss processes of Mars. Because of diffusive separation of lighter elements and isotopes, and because the gravitational binding energy is proportional to the mass, the erosion process preferentially removes the lighter components of the atmosphere. Solar wind sputtering could therefore compete with other erosion mechanisms in generating substantial fractionation effects.

Watson, C. C.↗

NASA NTRS1979

The erosion of planetary and satellite atmospheres by energetic atomic particles

Analytic expressions are presented which may be used to compute the sputter-erosion yield from any unimolecular gravitationally bound gas by any atomic charged particle of any energy. A calculation of solar wind proton and alpha particle induced erosion of the CO2 atmosphere of Mars predicts molecular sputtering yields. An expression for the emission yield of energetic molecular fragments produced in primary knock-on events is given in closed form; such fragment emission is of secondary importance for mass loss compared to the molecular yield itself. Erosion by radiation belt protons of a hypothetical thin O2 atmosphere associated with the Jovian satellite Ganymede is considered, and molecular sputtering yields for proton energies are determined.

Haff, P. K.↗

NASA NTRS1979

Solar wind sputtering effects in the Martian atmosphere

A Monte Carlo simulation of the sputtering of the upper atmosphere of Mars by the solar wind was performed. The calculated sputtering yields imply loss rates (molecules/cm square - sec escaping the planet) for carbon dioxide, carbon, and oxygen of R(CO2) = 2.6 X 1000000/cm square - sec, R(C) = 6.6 X 1000000/cm square - sec, and R(O) = 7.7 X 1000000/cm - sec. The total mass loss by sputtering is only about 10% of that due to chemical and photo-chemical processes, but sputtering provides a major exospheric sink for carbon. The erosion process described here preferentially removes the lighter components of the atmosphere. Calculations based on a Monte Carlo simulation suggest that for a model atmosphere, 97% of the N2 and 33% of the CO2 originally present may have been sputtered away over 4.5 X 10 to the 9th power y. In the same length of time the (15)N/(14)N isotopic ratio for the bulk atmosphere would have increased by a factor 1.7.

Watson, C. C.↗

NASA NTRS1979

Ion erosion on the Galilean satellites of Jupiter

Several different models of ion-induced erosion of both the rocky and icy Galilean satellites are considered. The conclusion is that erasure of surface relief via this mechanism is likely to occur at a rate not exceeding about 100 m per billion years, and therefore is not of global topographic significance. Another effect of ion bombardment is the possible growth of a thin mineral armor on the surfaces of the icy satellites. A volume concentration of 0.1% of 1 micron sized mineral grains can lead to 50% armoring of the surface on time scales ranging from thousands to millions of years.

Haff, P. K.↗

NASA NTRS1978

Solar-wind sputtering of the Martian atmosphere

The role of solar-wind sputtering in causing atmospheric mass loss from Mars is discussed. The atmospheric mass loss is studied by analogy to models of sputtering of solid surfaces and by use of empirical data for the lunar surface. Results show that Martian atmospheric mass loss, when integrated over the course of 4 times 10 to the ninth power years, amounts to 3.1 times 10 to the 41st power CO2 molecules. Preferential loss of light elements and isotopes by sputtering is also considered.

Haff, P. K.↗

NASA NTRS1977

Mass fractionation of the lunar surface by solar wind sputtering

An investigation is conducted concerning the mass-fractionation effects produced in connection with the bombardment of the moon by the solar wind. Most of the material ejected by sputtering escapes the moon's gravity, but some returning matter settles back onto the lunar surface. This material, which is somewhat richer in heavier atoms than the starting surface, is incorporated into the heavily radiation-damaged outer surfaces of grains. The investigation indicates that sputtering of the lunar surface by the solar wind will give rise to significant surface heavy atom enrichments if the grain surfaces are allowed to come into sputtering equilibrium.

Switkowski, Z. E.↗

NASA NTRS1977

Solar wind sputtering of the Martian atmosphere

The interaction between the solar wind and a planetary atmosphere is evaluated as a cause of atmospheric mass loss. For the case of Mars, calculations suggest that an amount of material has been sputtered which is of the same order as the mass of the present atmosphere.

Haff, P. K.↗

NASA NTRS1977

Ion-beam-induced atomic mixing

Calculations based on the diffusion model are presented of atomic mixing by ion bombardment. This mixing is assumed to have its basis, as does sputtering, in the collision cascades generated by the primary beam. Sharp interfaces within a target are seen to be smoothed by ion bombardment. Mixing may place fundamental limits on the resolution of ion microprobes.

Haff, P. K.↗