Engineering topics

Watson, C. C.

Publications and source records attributed to Watson, C. C..

8 records

NASA NTRS1982

The sputter-generation of planetary coronae - Galilean satellites of Jupiter

The structure and mechanisms of the sputter-induced coronae surrounding the Galilean satellites of Jupiter are considered. Calculations are presented of the particle flux distributions expected in sputtering by collisional cascade and thermal spike mechanisms, and the coronae produced by the two mechanisms are examined. Consideration is then given to the coronal structures generated by sputtering of the surface of Io by corotating sulphur and oxygen plasma, and the sputtering of ice on the surfaces of the outer Galilean satellites as represented by Europa.

Watson, C. C.↗

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.↗