Solar Wind Sputtering Research Articles

The -SiH vibrational stretch as an indicator of the oxidation state of silicon on a cometary or asteroidal regolith

Spectral features due to -SiH functional groups are observed to be extremely sensitive to the oxidation state of the “silicate” in which it is bound, ranging from 4.4 μm in highly oxidized grains to 4.74 μm in silicon carbide. Solar wind ions have been shown to chemically reduce exposed surfaces on the moon and it seems reasonable that such processes should also act on asteroidal surfaces, albeit at a reduced rate. If H-atoms in the solar wind are implanted in asteroidal surfaces undergoing solar wind sputtering, then the strength and position of the -SiH fundamental stretch could be used as an indicator of the relative age of the regolith. Observations to test this hypothesis could prove to be very exciting.

The atomic sodium exosphere/coma of the Moon

A Monte Carlo model is used to study the random walk process of sodium atoms created by meteoroid impacts on the lunar surface. The collisional surface interaction is parameterized by a thermal accomodation coefficient of 0.5. Sodium atoms launched with initial velocities well below the surface escape velocity of 2.38 km s−1 will be recycled and contribute to the formation of an exosphere with a temperature of about 500 K on the sunlit side. For large launch velocities (>2 km s−1) there will be systematic transport of the escaping sodium atoms to the antisolar direction as a result of the acceleration effect of the solar radiation pressure. An extended coma with a significant elongation in the antisolar direction can form this way. Recent spectroscopic and imaging observations are consistent with this picture. The present model calculations suggest that meteoroid impact effect (and solar wind sputtering) should be the major supplier of the lunar sodium exosphere; the possible existence of an efficient loss mechanism of the sodium atoms during surface interaction is also indicated.

Ion sputtering of minerals and glasses: a first step to the simulation of solar wind erosion

Abstract Selected rockforming minerals (plagioclase, augite, olivine, ilmenite, silicate and metal phases of the meteorite “Brenham”) as well as silicate and phosphate glasses were irradiated with heavy ions (4He+, 14N+, 20Ne+, 40Ar+, 56Fe+, Xe+ nat) in the energy range of 50-130 keV in order to study ion-induced sputtering. Sputtering yields were measured independently by means of multiple beam interferometry and particle track autoradiography. The theory of sputtering by Sigmund, modified by Smith, was used to convert experimental heavy ion sputtering yields to H+- and He+-sputtering yields of the same target. Taking into account solar wind irradiation conditions at the lunar surface, an estimate of lunar erosion rates due to solar wind sputtering is given for the targets studied.

Progress in defining the solar wind sputter rate on protoplanets and interplanetary matter

Solar wind sputtering of exposed surfaces in heliocentric orbits is considered for bodies ranging in size from planets down to micrometeoroids. Erosion rates derived from APOLLO sample lunar regolith studies are presented together with the results of computer modelling of surface topographical alteration in order to characterize the sputter erosion process on Solar System bodies bombarded by the solar wind. The consensus of recent measurements leads to a rate of between 0.025 to 0.045 Å yr −1 near the lunar equator.

The fractionation of Ca isotopes by sputtering

Using a double-spike, mass spectrometric technique, we have measured the absolute isotopic composition of Ca sputtered from several Ca minerals by normally incident, low energy, nitrogen beams (130 keV N^+ and 100 keV N^(+)_(2)). The precision in the ^(40)Ca/^(44)Ca ratio is 0.05% for samples as small as 10^(−7) g, corresponding to a resolution of possible mass dependent isotope fractionation effects of ∼0.01% per unit mass difference. In general, the material sputtered first was isotopically light by up to 2% in the isotopic ratio ^(40)Ca/^(44)Ca when compared with the measured isotopic composition of the target; as the bombardment progressed, the isotopic composition became less light and approached the initial composition of the target. We have demonstrated that, after prolonged bombardment, a surface layer develops on the target with a distinctly heavy isotopic composition; the thickness of the layer is a significant fraction of the range of the bombarding ions. Angular distributions of the sputtered material show that, throughout the bombardment, the material ejected back along the beam direction was isotopically light relative to the material ejected at oblique angles; the difference in the isotopic ratio ^(40)Ca/^(44)Ca over the angular range was ∼ 1.5%. This fractionation with angle of ejection persisted even when a quasi-steady state was reached after heavy bombardment, when the isotopic composition of the material averaged over angle had become essentially indistinguishable from that of the pristine target. These results indicate that sputtering produces isotope fractionation and that there is a marked angular dependence in the effects. This suggests that solar wind sputtering of the lunar surface may contribute to the isotopic and elemental enrichments observed for the surface layers of soil grains.

A surface related component of iron metal in lunar samples?

SOLAR wind elements implanted into lunar soil grains come to rest within a few hundred angstroms, consequently high concentrations should build up in near surface locations. It has become common practice to ‘identify’ surface correlated species by investigating sieved soil fractions; the increased concentrations in fine grain sizes being attributed to the greater surface area. The approach used1–4, infers a surface location for finely divided iron metal (Fe(0)) and associated hydrolysable carbon (Chyd) in lunar samples. These studies, together with other evidence5, imply solar wind induced reduction processes were involved in producing metal from ferrous iron (Fe(II)) in silicates. The experiments described here performed with a reagent (CuCl2/2KCl), which should only attack exposed metal, have failed to recognise a genuine exposed surface component. We believe that some effect associated with solar wind sputtering is responsible for iron formation and we must therefore question the validity of the model on which the initial conclusions in favour of surface located metal were based.

Reply to “Solar wind effects on the surface chemistry of lunar grains” by E. Bilson

Bilson (1978) has criticized our interpretation of ion probe measurements on lunar plagioclase grains by contending that the sputter action of solar wind H and He can account for observed surface enhancements of Mg and Fe. We think that an inappropriate analogy between lunar bulk soils and plagioclase grains led Bilson to this conclusion. While it is true that ion implantation is unlikely to have a dominant effect on the major element composition of the surface of lunar grains of average bulk soil composition, we show that chemical alteration by solar wind sputtering cannot explain our ion probe result on plagioclase grains. Glass splashes on shielded grains and solar wind implanted ions on some selected exposed crystals are the most likely source of Mg and Fe enhancements.

Is the Moon really as smooth as a billiard ball? Remarks concerning recent models of sputter‐fractionation on the lunar surface

Two recent discussions of chemical and isotopic fractionation by solar wind sputtering (Pillinger et al., 1976; Switkowski et al., 1977) are not applicable to the moon because the authors have failed to include major effects caused by the real geometry of the lunar surface. The complex reentrant micromorphology of the lunar soil drastically affects sputter‐fractionation because most of the sputtered species do not leave the moon, but strike surfaces which are partially shielded from further sputtering. At these sites fractionation is effectively controlled by desorption and angular sputtering effects rather than by sputtering yields. The mass ratio of matter which has been sputter‐deposited to that in the sputter‐altered transition layers on individual grains exceeds 100∶1.

Solar-wind sputtering of the martian atmosphere

IN the sputtering process an incident particle beam loses part of its energy to recoil motion of target atoms, some of which may escape through a nearby surface. The sputtering yield, S, is defined as the number of atoms ejected per incident particle. In the Solar System, sputtering will occur whenever the solar wind, consisting mainly of 1 keV AMU−1 hydrogen and helium ions, strikes a material body. Many years ago, Wehner et al.1 suggested that solar wind-induced sputtering of the lunar surface should be an important cause of erosion; recently, analyses2 of returned lunar material have been interpreted quantitatively3 in terms of such solar-wind sputtering. Mars provides another example of the interaction of the solar wind with a planetary body. However, in contrast to the lunar surface, the martian surface is largely protected from direct solar wind bombardment by its atmosphere. The primarily CO2 atmosphere is thin by terrestrial standards but still opaque to the solar wind. We discuss here whether solar-wind sputtering of the martian atmosphere is a mechanism leading to significant mass loss.

Mass fractionation of the lunar surface by solar wind sputtering

The sputtering of the lunar surface by the solar wind is examined as a possible mechanism of mass fractionation. Simple arguments based on current theories of sputtering and the ballistics of the sputtered atoms suggest that most ejected atoms will have sufficiently high energy to escape lunar gravity. However, the fraction of atoms which falls back to the surface is enriched in the heavier atomic components in relation to the lighter ones. This material is incorporated into the heavily radiation-damaged outer surfaces of grains, where it is subject to resputtering. Calculations predict that an equilibrium surface layer, enriched in heavier atoms, will form with δ(18O) ≈ +20‰ ≈ δ(30Si) and that oxygen will be depleted on the surface layers of grains relative to the bulk composition by about 12.5%. These results are in fair agreement with experiment. The dependence of the calculated results upon the energy spectrum of sputtered particles is investigated. We conclude that mass fractionation by solar wind sputtering is likely to be an important phenomenon on the lunar surface but that the complex isotopic variations observed in lunar soils cannot be completely explained by this mechanism.

Solar wind and micrometeorite effects in the lunar regolith

Using available data from the literature, we formulate an outline of the major physical and chemical effects expected during solar wind bombardment of the lunar regolith. In agreement with the results of Auger and e.s.c.a. analyses of the composition of lunar grain surfaces, this outline predicts that solar wind sputtering will tend to clean exposed grain surfaces by ejecting material at velocities exceeding lunar escape velocity. We also discuss results showing that Fe is partially reduced in the outer few 10 nm of grain surfaces and that this reduced Fe forms 10 nm diameter range metal spheres throughout the glass during agglutinate formation by micrometeorite impacts. These metal spheres give the agglutinates their distinctive optical and magnetic properties and are partially responsible for the decreasing albedo of the lunar surface with exposure age.

The relationship of surface chemistry and albedo of lunar soil samples

A relation between the albedo and the surface iron concentration (determined by Auger electron spectroscopy) of lunar soil samples is described. The effect of solar wind sputtering on the surface chemistry and albedo of the soil is discussed.

Electron Microscopy of Irradiation Effects in Space

The paper discusses several discoveries made in the study of lunar material. In the examination of the effects of solar wind implantations the topics covered include (1) solar wind radiation damage parameters and their aging characteristics, (2) the theory of the ancient solar wind, (3) the solar wind sputtering erosion rate, (4) the physicochemical properties of amorphous coatings, (5) maturity indexes and the macroscopic properties of the lunar regolith, (6) solar wind gas bubbles, (7) the composition of very heavy nuclei in the contemporary solar wind, and (8) track aging processes. Conclusions are drawn from the results about other extraterrestrial features such as the parent bodies of the meteorites, early solar nebulas, and interstellar clouds.

Effects of vapor-phase deposition processes on the optical, chemical, and magnetic properties OE the lunar regolith

The processes of solar wind sputtering and meteoritic impact vaporization have created materials in the lunar regolith which were deposited from a vapor phase. Although the quantity of such exotic condensed substances should theoretically be comparable with that of materials which have been melted by impacts, their existence in the fines has not been generally recognized. We have investigated the physical and chemical properties of materials deposited from vapors generated by hydrogen-ion sputtering and thermal evaporation of lunar and artificial ferrosilicates. Both processes are highly reducing. The deposits are enriched in Fe, have large, nonselective, optical absorptivities, and contain abundant sub-microscopic, superparamagnetic grains of metallic Fe which exhibit the characteristicg=2.1 ESR resonance. The sputter-deposited films are enriched in heavy elements. Thus the hypothesis that the lunar fines contain several percent of materials deposited from the vapor phase accounts in a natural manner for many of the unusual optical, physical and chemical properties of lunar soils. The vapor-deposits are probably concentrated in the agglutinate particles of the regolith.

Some applications of transmission electron microscopy to space physics

We have used the techniques of 1 MeV and 100 keV electron microscopy to compare various radiation-damage features in lunar dust grains and in mineral grains that have been exposed to artificial fluxes of low-energy nuclear particles; in particular, amorphous rims to the grains and etched ion tracks were examined. The results of these investigations clearly show the effects of solar wind sputtering and reveal the existence of high fluxes of low-energy particles in interplanetary space.

Darkening of silicate rock powders by solar wind sputtering

Hydrogen ion irradiation of powdered igneous rocks, including Apollo rocks, has been observed in the laboratory to darken the powders and to make their optical properties similar to the Moon's. An extensive series of investigations shows that this darkening is not spurious. These results are consistent with those of other investigators, including Nash (1967). Darkening of lunar igneous rock powders by the formation of solar wind-sputtered glass films is a real process which occurs on the Moon. The time scale for darkening of undisturbed lunar soil is of the order of 50000–100000 yr. Comparison of the rates of the formation of glasses on the lunar surface by solar wind sputter-deposition, meteorite impact melting and impact vaporization-deposition indicates that these processes are of comparable importance under the present flux of meteorites. Thus the formation of glass by sputter-deposition must be regarded as a major process on the lunar surface.

Modification of the lunar surface by the solar-wind bombardment

Long-duration, solar-wind sputtering conditions were simulated at a much accelerated scale with mass-separated hydrogen ion beams or in low-pressure, noble gas or hydrogen plasmas. Experiments with metal targets and metal-, oxide-, and rock-powder samples demonstrate the leveling and smoothing of macroscopic surface features, and cementing together of loose particles into a porous, brittle, fibrous crust. Certain oxide surfaces become enriched with metal atoms under the bombardment and sputtering action. It is concluded that many of the unusual properties of the lunar surface can be explained by the action of solar-wind bombardment.