The influence of space environment on the evolution of Mercury

It can be assumed that the composition of Mercury's thin gas envelope (exosphere) is related to the composition of the planets crustal materials. If this relationship is true, then inferences regarding the bulk chemistry of the planet might be made from a thorough exospheric study. The most vexing of all unsolved problems is the uncertainty in the source of each component. Historically, it has been believed that H and He come primarily from the solar wind, while Na and K originate from volatilized materials partitioned between Mercury's crust and meteoritic impactors. The processes that eject atoms and molecules into the exosphere of Mercury are generally considered to be thermal vaporization, photonstimulated desorption (PSD), impact vaporization, and ion sputtering. Each of these processes has its own temporal and spatial dependence. The exosphere is strongly influenced by Mercury's highly elliptical orbit and rapid orbital speed. As a consequence the surface undergoes large fluctuations in temperature and experiences differences of insolation with longitude. We will discuss these processes but focus more on the expected surface composition and solar wind particle sputtering which releases material like Ca and other elements from the surface minerals and discuss the relevance of composition modelling.

Space Science Reviews, 2014

Mercury's regolith, derived from the crustal bedrock, has been altered by a set of space weathering processes. Before we can interpret crustal composition, it is necessary to understand the nature of these surface alterations. The processes that space weather the surface are the same as those that form Mercury's exosphere (micrometeoroid flux and solar

Icarus, 2003

Mercury's close orbit around the Sun, its weak intrinsic magnetic field and the absence of an atmosphere (P surface < 1 × 10 −8 Pa) results in a strong direct exposure of the surface to energetic ions, electrons and UV radiation. Thermal processes and particle-surfacecollisions dominate the surface interaction processes leading to surface chemistry and physics, including the formation of an exosphere (N 10 14 cm −2 ) in which gravity is the dominant force affecting the trajectories of exospheric atoms. NASA's Mariner 10 spacecraft observed the existence of H, He, and O in Mercury's exosphere. In addition, the volatile components Na, K, and Ca have been observed by ground based instrumentation in the exosphere. We study the efficiency of several particle surface release processes by calculating stopping cross-sections, sputter yields and exospheric source rates. Our study indicates surface sputter yields for Na between values of about 0.27 and 0.35 in an energy range from 500 eV up to 2 keV if Na + ions are the sputter agents, and about 0.037 and 0.082 at an energy range between 500 eV up to 2 keV when H + are the sputter agents and a surface binding energy of about 2 eV to 2.65 eV. The sputter yields for Ca are about 0.032 to 0.06 and for K atoms between 0.054 to 0.1 in the same energy range. We found a sputter yield for O atoms between 0.025 and 0.04 for a particle energy range between 500 eV up to 2 keV protons. By taking the average solar wind proton surface flux at the open magnetic field line area of about 4 × 10 8 cm −2 s −1 calculated by Massetti et al. (2003, Icarus, in press) the resulting average sputtering flux for O is about 0.8-1.0 × 10 7 cm −2 s −1 and for Na approximately 1.3-1.6 × 10 5 cm −2 s −1 depending on the assumed Na binding energies, regolith content, sputtering agents and solar activity. By using lunar regolith values for K we obtain a sputtering flux of about 1.0-1.4 × 10 4 cm −2 s −1 . By taking an average open magnetic field line area of about 2.8 × 10 16 cm 2 modelled by Massetti et al. (2003, Icarus, in press) we derive an average surface sputter rate for Na of about 4.2 × 10 21 s −1 and for O of about 2.5 × 10 23 s −1 . The particle sputter rate for K atoms is about 3.0 × 10 20 s −1 assuming lunar regolith composition for K. The sputter rates depend on the particle content in the regolith and the open magnetic field line area on Mercury's surface. Further, the surface layer could be depleted in alkali. A UV model has been developed to yield the surface UV irradiance at any time and latitude over a Mercury year. Seasonal and diurnal variations are calculated, and Photon Stimulated Desorption (PSD) fluxes along Mercury's orbit are evaluated. A solar UV hotspot is created towards perihelion, with significant average PSD particle release rates and Na fluxes of about 3.0 × 10 6 cm −2 s −1 . The average source rates for Na particles released by PSD are about 1 × 10 24 s −1 . By using the laboratory obtained data of Madey et al. (1998, J. Geophys. Res. 103, 5873-5887) for the calculation of the PSD flux of K atoms we get fluxes in the order of about 10 4 cm −2 s −1 along Mercury's orbit. However, these values may be to high since they are based on idealized smooth surface conditions in the laboratory and do not include the roughness and porosity of Mercury's regolith. Further, the lack of an ionosphere and Mercury's small, temporally and spatially highly variable magnetosphere can result in a large and rapid increase of exospheric particles, especially Na in Mercury's exosphere. Our study suggests that the average total source rates for the exosphere from solar particle and radiation induced surface processes during quiet solar conditions may be of the same order as particles produced by micrometeoroid vaporization. We also discuss the capability of in situ measurements of Mercury's highly variable particle environment by the proposed NPA-SERENA instrument package on board ESA's BepiColombo Mercury Planetary Orbiter (MPO).

Science (New York, N.Y.), 2009

Mapping the distribution and extent of major terrain types on a planet&#39;s surface helps to constrain the origin and evolution of its crust. Together, MESSENGER and Mariner 10 observations of Mercury now provide a near-global look at the planet, revealing lateral and vertical heterogeneities in the color and thus composition of Mercury&#39;s crust. Smooth plains cover approximately 40% of the surface, and evidence for the volcanic origin of large expanses of plains suggests that a substantial portion of the crust originated volcanically. A low-reflectance, relatively blue component affects at least 15% of the surface and is concentrated in crater and basin ejecta. Its spectral characteristics and likely origin at depth are consistent with its apparent excavation from a lower crust or upper mantle enriched in iron- and titanium-bearing oxides.

Space Science Reviews, 2007

It has been speculated that the composition of the exosphere is related to the composition of Mercury's crustal materials. If this relationship is true, then inferences regarding the bulk chemistry of the planet might be made from a thorough exospheric study. The most vexing of all unsolved problems is the uncertainty in the source of each component. Historically, it has been believed that H and He come primarily from the solar wind (Goldstein, B.E., et al. in J. Geophys. Res. 86:5485-5499, 1981), Na and K come from volatilized materials partitioned between Mercury's crust and meteoritic impactors (Hunten, D.M., et al. in Mercury, pp. 562-612, 1988; Morgan, T.H., et al. in Icarus 74:156-170, 1988; Killen, R.M., et al. in Icarus 171:1-19, 2004b). The processes that eject atoms and molecules into the exosphere of Mercury are generally considered to be thermal vaporization, photon-stimulated desorption (PSD), impact vaporization, and ion sputtering. Each of these processes has its own temporal and spatial dependence. The exosphere is strongly influenced by Mercury's highly elliptical orbit and rapid orbital speed. As a consequence the surface undergoes large R. Killen ( )

Icarus, 2008

Chemical processes associated with meteoroid bombardment of Mercury are considered. Meteoroid impacts lead to production of metal atoms as well as metal oxides and hydroxides in the planetary exosphere. By using quenching theory, the abundances of the main Na-, K-, CaFe Fe-, Al-, Mg-, Si-, and Ti-containing species delivered to the exosphere during meteoroid impacts were estimated. Based on a correlation between the solar photo rates and the molecular constants of atmospheric diatomic molecules, photolysis lifetimes of metal oxides and SiO are estimated. Meteoroid impacts lead to the formation of hot metal atoms (0.2-0.4 eV) produced directly during impacts and of very hot metal atoms (1-2 eV) produced by the subsequent photolysis of oxides and hydroxides in the exosphere of Mercury. The concentrations of impact-produced atoms of the main elements in the exosphere are estimated relative to the observed concentrations of Ca, assumed to be produced mostly by ion sputtering. Condensation of dust grains can significantly reduce the concentrations of impact-produced atoms in the exosphere. Na, K, and Fe atoms are delivered to the exosphere directly by impacts while Ca, Al, Mg, Si, and Ti atoms are produced by the photolysis of their oxides and hydroxides. The chemistry of volatile elements such as H, S, C, and N during meteoroid bombardment is also considered. Our conclusions about the temperature and the concentrations of impact-produced atoms in the exosphere of Mercury may be checked by the Messenger spacecraft in the near future and by BepiColombo spacecraft some years later.

Journal of Geophysical Research, 1993

We examine the conditions under which ions impacting the Hermean surface can act as a regional source of enhanced atmospheric column through ion implantation and subsequent release at the surface. At most latitudes, energetic (a few keV) Na or K ions which impact the nightside surface, are released quickly (well before noon) upon warming of the surface. We show that the relative sunrise/sunset difference produced by ion implantation is [ [n]SR-[n]ss }/[n]^ve = f, where f is the fraction of the photo-ions recycled, [n]SR is the average zenith colunto above the sunrise portion of the illuminated hemisphere, [hiss is the average zenith column in the sunset portion, and [n]^v e is the average column over the sunlit disk. Thus, to produce a large sunrise/sunset difference via ion implantation and subsequent release requires efficient (close to total) recycling. We show that the most extensive set of available data reduced to Na column abundance does not show any sunrise enhancement. We argue that the K data do not permit an unambiguous interpretation in favor of sunrise/sunset differences. We thrther find that if an efficient surface loss process for the alkali is not operating after sunrise, the initial Na and K distribution will relax into the bulk of the solid. Preserved abundance gradients of Na and K in lunar glasses suggest that the Arrhenius coefficients for impact glasses are likely more modest than those for laboratory glasses derived from rock by nonimpact processes, but the effect of the more modest diffusion rates is only to delay the efficient loss of Na by a few Earth days. We argue that implantation can lead to observable regional increases in the observed Na or K column densities only if it occurs at very high latitudes, where diffusion is slow. It is typically lower energy ions which ixnpact at high latitude and these are both more numerous than the high energy ions and possess smaller average penetration depths; thus there are additional reasons to favor a high-latitude locus for any possible prmnpt return related increases in zenith colmnn. We find that we cannot rule out sputtering as a source process, as the low-energy ions are efficient sputterers. Finally, we argue that the observed Na/K ratio in the atmosphere may be the expression of their different azymptotic rates of loss from the interiors of the regolith grains. If so. this allows us to fix the importance of sputtering and photon stimulated desorption relative to impact vaporization. exponential function of temperature, which varies as cos'a(q>) (where q> is the angle from the subsolar point), these results are more accurate. Using the same initial conditions and boundary conditions, and solving the time-dependent diffusion equation tbr n(t), the density of Na or K in the implant layer as a function of time, we show (1) that the exhaustion of the implanted Na and K is very rapid, and that the resulting enhancement is closely confined to the sunrise terminator even for high latitudes; (2) that the difference in average column between the morning and the afternoon sides of the sunlit hemisphere due to diffusion of implanted Na and K is Jln^v•], where f is the recycling efficiency, and [n^v•] is the average global zenith column; (3) that there is a body of Na data which do not exhibit any sunrise/sunset column differences; (4) that with the diffusion coefficients and boundary condition used by Sprague, shallow (a few microns thick) concentrations of Na could not be Copyright 1993 by the American Geophysical lrnion. Paper number 93JE02617. 0148-0227/93/93 JE-02617505.00 show that differences between the asymptotic flux to the surface of Na and K in the bulk of the regolith may explain certain differences in the average zenith columns of Na and K. TIME DEPENDENT DIFFUSION Sprague [1992] argued that a portion of the Na and K photoions from the sunlit side of the Hermean exosphere would be returned to the darkside of the planet and would be implanted there in a thin (50 • wide), shallow layer at a mean depth of 300 • (Figure 1). It was argued that upon rotation into sunlight, the Na and K would diffuse to the surface and reenter the exosphere. First, it is neccessary to consider the the latitudinal extent of the region in which implantation may occur. There is considerable uncertainty as to the position and extent of the auroral ovals on Mercury. Goldstein et al. [1981] place them a quite low latitudes (20ø-30ø), while Baker et al. [1987] place them between 30 ø and 60 ø latitude. The longitudinal extent of the auroral oval is also uncertain [Gol&tein et al., 1981]. I p [1987] showed that the trajectories of photoions which reimpact the surface preferentially do so at low latitudes. Ions which begin at low latitudes tend to escape; however, it is possible that they will enter the general convection and return in the plasma sheet. Ti•erefore, we will consider implantation at a wide rano, ß of latitudes. 23,589 23,590 KILLEN AND MORGAN: Na AND K iN THE REGOLITH OF MERCURY

Icarus, 2004

The supply rates of Na and K to the atmosphere of Mercury by processes acting on the extreme surface-thermal vaporization, photonstimulated desorption (PSD), and ion-sputtering-are limited by the rates at which atoms can be supplied to the extreme surface by diffusion from inside the regolith grains. Supply rates to the atmosphere are further regulated by ion retention and by gardening rates that supply new grains to the surface. We consider the limits on supply of sodium and potassium atoms to the atmosphere, and rates of photoion recycling to the surface. Thermal vaporization rates are severely limited by the ability of atoms to diffuse to the surface of the grain. Therefore, the diffusion-limited thermal vaporization rates on Mercury's surface are comparable to or less than the PSD rates. Ion sputtering is primarily due to highly ionized heavy ions, even though they represent a small fraction of the solar wind. We have shown that up to 60% of the Na photoions are deposited on the surface of Mercury. Ion recycling to the surface can have a long-term effect on the regolith abundance if an average recycling pattern persists such that more ions return to a particular area than are launched there. It is unknown whether the formation of latitude bands of > 100% ion retention persist on average despite a rapidly changing magnetosphere. The total exospheric column of sodium observed at Mercury between 1997 to 2003 varied by a factor of 2-3 from perihelion to aphelion.

Advances in Space Research, 2004

Space weathering is a process where formation of nanophase iron particles causes darkening of overall reflectance, spectral reddening, and weakening of absorption bands on atmosphereless bodies such as the moon and asteroids. Using pulse laser irradiation, formation of nanophase iron particles by micrometeorite impact heating is simulated. Although Mercurian surface is poor in iron and rich in anorthite, microscopic process of nanophase iron particle formation can take place on Mercury. On the other hand, growth of nanophase iron particles through Ostwald ripening or repetitive dust impacts would moderate the weathering degree. Future MESSENGER and BepiColombo mission will unveil space weathering on Mercury through multispectral imaging observations.

Icarus, 1985

During recent years my research on the primitive solar nebular has followed two main themes: (1) Very early in the development of the nebula conditions probably favored the occurrence of major gaseous instabilities leading to the formation of giant gaseous protoplanets, but the rapid rise of the external temperature soon evaporated the envelopes of these protoplanets, possibly leaving behind precipitated solids which formed the cores and mantles of the terrestrial planets. (2) Models of the nebula indicate a later stage when conditions in the inner Solar System became very hot; at the position of Mercury the temperature was probably in the range 2500-3500°K. This leads to the hypothesis that the original protomercury was a body substantially more massive than the present planet and of normal composition, but that when it was immersed in the high-temperature field of the dissipating solar nebula, most of the rocky mantle was vaporized and mixed into the solar nebula gases and carried away by them. This hypothesis is investigated in the present paper. For simplicity the vaporization of a mantle composed of enstatite, MgSiO3, was computed for a planet with 2.25 the mass of Mercury at a temperature of 3000°K. It is argued that the mantle could probably be largely removed in the available time of 3 × 104 years. Subsequent accretion would restore some magnesium silicates to the mantle of the planet.