Mercury's low-reflectance material: Constraints from hollows

Unusually low reflectance material, within which depressions known as hollows appear to be actively forming by sublimation, is a major component of Mercury's surface geology. The observation that this material is exhumed from depth by large impacts has the intriguing implication that the planet's lower crust or upper mantle contains a significant volatile-rich, low-reflectance layer, the composition of which will be key for developing our understanding of Mercury's geochemical evolution and bulk composition. Hollows provide a means by which the composition of both the volatile and non-volatile components of the low-reflectance material (LRM) can be constrained, as they result from the loss of the volatile component, and any remaining lag can be expected to be formed of the non-volatile component. However, previous work has approached this by investigating the spectral character of hollows as a whole, including that of bright deposits surrounding the hollows, a unit of uncertain character. Here we use high-resolution multispectral images, obtained as the MESSENGER spacecraft approached Mercury at lower altitudes in the latter part of its mission, to investigate reflectance spectra of inactive hollow floors where sublimation appears to have ceased, and compare this to those of the bright surrounding products and the parent material. This analysis reveals that the final lag after hollow-formation has a flatter spectral slope than that of any other unit on the planet and reflectance approaching that of more space-weathered parent material. This indicates firstly that the volatile material lost has a steeper spectral slope and higher reflectance than the parent material, consistent with (Ca,Mg) sulfides, and secondly, that the low-reflectance component of LRM is non-volatile and may be graphite

Impact ionization mass spectra of anorthite cosmic dust analogue particles

Anorthite, the Ca-rich end-member of plagioclase feldspar, is a dominant mineral component of the Lunar highlands. Plagioclase feldspar is also found in comets, meteorites and stony asteroids. It is therefore expected to contribute to the population of interplanetary (and circumplanetary) dust grains within the solar system. After coating micron- and submicron-sized grains of Anorthite with a conductive layer of Platinum, the mineral was successfully accelerated to hypervelocity speeds in the Max Planck Institut für Kernphysik’s Van de Graaff accelerator. We present impact ionization mass spectra generated following the impacts of anorthite grains with a prototype mass spectrometer (the Large Area Mass Analyser, LAMA) designed for use in space, and discuss the behavior of the spectra with increasing impact energy. Correlation analysis is used to identify the compositions and sources of cations present in the spectra, enabling the identification of several molecular cations (e.g., CaAlO2, CaSiO2, Ca2AlO3/CaAlSi2O2) which identify anorthite as the progenitor bulk grain material

Asteroid taxonomic classification in the

Aims.We evaluate the prospects of performing automatic taxonomic classification of asteroids in a proposed broad and medium band photometric system of Gaia. Methods.The study is based on asteroid colors from the Eight-Color Asteroid Survey (ECAS) and CCD spectra from the Small Mainbelt Asteroid Spectral Survey II (SMASSII). The success of the Gaia photometric systems for taxonomic classification is evaluated using supervised classification techniques and mean taxonomic class spectra in the Tholen and Bus&Binzel taxonomic systems. Our supervised classification method is based on rms differences between individual asteroid spectra and taxonomic mean spectra and provides probability estimates of membership in all taxonomic classes. Results.We find that both photometric systems of Gaia are able to discriminate between all of the twelve Tholen asteroid classes for noise-free data. The medium band system is able to discriminate between the majority of the 26 SMASSII asteroid classes in case of high quality photometric data. For both the Tholen and Bus & Binzel taxonomies we find that about 25% of the asteroids are spectrally more similar to another taxonomic class in a best-fit sense, though the differences within the three major complexes (C, S and X) are 1–10%. Conclusions.Among the two main existing taxonomies, the Gaia photometric system is found to be best suited for the Bus & Binzel taxonomy. The medium band system is the preferred system for all but the faintest objects. The classification method employed here results in more concentrated taxonomic class domains in principal component space, and mean taxonomic spectra that are formed from less divergent class members, than the case for the nominal classification systems. It provides statistical probability estimates for class memberships and naturally reflects the fact that asteroid spectral shapes form a continuum in principal component space

Albedo–color distribution on Mercury

Aims.The variation of albedo and color of Mercury's surface is studied with disk-resolved image data obtained at six evenly spaced wavelengths in the optical to near infrared wavelength range (447–944 nm) with the 1-m Swedish Solar Telescope on La Palma in April, 2003. Methods.Disk images have been modeled and photometrically normalized with the light scattering theory of Hapke to derive albedo–color properties of a poorly known region (unimaged by Mariner 10) of Mercury's surface between longitudes 210°W and 270°W. Maps of relative abundances of ferrous iron, titanium and optical maturation are derived on the basis of a feldspathic model for the crustal composition and previous results for the Moon, assuming the validity of the general maturation model for mafic silicate regoliths of atmosphereless bodies. Results.The albedo–color scatterplot distributions of Mercury's surface are uniform with respect to wavelength in the near-ultraviolet to near-infrared due to the absence of strong absorption bands in the reflectance spectrum. The extents of the distributions are less than for the global Moon and similar to that of the lunar farside, which is related to the relatively subdued color contrasts of Mercury's primarily feldspathic surface. At the attained 500-km spatial resolution scale, these maps do not indicate the existence of surface regions chemically similar to the lunar maria, which have a high FeO and TiO2 content. Variations in abundances of ferrous iron and titanium are shown to be less than for the global Moon and similar to the lunar farside at the same spatial scale. Optically bright regions on Mercury are less mature and less opaque than their surroundings consistent with geologically recent immature crater ejecta, while localized dark regions generally have intermediate maturities and iron abundances and higher-than-average titanium abundances. The smaller relative intensity range of spatial variations of spectral parameters in the near infrared compared to the near ultraviolet may imply that relative abundance variations in ferrous iron are smaller than variations in opaque minerals. Conclusions.The results reinforce the similar natures of the Mariner 10-imaged and the poorly known hemispheres of Mercury, as well as their superficial similarity to the lunar farside, and demonstrate that geological interpretation of ground-based observations of albedo features on Mercury is possible

Evolution of Mercury-like orbits: A numerical study

The orbital evolution of ~$30\,000$ objects has been studied numerically for time scales of up to 11 Myr in order to investigate the dynamical importance of Mercury on objects with orbits similar to the planet's, as well as the existence of stable hermeocentric orbits. We find that Mercury, despite its small mass, exhibits a strong dynamical influence on objects with heliocentric Mercury-like orbits. The orbits of such objects may become temporarily and repeatedly Trojan-like for durations exceeding 20 kyr, exhibiting horseshoe orbits in the 1:1 resonance with periods of around 60 years. Objects with Mercury-like orbits may evolve dynamically to Venus crossers on time scales of 50–100 kyr by successive close encounters with Mercury which quasi-instantaneously increase the semimajor axis and cause subsequent capture in strong secular resonances, which force the eccentricity to high values. Initially, all particles were located in the $\nu_1$ and $\nu_{11}$ resonances which strongly affect their dynamical evolution. Only objects for which the semimajor axis and eccentricity were significantly increased due to close encounters with Mercury escaped these resonances. The effects on the orbital elements of close approaches within the Hill radius of Mercury are generally minor and their values are then unlikely to change significantly. The largest changes to the orbital elements at such events occur for Hill sphere traverse times of 2 days, when a, e and i may change by as much as 0.07 AU, 0.13 and 3.5°. However, secular resonances are generally considerably more efficient in affecting the orbital elements, particularly $\nu_2$ and $\nu_{12}$. We find retrograde and prograde hermeocentric orbits which are stable for durations in eccess of 4.5 Myr for initial semimajor axes <30 Mercury radii. Retrograde orbiters are stable for longer time scales than prograde. Collisions with Mercury occur for heliocentric objects, temporarily captured objects and for large hermeocentric orbits of both senses of motion. Gravitational scattering by Venus may constitute a possible route for transporting mercurian meteorites to Earth. We estimate the number of objects presently with orbits similar to that of Mercury and find that the current existence of such an object in the region has a low probability. We suggest an observational approach that would make the detection of hermeocentric objects larger than 1 km possible with a moderately sized telescope.

Constraints on Mercury’s surface composition from MESSENGER and ground-based spectroscopy

The composition and chemistry of Mercury’s regolith has been calculated from MESSENGER MASCS 0.3–1.3 μm spectra from the first flyby, using an implementation of Hapke’s radiative transfer-based photometric model for light scattering in semi-transparent porous media, and a linear spectral mixing algorithm. We combine this investigation with linear spectral fitting results from mid-infrared spectra and compare derived oxide abundances with mercurian formation models and lunar samples. Hapke modeling results indicate a regolith that is optically dominated by finely comminuted particles with average area weighted grain size near 20 lm. Mercury shows lunar-style space weathering, with maturation-produced microphase iron present at ~0.065 wt.% abundance, with only small variations between mature and immature sites, the amount of which is unable to explain Mercury’s low brightness relative to the Moon. The average modal mineralogies for the flyby 1 spectra derived from Hapke modeling are 35–70% Na-rich plagioclase or orthoclase, up to 30% Mg-rich clinopyroxene, <5% Mg-rich orthopyroxene, minute olivine, ~20–45% low-Fe, low-Ti agglutinitic glass, and <10% of one or more lunar-like opaque minerals. Mercurian average oxide abundances derived from Hapke models and mid-infrared linear fitting include 40–50 wt.% SiO2, 10–35 wt.% Al2O3, 1–8 wt.% FeO, and <25 wt.% TiO2; the inferred rock type is basalt. Lunar-like opaques or glasses with high Fe and/or Ti abundances cannot on their own, or in combination, explain Mercury’s low brightness. The linear mixing results indicate the presence of clinopyroxenes that contain up to 21 wt.% MnO and the presence of a Mn-rich hedenbergite. Mn in M1 crystalline lattice sites of hedenbergite suppresses the strong 1 and 2 lm crystal field absorption bands and may thus act as a strong darkening agent on Mercury. Also, one or more of thermally darkened silicates, Fepoor opaques and matured glasses, or Mercury-unique Ostwald-ripened microphase iron nickel may lower the albedo. A major part of the total microphase iron present in Mercury’s regolith is likely derived from FeO that is not intrinsic to the crust but has been subsequently delivered by exogenic sources

Conceptual Design of a Hydraulic Valve Train System

Variable valve train systems have been brought into focus during recent years as a means to decrease fuel consumption in tomorrow's combustion engines. In this paper an integrated approach, called simulation driven experiments, is utilised in order to aid the development of such highly dynamic systems. Through the use of systematic design methodology, a number of feasible concepts are developed. Critical components are subsequently identified using simulation. In this approach, component behaviour is simulated and validated by measurements on prototype components. These models are unified with complete system models of hydraulically actuated valve trains. In the case of the valve trains systems studied here component models could be validated using comparably simple test set-ups. These models enable the determination of non-critical design parameters in an optimal sense. This results in a number of optimised concepts facilitating an impartial functional concept selection

Dust continuum imaging of C/1995 O1 (Hale-Bopp):

Dust continuum imaging of comet C/1995 O1 (Hale-Bopp) was carried out with the Swedish Vacuum Solar Telescope (SVST) on La Palma, Spain, during the period April 21–25, 1997. CCD observations were made with $\lambda~550\ \rm nm$ and $\lambda~830\ \rm nm$ filters, with band-passes excluding major cometary gaseous emissions. Images were reduced according to standard procedure, aligned, averaged, navigated and enhanced with azimuthal renormalisation, rotational and temporal derivative and unsharp masking techniques. The rotation period of the nucleus was determined to 11\hbox{.\!\!^{\rm h}}46 and the mean projected dust outflow velocity to $0.41\ \rm km\ s^{-1}$ measured over a radius range of $5\,000~-~45\,000\ \rm km$. No difference in quasi-simultaneous dust morphology between the filter wavelengths was seen. Up to five shell envelopes were visible in the sun-ward side of the coma out to a radius of $80\,000\ \rm km$, separated by a projected distance of ~$15\,000-20\,000\ \rm km$ and spiralling towards smaller radii in the direction of local evening. Outer boundaries of shells were more sharply defined than inner, with shell intensity maxima occurring at ~35° past the sun-comet line in the direction of rotation. Small scale inhomogeneities of size $1\,000-2\,000\ \rm km$ were seen at radii $\le 20\,000 \rm km$. Two superposed shell systems with a relative lag angle of ~55° were evident during the period of observation. The sense of rotation of dust emission features indicates that the north pole of the nucleus was directed towards the Earth. The shape of the incomplete spiral shell pattern indicates that the angle between the line-of-sight and the rotation axis of the nucleus was not very large