[Show abstract][Hide abstract] ABSTRACT: We have characterized the spectral reflectance of geological units associated with 121 complex impact craters on Mercury. To do so, we have combined Mercury Atmospheric and Surface Composition Spectrometer (MASCS) Visible and Infrared Spectrograph (VIRS) data with Mercury Dual Imaging System (MDIS) images, both acquired from orbit by the MESSENGER spacecraft.
European Planetary Science Congress 2014, Cascais, Portugal; 09/2014
[Show abstract][Hide abstract] ABSTRACT: Observations of Mercury's internal magnetic field from the Magnetometer on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft have revealed a dipole moment of 190 nT RM3 offset about 480 km northward from the planetary equator. We have reanalyzed magnetic field observations acquired by the Mariner 10 spacecraft during its third flyby of Mercury (M10-III) in 1975 to constrain the secular variation in the internal field over the past 40 years. With the application of techniques developed in the analysis of MESSENGER data, we find that the dipole moment that best fits the M10-III data is 188 nT RM3 offset 475 km northward from the equator. Our results are consistent with no secular variation, although variations of up to 10%, 16%, and 35%, respectively, are permitted in the zonal coefficients g10, g20, and g30 in a spherical harmonic expansion of the internal field.
[Show abstract][Hide abstract] ABSTRACT: The structure of Mercury's dayside magnetosphere is investigated during three extreme solar-wind dynamic-pressure events. Two were the result of coronal mass ejections (CMEs), and one was from a high-speed stream (HSS). The inferred pressures for these events are ~ 45 to 65 nPa. The CME events produced thick, low-β (where β is the ratio of plasma thermal to magnetic pressure) plasma-depletion layers and high reconnection rates of 0.1–0.2, despite small magnetic shear angles across the magnetopause of only 27 to 60°. For one of the CME events, brief, ~ 1–2-s-long diamagnetic decreases, which we term cusp plasma filaments, were observed within and adjacent to the cusp. These filaments may map magnetically to flux transfer events at the magnetopause. The HSS event produced a high-β magnetosheath with no plasma-depletion layer and large magnetic shear angles of 148 to 166°, but low reconnection rates of 0.03 to 0.1. These results confirm that magnetic reconnection at Mercury is very intense, and its rate is primarily controlled by plasma β in the adjacent magnetosheath. The distance to the subsolar magnetopause is reduced during these events from its mean of 1.45 Mercury radii (RM) from the planetary magnetic dipole to between 1.03 and 1.12 RM. The shielding provided by induction currents in Mercury's interior, which temporarily increase Mercury's magnetic moment, was negated by reconnection-driven magnetic flux erosion.
Journal of Geophysical Research: Space Physics. 08/2014;
[Show abstract][Hide abstract] ABSTRACT: Data from the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) Neutron Spectrometer (NS) have been used to identify energetic neutrons (0.5–8 MeV energy) associated with solar events that occurred on 4 June 2011. Multiple lines of evidence, including measurements from the NS and the MESSENGER Gamma‐Ray Spectrometer, indicate that the detected neutrons have a solar origin. This evidence includes a lack of time‐coincident, energetic (>45 MeV) charged particles that could otherwise create local neutrons from nearby spacecraft material and a lack of proton‐induced gamma rays that should be seen if energetic protons were present. NS data cannot rule out the presence of lower‐energy ions ( Solar neutrons were detected by MESSENGER on 4 June 2011Energetic particles and gamma rays support a solar origin for the neutrons
Journal of Geophysical Research: Space Physics. 07/2014; 119(7).
[Show abstract][Hide abstract] ABSTRACT: The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft has observed the northern magnetospheric cusp of Mercury regularly since the probe was inserted into orbit about the innermost planet in March 2011. Observations from the Fast Imaging Plasma Spectrometer (FIPS) made at altitudes < 400 km in the planet's cusp have shown average proton densities (>10 cm−3) that are exceeded only by those observed in the magnetosheath. These high plasma densities are also associated with strong diamagnetic depressions observed by MESSENGER's Magnetometer. Plasma in the cusp may originate from several sources: (1) Direct inflow from the magnetosheath; (2) locally produced planetary photo-ions and ions sputtered off the surface from solar wind impact and then accelerated upward; and (3) flow of magnetosheath and magnetospheric plasma accelerated from dayside reconnection X-lines. We surveyed 518 cusp passes by MESSENGER, focusing on the spatial distribution, energy spectra, and pitch-angle distributions of protons and Na+-group ions. Of those, we selected 77 cusp passes during which substantial Na+-group ion populations were present for a more detailed analysis. We find that Mercury's cusp is a highly dynamic region, both in spatial extent and plasma composition and energies. From the three-dimensional plasma distributions observed by FIPS, protons with mean energies of 1 keV were found flowing down into the cusp (i.e., source 1 above). The distribution of pitch angles of these protons showed a depletion in the direction away from the surface, indicating that ions within 40° of the magnetic field direction are in the loss cone, lost to the surface rather than being reflected by the magnetic field. In contrast, Na+-group ions show two distinct behaviors depending on their energy. Low-energy (100–300 eV) ions appear to be streaming out of the cusp, showing pitch-angle distributions with a strong component anti-parallel to the magnetic field (away from the surface). These ions appear to have been generated in the cusp and accelerated locally (i.e., source 2 above). Higher-energy (≥1 keV) Na+-group ions in the cusp exhibit much larger perpendicular components in their energy distributions. During active times, as judged by frequent, large-amplitude magnetic field fluctuations, many more Na+-group ions are measured at latitudes south of the cusp. In several cases, these Na+-group ions in the dayside magnetosphere are flowing northward toward the cusp. Their high mean energy and pitch angle distributions and the large number of Na+-group ions on dayside magnetospheric field lines are inconsistent with direct transport into the cusp of sputtered ions from the surface or newly photo-ionized particles. Furthermore, the highest densities and mean energies often occur together with high-amplitude magnetic fluctuations, attributed to flux transfer events along the magnetopause. These results indicate that high-energy Na+-group ions in the cusp are likely formed by ionization of escaping neutral Na in the outer dayside magnetosphere and the magnetosheath followed by acceleration and transport into the cusp by reconnection at the subsolar magnetopause (i.e., source 3 above).
Journal of Geophysical Research: Space Physics. 06/2014;
[Show abstract][Hide abstract] ABSTRACT: With data from the Fast Imaging Plasma Spectrometer sensor on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging spacecraft, we demonstrate that the average distributions for both solar wind and planetary ions in Mercury's pre-midnight plasma sheet are well described by hot, near-isotropic Maxwell-Boltzmann distributions. Temperatures and densities of the H+-dominated plasma sheet, in the ranges ~1–10 cm-3 and ~5–30 MK, respectively, maintain thermal pressures of ~1 nPa. The dominant planetary ion, Na+, has number densities about 10% that of H+. Solar wind ions retain near-solar-wind abundances with respect to H+ and exhibit mass-proportional ion temperatures, indicative of a reconnection-dominated heating in the magnetosphere. Conversely, planetary ion species are accelerated to similar average energies greater by a factor of ~1.5 than that of H+. This energization is suggestive of acceleration in an electric potential, consistent with the presence of a strong centrifugal acceleration process in Mercury's magnetosphere.
[Show abstract][Hide abstract] ABSTRACT: Solar wind protons observed by the MESSENGER spacecraft in orbit about Mercury exhibit signatures of precipitation loss to Mercury's surface. We apply proton-reflection magnetometry to sense Mercury's surface magnetic field intensity in the planet's northern and southern hemispheres. The results are consistent with a dipole field offset to the north and show that the technique may be used to resolve regional-scale fields at the surface. The proton loss cones indicate persistent ion precipitation to the surface in the northern magnetospheric cusp region and in the southern hemisphere at low nightside latitudes. The latter observation implies that most of the surface in Mercury's southern hemisphere is continuously bombarded by plasma, in contrast with the premise that the global magnetic field largely protects the planetary surface from the solar wind.
[Show abstract][Hide abstract] ABSTRACT: The Gravity Recovery and Interior Laboratory (GRAIL) mission has sampled lunar gravity with unprecedented accuracy and resolution. The lunar GM, the product of the gravitational constant G and the mass M, is very well determined. However, uncertainties in the mass and mean density, 3345.56 ± 0.40 kg/m3, are limited by the accuracy of G. Values of the spherical harmonic degree-2 gravity coefficients J2 and C22, as well as the Love number k2 describing lunar degree-2 elastic response to tidal forces, come from two independent analyses of the 3-month GRAIL Primary Mission data at the Jet Propulsion Laboratory and the Goddard Space Flight Center. The two k2 determinations, with uncertainties of ~1%, differ by 1%; the average value is 0.02416 ± 0.00022 at a 1-month period with reference radius R = 1738 km. Lunar Laser Ranging (LLR) data analysis determines (C–A)/B and (B–A)/C, where A < B < C are the principal moments of inertia; the flattening of the fluid outer core; the dissipation at its solid boundaries; and the monthly tidal dissipation Q = 37.5 ± 4. The moment of inertia computation combines the GRAIL-determined J2 and C22 with LLR-derived (C–A)/B and (B–A)/C. The normalized mean moment of inertia of the solid Moon is Is/MR2 = 0.392728 ± 0.000012. Matching the density, moment, and Love number, calculated models have a fluid outer core with radius 200–380 km, a solid inner core with radius 0–280 km and mass fraction 0–1%, and a deep mantle zone of low seismic shear velocity. The mass fraction of the combined inner and outer core is ≤1.5%.
Journal of Geophysical Research: Planets. 06/2014;
[Show abstract][Hide abstract] ABSTRACT: During its interplanetary trajectory in 2007-2009, the MErcury Surface, Space ENvrionment, GEochemistry, and Ranging (MESSENGER) spacecraft passed through the gravitational focusing cone for interstellar helium multiple times at a heliocentric distance R 0.3 AU. Observations of He+ interstellar pickup ions made by the Fast Imaging Plasma Spectrometer sensor on MESSENGER during these transits provide a glimpse into the structure of newly formed inner heliospheric pickup-ion distributions. This close to the Sun, these ions are picked up in a nearly radial interplanetary magnetic field. Compared with the near-Earth environment, pickup ions observed near 0.3 AU will not have had sufficient time to be energized substantially. Such an environment results in a nearly pristine velocity distribution function that should depend only on pickup-ion injection velocities (related to the interstellar gas), pitch-angle scattering, and cooling processes. From measured energy-per-charge spectra obtained during multiple spacecraft observational geometries, we have deduced the phase-space density of He+ as a function of magnetic pitch angle. Our measurements are most consistent with a distribution that decreases nearly monotonically with increasing pitch angle, rather than the more commonly modeled isotropic or hemispherically symmetric forms. These results imply that pitch-angle scattering of He+ may not be instantaneous, as is often assumed, and instead may reflect the velocity distribution of initially injected particles. In a slow solar wind stream, we find a parallel-scattering mean free path of λ || ~ 0.1 AU and a He+ production rate of ~0.05 m–3 s–1 within 0.3 AU.
The Astrophysical Journal 05/2014; 788(2):124. · 6.73 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The large-scale dynamic behavior of Mercury's highly compressed magnetosphere is predominantly powered by magnetic reconnection, which transfers energy and momentum from the solar wind to the magnetosphere. The contribution of flux transfer events (FTEs) at the dayside magnetopause to the redistribution of magnetic flux in Mercury's magnetosphere is assessed with magnetic field data acquired in orbit about Mercury by the Magnetometer on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft. FTEs with core fields greater than the planetary field just inside the magnetopause are prevalent at Mercury. Fifty-eight such large-amplitude FTEs were identified during February and May 2012, when MESSENGER sampled the subsolar magnetosheath. The orientation of each FTE was determined by minimum variance analysis, and the magnetic flux content of each was estimated using a force-free flux rope model. The average flux content of the FTEs was 0.06 MWb, and their durations imply a transient increase in the cross-polar-cap potential of ~25 kV. For a substorm timescale of a 2–3 min, as indicated by magnetotail flux loading and unloading, the FTE repetition rate (10 s) and average flux content (assumed to be 0.03 MWb) imply that FTEs contribute at least ~30% of the flux transport required to drive the Mercury substorm cycle. At Earth, in contrast, FTEs are estimated to contribute less than 2% of the substorm flux transport. This result implies that whereas at Earth, at which steady-state dayside reconnection is prevalent, multiple X-line dayside reconnection and associated FTEs at Mercury are a dominant forcing for magnetospheric dynamics.
Journal of Geophysical Research: Space Physics. 05/2014;
[Show abstract][Hide abstract] ABSTRACT: We have characterized the spectral reflectance of 121 complex impact craters on Mercury. To do so, we have combined Mercury Atmospheric and Surface Composition Spectrometer (MASCS) Visible and Infrared Spectro-graph (VIRS) data with Mercury Dual Imaging System (MDIS) images, both acquired from orbit by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft. The MASCS spectra were taken from the DLR database, which currently contains several million MASCS VIRS spectra normalized at 700 nm wavelength to provide a first-order correction for variations in observing conditions. The craters for this study were selected on the basis of geological and physical criteria from the MDIS global dataset. For each impact crater, we mapped as independent geological units the central peak, floor deposits, wall deposits, and external ejecta from 1 to 10 crater radii outward of the crater rim (at a sampling step of 1 radius). From the DLR database, we retrieved MASCS VIRS observations for each geological unit of the 121 impact craters. We explored two different clas-sification schemes. In the first scheme we included all reflectance observations, even those shared between units for different craters. In the second scheme, we excluded spectra that are shared by multiple areas. Under the first scheme, therefore, the same spectral unit can be assigned to two or more craters, whereas under the second scheme spectral units are uniquely linked to a single crater. Preliminary results of our study show a range of distinctive spectra for the crater central peaks. Spectral variations are also seen among crater floor deposits. The goal of the study is to complete a global spectral map as a basis for improving our understanding of crustal stratigraphy on Mercury using impact craters as stratigraphic markers.
[Show abstract][Hide abstract] ABSTRACT: Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC), Ionosonde and Global Ultraviolet Imager (GUVI) data have been used to investigate the solar cycle changes in the winter anomaly (the winter anomaly is defined as the enhancement of the F2 peak electron density in the winter hemisphere over that in the summer hemisphere) in the last solar cycle. There is no winter anomaly in solar minimum, and an enhancement of about 50 % in winter over summer ones on the same day of the year at solar maximum. This solar cycle variation in the winter anomaly is primarily due to greater winter to summer differences of [O]/[N2] in solar maximum than in solar minimum, with a secondary contribution from the effects of temperature on the recombination coefficient between O+ and the molecular neutral gas. The greater winter increases in electron density in the northern hemisphere than in the southern hemisphere appear to be related to the greater annual variation of [O]/[N2] in the north than in the south.
Journal of Geophysical Research: Space Physics. 05/2014;
[Show abstract][Hide abstract] ABSTRACT: Mercury, a planet with a lithosphere that forms a single tectonic plate, is replete with tectonic structures interpreted to be the result of planetary cooling and contraction. However, the amount of global contraction inferred from spacecraft images has been far lower than that predicted by models of the thermal evolution of the planet's interior. Here we present a synthesis of the global contraction of Mercury from orbital observations acquired by the MESSENGER spacecraft. We show that Mercury's global contraction has been accommodated by a substantially greater number and variety of structures than previously recognized, including long belts of ridges and scarps where the crust has been folded and faulted. The tectonic features on Mercury are consistent with models for large-scale deformation proposed for a globally contracting Earth--now obsolete--that pre-date plate tectonics theory. We find that Mercury has contracted radially by as much as 7 km, well in excess of the 0.8-3 km previously reported from photogeology and resolving the discrepancy with thermal models. Our findings provide a key constraint for studies of Mercury's thermal history, bulk silicate abundances of heat-producing elements, mantle convection and the structure of its large metallic core.
[Show abstract][Hide abstract] ABSTRACT: 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 wind interactions) and are moderated by the local space environment and the presence of a global magnetic field. To comprehend how space weathering acts on Mercury's regolith, an understanding is needed of how contributing processes act as an interactive system. As no direct information (e.g., from returned samples) is available about how the system of space weathering affects Mercury's regolith, we use as a basis for comparison the current understanding of these same processes on lunar and asteroidal regoliths as well as laboratory simulations. These comparisons suggest that Mercury's regolith is overturned more frequently (though the characteristic surface time for a grain is unknown even relative to the lunar case), more than an order of magnitude more melt and vapor per unit time and unit area is produced by impact processes than on the Moon (creating a higher glass content via grain coatings and agglutinates), the degree of surface irradiation is comparable to or greater than that on the Moon, and photon irradiation is up to an order of magnitude greater (creating amorphous grain rims, chemically reducing the upper layers of grains to produce nanometer-scale particles of metallic iron, and depleting surface grains in volatile elements and alkali metals). The processes that chemically reduce the surface and produce nanometer-scale particles on Mercury are suggested to be more effective than similar processes on the Moon. Estimated abundances of nanometer-scale particles can account for Mercury's dark surface relative to that of the Moon without requiring macroscopic grains of opaque minerals. The presence of nanometer-scale particles may also account for Mercury's relatively featureless visible-near-infrared reflectance spectra. Characteristics of material returned from asteroid 25143 Itokawa demonstrate that this nanometer-scale material need not be pure iron, raising the possibility that the nanometer-scale material on Mercury may have a composition different from iron metal [such as (Fe,Mg)S]. The expected depletion of volatiles and particularly alkali metals from solar-wind interaction processes are inconsistent with the detection of sodium, potassium, and sulfur within the regolith. One plausible explanation invokes a larger fine fraction (grain size <45 μm) and more radiation-damaged grains than in the lunar surface material to create a regolith that is a more efficient reservoir for these volatiles. By this view the volatile elements detected are present not only within the grain structures, but also as adsorbates within the regolith and deposits on the surfaces of the regolith grains. The comparisons with findings from the Moon and asteroids provide a basis for predicting how compositional modifications induced by space weathering have affected Mercury's surface composition.
[Show abstract][Hide abstract] ABSTRACT: The combination of the radio tracking of the MESSENGER spacecraft and Earth-based radar measurements of the planet's spin state gives three fundamental quantities for the determination of the interior structure of Mercury: mean density ρ, moment of inertia C, and moment of inertia of the outer solid shell Cm. This work focuses on the additional information that can be gained by a determination of the change in gravitational potential due to planetary tides, as parametrized by the tidal potential Love number k2. We investigate the tidal response for sets of interior models that are compatible with the available constraints (ρ, C, and Cm). We show that the tidal response correlates with the size of the liquid core and the mean density of material below the outer solid shell, and that it is affected by the rheology of the outer solid shell of the planet, which depends on its temperature and mineralogy. For a mantle grain size of 1 cm, we calculate that the tidal k2 of Mercury is in the range 0.45 to 0.52. Some of the current models for the interior structure of Mercury are compatible with the existence of a solid FeS layer at the top of the core. Such a layer, if present, would increase the tidal response of the planet.
Journal of Geophysical Research: Planets. 03/2014;
[Show abstract][Hide abstract] ABSTRACT: We assess the statistical spatial distribution of plasma in Mercury's magnetosphere from observations of magnetic pressure deficits and plasma characteristics by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft. The statistical distributions of proton flux and pressure were derived from 10 months of Fast Imaging Plasma Spectrometer (FIPS) observations obtained during the orbital phase of the MESSENGER mission. The Magnetometer-derived pressure distributions compare favorably with those deduced from the FIPS observations at locations where depressions in the magnetic field associated with the presence of enhanced plasma pressures are discernible in the Magnetometer data. The magnitudes of the magnetic pressure deficit and the plasma pressure agree on average, although the two measures of plasma pressure may deviate for individual events by as much as a factor of ~3. The FIPS distributions provide better statistics in regions where the plasma is more tenuous and reveal an enhanced plasma population near the magnetopause flanks resulting from direct entry of magnetosheath plasma into the low-latitude boundary layer of the magnetosphere. The plasma observations also exhibit a pronounced north–south asymmetry on the nightside, with markedly lower fluxes at low altitudes in the northern hemisphere than at higher altitudes in the south on the same field line. This asymmetry is consistent with particle loss to the southern hemisphere surface during bounce motion in Mercury's offset dipole magnetic field.
Journal of Geophysical Research: Space Physics. 03/2014;