MErcury Surface, Space ENvironment, GEochemistry, And Ranging Spacecraft Research Articles

Processing of laser altimeter time-of-flight measurements to geodetic coordinates

Laser altimeters are commonly used in planetary research for their high geodetic accuracy. A key procedure in processing of laser altimeter data is the geolocation. In this process, the time-of-flight measurements are converted to coordinates of laser pulse footprints on the surface of the target body. Here, we present a consistent and systematic formulation of three commonly used geolocation models with increasing complexity: static model, spacecraft motion model, pointing aberration model and special relativity model. We show that for small velocities of the spacecraft relative to the target the special relativity model can be reduced to the pointing aberration model without significant loss in the geolocation accuracy. We then discuss the respective accuracies of the proposed models and apply them to time-of-flight measurements from the Mars Orbiter Laser Altimeter (MOLA) onboard the Mars Global Surveyor (MGS) spacecraft and the Mercury Laser Altimeter (MLA) onboard the MErcury Surface, Space ENvironment, GEochemistry and Ranging spacecraft (MESSENGER). While, the archived datasets had not considered the effect of pointing aberration, we demonstrate that a correction due to pointing aberration makes insignificant improvements of 4–5 m laterally and up to ± 3 cm radially for MOLA profiles, these figures enormously increase to up to about 150 m laterally and ± 25 m radially when applied to the MLA orbital profiles.

Precession of Mercury’s Perihelion from Ranging to the MESSENGER Spacecraft

Abstract The perihelion of Mercury’s orbit precesses due to perturbations from other solar system bodies, solar quadrupole moment (J 2), and relativistic gravitational effects that are proportional to linear combinations of the parametrized post-Newtonian parameters β and γ. The orbits and masses of the solar system bodies are quite well known, and thus the uncertainty in recovering the precession rate of Mercury’s perihelion is dominated by the uncertainties in the parameters J 2, β, and γ. Separating the effects due to these parameters is challenging since the secular precession rate has a linear dependence on each parameter. Here we use an analysis of radiometric range measurements to the MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) spacecraft in orbit about Mercury to estimate the precession of Mercury’s perihelion. We show that the MESSENGER ranging data allow us to measure not only the secular precession rate of Mercury’s perihelion with substantially improved accuracy, but also the periodic perturbation in the argument of perihelion sensitive to β and γ. When combined with the γ estimate from a Shapiro delay experiment from the Cassini mission, we can decouple the effects due to β and J 2 and estimate both parameters, yielding and J 2 = (2.25 ± 0.09) × 10−7. We also estimate the total precession rate of Mercury’s perihelion as 575.3100 ± 0.0015″/century and provide estimated contributions and uncertainties due to various perturbing effects.

Duration of activity on lobate‐scarp thrust faults on Mercury

Lobate scarps, landforms interpreted as the surface manifestation of thrust faults, are widely distributed across Mercury and preserve a record of its history of crustal deformation. Their formation is primarily attributed to the accommodation of horizontal shortening of Mercury's lithosphere in response to cooling and contraction of the planet's interior. Analyses of images acquired by the Mariner 10 and MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft during flybys of Mercury showed that thrust faults were active at least as far back in time as near the end of emplacement of the largest expanses of smooth plains. However, the full temporal extent of thrust fault activity on Mercury, particularly the duration of this activity following smooth plains emplacement, remained poorly constrained. Orbital images from the MESSENGER spacecraft reveal previously unrecognized stratigraphic relations between lobate scarps and impact craters of differing ages and degradation states. Analysis of these stratigraphic relations indicates that contraction has been a widespread and long-lived process on the surface of Mercury. Thrust fault activity had initiated by a time near the end of the late heavy bombardment of the inner solar system and continued through much or all of Mercury's subsequent history. Such deformation likely resulted from the continuing secular cooling of Mercury's interior.

The origin of boninites on Mercury: An experimental study of the northern volcanic plains lavas

Phase equilibrium experiments were conducted on a synthetic rock composition matching that of the northern volcanic plains of Mercury as measured by the MErcury Surface, Space ENvironment, GEochemistry and Ranging spacecraft (MESSENGER). The northern volcanic plains are smooth plains of suspected volcanic origin that cover more than 6% of the surface area of Mercury. The northern volcanic plains are less cratered than their surroundings and reported to be the product of flood volcanism, making them a prime candidate for experimental study. The bulk composition of the northern volcanic plains is that of an alkali-rich boninite and represents the first silica-enriched crustal terrane identified on an extraterrestrial planet from orbital data. Phase equilibrium experiments were conducted over the pressure range of the mercurian mantle (0.5–5GPa) at very low oxygen fugacity (∼ΔIW0 to −7) using a piston-cylinder apparatus (P 0.5–1.7GPa) and a Walker-style multi-anvil device (P⩾2.5GPa). Our results indicate the origin of the northern volcanic plains lavas (boninites) are best explained by high degrees of partial melting of an olivine-dominant, pyroxene- and plagioclase-bearing mantle source at low pressure (⩽1.4GPa) and does not require hydrous melting to achieve the silica-enriched melt composition. The formation mechanism for boninites on Mercury contrasts substantially with terrestrial boninites, which typically occur in oxidized and hydrous arc environments associated with subduction zones. Instead, mercurian boninites form at exceptionally low oxygen fugacity and do not require melting of hydrated source materials. The NVP lavas represent a novel mechanism by which planetary bodies can form silica-enriched secondary crusts without the aid of water.

Calibration of the Mercury Laser Altimeter on the MESSENGER Spacecraft

This paper gives a detailed description of the prelaunch and in-orbit calibrations of the Mercury Laser Altimeter (MLA) on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission, which was launched on August 3, 2004 and has been operating in orbit about Mercury since March 2011. A brief summary of the MLA instrument is given, followed by the instrument measurement model and calibration formulas. The prelaunch tests used to determine the values of various calibration coefficients are described. The boresight alignment parameters were verified and recalibrated by special tests, with the MESSENGER spacecraft en route to Mercury. The MLA instrument model and the calibration methods were largely derived from airborne and spaceborne lidar for Earth science observation at the NASA Goddard Space Flight Center and will benefit future space lidar developments for Earth and space science.

Detection and characterization of 0.5–8 MeV neutrons near Mercury: Evidence for a solar origin

AbstractData 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 (<30 MeV) that can produce local neutrons. However, the ion spectral shape required to produce the measured neutron count rate locally is softer than any known ion spectral shape. The neutron energy spectrum shows a relative enhancement in the energy range 0.8–3 MeV compared with cosmic‐ray‐generated neutrons from the spacecraft or Mercury. The spectral shape of the measured neutron fluence spectrum is consistent with a previously modeled fluence spectrum of neutrons that originate at the Sun and are propagated through the MESSENGER spacecraft to the NS. These measurements provide strong evidence for a solar origin of the detected neutrons and suggest that a large number of low‐energy threshold ion evaporation reactions were taking place on the Sun during the neutron event.

Distribution and compositional variations of plasma ions in Mercury's space environment: The first three Mercury years of MESSENGER observations

AbstractWe have analyzed measurements of planetary ions near Mercury made by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) Fast Imaging Plasma Spectrometer (FIPS) over the first three Mercury years of orbital observations (25 March 2011 through 31 December 2011). We determined the composition and spatial distributions of the most abundant species in the regions sampled by the MESSENGER spacecraft during that period. In particular, we here focus on altitude dependence and relative abundances of species in a variety of spatial domains. We used observed density as a proxy for ambient plasma density, because of limitations to the FIPS field of view. We find that the average observed density is 3.9 × 10–2 cm–3 for He2+, 3.4 × 10–4 cm–3 for He+, 8.0 × 10–4 cm–3 for O+‐group ions, and 5.1 × 10–3 cm–3 for Na+‐group ions. Na+‐group ions are particularly enhanced over other planetary ions (He+ and O+ group) in the northern magnetospheric cusp (by a factor of ~2.0) and in the premidnight sector on the nightside (by a factor of ~1.6). Within 30° of the equator, the average densities of all planetary ions are depressed at the subsolar point relative to the dawn and dusk terminators. The effect is largest for Na+‐group ions, which are 49% lower in density at the subsolar point than at the terminators. This depression could be an effect of the FIPS energy threshold. The three planetary ion species considered show distinct dependences on altitude and local time. The Na+ group has the smallest e‐folding height at all dayside local times, whereas He+ has the largest. At the subsolar point, the e‐folding height for Na+‐group ions is 590 km, and that for the O+ group and He+ is 1100 km. On the nightside and within 750 km of the geographic equator, Na+‐group ions are enhanced in the premidnight sector. This enhancement is consistent with nonadiabatic motion and may be observational evidence that nonadiabatic effects are important in Mercury's magnetosphere.

An MHD simulation of the inner heliosphere during Carrington rotations 2060 and 2068: Comparison with MESSENGER and ACE spacecraft observations

We present results from a new magnetohydrodynamic (MHD) model of the inner heliosphere. The model is adapted from the well-established Lyon–Fedder–Mobarry (LFM) MHD simulation code, which until recently mostly applied to studies of the terrestrial magnetosphere. We perform quasi steady-state simulations of two Carrington rotations: 2060 and 2068. During both of these periods, the heliosphere remained quiet and undisturbed by transient phenomena, making them well-suited for simulation studies of Corotating Interaction Regions (CIRs). The MHD model of the solar wind is driven at the inner boundary by the Wang–Sheeley–Arge (WSA) model of the corona augmented with empirical relations to infer the solar wind velocity, density, and temperature. Here we report on a validation exercise whereby LFM-helio simulation results are compared with in situ data from the Advanced Composition Explorer (ACE) and MErcury Surface, Space ENvironment, GEochemistry and Ranging (MESSENGER) spacecraft. We find that the model successfully reproduces the large-scale configuration of the inner heliosphere, namely timing and duration of high-speed streams and heliospheric current sheet crossings, as reflected in ACE and MESSENGER observations. Discrepancies between in situ measurements and simulations, such as 1–2 day errors in the time of arrival of a CIR or the strength of the simulated magnetic field at the spacecraft, are attributed to the uncertainty in the specification of the coronal conditions, rather than a poor performance of the solar wind model. More comparisons between different inner heliosphere models driven with identical coronal conditions are suggested as a way to explore their comparative strengths and weaknesses.

The transition from complex crater to peak-ring basin on Mercury: New observations from MESSENGER flyby data and constraints on basin formation models

The study of peak-ring basins and other impact crater morphologies transitional between complex craters and multi-ring basins is important to our understanding of the mechanisms for basin formation on the terrestrial planets. Mercury has the largest population, and the largest population per area, of peak-ring basins and protobasins in the inner solar system and thus provides important data for examining questions surrounding peak-ring basin formation. New flyby images from the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft have more than doubled the area of Mercury viewed at close range, providing nearly complete global coverage of the planet's surface when combined with flyby data from Mariner 10. We use this new near-global dataset to compile a catalog of peak-ring basins and protobasins on Mercury, including measurements of the diameters of the basin rim crest, interior ring, and central peak (if present). Our catalog increases the population of peak-ring basins by ∼150% and protobasins by ∼100% over previous catalogs, including 44 newly identified peak-ring basins (total=74) and 17 newly identified protobasins (total=32). A newly defined transitional basin type, the ringed peak-cluster basin (total=9), is also described. The new basin catalog confirms that Mercury has the largest population of peak-ring basins of the terrestrial planets and also places the onset rim-crest diameter for peak-ring basins at 126 − 26 + 33 km , which is intermediate between the onset diameter for peak-ring basins on the Moon and those for the other terrestrial planets. The ratios of ring diameter to rim-crest diameter further emphasize that protobasins and peak-ring basins are parts of a continuum of basin morphologies relating to their processes of formation, in contrast to previous views that these forms are distinct. Comparisons of the predictions of peak-ring basin-formation models with the characteristics of the basin catalog for Mercury suggest that formation and modification of an interior melt cavity and nonlinear scaling of impact melt volume with crater diameter provide important controls on the development of peak rings. The relationship between impact-melt production and peak-ring formation is strengthened further by agreement between power laws fit to ratios of ring diameter to rim-crest diameter for peak-ring basins and protobasins and the power-law relation between the dimension of a melt cavity and the crater diameter. More detailed examination of Mercury's peak-ring basins awaits the planned insertion of the MESSENGER spacecraft into orbit about Mercury in 2011.

Electron-stimulated desorption of silicates: A potential source for ions in Mercury's space environment

[1] The potential role of electron-stimulated desorption (ESD) in the formation of Mercury's exosphere has been examined. Experimental results involving electron irradiation of Na- and K-bearing silicate glasses yielded direct desorption of H+, H2+, O+, H3O+, Na+, K+, and O2+. A simulation has also been performed to calculate electron precipitation fluxes and energies that may be used in the interpretation of measurements made by MESSENGER (Mercury Surface, Space Environment, Geochemistry, and Ranging) spacecraft instruments and to better understand the formation of the ion and neutral exosphere at Mercury. The electron fluxes are estimated to be ∼1010 cm−2 s−1 with average energies up to 500 eV. The threshold energies for production/release of ions have been measured to be 25 ± 2 eV for H+; 30 ± 2 eV for O+, Na+, and K+; 40 ± 2 eV for H2+ and H3O+; and 90 ± 2 eV for O2+. The 25–30 eV thresholds correlate with deep valence holes in the O 2s levels that undergo Auger decay. The thresholds for H2+ and H3O+ correspond to two-hole states in chemisorbed water, which produce energetic protons that undergo reactive scattering on the surface. A significant increase in ESD yield of all ions is observed above a substrate temperature of 350 K. The estimated total cross section for ESD at a surface temperature of 400 K is ≥10−19 cm2, much larger than neutral production followed by electron impact ionization or photoionization in the gas phase. These results indicate that ESD may contribute to the production and release of regolith constituents, particularly in ionic form, directly into the exosphere of Mercury.

The Study of Mercury

AbstractWhen the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft enters orbit about Mercury in March 2011 it will begin a new phase in an age-old scientific study of the innermost planet. Despite being visible to the unaided eye, Mercury's proximity to the Sun makes it extremely difficult to observe from Earth. Nonetheless, over the centuries man has pursued a quest to understand the elusive planet, and has teased out information about its motions in the sky, its relation to the other planets, and its physical characteristics. A great leap was made in our understanding of Mercury when the Mariner 10 spacecraft flew past it three times in the mid-1970s, providing a rich set of close-up observations. Now, three decades later, The MESSENGER spacecraft has also visited the planet three times, and is poised to add significantly to the study with a year-long orbital observation campaign.

The X-Ray Spectrometer on the MESSENGER Spacecraft

NASA’s MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) mission will further the understanding of the formation of the planets by examining the least studied of the terrestrial planets, Mercury. During the one-year orbital phase (beginning in 2011) and three earlier flybys (2008 and 2009), the X-Ray Spectrometer (XRS) onboard the MESSENGER spacecraft will measure the surface elemental composition. XRS will measure the characteristic X-ray emissions induced on the surface of Mercury by the incident solar flux. The Kα lines for the elements Mg, Al, Si, S, Ca, Ti, and Fe will be detected. The 12° field-of-view of the instrument will allow a spatial resolution that ranges from 42 km at periapsis to 3200 km at apoapsis due to the spacecraft’s highly elliptical orbit. XRS will provide elemental composition measurements covering the majority of Mercury’s surface, as well as potential high-spatial-resolution measurements of features of interest. This paper summarizes XRS’s science objectives, technical design, calibration, and mission observation strategy.

MESSENGER Mission Design and Navigation

Nearly three decades after the Mariner 10 spacecraft’s third and final targeted Mercury flyby, the 3 August 2004 launch of the MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) spacecraft began a new phase of exploration of the closest planet to our Sun. In order to ensure that the spacecraft had sufficient time for pre-launch testing, the NASA Discovery Program mission to orbit Mercury experienced launch delays that required utilization of the most complex of three possible mission profiles in 2004. During the 7.6-year mission, the spacecraft’s trajectory will include six planetary flybys (including three of Mercury between January 2008 and September 2009), dozens of trajectory-correction maneuvers (TCMs), and a year in orbit around Mercury. Members of the mission design and navigation teams optimize the spacecraft’s trajectory, specify TCM requirements, and predict and reconstruct the spacecraft’s orbit. These primary mission design and navigation responsibilities are closely coordinated with spacecraft design limitations, operational constraints, availability of ground-based tracking stations, and science objectives. A few days after the spacecraft enters Mercury orbit in mid-March 2011, the orbit will have an 80° inclination relative to Mercury’s equator, a 200-km minimum altitude over 60°N latitude, and a 12-hour period. In order to accommodate science goals that require long durations during Mercury orbit without trajectory adjustments, pairs of orbit-correction maneuvers are scheduled every 88 days (once per Mercury year).

Two-Way Laser Link over Interplanetary Distance

Here we report timed observations with subnanosecond precision of short laser pulses at a distance of nearly 24 million kilometers between the Mercury Laser Altimeter (MLA) aboard the MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) spacecraft and the NASA Goddard Geophysical and Astronomical Observatory (GGAO). Forty MLA downlink observations and 90 uplink observations were obtained during observing sessions on 27 and 31 May 2005. Precise standard ground timing allowed a solution for spacecraft range, range rate, and acceleration, as well as clock bias. This experiment established a new distance record for laser detection and accomplished a two-way laser link at an interplanetary distance.