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Background on mercury exposure is presented including forms, sources, permissible exposure limits, and physiological effects. The purpose of the Mercury Surveillance Program at LeRC is outlined, and the specifics of the Medical Surveillance Program for Mercury Exposure at LeRC are discussed.
A general theoretical overview for the sources, sinks, gas-surface interactions, and transport dynamics of sodium and potassium in the exospheric atmsophere of Mercury is given. Information for these four factors, which control the spatial distribution of these two alkali-group gases about the planet, is incorporated in numerical models. The spatial nature and relative importance of the initial source atom atmosphere and the ambient (ballistic hopping) atom atmosphere are then examined and are shown to be controlled and coupled to a great extent by the extremely large and variable solar radiation acceleration experienced by sodium and potassium as they resonantly scatter solar photons. The lateral (antisunward) transport rate of thermally accommodated sodium and potassium ambient atoms is shown to be driven by the solar radiation acceleration and, over a significant portion of Mercury's orbit about the Sun, is sufficiently rapid to be competitive with the short photoionization lifetimes for these atoms when they are located on the summit surface near or within about 30 deg of the terminator. The lateral transport rate is characterized by a migration time determined by model calculations for an ensemble of atoms initially starting at a point source on the surface (i.e., a numerical spacetime dependent Green's function). Four animations for the spacetime evolution of the sodium (or potassium) atmosphere produced by a point source on the surface are presented on a videotape format. For extended surface sources for sodium and potassium, the local column density is determined by competition between the photoionization lifetimes and the lateral transport times of atoms originating from different surface source locations. Sodium surface source fluxes (referenced to Mercury at perihelion) that are required on the sunlit hemisphere to reproduce the typically observed several megarayleighs of D2 emission-line brightness and the inferred column densities of 1-2 x 10(exp 11) atoms per sq cm range from approximately 2-5 x 10(exp 7) atoms/sq cm/sec. The sodium model is applied to study observational data that document an anticorrelation in the average sodium column density and solar radiation acceleration. Lateral transport driven by the solar radiation acceleration is shown to produce this behavior for combinations of different sources and surface accomodation coefficients. The best fit model fits to the observational data require a significant degree of thermal accommodation of the ambient sodium atoms to the surface and a source rate that decreases as an inverse power of 1.5 to 2 in heliocentric distance.
If Mercury's radius is expanded in Legendre functions to the second degree and order, the systematic error in radar ranging can be reduced significantly. We interpret the expansion coefficients in terms of a best-fit ellipsoid displaced with respect to the center of mass in the equatorial plane. The ellipsoid's principal axes are rotated in the equatorial plane such that the long axis is aligned with cartographic longitude 15.3 deg +/- 2.9 deg (west). The pole location is consistent with the IAU pole, normal to Mercury's orbital plane. There is a significant equatorial ellipticity (a - b)/a = (540 +/- 54) x 10(exp -6). The center of figure is offset from the center of mass (C.F.-C.M.) by 640 +/- 78 m in the equatorial plane in the direction of cartographic longitude 319.5 deg +/- 6.9 deg. The magnitude of the equatorial center of figure offset implies an excess crustal thickness of 12 km or less, comparable to the Moon's excess. By comparing the equatorial ellipticity with the Mariner 10 gravity coefficient C(sub 22) and assuming Airy isostatic compensation, we conclude that Mercury's crustal thickness is in the range from 100 to 300 km.
Sodium in the atmosphere of Mercury can be detected by sunlight scattered in the D1 and D2 resonance lines. Images of the sodium emission show that the sodium density changes from day to day and is often concentrated in regions at high or mid latitudes. Drew Potter (NASA/JSC) and Tom Morgan (SWRI) suggested that sputtering by magnetospheric particles was the origin of the sodium. A problem with this is that the magnetic field of Mercury is strong enough that it is believed to shield the surface from solar particles much of the time, although particle precipitation at the magnetospheric cusps could deposit particles to the surface at high latitudes. Ann Sprague (UA/LPL) noted that the "spots" of sodium emission tended to coincide with major geologic features, such as the Caloris Basin. She proposed that the sodium is released from sodiumrich surface rocks that are associated with these features; however, some spots have appeared where there are no obvious geologic features. Some of the difficulty in ascribing a source for the sodium arises from the effect of terrestrial atmospheric blurring of the image. It is hard to tell exactly where the sodium emission originates after the atmosphere has blurred the image. Potter, Killen (SWRI), and Morgan recently developed a technique for correcting sodium images for atmospheric blurring, using images made with a large-area image slicer. They applied this technique to a series of Mercury sodium observations made in November, 1997 at the McMath-Pierce Solar Telescope. Their technique for producing images from the spectroscopic data provides images of both the sodium emission and of the sunlight reflected from the surface.
Remote sensing measurements of the Moon and Mercury in the thermal infrared have been plagued with problems due to atmospheric absorption and other effects. Controversial results have been discussed by Nash et al. and by Salisbury et al.. We have developed a technique to obtain thermal infrared spectra of Mercury together with solar and sky spectra in the 8-13 micrometer region suitable for determining the presence or absence of mineralogical features. High-resolution (0.05 cm(exp -1) between data points) spectra were obtained during daylight using the Fourier Transform Spectrograph (FTS) at the McMath-Pierce Solar Telescope Facility at Kitt Peak National Observatory, Arizona. The reduced data were independent of any model of the transmission of the Earth's atmosphere and rest on a calibration using sky spectra along the same line-of-sight through the atmosphere as the actual Mercury raw spectrum. This method can be applied at all solar elongation angles including small elongation angles (5 deg or less). The basic sequence and the technique are robust.
We examine the possibilities of sustaining an argon atmosphere by diffusion from the upper 10 km of crust, and alternatively by effusion from a molten or previously molten area at great depth . Ar-40 in the atmospheres of the planets is a measure of potassium abundance in the interiors since Ar-40 is a product of radiogenic decay of K-40 by electron capture with the subsequent emission of a 1.46 eV gamma-ray. Although the Ar-40 in the earth's atmosphere is expected to have accumulated since the late bombardment, Ar-40 in surface-bounded exospheres is eroded quickly by photoionization and electron impact ionization. Thus, the argon content in the exospheres of the Moon, Mercury and probably Europa is representative of current effusion rather than accumulation over the lifetime of the body. Argon content will be a function of K content, temperature, grain size distribution, connected pore volume and possible seismic activity. Although Mercury and the Moon differ in many details, we can train the solutions to diffusion equations to predict the average lunar atmosphere. Then these parameters can be varied for Hermean conditions. Assuming a lunar crustal potassium abundance of 300 ppm, the observed argon atmosphere requires equilibrium between the argon production in the upper 9 Km of the moon (1.135 x 10(exp -3) cm(exp -3) s(exp -1)) and its loss. Hodges et al. conclude that this loss rate and the observed time variability requires argon release through seismic activity, tapping a deep argon source. An important observation is that the extreme surface of the Moon is enhanced in argon rather than depleted, as one would expect from outgassing of radiogenic argon. Manka and Michel concluded that ion implantation explains the surface enhancement of Ar-40. About half of the argon ions produced in the lunar atmosphere would return to the surface, where they would become embedded in the rocks. Similarly, at Mercury we expect the surface rocks to be enhanced in Ar-40 wherever the magnetosphere has been open over time. Thus the measurement of surface composition will reveal the long-term effects of solar wind-magnetosphere interaction. Additional information is contained in the original extended abstract.
Evidence for a global magnetic field at Mercury has raised perplexing questions regarding the existence and nature of the planet's core. The problems related to Mercury's core are of great interest because of their implications on how planets evolve thermally and on how they generate magnetic fields. Peale showed that the measurement of four quantities could place constraints on the size and state of Mercury's core. The required parameters in Peale's experiment are the C(sub 20) and C(sub 22) coefficients in the spherical harmonic expansion of the gravity field, the planet's obliquity theta, and the amplitude phi of the forced libration in longitude. While the gravitational harmonics are best determined from an orbiting spacecraft, Earth-based radar observations may provide refined estimates of the obliquity and amplitude of the libration. We describe several techniques and attempt to quantify their potential for the proposed measurements. Additional information is contained in the original extended abstract.
NASA's requirements for high reliability, high performance satellite laser instruments have driven the investigation of many critical components; specifically, 808 nm laser diode array (LDA) pump devices. The MESSENGER mission is flying the Mercury Laser Altimeter (MLA) which is a diode-pumped Nd:YAG laser instrument designed to map the topography of Mercury. The environment imposed on the instrument by the orbital dynamics places special requirements on the laser diode arrays. In order to limit the radiative heating of the satellite from the surface of Mercury, the satellite is designed to have a highly elliptical orbit. The satellite will heat near perigee and cool near apogee. The laser power is cycled during these orbits so that the laser is on for only 30 minutes (perigee) in a 12 hour orbit. The laser heats 10 C while powered up and cools while powered down. In order to simulate these operational conditions, we designed a test to measure the LDA performance while being temperature and power cycled. Though the mission requirements are specific to NASA and performance requirements are derived from unique operating conditions, the results are general and widely applicable. We present results on the performance of twelve LDAs operating for several hundred million pulses. The arrays are 100 watt, quasi-CW, conductively-cooled, 808 nm devices. Prior to testing, we fully characterize each device to establish a baseline for individual array performance and status. Details of this characterization can be found in reference. Arrays are divided into four groups and subjected to the temperature and power cycling matrix are shown.
A compact, passively Q-switched Nd:YAG laser has been developed for the Mercury Laser Altimeter (MLA) instrument which is an instrument on the MESSENGER mission to the planet Mercury. The laser achieves 5.4 percent efficiency with a near diffraction limited beam. It has passed all space flight environmental tests at system, instrument, and satellite integration. The laser design draws on a heritage of previous laser altimetry missions, specifically ISESAT and Mars Global Surveyor; but incorporates thermal management features unique to the requirements of an orbit of the planet Mercury.
The Mercury Laser Altimeter (MLA) is one of the payload science instruments on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission, which launched on 3 August 2004. The altimeter will measure the round trip time-of-flight of transmitted laser pulses reflected from the surface of the planet that, in combination with the spacecraft orbit position and pointing data, gives a high-precision measurement of surface topography referenced to Mercury's center of mass. The altimeter measurements will be used to determine the planet's forced librations by tracking the motion of large-scale topographic features as a function of time. MLA's laser pulse energy monitor and the echo pulse energy estimate will provide an active measurement of the surface reflectivity at 1064 nm. This paper describes the instrument design, prelaunch testing, calibration, and results of post-launch testing.
Aboard every U.S. piloted spacecraft, from Mercury through Apollo, NASA installed tape recorders that captured nearly every word spoken by the astronauts during their history-making flights into space. For the first time ever, NASA has digitally scanned all of the transcripts made from both the onboard tapes and those tape recordings made on the ground from the air-to-ground transmissions and placed them on this two CD-ROM set. Gathered in this special collection are 80 transcripts totaling nearly 45,000 pages of text that cover every US human spaceflight from the first human Mercury mission through the last lunar landing flight of Apollo 17. Users of this CD will note that the quantity and type of transcripts made for each mission vary. For example, the Mercury flights each had one transcript whereas the Gemini missions produced several. Starting with the Gemini flights, NASA produced a Public Affairs Office (PAO) commentary version, as well as at least one "technical" air-to-ground transcript version, per mission. Most of the Apollo missions produced four transcripts per flight. These included the onboard voice data recorder transcripts made from the Data Storage Equipment (DSE) on the Command Module (CM), and the Data Storage Electronics Assembly (DSEA) onboard the Lunar Module (LM), in addition to the PAO commentary and air-to-ground technical transcripts. The CD set includes an index listing each transcript file by name. Some of the transcripts include a detailed explanation of their contents and how they were made. Also included in this collection is a listing of all the original air-to-ground audiotapes housed in NASA's archives from which many of these transcripts were made. We hope you find this collection of transcripts interesting and useful.
The MESSENGER mission to Mercury offers our first opportunity to explore this planet's miniature magnetosphere since Mariner 10's brief fly-bys in 1974-5. The magnetosphere of Mercury is the smallest in the solar system with its magnetic field typically standing off the solar wind only - 1000 to 2000 km above the surface. An overview of the MESSENGER mission and its January 14th close flyby of Mercury will be provided. Primary science objectives and the science instrumentation will be described. Initial results from MESSENGER'S first flyby on January 14th, 2008 will be discussed with an emphasis on the magnetic field and charged particle measurements.
Satellite observations at the Earth, supported by theory and modeling, have established a close connection between the episodes of intense magnetospheric convection termed substorms and the occurrence of magnetic reconnection. Magnetic reconnection at the dayside magnetopause results in strong energy input to the magnetosphere. This energy can either be stored or used immediately to power the magnetospheric convection that produces the phenomena that collectively define the 'substorm.' However, many aspects of magnetic reconnection and the dynamic response of the coupled solar wind - magnetosphere - ionosphere system at the Earth during substorms remain poorly understood. For example, the rate of magnetic reconnection is thought to be proportional to the local Alfven speed, but the limited range of changes in this solar wind parameter at 1 AU have made it difficult to detect its influence over energy input to the Earth's magnetosphere. In addition, the electrical conductance of the ionosphere and how it changes in response to auroral charged particle precipitation are hypothesized to play a critical role in the development of substorms, but the nature of this electrodynamic interaction remain difficult to deduce from Earth observations alone. The amount of energy the terrestrial magnetosphere can store in its tail, the duration of the storage, and the trigger(s) for its dissipation are all thought to be determined by not only the microphysics of the cross-tail current layer, but also the properties of the coupled magnetosphere - ionosphere system. Again, the separation of microphysics effects from system response has proved very difficult using measurements taken only at the Earth. If MESSENGER'S charged particle and magnetic field measurements confirm the occurrence of terrestrial-style substorms in Mercury's miniature magnetosphere, then it may be possible to determine how magnetospheric convection, field-aligned currents, charged particle acceleration, reconnection, and tail energy storage are influenced by the intense magnetic reconnection expected to be associated with solar wind conditions at 0.3 - 0.5 AU from the Sun and the simplified electrodynamic feed-back anticipated for a planet lacking an ionosphere. MESSENGER observations from its 14 January 2008 Mercury flyby relevant to the occurrence of magnetic reconnection and substorms at Mercury will be discussed.
MESSENGER'S 14 January 2008 encounter with Mercury has provided new observations of the magnetopause of this small magnetosphere, particularly concerning the effect of the direction of the interplanetary magnetic field (IMF) on the structure and dynamics of this boundary. The IMF was northward immediately prior to and following the passage of the MESSENGER spacecraft through Mercury's magnetosphere. However, several-minute episodes of southward IMF were observed in the magnetosheath during the inbound portion of the encounter. Evidence for reconnection at the dayside magnetopause in the form of well-developed flux transfer events (FTEs) was observed in the magnetosheath following some of these southward-B, intervals. The inbound magnetopause crossing seen in the magnetic field measurements is consistent with a transition from the magnetosheath into the plasma sheet. Immediately following MESSENGER'S entry into the magnetosphere, rotational perturbations in the magnetic field similar to those seen at the Earth in association with large-scale plasma sheet vortices driven by Kelvin-Helmholtz waves along the magnetotail boundary at the Earth were observed. The outbound magnetopause occurred during northward IMF B(sub z) and had the characteristics of a tangential discontinuity. These new observations by MESSENGER may be combined and compared with the magnetopause measurements collected by Mariner 10 to derive new understanding of the response of Mercury's magnetopause to IMF direction and its effect on the rate of solar wind energy and mass input to this small magnetosphere.
The Mercury Laser Altimeter performed the first laser ranging measurements to Mercury during the Mercury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) flyby in January 2008. The instrument successfully ranged to 600 km at an off-nadir angle >60 and to >1600 km in the nadir direction.
The MESSENGER mission to Mercury offers our first opportunity to explore this planet's miniature magnetosphere since Mariner 10's brief fly-bys in 1974-5. The magnetosphere of Mercury is the smallest in the solar system with its magnetic field typically standing off the solar wind only approximately 1000 km above the surface. An overview of the MESSENGER mission and its January 14th and October 6th, 2008 close flybys of Mercury will be provided. Primary science objectives and the science instrumentation will be described. Initial results from MESSENGER will be discussed with an emphasis on the magnetic field and charged particle measurements.
The MErcury, Surface, Space, ENvironment GEochemistry and Ranging (MESSENGER) spacecraft is a NASA Discovery Mission spacecraft developed and operated by the Johns Hopkins University Applied Physics Laboratory. It was launched on August 3, 2004 and is currently on a course for Mercury orbit insertion in March 2011. To date the mission trajectory has taken the spacecraft to minimum solar distances of 0.332 and 0.313 AU and on January 14, 2008 the first flyby of Mercury in 33 years. From launch through the latest perihelion passage temperature performance data has been collected for the sun facing Digital Sun Sensors (DSS), the sun facing phased array and low gain (omni) antennas, the solar arrays, the sunshade and the two sun facing attitude control 4.4 N thrusters. Prior to launch, extensive solar simulation testing was conducted at the Glenn Research Center, Tank 6 solar simulation facility in Cleveland Ohio. Flight hardware qualification units representing these Sun exposed components were tested in solar environments that represented near mission minimum solar distance as to verify the thermal designs and the material used in fabrication. The paper will review the thermal designs of these components and their thermal performance to date as compared to the solar simulation testing.
Like the palladium chloride (PdCl2) films described in the immediately preceding article, gold nanowire sensors have been found to be useful for detecting airborne elemental mercury at concentrations on the order of parts per billion (ppb). Also like the PdCl2 films, gold nanowire sensors can be regenerated under conditions much milder than those necessary for regeneration of gold films that have been used as airborne-Hg sensors. The interest in nanowire sensors in general is prompted by the expectation that nanowires of a given material covering a given surface may exhibit greater sensitivity than does a film of the same material because nanowires have a greater surface area. In preparation for experiments to demonstrate this sensor concept, sensors were fabricated by depositing gold nanowires, variously, on microhotplate or microarray sensor substrates. In the experiments, the electrical resistances were measured while the sensors were exposed to air at a temperature of 25 C and relative humidity of about 30 percent containing mercury at various concentrations from 2 to 70 ppb (see figure). The results of this and other experiments have been interpreted as signifying that sensors of this type can detect mercury at ppb concentrations in room-temperature air and can be regenerated by exposure to clean flowing air at temperatures <40 C.