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At least 613 records · Page 34

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NASA NTRS1985

The partial volatilization of Mercury

During recent years research on the primitive solar nebula 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 30,000 yrs. Subsequent accretion would restore some magnesium silicates to the mantle of the planet.

Cameron, A. G. W.↗

NASA NTRS1985

The use of radar and visual observations to characterize the surface structure of the planet Mercury

An analysis is conducted of available topographic profiles and scattering parameters derived from earth-based S- and X-band radar observations of Mercury, in order to determine the nature and origin of regional surface variations and structures that are typical of the planet. Attention is given to the proposal that intercrater plains on Mercury formed from extensive volcanic flooding during bombardment, so that most craters were formed on a partially molten surface and were thus obliterated, together with previously formed tectonic features.

Clark, P. E.↗

NASA NTRS1986

Radar altimetry of Mercury - A preliminary analysis

Altitude measurements derived from 2380 MHz radar observations of the equatorial zone of Mercury collected between 1978-1984 at Arecibo Observatory are analyzed. The altitude profiles, which span from 20-90 deg longitude at a resolution of 0.15 deg (longitude) by 2.5 deg (latitude), are compared with geologic maps of Mercury. The depths and shapes of the craters and basins are examined; it is observed that Mercurian craters are shallower than lunar craters. The plains of Tir Planitia, Caloris Basin, and circum-Caloris are studied and it is hypothesized that the plains are of a volanic origin. The ridges, scarps, and fault zones detected in the altimetry data are described.

Harmon, J. K.↗

NASA NTRS1986

Acceleration of charged particles in Mercury's magnetosphere

A detailed analysis is presented of the measurements of the high-intensity bursts of electrons with energies of up to 600 keV, discovered in Mercury's magnetosphere during the two nightside encounters of the Mariner 10 with the planet in 1974 and 1975 (Simpson et al., 1974; Simpson, 1975; and Christon and Simpson, 1979). The results provide strong evidence for particle acceleration during explosive magnetic field reconnection within Mercury's magnetotail and suggest a rapid release of magnetic free energy through instabilities occurring in regions of magnetic field reconnection in the planetary magnetotail.

Eraker, J. H.↗

NASA NTRS1987

Mercury's magnetic field - A thermoelectric dynamo?

Permanent magnetism and conventional dynamo theory are possible but problematic explanations for the magnitude of the Mercurian magnetic field. A new model is proposed in which thermoelectric currents driven by temperature differences at a bumpy core-mantle boundary are responsible for the (unobserved) toroidal field, and the helicity of convective motions in a thin outer core (thickness of about 100 km) induces the observed poloidal field from the toroidal field. The observed field of about 3 x 10 to the -7th T can be reproduced provided the electrical conductivity of Mercury's semiconducting mantle approaches 1000/ohm per m. This model may be testable by future missions to Mercury because it predicts a more complicated field geometry than conventional dynamo theories. However, it is argued that polar wander may cause the core-mantle topography to migrate so that some aspects of the rotational symmetry may be reflected in the observed field.

Stevenson, D. J.↗

NASA NTRS1987

A vaporization model for iron/silicate fractionation in the Mercury protoplanet

A study has been carried out on the vaporization of a totally molten silicate magma of chondritic composition heated into the range 2500-3500 K. The motivation for this was to determine the changes in the composition of the mantle that would occur in the Mercury protoplanet should that body have been subjected to the high-temperature phase in the evolution of the primitive solar nebula, but the results are of more general interest. An empirical model based on ideal mixing of complex components was used to describe the nonideal magma. It is found that vaporization of about 70-80 percent of the original amount of silicate from a chondritic planet is required to produce an iron-rich body with a mean uncompressed density equal to that deduced for Mercury. At this point the silicate is depleted in the alkalis, FeO, and SiO2, and enriched in CaO, MgO, Al2O3, and TiO2 relative to chondritic material.

Fegley, Bruce, Jr.↗

NASA NTRS1987

Radio-interferometric imaging of the subsurface emissions from the planet Mercury

The distribution of total and polarized intensities from Mercury's subsurface layers have been mapped using VLA observations. The first detection of a hot pole along the Hermean equator is reported and modeled as black-body reradiation from preferential diurnal heating. These observations appear to rule out any internal sources of heat within Mercury. Polarized emission from the limb of the planet is also found, and is understood in terms of the dielectric properties of the Hermean surface.

Burns, J. O.↗

NASA NTRS1987

The mass, gravity field, and ephemeris of Mercury

In the present gravity analysis of Mariner 10/Deep Space Network radio Doppler and range data for Mercury encounters in March 1974 and March 1975, a combined least-squares fit to the Doppler data has determined two second-degree gravity harmonics that are referred to a 2439-km equatorial radius. It is noted that the 1-sigma error limits on the gravity results encompass the possibility that harmonics other than J2 and C22 significantly differ from zero. The Deep Space Network radio range data obtained with Mariner 10 are primarily applicable to such improvements of Mercury's ephemeris as the more precise determination of perihelion precession.

Anderson, John D.↗

NASA NTRS1987

Mercury - Wavelength and longitude dependence of polarization

The polarization-time variations noted in the present linear polarization observations of the integrated Mercury disk, with filters covering the 0.3-1.0 micron bandpass and between 53-130 deg of phase angle, may be accounted for in terms of longitude dependence through the variation of brightness or other properties over the surface. Kepler's equation for the eccentric anomaly was solved for each observation, and the true anomaly was used to find the subsolar longitude. The difference found in the polarization-albedo relations of Mercury and the moon indicates a difference in surface texture and/or composition.

Gehrels, T.↗

NASA NTRS1987

The formation of Mercury's smooth plains

Indirect evidence is presented for a volcanic origin in the case of two smooth plains on Mercury, the Borealis Planitia and the Hilly and Lineated Terrain. These results, in conjunction with those previously obtained for the circum-Caloris plains and the Tolstoj basin, indicate that smooth plains volcanism was a global process on Mercury. It is further suggested that the smooth and intercrater plains may have resulted from two separate volcanic episodes, with smooth plains volcanism being a consequence of Caloris impact-triggering.

Kiefer, Walter S.↗

NASA NTRS1987

Dynamic substorm injections - Similar magnetospheric phenomena at earth and Mercury

Correlations between energetic electrons, plasma electrons, and magnetic fields during the Mercury 1 energetic particle events are examined and comparisons are made with several well-documented substorm injections at the earth. The data reveal that the B and B-prime events possess the same characteristics as single-point observations of terrestrial dynamic injections. Several recently discovered correlations between the energetic electrons, plasma electrons, and magnetic fields at Mercury are discussed.

Christon, S. P.↗

NASA NTRS1988

Determination of rock type on Mercury and the moon through remote sensing in the thermal infrared

Thermal infrared emission spectra of the moon and Mercury have been obtained using the Si:As photoconductor and circular variable filter at the NASA Infrared Telescope Facility on Mauna Kea, Hawaii. Lunar spectra from 7.2 to 12.2 microns for two different locations in the south polar highlands have Christiansen frequency peaks at 8.1 microns and 7.9 microns, respectively. This indicates different compositions at the two locations; mafic in the first case, more felsic in the second. Emission spectra from Mercury are not as spatially localized,; however, the longitude of maximum contribution to the spectrum can be calculated from thermal models of the earth-facing disk. Results for areas centered at two longitudes have been obtained. Two locations in the intercrater plains were observed. At 40-deg longitude (very near the crater Homer), a peak at 7.9 microns indicates mafic igneous rock type. Spectra emanating from 46-deg longitude have peaks at 7.8 microns, indicating a region borderline between mafic and intermediate composition.

Tyler, Ann L.↗

NASA NTRS1989

Mercury ion thruster technology

The Mercury Ion Thruster Technology program was an investigation for improving the understanding of state-of-the-art mercury ion thrusters. Emphasis was placed on optimizing the performance and simplifying the design of the 30 cm diameter ring-cusp discharge chamber. Thruster performance was improved considerably; the baseline beam-ion production cost of the optimized configuration was reduced to Epsilon (sub i) perspective to 130 eV/ion. At a discharge propellant-utilization efficiency of 95 percent, the beam-ion production cost was reduced to about 155 eV/ion, representing a reduction of about 40 eV/ion over the corresponding value for the 30 cm diameter J-series thruster. Comprehensive Langmuir-probe surveys were obtained and compared with similar measurements for a J-series thruster. A successful volume-averaging scheme was developed to correlate thruster performance with the dominant plasma processes that prevail in the two thruster designs. The average Maxwellian electron temperature in the optimized ring-cusp design is as much as 1 eV higher than it is in the J-series thruster. Advances in ion-extraction electrode fabrication technology were made by improving materials selection criteria, hydroforming and stress-relieving tooling, and fabrications procedures. An ion-extraction performance study was conducted to assess the effect of screen aperture size on ion-optics performance and to verify the effectiveness of a beam-vectoring model for three-grid ion optics. An assessment of the technology readiness of the J-series thruster was completed, and operation of an 8 cm IAPS thruster using a simplified power processor was demonstrated.

Beattie, J. R.↗

NASA NTRS1989

Plasma and energetic electron flux variations in the Mercury 1 C event - Evidence for a magnetospheric boundary layer

Charge-particle and magnetic-field data obtained during the first encounter (on March 29, 1974) of Mariner 10 with the planet Mercury are reexamined, and a new interpretation of the Mariner 10 energetic electron, plasma electron, and magnetic field data near the outbound magnetopause at Mercury is presented. It is shown that Mariner 10 sampled the hot substorm energized magnetospheric plasma sheet for the first 36 sec of the C event and, for the next 48 sec, alternatively sampled hot (plasma sheet) and cold (boundary-layer magnetosheathlike) plasma regions. It is argued that the counting rate of the ID1 event (i.e., a particle event triggering detector D1 but not the D2, D3, or D7 detectors) throughout the C event most probably represents a pulse pileup response to about 35-175 keV electrons, rather than the nominal above-175 keV electrons presumed in the earlier interpretations.

Christon, S. P.↗

NASA NTRS1988

Stratigraphy and geologic history of Mercury

The geologic evolution of Mercury based on the Mariner-10 mission data is discussed. As reconstructed through photogeological analysis of global geologic relations of rock-stratigraphic units, Mercury's geologic history is shown to involve intensive early impact bombardment and widespread resurfacing by volcanic lavas. Evidence is presented to indicate that this volcanic activity essentially ended as much as 3 Gyr ago, with most of the major geologic events being completed within the first 1 to 1.5 Gyr of Mercurian history.

Spudis, Paul D.↗

NASA NTRS1988

The cratering record on Mercury and the origin of impacting objects

The cratering records of terrestrial planets are discussed with special consideration given to Mercury. The geologic units on Mercury most relevant to its cratering record are reviewed, and new observations are presented on the issues of equilibrium and saturation. The implications of the Mercurian cratering record for geologic processes are examined. Particular attention is given to the origin of objects responsible for the period of late heavy bombardment on terrestrial planets; it is suggested that the impactors were accretional remnants left over from the formation of the terrestrial planets and confined to the inner solar system. The cratering record in the outer solar system may have been produced largely by objects in planetocentric orbits.

Strom, Robert G.↗

NASA NTRS1988

The Mercury atmosphere

Data available on the composition of Mercury's atmosphere are reviewed, and the sources of these atmospheric components are considered. The known gaseous components in Mercury's atmosphere are H, He, O, Na, and K; among other gases likely to be present are H2 and H2O. Probable sources of these components are the solar wind for hydrogen and helium. The alkalis and water are considered to come from the evaporation of meteoroidal material, with a possible contribution to the former by sputtering and photosputtering.

Hunten, D. M.↗

NASA NTRS1988

The strange density of Mercury - Theoretical considerations

Two classes of models which have been advanced to explain the high density of Mercury are reviewed and contrasted. These models invoke either the differing volatilities of iron and silicates or disruptive collisions to fractionate the two phases. Also contrasted are equilibrium condensation and planetary vaporization models, both of which fall within the first broad class considered. Results indicate that equilibrium condensation models are unable to account for the observed high density of Mercury without invoking special mechanisms such as unrealistically narrow planetary accretion zones. However, it is found that distinctive chemical differences, which are potentially testable by spacecraft experiments, provide means for distinguishing between planetary vaporization and large impact scenarios.

Cameron, A. G. W.↗