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At least 559 records · Page 31
All sourcesNASA NTRSDOE OSTI
NASA NTRS1975

Surface history of Mercury - Implications for terrestrial planets

A plausible surface history of Mercury is presented which is suggested by Mariner 10 television pictures. Five periods are postulated which are delineated by successive variations in the modification of the surface by external and internal processes: accretion and differentiation, terminal heavy bombardment, formation of the Caloris basin, flooding of that basin and other areas, and light cratering accumulated on the smooth plains. Each period is described in detail; the overall history is compared with the surface histories of Venus, Mars, and the moon; and the implications of this history for earth are discussed. It is tentatively concluded that: Mercury is a differentiated planet most likely composed of a large iron core enclosed by a relatively thin silicate layer; heavy surface bombardment occurred about four billion years ago, which probably affected all the inner planets, and was followed by a period of volcanic activity; no surface modifications caused by tectonic, volcanic, or atmospheric processes took place after the volcanic period.

Murray, B. C.↗
NASA NTRS1975

Dynamo generation in Mercury

Models of the interior and thermal evolution of Mercury calculated by Siegfried and Solomon (1974) using Lewis' (1972) cosmochemical calculations to constrain the composition are tested for four necessary conditions for MHD dynamo generation. It is shown that dynamo generation requires at least a partially fluid interior, an energy source that drives a flow of core fluid relative to the rigidly rotating planet, a magnetic-field diffusion time in excess of the characteristic fluid-flow time scale, and a fluid flow of sufficient complexity to satisfy Cowling's (1934) theorem. It is concluded that a literal interpretation of Lewis' calculations implies that dynamo generation in Mercury is improbable. Generation would be possible only if the metallic core were contaminated with substantial amounts of radioactive material.

Stevenson, D. J.↗
NASA NTRS1975

Surface properties and effective resolution from ground-based observations of Mercury

A detailed study to evaluate ground-based photographs of Mercury has been carried out. Models of the surface scattering properties have been assumed and smeared with a Gaussian function for direct comparison with center-to-limb scans along Mercury's intensity equator. Data from a range of phase angles from 31 to 92 deg have been compared with smeared models assuming a Lambert surface, a surface which obeys the Lommel-Seeliger law and one which is Minnaertian, having a variable coefficient. Within the limits of the observations a lunar Minnaert surface yields the most consistent interpretation. An objective evaluation of the resolution of the photographs is obtained in terms of Gaussian half-widths.

Beebe, R. F.↗
NASA NTRS1975

The 15 cm mercury ion thruster research 1975

Doubly charged ion current measurements in the beam of a SERT II thruster are shown to introduce corrections which bring its calculated thrust into close agreement with that measured during flight testing. A theoretical model of doubly charged ion production and loss in mercury electron bombardment thrusters is discussed and is shown to yield doubly-to-singly charged ion density ratios that agree with experimental measurements obtained on a 15 cm diameter thruster over a range of operating conditions. Single cusp magnetic field thruster operation is discussed and measured ion beam profiles, performance data, doubly charged ion densities, and discharge plasma characteristics are presented for a range of operating conditions and thruster geometries. Variations in the characteristics of this thruster are compared to those observed in the divergent field thruster and the cusped field thruster is shown to yield flatter ion beam profiles at about the same discharge power and propellant utilization operating point. An ion optics test program is described and the measured effects of grid system dimensions on ion beamlet half angle and diameter are examined. The effectiveness of hollow cathode startup using a thermionically emitting filament within the cathode is examined over a range of mercury flow rates and compared to results obtained with a high voltage tickler startup technique. Results of cathode plasma property measurement tests conducted within the cathode are presented.

Wilbur, P. J.↗
NASA NTRS1975

Observations of Mercury's magnetic field

Magnetic field data obtained by Mariner 10 during the third and final encounter with the planet Mercury on 16 March 1975 were studied. A well developed bow shock and modest magnetosphere, previously observed at first encounter on 29 March 1974, were again observed. In addition, a much stronger magnetic field near closest approach, 400 gamma versus 98 gamma, was observed at an altitude of 327 km and approximately 70 deg north Mercurian latitude. Spherical harmonic analysis of the data provide an estimate of the centered planetary magnetic dipole of 4.7 x 10 to the 22nd power Gauss/cu cm with the axis tilted 12 deg to the rotation axis and in the same sense as Earth's. The interplanetary field was sufficiently different between first and third encounters that in addition to the very large field magnitude observed, it argues strongly against a complex induction process generating the observed planetary field. While a possibility exists that Mercury possesses a remanent field due to magnetization early in its formation, a present day active dynamo seems to be a more likely candidate for its origin.

Ness, N. F.↗
NASA NTRS1975

Substorms on Mercury

The Mariner 10 encounter of Mercury provided data showing a stron interaction between the solar wind and the planet similar to a scaled down version of that producing the earth's magnetosphere. Some of the features observed in Mercury's night side magnetosphere suggest time-dependent processes occurring there. Interpreted as temporal events, these features bear striking resemblances to substorm phenomena in the earth's magnetosphere.

Siscoe, G. L.↗
NASA NTRS1976

Comments on the origin of Mercury

The ratio between the mass of condensed matter in Mercury's nebular zone and the final mass of condensed matter in the planet is calculated, and the result suggests that condensed matter equal to more than 10 times the planet's mass was lost from that zone. Five hypotheses to account for this loss are considered: (1) the temperature in Mercury's zone was appreciably higher than the suggested value of 1400 K, (2) the excess was expelled by solar mass outflow, (3) the excess was dragged down by the sun during its contraction, (4) the excess was knocked out by Jupiter-perturbed planetesimals, and (5) the excess was knocked out by earth- and Venus-perturbed planetesimals. The plausibility of each hypothesis is examined, and it is concluded that only planetesimal scattering by earth and Venus appears to be ruled out

Kaula, W. M.↗
NASA NTRS1976

Observations of Mercury's magnetic field

Magnetic-field observations made during the third encounter of Mariner 10 with Mercury are discussed along with implications of an intrinsic field for the planetary interior. The data obtained confirm the presence of characteristic bow-shock, magnetosheath, and magnetosphere regions surrounding the planet and also indicate that Mercury occupies a much larger portion of its magnetosphere than does earth. Combined plasma, charged-particle, and magnetic-field data establish that the origin of the field is intrinsic to the planet rather than associated with an induction process due to solar-wind flow. Spherical harmonic analysis of the field data shows that the internal field of the planet is well described by a centered dipole with a moment of 5.0 by 10 to the 22nd power gauss-cu cm and oriented within 12 deg of the normal to the orbital plane in the same sense as earth. Surface intensities are deduced to be between about 300 and 800 gammas. It is concluded that an active dynamo is a more likely candidate than fossil magnetization for the origin of the field.

Ness, N. F.↗
NASA NTRS1976

Some aspects of core formation in Mercury

Some questions dealing with the nature and history of a large metallic core within Mercury are considered. These include the existence of a core, its size, whether it is fluid or solid, the timescale for core formation, the geological consequences of core formation, and whether such consequences are consistent with the surface geology. Several indirect lines of evidence are discussed which suggest the presence of a large iron-rich core. A core-formation model is examined in which core infall is accompanied by an increase of 17 km in planetary radius, an increase of 700 K in mean internal temperature, and substantial melting of the mantle. It is argued that if the core differentiated from an originally homogeneous planet, that event must have predated the oldest geological units comprising most of the planetary surface. A convective dynamo model for the source of Mercury's magnetic field is shown to conflict with cosmochemical models that do not predict a substantial radiogenic heat source in the core.

Solomon, S. C.↗
NASA NTRS1976

Mercury's atmosphere - A perspective after Mariner 10

Implications of Mariner 10 observations are discussed regarding the composition and structure of Mercury's atmosphere as well as its interaction with the solar wind. The Mariner 10 data cited indicate that Mercury's atmosphere is apparently similar to that on the moon. It is shown that helium could be supplied to the atmosphere either by the radioactive decay of uranium and thorium in the planetary crust or by accretion of a small fraction of the solar-wind flux. It is noted that the absence of argon could be explained by a potassium-deficient crust, but this conclusion is by no means firm. Upper limits for outgassing of H2O and CO2 are estimated which suggest that either the planet is deficient in volatiles or is relatively inactive in comparison with earth.

Kumar, S.↗
NASA NTRS1976

Does Mercury have a molten core

A feasible nonseismic observational experiment is proposed for determining the existence and extent of a conducting molten core within Mercury. This experiment would utilize the effects of a liquid core on the dynamics of Mercury's rotation; two necessary conditions for performing it are that the core must not follow the mantle's forced librations in longitude but must follow the mantle on the timescale of the 250,000-yr precession. A method is developed by assuming these conditions to be satisfied, and bounds are established on the core viscosity for which they are satisfied. It is shown that the value of the ratio of the moment of inertia of the mantle to the largest principal moment of inertia of the entire planet would indicate whether the core is most probably solid, partially fluid, or entirely fluid. Techniques are suggested for determining the unknowns required to compute the necessary ratio.

Peale, S. J.↗
NASA NTRS1977

Geologic investigation of Mercury data and the subsequent mapping and explanation of the quadrangle area H-12

The origin of scraps and ridges mapped in the H-12 region of Mercury are discussed. These features apparently are of non-impact origin and show distinctly non-random patterns of distribution and orientation. These patterns cannot be simply related to selective enhancement of visibility by lighting conditions, global stresses of spin down or interior cooling, or local uplifts or subsidences. Suggestions are made that the origin of these structures may lie outside the region of Mercury photographed or the scarps and ridges may have formed by reactivation of faults or joints bounding pre-existing crystal blocks.

Blasius, K. R.↗
NASA NTRS1977

Report of the Terrestrial Bodies Science Working Group. Volume 2: Mercury

The objectives and rationale for scientific investigation of Mercury are explored. Knowledge already obtained by astronomical observations and Mariner 10 spacecraft is reviewed and measurements required for the principal scientific goals are described. The use of low thrust propulsion systems is recommended so that maximum scientific return may be achieved and the reconnaissance phase of Mercury exploration may be completed in a single mission. Accelerated development is recommended on solar electric propulsion, solar sails, passive and active cooling mechanisms, and single rough landers.

Albee, A. L.↗
NASA NTRS1976

Observations of intercrater plains on Mercury

Analysis of Mariner 10 stereoscopic images of the surface of Mercury suggest that ancient, intercrater plains consist in part of visually indistinct but topographically defined circular depressions inferred to be highly degraded craters and basins. Embayment of some craters within the heavily cratered terrain and transection of basins and craters by intercrater plains suggest that formation of at least some intercrater plains postdates the later portions of the bombardment of Mercury by large objects. On the other hand, superposition of crater ejecta onto portions of intercrater plains in other areas indicates that some large craters formed into a pre-existing, intercrater plains unit. A single episode of obliteration (to form intercrater plains) or bombardment (to produce the large craters) cannot be reconciled with available observations. A more complex history of contemporaneous crater and plains formation is suggested.

Malin, M. C.↗
NASA NTRS1976

Experimental determination of Mercury's mass and oblateness

The ratio of the mass of the sun to that of Mercury deduced from the first and third flybys of Mercury by Mariner 10 is 6,023,600 plus or minus 600 and 6,023,700 plus or minus 300, respectively. Additional analysis should improve these uncertainties by a factor of 2 to 5. Assuming an equatorial radius of 2439 km, the mean density of the planet is 5.44 g/cu cm. The gravitational oblateness deduced from the first encounter is discussed. Data received from the third encounter are better suited for this determination and yield J2 of 0.00008 plus or minus 0.00006. In addition, the third-encounter data are sensitive to local gravity effects or anomalies.

Esposito, P. B.↗
NASA NTRS1977

Magnetospheric magnetic field of Mercury

This paper presents a model for the magnetospheric magnetic field of Mercury in which the external field is represented by an image dipole and a tail field and the internal field includes a dipole, a quadrupole, and an octupole. The dipole moment estimated by this model is approximately 2.4 x 10 to the 22nd G cu cm, tilted 2.3 degrees from the normal to the planetary orbital plane and having the same directional sense as that of the earth. The dipole, quadrupole, and octupole moment intensities are in the approximate ratios 1:0.4:0.3, respectively. All planetary field lines of the model magnetosphere are confined to a magnetospherelike region. Results are obtained which show the geometry, field line configuration, and field isointensity contours inside the magnetosphere of Mercury.

Whang, Y. C.↗
NASA NTRS1977

Flight to Mercury

The project development and active flight of the Mariner 10 deep-space probe mission is recounted in detail, with sequential blow-by-blow coverage. Early studies and speculation on the planet Mercury are reviewed, and the spin-orbit coupling and near-synchronous rotation of the planet are described. Use of Venus as a slingshot in a gravity-assist maneuver is described, and Mariner 10 records of Venus are shown. The three encounters of Mariner 10 with Mercury (March 1974, Aug. 1974, March 1975) are described in detail, with purposes, problems, mishaps, and glossy photographs recovered from Mariner 10 data. Information on the planet's magnetosphere, surface topography, inferred internal structure, and IR signature is provided, and the end-of-mission improvised solar-sail experiment is outlined.

Murray, B.↗