共查询到20条相似文献,搜索用时 46 毫秒
1.
Maria T. Zuber Oded Aharonson Jonathan M. Aurnou Andrew F. Cheng Steven A. Hauck II Moritz H. Heimpel Gregory A. Neumann Stanton J. Peale Roger J. Phillips David E. Smith Sean C. Solomon Sabine Stanley 《Space Science Reviews》2007,131(1-4):105-132
Current geophysical knowledge of the planet Mercury is based upon observations from ground-based astronomy and flybys of the
Mariner 10 spacecraft, along with theoretical and computational studies. Mercury has the highest uncompressed density of the
terrestrial planets and by implication has a metallic core with a radius approximately 75% of the planetary radius. Mercury’s
spin rate is stably locked at 1.5 times the orbital mean motion. Capture into this state is the natural result of tidal evolution
if this is the only dissipative process affecting the spin, but the capture probability is enhanced if Mercury’s core were
molten at the time of capture. The discovery of Mercury’s magnetic field by Mariner 10 suggests the possibility that the core
is partially molten to the present, a result that is surprising given the planet’s size and a surface crater density indicative
of early cessation of significant volcanic activity. A present-day liquid outer core within Mercury would require either a
core sulfur content of at least several weight percent or an unusual history of heat loss from the planet’s core and silicate
fraction. A crustal remanent contribution to Mercury’s observed magnetic field cannot be ruled out on the basis of current
knowledge. Measurements from the MESSENGER orbiter, in combination with continued ground-based observations, hold the promise
of setting on a firmer basis our understanding of the structure and evolution of Mercury’s interior and the relationship of
that evolution to the planet’s geological history. 相似文献
2.
MESSENGER: Exploring Mercury’s Magnetosphere 总被引:1,自引:0,他引:1
James A. Slavin Stamatios M. Krimigis Mario H. Acuña Brian J. Anderson Daniel N. Baker Patrick L. Koehn Haje Korth Stefano Livi Barry H. Mauk Sean C. Solomon Thomas H. Zurbuchen 《Space Science Reviews》2007,131(1-4):133-160
The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission to Mercury offers our first opportunity
to explore this planet’s miniature magnetosphere since the brief flybys of Mariner 10. Mercury’s magnetosphere is unique in
many respects. The magnetosphere of Mercury is among the smallest in the solar system; its magnetic field typically stands
off the solar wind only ∼1000 to 2000 km above the surface. For this reason there are no closed drift paths for energetic
particles and, hence, no radiation belts. Magnetic reconnection at the dayside magnetopause may erode the subsolar magnetosphere,
allowing solar wind ions to impact directly the regolith. Inductive currents in Mercury’s interior may act to modify the solar
wind interaction by resisting changes due to solar wind pressure variations. Indeed, observations of these induction effects
may be an important source of information on the state of Mercury’s interior. In addition, Mercury’s magnetosphere is the
only one with its defining magnetic flux tubes rooted beneath the solid surface as opposed to an atmosphere with a conductive
ionospheric layer. This lack of an ionosphere is probably the underlying reason for the brevity of the very intense, but short-lived,
∼1–2 min, substorm-like energetic particle events observed by Mariner 10 during its first traversal of Mercury’s magnetic
tail. Because of Mercury’s proximity to the sun, 0.3–0.5 AU, this magnetosphere experiences the most extreme driving forces
in the solar system. All of these factors are expected to produce complicated interactions involving the exchange and recycling
of neutrals and ions among the solar wind, magnetosphere, and regolith. The electrodynamics of Mercury’s magnetosphere are
expected to be equally complex, with strong forcing by the solar wind, magnetic reconnection, and pick-up of planetary ions
all playing roles in the generation of field-aligned electric currents. However, these field-aligned currents do not close
in an ionosphere, but in some other manner. In addition to the insights into magnetospheric physics offered by study of the
solar wind–Mercury system, quantitative specification of the “external” magnetic field generated by magnetospheric currents
is necessary for accurate determination of the strength and multi-polar decomposition of Mercury’s intrinsic magnetic field.
MESSENGER’s highly capable instrumentation and broad orbital coverage will greatly advance our understanding of both the origin
of Mercury’s magnetic field and the acceleration of charged particles in small magnetospheres. In this article, we review
what is known about Mercury’s magnetosphere and describe the MESSENGER science team’s strategy for obtaining answers to the
outstanding science questions surrounding the interaction of the solar wind with Mercury and its small, but dynamic, magnetosphere. 相似文献
3.
The Magnetic Field of Mercury 总被引:1,自引:0,他引:1
Brian J. Anderson Mario H. Acuña Haje Korth James A. Slavin Hideharu Uno Catherine L. Johnson Michael E. Purucker Sean C. Solomon Jim M. Raines Thomas H. Zurbuchen George Gloeckler Ralph L. McNutt Jr. 《Space Science Reviews》2010,152(1-4):307-339
The magnetic field strength of Mercury at the planet’s surface is approximately 1% that of Earth’s surface field. This comparatively low field strength presents a number of challenges, both theoretically to understand how it is generated and observationally to distinguish the internal field from that due to the solar wind interaction. Conversely, the small field also means that Mercury offers an important opportunity to advance our understanding both of planetary magnetic field generation and magnetosphere-solar wind interactions. The observations from the Mariner 10 magnetometer in 1974 and 1975, and the MESSENGER Magnetometer and plasma instruments during the probe’s first two flybys of Mercury on 14 January and 6 October 2008, provide the basis for our current knowledge of the internal field. The external field arising from the interaction of the magnetosphere with the solar wind is more prominent near Mercury than for any other magnetized planet in the Solar System, and particular attention is therefore paid to indications in the observations of deficiencies in our understanding of the external field. The second MESSENGER flyby occurred over the opposite hemisphere from the other flybys, and these newest data constrain the tilt of the planetary moment from the planet’s spin axis to be less than 5°. Considered as a dipole field, the moment is in the range 240 to 270 nT-R M 3 , where R M is Mercury’s radius. Multipole solutions for the planetary field yield a smaller dipole term, 180 to 220 nT-R M 3 , and higher-order terms that together yield an equatorial surface field from 250 to 290 nT. From the spatial distribution of the fit residuals, the equatorial data are seen to reflect a weaker northward field and a strongly radial field, neither of which can be explained by a centered-dipole matched to the field measured near the pole by Mariner 10. This disparity is a major factor controlling the higher-order terms in the multipole solutions. The residuals are not largest close to the planet, and when considered in magnetospheric coordinates the residuals indicate the presence of a cross-tail current extending to within 0.5R M altitude on the nightside. A near-tail current with a density of 0.1 μA/m2 could account for the low field intensities recorded near the equator. In addition, the MESSENGER flybys include the first plasma observations from Mercury and demonstrate that solar wind plasma is present at low altitudes, below 500 km. Although we can be confident in the dipole-only moment estimates, the data in hand remain subject to ambiguities for distinguishing internal from external contributions. The anticipated observations from orbit at Mercury, first from MESSENGER beginning in March 2011 and later from the dual-spacecraft BepiColombo mission, will be essential to elucidate the higher-order structure in the magnetic field of Mercury that will reveal the telltale signatures of the physics responsible for its generation. 相似文献
4.
James V. McAdams Robert W. Farquhar Anthony H. Taylor Bobby G. Williams 《Space Science Reviews》2007,131(1-4):219-246
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). 相似文献
5.
J. Wicht M. Mandea F. Takahashi U. R. Christensen M. Matsushima B. Langlais 《Space Science Reviews》2007,132(2-4):261-290
Mariner 10 measurements proved the existence of a large-scale internal magnetic field on Mercury. The observed field amplitude,
however, is too weak to be compatible with typical convective planetary dynamos. The Lorentz force based on an extrapolation
of Mariner 10 data to the dynamo region is 10−4 times smaller than the Coriolis force. This is at odds with the idea that planetary dynamos are thought to work in the so-called
magnetostrophic regime, where Coriolis force and Lorentz force should be of comparable magnitude. Recent convective dynamo
simulations reviewed here seem to resolve this caveat. We show that the available convective power indeed suffices to drive
a magnetostrophic dynamo even when the heat flow though Mercury’s core–mantle boundary is subadiabatic, as suggested by thermal
evolution models. Two possible causes are analyzed that could explain why the observations do not reflect a stronger internal
field. First, toroidal magnetic fields can be strong but are confined to the conductive core, and second, the observations
do not resolve potentially strong small-scale contributions. We review different dynamo simulations that promote either or
both effects by (1) strongly driving convection, (2) assuming a particularly small inner core, or (3) assuming a very large
inner core. These models still fall somewhat short of explaining the low amplitude of Mariner 10 observations, but the incorporation
of an additional effect helps to reach this goal: The subadiabatic heat flow through Mercury’s core–mantle boundary may cause
the outer part of the core to be stably stratified, which would largely exclude convective motions in this region. The magnetic
field, which is small scale, strong, and very time dependent in the lower convective part of the core, must diffuse through
the stagnant layer. Here, the electromagnetic skin effect filters out the more rapidly varying high-order contributions and
mainly leaves behind the weaker and slower varying dipole and quadrupole components (Christensen in Nature 444:1056–1058,
2006). Messenger and BepiColombo data will allow us to discriminate between the various models in terms of the magnetic fields
spatial structure, its degree of axisymmetry, and its secular variation. 相似文献
6.
Dipak K. Srinivasan Mark E. Perry Karl B. Fielhauer David E. Smith Maria T. Zuber 《Space Science Reviews》2007,131(1-4):557-571
The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) Radio Frequency (RF) Telecommunications Subsystem
is used to send commands to the spacecraft, transmit information on the state of the spacecraft and science-related observations,
and assist in navigating the spacecraft to and in orbit about Mercury by providing precise observations of the spacecraft’s
Doppler velocity and range in the line of sight to Earth. The RF signal is transmitted and received at X-band frequencies
(7.2 GHz uplink, 8.4 GHz downlink) by the NASA Deep Space Network. The tracking data from MESSENGER will contribute significantly
to achieving the mission’s geophysics objectives. The RF subsystem, as the radio science instrument, will help determine Mercury’s
gravitational field and, in conjunction with the Mercury Laser Altimeter instrument, help determine the topography of the
planet. Further analysis of the data will improve the knowledge of the planet’s orbital ephemeris and rotation state. The
rotational state determination includes refined measurements of the obliquity and forced physical libration, which are necessary
to characterize Mercury’s core state. 相似文献
7.
Deborah L. Domingue Patrick L. Koehn Rosemary M. Killen Ann L. Sprague Menelaos Sarantos Andrew F. Cheng Eric T. Bradley William E. McClintock 《Space Science Reviews》2007,131(1-4):161-186
The existence of a surface-bounded exosphere about Mercury was discovered through the Mariner 10 airglow and occultation experiments.
Most of what is currently known or understood about this very tenuous atmosphere, however, comes from ground-based telescopic
observations. It is likely that only a subset of the exospheric constituents have been identified, but their variable abundance
with location, time, and space weather events demonstrate that Mercury’s exosphere is part of a complex system involving the
planet’s surface, magnetosphere, and the surrounding space environment (the solar wind and interplanetary magnetic field).
This paper reviews the current hypotheses and supporting observations concerning the processes that form and support the exosphere.
The outstanding questions and issues regarding Mercury’s exosphere stem from our current lack of knowledge concerning the
surface composition, the magnetic field behavior within the local space environment, and the character of the local space
environment. 相似文献
8.
A. Milillo P. Wurz S. Orsini D. Delcourt E. Kallio R. M. KILLEN H. Lammer S. Massetti A. Mura S. Barabash G. Cremonese I. A. Daglis E. De Angelis A. M. Di Lellis S. Livi V. Mangano K. Torkar 《Space Science Reviews》2005,117(3-4):397-443
Mercury is a poorly known planet, since the only space-based information comes from the three fly-bys performed in 1974 by
the Mariner 10 spacecraft. Ground-based observations also provided some interesting results, but they are particularly difficult
to obtain due to the planet’s proximity to the Sun. Nevertheless, the fact that the planet’s orbit is so close to the Sun
makes Mercury a particularly interesting subject for extreme environmental conditions. Among a number of crucial scientific
topics to be addressed, Mercury’s exosphere, its interaction with the solar wind and its origin from the surface of the planet,
can provide important clues about planetary evolution. In fact, the Hermean exosphere is continuously eroded and refilled
by these interactions, so that it would be more proper to consider the Hermean environment as a single, unified system – surface-exosphere-magnetosphere.
These three parts are indeed strongly linked to each other. In recent years, the two missions scheduled to explore the iron
planet, the NASA MESSENGER mission (launched in March 2004) and the ESA cornerstone mission (jointly with JAXA) BepiColombo
(to be launched in 2012), have stimulated new interest in the many unresolved mysteries related to it. New ground-based observations,
made possible by new technologies, have been obtained, and new simulation studies have been performed. In this paper some
old as well as the very latest observations and studies related to the surface-exosphere-magnetosphere system are reviewed,
outlining the investigations achievable by the planned space-based observations. This review intends to support the studies,
in preparation of future data, and the definition of specific instrumentation. 相似文献
9.
André Balogh Réjean Grard Sean C. Solomon Rita Schulz Yves Langevin Yasumasa Kasaba Masaki Fujimoto 《Space Science Reviews》2007,132(2-4):611-645
Mercury is a very difficult planet to observe from the Earth, and space missions that target Mercury are essential for a comprehensive
understanding of the planet. At the same time, it is also difficult to orbit because it is deep inside the Sun’s gravitational
well. Only one mission has visited Mercury; that was Mariner 10 in the 1970s. This paper provides a brief history of Mariner
10 and the numerous imaginative but unsuccessful mission proposals since the 1970s for another Mercury mission. In the late
1990s, two missions—MESSENGER and BepiColombo—received the go-ahead; MESSENGER is on its way to its first encounter with Mercury
in January 2008. The history, scientific objectives, mission designs, and payloads of both these missions are described in
detail. 相似文献
10.
Doris Breuer Steven A. Hauck II Monika Buske Martin Pauer Tilman Spohn 《Space Science Reviews》2007,132(2-4):229-260
The interior evolution of Mercury—the innermost planet in the solar system, with its exceptional high density—is poorly known.
Our current knowledge of Mercury is based on observations from Mariner 10’s three flybys. That knowledge includes the important
discoveries of a weak, active magnetic field and a system of lobate scarps that suggests limited radial contraction of the
planet during the last 4 billion years. We review existing models of Mercury’s interior evolution and further present new
2D and 3D convection models that consider both a strongly temperature-dependent viscosity and core cooling. These studies
provide a framework for understanding the basic characteristics of the planet’s internal evolution as well as the role of
the amount and distribution of radiogenic heat production, mantle viscosity, and sulfur content of the core have had on the
history of Mercury’s interior.
The existence of a dynamo-generated magnetic field suggests a growing inner core, as model calculations show that a thermally
driven dynamo for Mercury is unlikely. Thermal evolution models suggest a range of possible upper limits for the sulfur content
in the core. For large sulfur contents the model cores would be entirely fluid. The observation of limited planetary contraction
(∼1–2 km)—if confirmed by future missions—may provide a lower limit for the core sulfur content. For smaller sulfur contents,
the planetary contraction obtained after the end of the heavy bombardment due to inner core growth is larger than the observed
value. Due to the present poor knowledge of various parameters, for example, the mantle rheology, the thermal conductivity
of mantle and crust, and the amount and distribution of radiogenic heat production, it is not possible to constrain the core
sulfur content nor the present state of the mantle. Therefore, it is difficult to robustly predict whether or not the mantle
is conductive or in the convective regime. For instance, in the case of very inefficient planetary cooling—for example, as
a consequence of a strong thermal insulation by a low conductivity crust and a stiff Newtonian mantle rheology—the predicted
sulfur content can be as low as 1 wt% to match current estimates of planetary contraction, making deep mantle convection likely.
Efficient cooling—for example, caused by the growth of a crust strongly in enriched in radiogenic elements—requires more than
6.5 wt% S. These latter models also predict a transition from a convective to a conductive mantle during the planet’s history.
Data from future missions to Mercury will aid considerably our understanding of the evolution of its interior. 相似文献
11.
John O. Goldsten Edgar A. Rhodes William V. Boynton William C. Feldman David J. Lawrence Jacob I. Trombka David M. Smith Larry G. Evans Jack White Norman W. Madden Peter C. Berg Graham A. Murphy Reid S. Gurnee Kim Strohbehn Bruce D. Williams Edward D. Schaefer Christopher A. Monaco Christopher P. Cork J. Del Eckels Wayne O. Miller Morgan T. Burks Lisle B. Hagler Steve J. DeTeresa Monika C. Witte 《Space Science Reviews》2007,131(1-4):339-391
A Gamma-Ray and Neutron Spectrometer (GRNS) instrument has been developed as part of the science payload for NASA’s Discovery
Program mission to the planet Mercury. Mercury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) launched
successfully in 2004 and will journey more than six years before entering Mercury orbit to begin a one-year investigation.
The GRNS instrument forms part of the geochemistry investigation and will yield maps of the elemental composition of the planet
surface. Major elements include H, O, Na, Mg, Si, Ca, Ti, Fe, K, and Th. The Gamma-Ray Spectrometer (GRS) portion detects
gamma-ray emissions in the 0.1- to 10-MeV energy range and achieves an energy resolution of 3.5 keV full-width at half-maximum
for 60Co (1332 keV). It is the first interplanetary use of a mechanically cooled Ge detector. Special construction techniques provide
the necessary thermal isolation to maintain the sensor’s encapsulated detector at cryogenic temperatures (90 K) despite the
intense thermal environment. Given the mission constraints, the GRS sensor is necessarily body-mounted to the spacecraft,
but the outer housing is equipped with an anticoincidence shield to reduce the background from charged particles. The Neutron
Spectrometer (NS) sensor consists of a sandwich of three scintillation detectors working in concert to measure the flux of
ejected neutrons in three energy ranges from thermal to ∼7 MeV. The NS is particularly sensitive to H content and will help
resolve the composition of Mercury’s polar deposits. This paper provides an overview of the Gamma-Ray and Neutron Spectrometer
and describes its science and measurement objectives, the design and operation of the instrument, the ground calibration effort,
and a look at some early in-flight data. 相似文献
12.
Charles E. Schlemm II Richard D. Starr George C. Ho Kathryn E. Bechtold Sarah A. Hamilton John D. Boldt William V. Boynton Walter Bradley Martin E. Fraeman Robert E. Gold John O. Goldsten John R. Hayes Stephen E. Jaskulek Egidio Rossano Robert A. Rumpf Edward D. Schaefer Kim Strohbehn Richard G. Shelton Raymond E. Thompson Jacob I. Trombka Bruce D. Williams 《Space Science Reviews》2007,131(1-4):393-415
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. 相似文献
13.
A. Sprague J. Warell G. Cremonese Y. Langevin J. Helbert P. Wurz I. Veselovsky S. Orsini A. Milillo 《Space Science Reviews》2007,132(2-4):399-431
Mercury’s surface is thought to be covered with highly space-weathered silicate material. The regolith is composed of material
accumulated during the time of planetary formation, and subsequently from comets, meteorites, and the Sun. Ground-based observations
indicate a heterogeneous surface composition with SiO2 content ranging from 39 to 57 wt%. Visible and near-infrared spectra, multi-spectral imaging, and modeling indicate expanses
of feldspathic, well-comminuted surface with some smooth regions that are likely to be magmatic in origin with many widely
distributed crystalline impact ejecta rays and blocky deposits. Pyroxene spectral signatures have been recorded at four locations.
Although highly space weathered, there is little evidence for the conversion of FeO to nanophase metallic iron particles (npFe0), or “iron blebs,” as at the Moon. Near- and mid-infrared spectroscopy indicate clino- and ortho-pyroxene are present at
different locations. There is some evidence for no- or low-iron alkali basalts and feldspathoids. All evidence, including
microwave studies, point to a low iron and low titanium surface. There may be a link between the surface and the exosphere
that may be diagnostic of the true crustal composition of Mercury. A structural global dichotomy exists with a huge basin
on the side not imaged by Mariner 10. This paper briefly describes the implications for this dichotomy on the magnetic field
and the 3 : 2 spin : orbit coupling. All other points made above are detailed here with an account of the observations, the
analysis of the observations, and theoretical modeling, where appropriate, that supports the stated conclusions. 相似文献
14.
The Geology of Mercury: The View Prior to the MESSENGER Mission 总被引:1,自引:0,他引:1
James W. Head Clark R. Chapman Deborah L. Domingue S. Edward Hawkins III William E. McClintock Scott L. Murchie Louise M. Prockter Mark S. Robinson Robert G. Strom Thomas R. Watters 《Space Science Reviews》2007,131(1-4):41-84
Mariner 10 and Earth-based observations have revealed Mercury, the innermost of the terrestrial planetary bodies, to be an
exciting laboratory for the study of Solar System geological processes. Mercury is characterized by a lunar-like surface,
a global magnetic field, and an interior dominated by an iron core having a radius at least three-quarters of the radius of
the planet. The 45% of the surface imaged by Mariner 10 reveals some distinctive differences from the Moon, however, with
major contractional fault scarps and huge expanses of moderate-albedo Cayley-like smooth plains of uncertain origin. Our current
image coverage of Mercury is comparable to that of telescopic photographs of the Earth’s Moon prior to the launch of Sputnik
in 1957. We have no photographic images of one-half of the surface, the resolution of the images we do have is generally poor
(∼1 km), and as with many lunar telescopic photographs, much of the available surface of Mercury is distorted by foreshortening
due to viewing geometry, or poorly suited for geological analysis and impact-crater counting for age determinations because
of high-Sun illumination conditions. Currently available topographic information is also very limited. Nonetheless, Mercury
is a geological laboratory that represents (1) a planet where the presence of a huge iron core may be due to impact stripping
of the crust and upper mantle, or alternatively, where formation of a huge core may have resulted in a residual mantle and
crust of potentially unusual composition and structure; (2) a planet with an internal chemical and mechanical structure that
provides new insights into planetary thermal history and the relative roles of conduction and convection in planetary heat
loss; (3) a one-tectonic-plate planet where constraints on major interior processes can be deduced from the geology of the
global tectonic system; (4) a planet where volcanic resurfacing may not have played a significant role in planetary history
and internally generated volcanic resurfacing may have ceased at ∼3.8 Ga; (5) a planet where impact craters can be used to
disentangle the fundamental roles of gravity and mean impactor velocity in determining impact crater morphology and morphometry;
(6) an environment where global impact crater counts can test fundamental concepts of the distribution of impactor populations
in space and time; (7) an extreme environment in which highly radar-reflective polar deposits, much more extensive than those
on the Moon, can be better understood; (8) an extreme environment in which the basic processes of space weathering can be
further deduced; and (9) a potential end-member in terrestrial planetary body geological evolution in which the relationships
of internal and surface evolution can be clearly assessed from both a tectonic and volcanic point of view. In the half-century
since the launch of Sputnik, more than 30 spacecraft have been sent to the Moon, yet only now is a second spacecraft en route
to Mercury. The MESSENGER mission will address key questions about the geologic evolution of Mercury; the depth and breadth
of the MESSENGER data will permit the confident reconstruction of the geological history and thermal evolution of Mercury
using new imaging, topography, chemistry, mineralogy, gravity, magnetic, and environmental data. 相似文献
15.
The Mercury Dual Imaging System on the MESSENGER Spacecraft 总被引:1,自引:0,他引:1
S. Edward Hawkins III John D. Boldt Edward H. Darlington Raymond Espiritu Robert E. Gold Bruce Gotwols Matthew P. Grey Christopher D. Hash John R. Hayes Steven E. Jaskulek Charles J. Kardian Jr. Mary R. Keller Erick R. Malaret Scott L. Murchie Patricia K. Murphy Keith Peacock Louise M. Prockter R. Alan Reiter Mark S. Robinson Edward D. Schaefer Richard G. Shelton Raymond E. Sterner II Howard W. Taylor Thomas R. Watters Bruce D. Williams 《Space Science Reviews》2007,131(1-4):247-338
The Mercury Dual Imaging System (MDIS) on the MESSENGER spacecraft will provide critical measurements tracing Mercury’s origin
and evolution. MDIS consists of a monochrome narrow-angle camera (NAC) and a multispectral wide-angle camera (WAC). The NAC
is a 1.5° field-of-view (FOV) off-axis reflector, coaligned with the WAC, a four-element refractor with a 10.5° FOV and 12-color
filter wheel. The focal plane electronics of each camera are identical and use a 1,024×1,024 Atmel (Thomson) TH7888A charge-coupled
device detector. Only one camera operates at a time, allowing them to share a common set of control electronics. The NAC and
the WAC are mounted on a pivoting platform that provides a 90° field-of-regard, extending 40° sunward and 50° anti-sunward
from the spacecraft +Z-axis—the boresight direction of most of MESSENGER’s instruments. Onboard data compression provides capabilities for pixel
binning, remapping of 12-bit data into 8 bits, and lossless or lossy compression. MDIS will acquire four main data sets at
Mercury during three flybys and the two-Mercury-solar-day nominal mission: a monochrome global image mosaic at near-zero emission
angles and moderate incidence angles, a stereo-complement map at off-nadir geometry and near-identical lighting, multicolor
images at low incidence angles, and targeted high-resolution images of key surface features. These data will be used to construct
a global image base map, a digital terrain model, global maps of color properties, and mosaics of high-resolution image strips.
Analysis of these data will provide information on Mercury’s impact history, tectonic processes, the composition and emplacement
history of volcanic materials, and the thickness distribution and compositional variations of crustal materials. This paper
summarizes MDIS’s science objectives and technical design, including the common payload design of the MDIS data processing
units, as well as detailed results from ground and early flight calibrations and plans for Mercury image products to be generated
from MDIS data. 相似文献
16.
Tim Van Hoolst Frank Sohl Igor Holin Olivier Verhoeven Véronique Dehant Tilman Spohn 《Space Science Reviews》2007,132(2-4):203-227
This review addresses the deep interior structure of Mercury. Mercury is thought to consist of similar chemical reservoirs
(core, mantle, crust) as the other terrestrial planets, but with a relatively much larger core. Constraints on Mercury’s composition
and internal structure are reviewed, and possible interior models are described. Large advances in our knowledge of Mercury’s
interior are not only expected from imaging of characteristic surface features but particularly from geodetic observations
of the gravity field, the rotation, and the tides of Mercury. The low-degree gravity field of Mercury gives information on
the differences of the principal moments of inertia, which are a measure of the mass concentration toward the center of the
planet. Mercury’s unique rotation presents several clues to the deep interior. From observations of the mean obliquity of
Mercury and the low-degree gravity data, the moments of inertia can be obtained, and deviations from the mean rotation speed
(librations) offer an exciting possibility to determine the moment of inertia of the mantle. Due to its proximity to the Sun,
Mercury has the largest tides of the Solar System planets. Since tides are sensitive to the existence and location of liquid
layers, tidal observations are ideally suited to study the physical state and size of the core of Mercury. 相似文献
17.
William V. Boynton Ann L. Sprague Sean C. Solomon Richard D. Starr Larry G. Evans William C. Feldman Jacob I. Trombka Edgar A. Rhodes 《Space Science Reviews》2007,131(1-4):85-104
The instrument suite on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft is well suited
to address several of Mercury’s outstanding geochemical problems. A combination of data from the Gamma-Ray and Neutron Spectrometer
(GRNS) and X-Ray Spectrometer (XRS) instruments will yield the surface abundances of both volatile (K) and refractory (Al,
Ca, and Th) elements, which will test the three competing hypotheses for the origin of Mercury’s high bulk metal fraction:
aerodynamic drag in the early solar nebula, preferential vaporization of silicates, or giant impact. These same elements,
with the addition of Mg, Si, and Fe, will put significant constraints on geochemical processes that have formed the crust
and produced any later volcanism. The Neutron Spectrometer sensor on the GRNS instrument will yield estimates of the amount
of H in surface materials and may ascertain if the permanently shadowed polar craters have a significant excess of H due to
water ice. A comparison of the FeO content of olivine and pyroxene determined by the Mercury Atmospheric and Surface Composition
Spectrometer (MASCS) instrument with the total Fe determined through both GRNS and XRS will permit an estimate of the amount
of Fe present in other forms, including metal and sulfides. 相似文献
18.
John F. Cavanaugh James C. Smith Xiaoli Sun Arlin E. Bartels Luis Ramos-Izquierdo Danny J. Krebs Jan F. McGarry Raymond Trunzo Anne Marie Novo-Gradac Jamie L. Britt Jerry Karsh Richard B. Katz Alan T. Lukemire Richard Szymkiewicz Daniel L. Berry Joseph P. Swinski Gregory A. Neumann Maria T. Zuber David E. Smith 《Space Science Reviews》2007,131(1-4):451-479
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 August 3, 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. MLA will
sample the planet’s surface to within a 1-m range error when the line-of-sight range to Mercury is less than 1,200 km under
spacecraft nadir pointing or the slant range is less than 800 km. The altimeter measurements will be used to determine the
planet’s forced physical 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 1,064 nm. This paper describes the instrument design, prelaunch testing, calibration, and results of postlaunch testing. 相似文献
19.
G. Cremonese A. Sprague J. Warell N. Thomas L. Ksamfomality 《Space Science Reviews》2007,132(2-4):291-306
The Mariner 10 spacecraft made three flyby passes of Mercury in 1974 and 1975. It imaged a little less than half of the surface
and discovered Mercury had an intrinsic magnetic field. This paper briefly describes the surface of Mercury as seen by Mariner
10 as a backdrop to the discoveries made since then by ground-based observations and the optimistic anticipation of new discoveries
by MESSENGER and BepiColombo spacecraft that are scheduled for encounter in the next decade. 相似文献
20.
Leonid Ksanfomality John Harmon Elena Petrova Nicolas Thomas Igor Veselovsky Johan Warell 《Space Science Reviews》2007,132(2-4):351-397
New planned orbiter missions to Mercury have prompted renewed efforts to investigate the surface of Mercury via ground-based
remote sensing. While the highest resolution instrumentation optical telescopes (e.g., HST) cannot be used at angular distances
close to the Sun, advanced ground-based astronomical techniques and modern analytical and software can be used to obtain the
resolved images of the poorly known or unknown part of Mercury. Our observations of the planet presented here were carried
out in many observatories at morning and evening elongation of the planet. Stacking the acquired images of the hemisphere
of Mercury, which was not observed by the Mariner 10 mission (1974–1975), is presented. Huge features found there change radically
the existing hypothesis that the “continental” character of a surface may be attributed to the whole planet. We present the
observational method, the data analysis approach, the resulting images and obtained properties of the Mercury’s surface. 相似文献