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ScienceWeek
PLANETARY SCIENCE: ON THE INTERIOR OF MARS
The following points are made by Y. Fei and C. Bertka (Science 2005 308:1120):
1) The planetary core is the engine of a planet: It drives convection of the mantle, shapes the planet's surface, and -- if the core contains convective molten metal that creates a dynamo -- generates a global magnetic field. Based on its mean density and the bulk chemistry of terrestrial planets, Mars is believed to have a dense metallic core and a silicate mantle. However, because no seismic data exist for Mars, the density profile of its interior and the depth of the core-mantle boundary are not known precisely, and it remains unclear whether the martian core is solid or liquid.
2) For the past decade, space missions to Mars have provided important constraints on the physics and chemistry of its interior, although these missions were primarily designed to map and understand surface or near-surface features of the planet. From a combined analysis of Mars Global Surveyor tracking data and Mars Pathfinder and Viking Lander range and Doppler data, the planet's moment of inertia -- an important geophysical parameter for understanding the planet's internal density distribution --has been determined at high precision [1,2]. Topography and gravity data collected by Mars Global Surveyor have constrained the global average thickness of the martian crust to between 30 and 80 km [3,4].
3) Models that combine these data with a range of possible core compositions allow the boundaries for mantle density to be defined. The results indicate that the martian mantle is more iron-rich than that of Earth [5]. Interpretations of the chemistry and mineralogy of martian meteorites and of the basaltic rocks at the landing sites of the Mars Exploration Rover Mission, although model dependent, are consistent with this conclusion.
4) Several lines of independent evidence suggest that the martian core has been liquid throughout its history. The first line of evidence comes from the discovery of strongly magnetized ancient crust by Mars Global Surveyor. The magnetization was acquired more than 4 billion years ago, implying a short-lived (~0.5 billion years) early martian core dynamo. Such a core dynamo may be driven either by compositional convection (which is set in motion by a composition gradient) in a liquid outer core due to solidification of an inner core, or by thermal convection in a fully liquid core due to high heat flux out of the core.
References (abridged):
1. W. M. Folkner et al., Science 278, [1749] (1997)
2. C. F. Yoder et al., Science 300, [299] (2003)
3. M. T. Zuber et al., Science 287, [1788] (2000)
4. M. A. Wieczorek, M. T. Zuber, J. Geophys. Res. 109, 10.1029/2003JE002153 (2004)
5. C. M. Bertka, Y. Fei, Earth Planet. Sci. Lett. 157, 79 (1998)
Science http://www.sciencemag.org
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PLANETARY SCIENCE: ON ANCIENT MARS
The following points are made by S.C. Solomon et al (Science 2005 307:1214):
1) Observations from spacecraft and analyses of martian meteorites suggest that the first billion years after Solar System formation was a time of intense surficial and internal activity on Mars. The heavy impact bombardment of the inner Solar System obscured much of the early record, but signatures of that era remain in the structure and magnetization of the martian crust and in isotope anomalies in martian meteorites. Understanding the comparative evolution of the terrestrial planets offers the potential to extract generalizations on common processes. Furthermore, the history of early Mars may offer clues to the earliest history of the larger Earth, from which even fewer remnants have been preserved. Finally, Mars has special interest as a potential habitat for past or present life, and it is during the earliest era when the boundary conditions pertinent to life were established.
2) Martian history has been divided into three major epochs on the basis of stratigraphic relationships and the density of impact craters [1-3]. The Noachian Epoch dates terrain older than about 3.7 billion years ago (Ga) and coincides approximately with the time of heavy impact bombardment of the inner Solar System [4]. About 40% of the surface of Mars [5] is Noachian in age, as are all of the largest impact basins. Progressively younger epochs are the Hesperian (3.7 to 3 Ga) and Amazonian (3 Ga to present). Of more than 30 martian meteorites -- so identified on the basis of generally young crystallization ages, distinctive rock chemical and isotopic signatures, and molecular and isotopic compositions of trapped gases similar to those of the martian atmosphere -- only one (ALH84001 at about 4.5 Ga) is Noachian in age; all of the others have ages of 1.3 Ga or less and sample the Middle to Late Amazonian.
3) Dynamical simulations of the accretion of the terrestrial planets hint that Mars may have formed in as short an interval as several million years. Calculations of the final stages of terrestrial planet formation -- the gravitational interaction over 10^(8) years of planetary embryos the size of Mercury to Mars formed in the solar nebula disk during the first 10^(6) years of solar system history -- can account for planets with masses and semimajor axes similar to those of Earth and Venus, but any final body at the orbit of Mars tends to be too large. This difficulty may indicate an unusually low initial density of disk material in the vicinity of Mars's orbit. Alternatively, Mars may be a surviving embryo that escaped either accretion or ejection. This possibility would reflect the highly chaotic nature of the late-stage planetary formation process and would imply that Mars was nearly fully formed before the final stage of formation of the larger inner planets.
4) In summary: Mars was most active during its first billion years. The core, mantle, and crust formed within 50 million years of Solar System formation. A magnetic dynamo in a convecting fluid core magnetized the crust, and the global field shielded a more massive early atmosphere against solar wind stripping. The Tharsis province became a focus for volcanism, deformation, and outgassing of water and carbon dioxide in quantities possibly sufficient to induce episodes of climate warming. Surficial and near-surface water contributed to regionally extensive erosion, sediment transport, and chemical alteration. Deep hydrothermal circulation accelerated crustal cooling, preserved variations in crustal thickness, and modified patterns of crustal magnetization.
References (abridged):
1. D. H. Scott, K. L. Tanaka, U.S. Geol. Surv. Misc. Geol. Invest. Map I-1802-A (1986)
2. R. Greeley, J. E. Guest, U.S. Geol. Surv. Misc. Geol. Invest. Map I-1802-B (1987)
3. K. L. Tanaka, D. H. Scott, U.S. Geol. Surv. Misc. Geol. Invest. Map I-1802-C (1987)
4. W. K. Hartmann, G. Neukum, Space Sci. Rev. 96, 165 (2001)
5. K. L. Tanaka, N. K. Isbell, D. H. Scott, R. Greeley, J. E. Guest, in Proceedings of the 18th Lunar and Planetary Science Conference, G. Ryder, Ed. (Cambridge Univ. Press, Cambridge, 1988), pp. 665-678
Science http://www.sciencemag.org
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GEOCHEMICAL EVIDENCE FOR WATER WITHIN MARS
Notes by ScienceWeek:
Mars is now a desert world on which liquid water, because of ambient conditions, is not likely to be found at the surface: average temperatures are below 273 degrees kelvin and atmospheric pressures are at or below water's triple-point vapor pressure of 6.1 millibars. However, in 1972 the Mariner 9 orbiter mission photographed evidence -- in the form of apparent giant flood channels and arborized networks of small valleys -- that liquid water might have been stable in the surface environment at some time in the past. Analysis of Mars 4 and Mars 5 data, Viking orbiter images (1976-1980), and observations of flood terrain by Mars Pathfinder in 1997 supported this conclusion.
In general, the term "metamorphism" refers to the process of changing the characteristics of a rock in response to changes in temperature, pressure, or volatile content. Most metamorphic changes of rocks do not include bulk chemical changes, but merely the crystallization of new mineral phases. Examples of the transformation of sediments through metamorphism are sand to sandstone and peat to coal. The term "regional metamorphism" refers to the recrystallization of pre-existing rocks in response to simultaneous changes of ambient conditions that include temperature and the weight of overlying rocks (lithostatic pressure). The term "contact metamorphism" (thermal metamorphism) refers to the recrystallization of rocks surrounding an igneous intrusion in response to the heat supplied by that intrusion.
In general, the term "magma" refers to molten rock generated by partial melting of a planet's crust. Igneous rocks are rocks that have congealed from a molten mass, particularly from magma.
The term "pyroxene" refers to any of a group of rock-forming minerals of variable composition, among which calcium-, magnesium-, and iron-rich varieties predominate. Pyroxenes are found in almost every variety of igneous rock, and they also occur in rocks of widely different compositions formed under conditions of regional and contact metamorphism.
Meteorites are divided roughly into 3 main classes according to their composition. "Iron meteorites" consist of an alloy of iron and nickel; "stony meteorites" consist of silicate minerals; and "iron-stony meteorites" are a mixture of the two previous types. The stony meteorites are further divided into "chondrites" and "achondrites". Chondrites contain small spherules of high-temperature silicates ("chondrules") and constitute more than 85 percent of recovered meteorites. The achondrites (which contain no chondrules) range in composition from rocks made up essentially of single minerals (e.g., olivine) to rocks resembling basaltic lava. Each category is further subdivided on the basis of chemical composition. (The olivines are a group of metal-silicate minerals, with the metal as magnesium, iron, manganese, and calcium, with minor amounts of nickel. The most common form is a solid solution of magnesium silicate and iron silicate.)
Basalt is a dark gray to black igneous rock of volcanic origin that cools rapidly.
In this context, the term "outgassing" refers to the release of gases by volcanic activity.
"Shergottite" is a very rare type of achondrite meteorite, named after the meteorite that fell at Shergotty, India, in 1865, the first known meteorite fall of this type. The Shergotty meteorite consists primarily of pyroxene, and it belongs to the class of meteorites known as SNC meteorites (shergottites, *nakhlites, *chassignites; called "snick" meteorites") which on the basis of their composition are considered to be from Mars. The SNC meteorites are igneous rocks that apparently solidified from a cooling magma near the surface of their parent body. All but one are relatively young (less than 1.3 billion years old); the single ancient SNC meteorite found to date was apparently formed 4.5 billion years ago. The proportions and isotopic abundance of noble gases trapped in one shergattite resemble the composition of the Martian atmosphere as analyzed by the Viking landers. The current consensus is that the SNC meteorites were ejected from Mars by impacts, and entered orbits around the Sun before falling to Earth.
The following points are made by H.Y. McSween Jr et al (Nature 25 Jan 01 409:487):
1) The authors point out that observations of Martian surface morphology have been used to argue that an ancient ocean once existed on Mars. It has been proposed that significant quantities of such water could have been supplied to the Martian surface via outgassing, but this suggestion is apparently contradicted by the low magmatic water content that is generally inferred from chemical analysis of igneous Martian meteorites.
2) The authors now report the distributions of trace elements within pyroxenes of the Shergotty meteorite -- a basalt body apparently ejected 175 million years ago from Mars -- as well as hydrous and anhydrous crystallization experiments that together imply that water contents of pre-eruptive magma on Mars could have been up to 1.8 percent. The authors report they have found that in the Shergotty meteorite, the inner cores of pyroxene minerals (which are believed to have formed at depths in the Martian crust) are enriched in soluble trace elements when compared to the outer rims of the minerals (which are believed to have crystallized on or near to the Martian surface). The authors suggest this implies that water was present in pyroxenes at depth, but was largely lost as pyroxenes were carried to the surface during magma ascent. The authors conclude that ascending magmas possibly delivered significant quantities of water to the Martian surface in recent times, this conclusion reconciling geologic and petrologic views of the outgassing history of Mars.
Nature http://www.nature.com/nature
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Notes by ScienceWeek:
nakhlites: A rare type of achondrite meteorite, named after the meteorite that fell at Nakla, Egypt, in 1911, the first known meteorite fall of this type. The nakhlites (also called "augite-olivine achondrites) consist of approximately 80 percent by weight of augite (a type of pyroxene) and approximately 14 percent by weight of iron-rich olivine. The textures of nakhlites suggest they formed within cooling magmas.
chassignites: Another rare type of achondrite meteorite, named after the meteorite that fell at Chassigny, France, in 1815, the only known meteorite fall of this type. Chassignite (also called "olivine achondrite) is an olivine-rich rock: the Chassigny meteorite contains 92 percent olivine.
ScienceWeek http://scienceweek.com
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