The history of the core dynamos of Mars and the Moon inferred from their crustal magnetization: a brief review

2019 ◽  
Vol 56 (9) ◽  
pp. 917-931
Author(s):  
Jafar Arkani-Hamed
Keyword(s):  
The Moon ◽  
Positive Mass ◽  
Polar Wander ◽  
The Core ◽  
True Polar Wander ◽  
History Of ◽  
Giant Impacts ◽  
Crustal Magnetization ◽  
Multiple Impact ◽  
Thermochemical Convection

The core dynamos of Mars and the Moon have distinctly different histories. Mars had no core dynamo at the end of accretion. It took ∼100 Myr for the core to create a strong dynamo that magnetized the martian crust. Giant impacts during 4.2–4.0 Ga crippled the core dynamo intermittently until a thick stagnant lithosphere developed on the surface and reduced the heat flux at the core–mantle boundary, killing the dynamo at ∼3.8 Ga. On the other hand, the Moon had a strong core dynamo at the end of accretion that lasted ∼100 Myr and magnetized its primordial crust. Either precession of the core or thermochemical convection in the mantle or chemical convection in the core created a strong core dynamo that magnetized the sources of the isolated magnetic anomalies in later times. Mars and the Moon indicate dynamo reversals and true polar wander. The polar wander of the Moon is easier to explain compared to that of Mars. It was initiated by the mass deficiency at South Pole Aitken basin, which moved the basin southward by ∼68° relative to the dipole axis of the core field. The formation of mascon maria at later times introduced positive mass anomalies at the surface, forcing the Moon to make an additional ∼52° degree polar wander. Interaction of multiple impact shock waves with the dynamo, the abrupt angular momentum transfer to the mantle by the impactors, and the global overturn of the core after each impact were probably the factors causing the dynamo reversal.

Introductory remarks

1982 ◽  
Vol 306 (1492) ◽  
pp. 9-10 ◽  
Keyword(s):  
Seismic Data ◽  
Geomagnetic Field ◽  
Early History ◽  
The Moon ◽  
The Core ◽  
The Past ◽  
The Earth ◽  
History Of ◽  
The Subject ◽  
Additional Constraints

The suggestion for this Discussion Meeting was put forward more than three years ago. The format of the programme has changed many times since the original version, reflecting in part changing interests in different aspects of the subject. Of the 25 papers to be presented, only 5 discuss the constitution of the core, 13 deal with the geomagnetic field (including the secular variation and reversals) and all but 1 of the remaining 7 on geophysical interpretations are also concerned with the geomagnetic field. This emphasis on geomagnetism reflects the additional constraints that the absence or presence of a magnetic field may put on the constitution of all the planets and the Moon. In contrast to the Earth, the record of the first 10 9 years of planetary history is still at least partly preserved on the Moon, Mercury and Mars (and perhaps on Venus), and a study of this record on these other bodies may yield some information on the early history of the Earth. We have some seismic data for the Moon, but it is only for the Earth that we have a rich store of such data. In this connection, a word of caution is in order. It must not be forgotten that the structure of the Earth as revealed by seismic data is only a snapshot of what it is like today, and in many ways a very imperfect snapshot. There is no science of palaeoseismology, and seismic data tell us nothing about the structure of the Earth in the past nor of its evolution.

scholarly journals Crustal porosity reveals the bombardment history of the Moon

2021 ◽  
Author(s):  
Ya Huei Huang ◽  
Jason Soderblom ◽  
David Minton ◽  
Masatoshi Hirabayashi ◽  
Jay Melosh
Keyword(s):  
Solar System ◽  
Surface Expression ◽  
Orbital Evolution ◽  
Terrestrial Planets ◽  
The Moon ◽  
Planetary Evolution ◽  
History Of ◽  
Giant Impacts ◽  
Formation And Evolution ◽  
Lunar Basins

Abstract Planetary bombardment histories provide critical information regarding the formation and evolution of the Solar System and of the planets within it. These records evidence transient instabilities in the Solar System’s orbital evolution, giant impacts such as the Moon-forming impact, and material redistribution. Such records provide insight into planetary evolution, including the deposition of energy, delivery of materials, and crustal processing, specifically the modification of porosity. Bombardment histories are traditionally constrained from the surface expression of impacts — these records, however, are degraded by various geologic processes. Here we show that the Moon’s porosity contains a more complete record of its bombardment history. We find that the terrestrial planets were subject to double the number of ≥20-km-diameter-crater-forming impacts than are recorded on the lunar highlands, fewer than previously thought to have occurred. We show that crustal porosity doesn’t slowly increase as planets evolve, but instead is generated early in a planet’s evolution when most basins formed and decreases as planets evolve. We show that porosity constrains the relative ages of basins formed early in a planet’s evolution, a timeframe for which little information exists. These findings demonstrate that the Solar System was less violent than previously thought. Fewer volatiles and other materials were delivered to the terrestrial planets, consistent with estimates of the delivery of siderophiles and water to the Moon. High crustal porosity early in the terrestrial planets’ evolution slowed their cooling and enhanced their habitability. Several lunar basins formed early than previously considered, casting doubt on the existence of a late heavy bombardment.

Geochemical Constraints on the Early Thermal History of the Earth

1989 ◽  
Vol 44 (10) ◽  
pp. 883-890 ◽  
Author(s):  
Michael J. Drake
Keyword(s):  
Upper Mantle ◽  
Partition Coefficients ◽  
Current Theory ◽  
Gravitational Potential Energy ◽  
The Moon ◽  
The Core ◽  
The Earth ◽  
History Of ◽  
Geochemical Constraints ◽  
Origin Of The Moon

Abstract Theories of the formation of the Earth strongly suggest that the Earth should have been substantially molten during and immediately after accretion. Estimates of the composition of the upper mantle indicate that many elements are present in chondritic ratios. Experimental measurements of element partition coefficients show that segregation of perovskite, majorite garnet, or olivine would fractionate the ratios of these elements away from chondritic values. The implication of these geochemical observations is that the Earth did not undergo extensive fractionation during and immediately following accretion. One possibility is that the Earth did not become substantially molten. Alternatively, if the Earth was indeed substantially molten, then it is possible that minerals were entrained in magma and were unable to segregate. In the former case, the accretional process must have delivered gravitational potential energy more slowly than current theory predicts, and an origin of the Moon in a giant impact would be unlikely. In the latter case, the high Mg/Si ratio in the upper mantle of the Earth relative to most classes of chondrites would be intrinsic to the silicate portion of the Earth. Unless significant amounts of Si exist in the core, the high Mg/Si ratio is a bulk planetary property, implying that the accretional process did not mix material between 1 AU and 2-4 AU.

scholarly journals The paleoinclination of the ancient lunar magnetic field from an Apollo 17 basalt

2021 ◽  
Author(s):  
Claire Nichols ◽  
Benjamin Weiss ◽  
Brenna Getzin ◽  
Harrison Schmitt ◽  
Annemarieke Beguin ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Lunar Surface ◽  
Magnetic Anomalies ◽  
Wall Rock ◽  
The Moon ◽  
Polar Wander ◽  
True Polar Wander ◽  
Field Geometry ◽  
Mare Basalts ◽  
Over Time

Abstract Paleomagnetic studies of Apollo samples indicate that the Moon generated a core dynamo lasting for at least 2 billion years. However, the geometry of the lunar magnetic field is still largely unknown because the original orientations of essentially all Apollo samples have not been well-constrained. Determining the direction of the lunar magnetic field over time could elucidate the mechanism by which the lunar dynamo was powered and whether the Moon experienced true polar wander. Here we present measurements of the lunar magnetic field 3.7 billion years (Ga) ago as recorded by Apollo 17 mare basalts 75035 and 75055. These samples formed as part of basalt flows in the Taurus-Littrow valley that make up wall-rock within Camelot crater, now exposed at the rim of the crater. Using apparent layering in the parent boulder for 75055, we inferred its original paleohorizontal orientation on the lunar surface at the time of magnetization. We find that 75035 and 75055 record a mean paleointensity of ~50 µT. Furthermore, 75055 records a paleoinclination of 34 ± 11°. This inclination is consistent with, but does not require, a selenocentric axial dipole field geometry (i.e., a dipole in the center of the Moon and aligned along the spin axis). Additionally, although true polar wander is also not required by our data, true polar wander paths inferred from some independent studies of lunar hydrogen deposits and crustal magnetic anomalies are consistent with our measured paleoinclination.

THE CORE AND PERIPHERY OF THE CURRENT CULTURE OF ARTISTIC CREATION ABOUT THE GREAT PATRIOTIC WAR IN THE STUDENTS’ VIEWS

2020 ◽  
Vol 6 (3) ◽  
pp. 130-149
Author(s):  
Elena N. NARKHOVA ◽  
Dmitry Yu. NARKHOV
Keyword(s):  
Historical Memory ◽  
Soviet Period ◽  
Russian Society ◽  
Student Culture ◽  
The Core ◽  
Musical Works ◽  
Great Patriotic War ◽  
History Of ◽  
Current Student ◽  
Historical Periods

This article analyzes the degree of demand for works of art (films and television films and series, literary and musical works, works of monumental art) associated with the history of the Great Patriotic War among contemporary students. This research is based on the combination of two theories, which study the dynamics and statics of culture in the society — the theory of the nucleus and periphery by Yu. M. Lotman and the theory of actual culture by L. N. Kogan. The four waves of research (2005, 2010, 2015, 2020) by the Russian Society of Socio¬logists (ROS) have revealed a series of works in various genres on this topic in the core structure and on the periphery of the current student culture; this has also allowed tracing the dynamics of demand and the “movement” of these works in the sociocultural space. The authors introduce the concept of the archetype of the echo of war. The high student recognition of works of all historical periods (from wartime to the present day) is shown. A significant complex of works has been identified, forming two contours of the periphery. Attention is drawn to the artistic work of contemporary students as a way to preserve the historical memory of the Great Patriotic War. This article explains the necessity of preserving the layer of national culture in order to reproduce the national identity in the conditions of informational and ideological pluralism of the post-Soviet period. The authors note the differentiation of youth due to the conditions and specifics of socialization in the polysemantic sociocultural space.

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