How was the Earth–Moon system formed? New insights from the geodynamo

The most widely accepted scenario for the formation of the Earth–Moon system involves a dramatic impact between the proto-Earth and some other cosmic body. Many features of the present-day Earth–Moon system provide constraints on the nature of this impact. Any model of the history of the Earth must account for the physical, geochemical, petrological, and dynamical evidence. These constraints notwithstanding, there are several radically different impact models that could in principle account for all the evidence. Thus, in the absence of further constraints, we may never know for sure how the Earth–Moon system was formed. Here, we put forward the idea that additional constraints are indeed provided by the fact that the Earth is strongly magnetized. It is universally accepted that the Earth’s magnetic field is maintained by a dynamo operating in the outer liquid core. However, because of the rapid rotation of the Earth, this dynamo has the peculiar property that it can maintain a strong field but cannot amplify a weak one. Therefore, the Earth must have been magnetized at a very early epoch, either preimpact or as a result of the impact itself. Either way, any realistic model of the formation of the Earth–Moon system must include magnetic field evolution. This requirement may ultimately constrain the models sufficiently to discriminate between the various candidates.

Timing of the martian dynamo: New constraints for a core field 4.5 and 3.7 Ga ago

The absence of crustal magnetic fields above the martian basins Hellas, Argyre, and Isidis is often interpreted as proof of an early, before 4.1 billion years (Ga) ago, or late, after 3.9 Ga ago, dynamo. We revisit these interpretations using new MAVEN magnetic field data. Weak fields are present over the 4.5-Ga old Borealis basin, with the transition to strong fields correlated with the basin edge. Magnetic fields, confined to a near-surface layer, are also detected above the 3.7-Ga old Lucus Planum. We conclude that a dynamo was present both before and after the formation of the basins Hellas, Utopia, Argyre, and Isidis. A long-lived, Earth-like dynamo is consistent with the absence of magnetization within large basins if the impacts excavated large portions of strongly magnetic crust and exposed deeper material with lower concentrations of magnetic minerals.

A new model of the crustal magnetic field of Mars using MGS and MAVEN

While devoid of an active magnetic field today, Mars possesses a remanent magnetic field which may reach several thousand nT locally. The exact origin, and the events which have shaped the crustal magnetization remain largely enigmatic. Three magnetic field datasets from two spacecraft collected over 13 cumulative years have sampled the martian magnetic field over a range of altitudes from 90 km up to 6000 km: a- Mars Global Surveyor (MGS) magnetometer (1997-2006); b- MGS Electron Reflectometer (1999-2006); c- MAVEN magnetometer (2014-today). In this paper we combine these complementary datasets for the first time to build a new model of the martian internal magnetic field. This new model improves upon previous ones in several aspects: comprehensive data coverage; refined data selection scheme; modified modeling scheme; discrete-to-continuous transformation of the model; increased model resolution. The new model has a spatial resolution of ~ 160 km at the surface, corresponding to spherical harmonic degree 134. It shows small scales and well defined features, which can now be associated with geological signatures.

The inception of plate tectonics: a record of failure

The development of plate tectonics from a pre-plate tectonics regime requires both the initiation of subduction and the development of nascent subduction zones into long-lived contiguous features. Subduction itself has been shown to be sensitive to system parameters such as thermal state and the specific rheology. While generally it has been shown that cold-interior high-Rayleigh-number convection (such as on the Earth today) favours plates and subduction, due to the ability of the interior stresses to couple with the lid, a given system may or may not have plate tectonics depending on its initial conditions. This has led to the idea that there is a strong history dependence to tectonic evolution-and the details of tectonic transitions, including whether they even occur, may depend on the early history of a planet. However, intrinsic convective stresses are not the only dynamic drivers of early planetary evolution. Early planetary geological evolution is dominated by volcanic processes and impacting. These have rarely been considered in thermal evolution models. Recent models exploring the details of plate tectonic initiation have explored the effect of strong thermal plumes or large impacts on surface tectonism, and found that these 'primary drivers' can initiate subduction, and, in some cases, over-ride the initial state of the planet. The corollary of this, of course, is that, in the absence of such ongoing drivers, existing or incipient subduction systems under early Earth conditions might fail. The only detailed planetary record we have of this development comes from Earth, and is restricted by the limited geological record of its earliest history. Many recent estimates have suggested an origin of plate tectonics at approximately 3.0 Ga, inferring a monotonically increasing transition from pre-plates, through subduction initiation, to continuous subduction and a modern plate tectonic regime around that time. However, both numerical modelling and the geological record itself suggest a strong nonlinearity in the dynamics of the transition, and it has been noted that the early history of Archaean greenstone belts and trondhjemite-tonalite-granodiorite record many instances of failed subduction. Here, we explore the history of subduction failure on the early Earth, and couple these with insights from numerical models of the geodynamic regime at the time.This article is part of a discussion meeting issue 'Earth dynamics and the development of plate tectonics'.

Could giant basin-forming impacts have killed Martian dynamo?

The observed strong remanent crustal magnetization at the surface of Mars suggests an active dynamo in the past and ceased to exist around early to middle Noachian era, estimated by examining remagnetization strengths in extant and buried impact basins. We investigate whether the Martian dynamo could have been killed by these large basin-forming impacts, via numerical simulation of subcritical dynamos with impact-induced thermal heterogeneity across the core-mantle boundary. We find that subcritical dynamos are prone to the impacts centered on locations within 30° of the equator but can easily survive those at higher latitudes. Our results further suggest that magnetic timing places a strong constraint on postimpact polar reorientation, e.g., a minimum 16° polar reorientation is needed if Utopia is the dynamo killer.

A younger age for ALH84001 and its geochemical link to shergottite sources in Mars

Martian meteorite ALH84001 (ALH) is the oldest known igneous rock from Mars and has been used to constrain its early history. Lutetium-hafnium (Lu-Hf) isotope data for ALH indicate an igneous age of 4.091 +/- 0.030 billion years, nearly coeval with an interval of heavy bombardment and cessation of the martian core dynamo and magnetic field. The calculated Lu/Hf and Sm/Nd (samarium/neodymium) ratios of the ALH parental magma source indicate that it must have undergone extensive igneous processing associated with the crystallization of a deep magma ocean. This same mantle source region also produced the shergottite magmas (dated 150 to 570 million years ago), possibly indicating uniform igneous processes in Mars for nearly 4 billion years.

The Mars Astrobiology Explorer-Cacher (MAX-C): a potential rover mission for 2018. Final report of the Mars Mid-Range Rover Science Analysis Group (MRR-SAG) October 14, 2009

This report documents the work of the Mid-Range Rover Science Analysis Group (MRR-SAG), which was assigned to formulate a concept for a potential rover mission that could be launched to Mars in 2018. Based on programmatic and engineering considerations as of April 2009, our deliberations assumed that the potential mission would use the Mars Science Laboratory (MSL) sky-crane landing system and include a single solar-powered rover. The mission would also have a targeting accuracy of approximately 7 km (semimajor axis landing ellipse), a mobility range of at least 10 km, and a lifetime on the martian surface of at least 1 Earth year. An additional key consideration, given recently declining budgets and cost growth issues with MSL, is that the proposed rover must have lower cost and cost risk than those of MSL--this is an essential consideration for the Mars Exploration Program Analysis Group (MEPAG). The MRR-SAG was asked to formulate a mission concept that would address two general objectives: (1) conduct high priority in situ science and (2) make concrete steps toward the potential return of samples to Earth. The proposed means of achieving these two goals while balancing the trade-offs between them are described here in detail. We propose the name Mars Astrobiology Explorer-Cacher(MAX-C) to reflect the dual purpose of this potential 2018 rover mission.