“Earth”

The scientific approach taken by the LPG since its creation is based on the motto “Understanding the planets using tools and knowledge developed on Earth and vice versa”. Over the past few years, we have been able to contribute new results in relation to other planets and their satellites using skills acquired across a range of different Earth Science domains. The work carried out within the theme Earth corresponds to the reciprocity referred to in the motto: we use expertise developed within the field of planetology in order to devise new methods of study and analysis on Earth. The aim for this theme, therefore, is to promote the emergence of original study topics through our experience in planetology over the coming years, with this ranging from internal dynamics to the evolution of the surface of the Earth. Our work currently has two primary areas of focus: the structure and dynamics of the Earth and Interactions between liquids/rocks/life. These two areas of focus draw substantially on the previous themes Planetary interiors and Planetary Surfaces (2012-2016)

1. The structure and dynamics of the Earth

This area of focus groups together our work on the physical and chemical structure of the interior of the Earth and on the way in which its different layers interact. We develop models of the interior of the Earth in terms of its elastic and/or magnetic properties, which impact upon the processes for exchanging matter and energy. Digital modelling and methodological developments play a significant role in this regard. Equally, the work carried out in order to analyse chemical transfers has made it possible to quantify exchanges of matter and to map out the history of the interior of our planet.

1.1 Observing and modelling the Earth’s internal structure

We study the internal structure of our planet, primarily using seismological and magnetic observations in addition to digital modelling.

The elastic, isotropic and anisotropic properties of the Earth’s crust and mantle are impacted upon by the seismic tomography at both a regional and global level. More specifically, we apply this technique to the data acquired as part of our contribution to the RESIF research body (part of the OSUNA). In principally heterogeneous structures such as the crust, for example, probabilistic characterisation of natural seismicity or speed anomalies can be used in order to better quantify any uncertainties with regards to the models obtained. The methods used in homogenising wave fields make it possible to effectively model complex objects and thus to arrive at a better understanding of what the waves actually see during their propagation. In less heterogeneous structures such as the mantle, there is still the same desire to provide a probabilistic approach. With this in mind, homogenisation is a case apart, given that it makes it possible to understand the inverse problem in a way that is more consistent with what seismic waves are able to provide in terms of information, particularly with regards to isotropic and anisotropic models. In concert with the theme Planets and Moons, certain methods of probabilistic tomography developed for Earth will be put to concrete use when the seismometer used as part of NASA’s InSight mission lands on the surface of Mars.

The physics of the internal magnetic field, its fine spatial structures and its rapid time variations will be analysed using the data from the SWARM mission. This mission (3 identical satellites in dedicated orbits used to measure the geomagnetic field, an ESA mission in which we are partners [co-1]) was launched towards the end of 2013, with particular attention to be paid to the mathematical separation of internal and external contributions in signals. More extensive studies on the geometric properties of the external magnetospheric and ionospheric fields, particularly in auroral zones, will make it possible to highlight the seasonal variations of the main field’s ancient variation as well as the fine spatial structures of the original lithospheric field. A better representation of the geometric properties of the external field, meanwhile, will enable more robust estimates of the electrical conductivity of the Earth’s mantle to be made, at depths which, today, are still largely inaccessible. Developing high-resolution models of the lithospheric field by combining satellite data from the SWARM mission and near-surface data (within the framework of the World Digital Magnetic Anomaly map project) using regional models, is one of the projects within this theme. These models will be interpreted in terms of the magnetisation and the depth of equivalent sources and will be used for the purposes of continental reconstruction over geological time. These studies are all part of a research project that we submitted to the European Space Agency (ESA). The researchers involved in the project are also in charge of a level 2 product regarding models of the Earth’s crust that is to be launched by the ESA.

1.2 Internal interactions and dynamics

The Earth’s internal interactions and dynamics are assessed using their mechanical, thermal, chemical and magnetic properties. The questions raised and the methods used will be linked to the work undertaken in Planets and Moons.

In order to understand, for example, the underlying origins of low-intensity anomalies in the magnetic field on the surface of the Earth, as observed above the South Atlantic, we model the kinematics of geomagnetic flow patches using secular variation models. The influence of the inner core on geodynamics is also assessed, with a particular focus on the role played by fusion/solidification, using a new digital dynamo code based on thermochemical convection with a method involving plotters.

Studying chemical transfers between the different layers of the Earth over geological time makes it possible to understand the constraints on the Earth’s internal dynamics. The issue of the cycle of volatile and chalcophile components is of primary importance with regards to our activities in this field. The objectives are i) to study the long-term effects of the recycling of volatile components in the Earth’s mantle, with a particular emphasis placed on redox interaction between the surface and the interior and ii) to characterise volcanic degassing. In order to do so, our aim is i) to establish robust solubility laws governing compounds of volatile components and ii) to precisely determine the dissolution mechanisms and the role these components play in the physicochemical properties of magma.

2. Interactions between Liquids/Rocks/Life

This area of focus for research is concerned with the role of physical and chemical interactions between liquids, rocks and living organisms as well as the impact they have on the evolution of the surface and the sub-surface of the Earth. This includes, on one hand, analysing the mechanical aspects of these interactions (the geomorphological and structural transformation of geological materials caused by liquids) and, on the other hand, analysing their chemical and mineralogical aspects (physicochemical transfers, deterioration and mineralisation in response to interactions between liquids, rocks and living organisms).

2.1 The geomorphological and structural transformation of geological materials caused by liquids

Our experience with regards to the mechanisms of deformation, erosion and sedimentation on the surface of different bodies within the Solar System (Mars, Titan, Europa, Ganymede and Enceladus) puts us in a position where we are able to consider new experiments on the mechanical interactions between liquids, deformations and transfers of matter in porous materials on the surface of the Earth. This is a recurring issue across a number of fields of geoscience, whether fundamental or applied. Geological materials in which liquids circulate (potentially when pressurised) may experience phenomena involving destructuring, clogging, channelling and, more generally, changes to their rheology (fracturing, brecciation, fluidisation, dissolution, etc.). The effects of these phenomena are known, as can be seen in reservoir geology and geotechnics (internal erosion of dykes and dams, changes to the permeability of reservoirs), for example. However, they remain misunderstood and their role in the structural and morphological evolution of both the surface and the sub-surface of the Earth as well as other planets is still to be evaluated. This research draws on experimental modelling equipment located at the University of Maine as well as structural, sedimentological and geomorphological analysis using natural examples. Field analysis requires the use of topographic measurement tools and both stereoscopic and spectroscopic imaging, all of which are available at the LPG and the OSUNA. Among the natural examples used, we are primarily concerned with (1) ancient sedimentary basins, the internal architecture of which can be substantially altered by liquid overpressure linked to sedimentary overloading and to phase changes in the organic matter, (2) sub-glacial environments, in which the circulation of pressurised interstitial liquids plays a fundamental role in erosion, sedimentary transfer and the creation of hilly areas and (3) karst environments, the morphological evolution of which is controlled in the first instance by the circulation of both surface and underground water.

2.2 Chemical transfers, deterioration and mineralisation

This theme brings together two of the LPG’s areas of expertise (processes of deterioration and mineralisation on the surface of planets and bio-physico-chemical transfers on the surface of the Earth) and focuses on the physicochemical evolution of surface geological materials (rocks, regolith, sediment, soil, ice). Among these various interactions, we are working on three specific issues, the primary motivation for which is their originality and their significance with regards to planet Earth, in addition to their planetological implications.

We are working on spectral criteria (infrared absorption), which can be used to differentiate between certain minerals (phyllosilicates and opals) formed as a result of hydrothermal circulation and those formed as a result of supergene deterioration. LPG’s analytical equipment (near-infrared spectrometry, Raman, DRX, MEB, MET, ICP-MS, isotope geochemistry). We are also exploring the role of microbial complexing agents over the course of the deterioration process (silicon released into the liquid) and over the course of precipitation in the form of opal. This work is linked to the work carried out on alteration minerals on the surface of Mars as part of the Planets and Moons theme.
We focus on the role of glaciers and their meltwater as crystallisation environments for certain minerals (sulphates, chlorides, nitrates, carbonates, etc.). The objective is to synthesise some of these minerals in the laboratory in a simulated cryogenic environment and to characterise the conditions under which they are formed. Aside from their significance for terrestrial mineralogy and glaciology, these experiments also contribute constraints to the process for forming minerals on the surface of Martian glaciers and icy bodies in the Solar System, including icy moons and comets.
Some of the metabolites emitted by microorganisms (including siderophores), known for their iron-complexing capacity, have an impact on various other chemical elements. The objective of this work is to specify the way in which these elements (currently Cs, Cu and Pb, to which can be added Ce, La and Y) are mobilised, with particular attention paid to complexation by siderophores, including pyoverdine and pyochelin. Analysis is made possible through the use of passive sampling agents developed in Nantes and Angers from 2D diffusive gels (DGT), combined with spectral-imaging. The studies aim to determine the environmental impact of the microbial mobilisation of metals (contamination of water and higher plant species) using continental (urban and vine-growing soil), estuarine (sediment from the Loire) and coastal (beaches) substrata. This work is linked to the work carried out on coastal environments as part of the theme Coastal and Marine Systems.