The core

The core, of radius 3.500 km, consists of liquid iron for its external two thirds but shelters a inner core in its center. The core is the seat of the internal magnetic field which appears to us on surface like primarily dipolar.

Only measurements by satellite can enable us to chart in totality this field actually more complex, like its temporal derivative. After Magsat, 25 years ago, the magnetic community had to await the beginning of the century (Ørsted, 1999, Champ, 2000, Sac C, 2000) to measure variations which reach nearly 10% of the average field in 20 years.

Amplitudes of magnetic field (top map) and of its variations (bottom map) in 20 years. In South Atlantic the variations exceed 10%. The geophysicists still do not have been able to chart the variations (jerks magnetic) of smaller temporal scale. ©IPGP

The equations of electromagnetism connect the magnetic field and its temporal derivative to the rates of flow of iron in the core. It is one of our rare means to understand this fundamental phenomenon which is the terrestrial dynamo. The magnetic field has temporal variations on all scales. In particular magnetic jerks interesting a part or all the core on scales of a few years were observed. The objective of Esa's Swarm mission, for which CNES contributes to the absolute magnetometers, is to measure the magnetic field in continuity.

The other means of looking further into our knowledge of the core by the space techniques are more indirect. The rotation of the Earth is affected by the presence of a fluid core, by the presence of a inner core in its center and by the nature of the coupling mantle/core. The follow-up of terrestrial rotation is a product of measurements of positioning (GPS, Doris, VLBI, soon Galiléo). The improvement of the parameters of terrestrial rotation asks for precise details of global positioning about the millimetre. This is why it is necessary to take care of the improvement of the systems of positioning and the generalization of the concept of geodetic observatory.

Finally the movements in the liquid core, like those of inner core within this one, induce variations of the field of gravity. These variations are still quite lower than the precise details than one can hope with short term (Grace) or medium term (Micromega project). They are however comparable with those that can be reach in certain projects of fundamental physics (tests of relativity and equivalence), which suggests a possible synergy.

The mantle

The first 3.000 kilometers under our feet constitute the mantle composed of silicate. Although in a solid state, it is the seat of slow movements which control the plate tectonics.

In the last twenty years, our comprehension of the dynamics of the mantle enormously profited from our knowledge of the terrestrial field of gravity, often represented by its potential gravity, the geoid. Indeed, the field of gravity at large wavelength cartography the variations of density of the mantle which, themselves, are the engine of the convection. Satellites such as Champ (2000) or Goce (2007) will make it possible to improve our models but their main contributions will relate to the dynamics of the lithosphere.

The field of gravity has also temporal variations. These are the ones that are tracked the Grace mission (2002-2007). This mission is showing (or will show...) primarily the sub-annual variations related to climatic and hydrous effects (variations of the water contents), or instantaneous variations (co-seismic deformations). The field of gravity has also secular temporal variations. Some are due to tectonic causes (subsidence, pre-seismic deformations), to the deformations induced by the last deglaciation (uplift of Scandinavia and Canada), to current climatic causes (the glaciers melting, hydrous variations). These variations (10³ mgal/an with a resolution of 1.000 km) could be mapped during a dedicated mission, longer than Grace, or by a repetition of missions of the Goce kind. The interest for the studies of the lithosphere or the subsurface will be developed further. For the study of the mantle, the slow variations of gravity enable us to probe the mechanical properties of the mantle (viscosity, elasticity, threshold of rupture, stratification).

Anomalies of gravity global map (at the surface) computed from EGM08 global model (model coming from the combination of space and ground gravimetry data). Source BGI/CNES.

The missions measuring the magnetic field, already evoked, also make it possible to probe the mantle. It consists in using the variations of external origin of this field, to probe the electric conductivity of the mantle. That is about the only means of knowing the lateral and radial variations of its temperature. Taking into account the conductivity of the mantle (0,1-1 S/m) and the difficulty of an electromagnetic wave to penetrate a conductor (skin effect), measurements uninterrupted of the magnetic field over several years will be necessary.

The main tool of internal geophysics was, and remains probably seismology. It is still possible, that space techniques become a partner impossible to circumvent. Among the seismic waves, some imply vertical movements of the ground (waves of Rayleigh), comparable with a swell.
These movements have a oceanic equivalent, the waves of tsunamis. The vertical movement of the ground is transmitted to the atmosphere where, due to the reduction in the density, the amplitude of the induced wave increases with altitude. This shock affects the ionosphere. There is thus the possibility of mapping a seismic front of wave from space. If very encouraging observations were made, the concept must prove its quantitative interest for the study of the Earth. To make seismology from space seems an interesting and original approach to be realized within a prospective framework.

The lithosphere

The lithosphere is the external part of the terrestrial mantle, of a hundred kilometers thickness. It is not distinguished from the mantle by its chemical properties but by its mechanical (more rigid) and thermal (colder) properties. It seems to us relatively indeformable, cut out in plates, whose borders are the place of earthquakes.

All positioning techniques (Doris, GPS, VLBI, SLR (Satellite Laser Ranging), Galileo) are brought into play to improve our knowledge of the movements and the deformations of the lithosphere. These measurements made it possible to confirm the models of the plate tectonics. It is now a question of going beyond and to measure the deformations which are not taken into account by the models of plate tectonics : diffuse, slow, pre or post-seismic deformations. The positioning techniques must aim at the millimetre in lateral precision and improve measurement of the vertical component under the centimetre.

Measurements of the field of gravity are also fundamental to understand heterogeneities of the lithosphere. Currently, some models of gravity include a priori estimated based on topography, while oceanic bathymetry itself is calculated based on gravity (from altimetric measurements). There is thus an artificial coupling between topographic models and of gravity which makes delicate a general study. The Grace mission will largely improve the situation but will not allow to fill the gap between gravity from ground and gravity measured by satellite. It will be necessary for that to improve the resolution of the models of gravity up to ten kilometers.

The magnetic field has also a crustal/lithospheric component which carries the trace of tectonics, thermal events and composition of the lithosphere. The current models are extremely poor. The modeling of the lithospheric magnetic field to a resolution of a few 100 kilometers is necessary and will benefit from the Swarm mission.

The joint study of electromagnetic variability allows, as for the mantle, to retrieve the conductivity of the lithosphere, related on the presence of fluids and the temperature. Electromagnetic variations of unknown origin were several times observed, and some appeared to be precursors of earthquakes. It is this kind of phenomena that are researched by the Demeter mission (2004).

The subsurface

This field gathers more conventional geology, the interactions between the man, the climate and the ground (matching some aspects covered by continental surface research), as well as the monitoring of natural risks (volcanology, landslides). These aspects thus have a strong component GMES, but cannot be reduced to this single component because an effort of research on the geophysical risks is essential (see figure below).

The basic data of any geological work is the knowledge of topography. However this one is poorly known. There is a strong scientific and social applications potential in the development of digital terrain models (see the success of the mission SRTM, Space Radar Topography Mission).

Thanks to SAR interferometry, the seismologists can study the ground deformations induced by earthquakes. Coloured fringes follow the lines of equal deformations. © CNRS Montpellier

The bathymetry, often estimated starting from the altimetric observations, can significantly be improved in precision as well as resolution. The lack of altimetric data in coastal domain also largely penalizes the continuity of the global models of gravity. Space missions are currently being studied to remedy this gap.

The various techniques of Earth observations (optical, radar, multispectral) remain, of course, fundamental tools of Earth sciences. But, beyond the static observations, they are the observations of changes which are now fundamental. In particular, the follow-up the continuous deformations (post-seismic as well as related to landslide, to volcanologic risk or to public works) by low frequency radar opened new prospects with Earth sciences. The continuity of these data in the future is not completely insured.

The Grace mission must enable us to highlight variations of gravity related to changes of masses distribution close to surface (change of the contents in water, ice, tectonic deformations, etc). It is a very new way of using the gravimetric data. By using the results of Grace, we must already project us in the future to conceive the missions which will make it possible to increase the temporal and spatial resolution as well as the continuity of this kind of measurement.