MAgnetic inteRactions at Mercury between the InTerior and the Exosphere – MARMITE

The MARMITE project is primarily based on the measurements made by the MESSENGER mission, launched by NASA in 2004 and in orbit around Mercury between 2011 and 2015. These observations were used to constrain the models of the Hermean magnetic field, highlighting for example the strong hemisphericity of the field, but also the periodic temporal variations of internal and external origin, coupled with the Mercury orbit. The dynamo has a very low intensity, and does not show any intrinsic temporal variability so far. By combining these in situ measurements by MESSENGER with those made from Earth, particularly during the transit of Mercury in 2016, the composition and dynamics of the exosphere is now better understood. We have completed and extended a numerical model to account for the different interactions, with trapped and/or ejected particles, between the surface, the exosphere and the ionosphere. At a greater distance, in the magnetosphere, these same particles undergo trajectory changes according to their energy and the rapid reconfiguration of the field lines, linked to the internal field but also to the solar wind. The spatial distribution of available measurements remains incomplete and insufficient to describe and validate all processes, but the BepiColombo mission, launched in October 2018 by ESA and JAXA (expected to arrive at the end of 2025) will make it possible to answer unresolved questions.

- Detailed description of Mercury's internal magnetic field over the northern hemisphere
- Highlighting of the internal structure, with a radius of the core equal to 85% of the radius of the planet
- First description of a dynamic exosphere, coupled with magnetosphere and surface
- New perspectives on dynamic regimes in the magnetospheric tail and comparison with the Earth's

Our work will continue, outside the framework of the ANR project. The members of this ANR collaborate together on many other more or less long-term projects. The BepiColombo mission (entering in orbit in 2025), and its measurements of the magnetic field (by MERMAG,: Glassmeier et al., 2010), and those of the MSA mass spectrometer (of which D. Delcourt is Co-PI, Delcourt et al., 2016), will be very important. In the shorter term, the next transit of Mercury will take place in November 2019. It should be observable from the Canary Islands and therefore using the THEMIS solar telescope that we used during observation campaigns that lasted several years. The next transit will only take place in 2032. This year's transit is therefore a very important opportunity to observe the Mercury exosphere on a relatively long time scale (a few hours) with potentially very short time resolution compared to the typical time scales of the exosphere (30 mn). This transit will be one of the longest ever observed from the Canary Islands (almost 7 hours). In general, the work carried out during the ANR MARMITE applies to moons and planets other than Mercury. As stated above, the same research is being conducted around Mars (including with the MAVEN mission), but also on Jupiter systems, including its moons Ganymede and Io.

The work carried out within the framework of the ANR MARMITE has led to the publication of numerous scientific papers, on the analysis of MESSENGER measurements, the development of numerical codes for MHD simulation, the inclusion of interactions between the different particles, the application of these codes to predict observables, and the interpretation of the different models, for Mercury or more generally for the comparison of planetary processes.

Understanding the similarities and differences among bodies within the solar system is a very powerful tool for unraveling their origin and evolution, and by comparison, for improving our knowledge about our unique Earth planet. Here we propose to describe, model and understand the magnetic environment of Mercury and its sources, internal (i.e., from the core to the lithosphere, or external (i.e., resulting from the interaction of the planet with the solar wind). These environments indeed bring crucial and otherwise inaccessible constraints to the internal structure and dynamics of the body and of its envelopes.
Mercury is the smallest, innermost and possibly most enigmatic of the telluric planets. Prior to space age, its small radius and slow rotation suggested a frozen interior bearing no internal dynamics. The Mariner 10 mission shook this paradigm and showed that Mercury it is the only other known telluric planet besides the Earth with an active dynamo. Mercury indeed possesses a weak internal magnetic field governed by largely unknown processes. Its exosphere is populated by numerous species, with great spatial and temporal variabilities. The interactions of the internal field, the exosphere and the solar wind lead to a very dynamical magnetosphere, which in turn induces electromagnetic currents in the interior and exosphere.
MARMITE stands for MAgnetic inteRactions at Mercury between the InTerior and the Exosphere. A MARMITE is a large cooking pot where different and sometimes unexpected but simple ingredients are mixed and stewed. This research program proposes a scientific recipe, which will provide a new flavor for understanding Mercury’s structure and dynamics through the merging of tools which are traditionally dedicated to independent studies of the convecting core, the (thin) mantle, the surface, the exosphere, the ionosphere and the .
We will analyze the magnetic field measurements performed by the currently orbiting-around-Mercury MESSENGER spacecraft. We aim at characterizing the internal static magnetic field and estimating its possible secular variation. We will resort to new modeling schemes to take into account the very elliptical orbit of MESSENGER and the fact that only a portion of the northern hemisphere is flown close enough to the surface. Once this internal field is described, we will interpret it in terms of internal dynamics and structure. We will perform numerical simulations of the dynamo to both explain the observed morphology of the field where it is known, and predict what may be its morphology above the southern hemisphere. We will also constrain the internal structure with spectral tools as well as with induction study, and compare this structure with large impact related mantle stripping processes. We will couple exospheric models with a hybrid magnetospheric model, which requires both parameterizing the internal field and estimating the ion production rate of all species so far identified around Mercury. This exospheric-magnetospheric hybrid model will be coupled to the surface and interior electrical conductivity, which will lead to a better characterization of induction effects. We finally intend to gather all these elements into a single model which would take into account all magnetic sources and interactions, taking advantage of the original collaboration between fields of research that are usually often disconnected.
This research proposal largely deals with the analysis of the NASA mission MESSENGER, in orbit around Mercury since 2011. We will develop theoretical and analytical tools to model the magnetic environment of a weakly magnetized body such as Mercury. This will especially contribute to a better preparation of our team for the next ESA-JAXA Cornerstone mission BepiColombo, to be launched in 2015. We could also consider applying such tools on other planets and moons like Ganymede, the Earth’s Moon, and Mars, to provide case studies for a better understanding of our planet.