Center for Planetary Origin A Material approach

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RESEARCH OVERVIEW

Our research project is based on the synergy between (i) high-performance observations, ranging from laboratory scale (on materials, rocks and meteorites) to deep space (in-situ observations by space missions, remote observations of extrasolar planets and disks) and (ii) theoretical and experimental modeling, covering all major stages of the “Origins” problem, from the formation of the first solids in the protoplanetary disk to the geological evolution of planets and small bodies.

We present here the global structure of our research project. This section focuses on the coherence of the ensemble and its multidisciplinary aspect. A more detailed description of the individual research themes is provided in the research page.

The protoplanetary disk
a ALMA image of the HL Tau disk

Planet formation starts with the formation of a protoplanetary disk. We will model the structure and the evolution of these disks using hydro-dynamical and magneto-hydro-dynamical (MHD) simulations. The former adopt a prescribed viscosity, but describe the whole disk, while MHD simulations are more computational expensive and therefore treat typically only a local portion of the disk, but show important processes such as the onset of turbulence and the formation of disk winds. From all these simulations we derive the evolution of density and temperature as a function of distance from the star, vertical distance from the mid-plane and time. Using this information, and thanks to the synergy with the team of chemistry at ICN, we will model the chemical evolution of the disk, with particular emphasis on organic material (see Theme 1)

Fortunately, the modeling of protoplanetary disks is not unconstrained. Disks are observed around young stars, and their properties can be unveiled with new high-resolution instruments such as ALMA and MATISSE. ALMA is a radio-interferometer particularly suitable to study the outer and colder part of the disks. Several of us participate to the proposal of a “large program” led by A. Dutrey in Bordeaux, and therefore will have access to the observational results. MATISSE is a spectral interferometer for the VLTI, combining the light of 4 telescopes in the near-IR. A member of our Center is the PI of the instrument, built with a large international consortium. MATISSE will start science operation by the end of 2018. It will allow imaging the inner parts of the disks, the site of terrestrial planet formation (see Theme 2). MATISSE will constrain the disk’s composition (for instance detecting silicates in both crystalline and amorphous forms and PAHs, the building blocks of organic chemistry) as well as the disk-wind interactions, which are so crucial in modern disk models. MATISSE and ALMA will allow also the localization of the main condensation lines (water, CO,…) which characterize the global temperature structure of the disk. Thus, MATISSE is a clear example of convergence between optics and planetary science, which is one of the assets of our Center. Numerical models and observations will be confronted with each other, in order to constrain the global structure of the disks from their observed characteristics, deduce the existence of vortices, gaps and cavities, which might reveal the presence of embedded planets and constraining the relative roles of turbulence and winds in the functioning of the “disk engine” (Theme 1).

MATISSE, however, is quite limited in the number of disks that it can image. The reason is that the interference fringes can be obtained only for a very short time, due to the turbulence of the atmosphere. This severely limits the exposure time and hence the limiting magnitude. To alleviate the problem it is necessary to couple MATISSE with a fringe tracker. At the beginning of the operations, MATISSE will be coupled to the fringe tracker of another VLTI instrument, GRAVITY, but the solution is far from optimal. Thanks to the world-wide recognized competences in interferometry in our Center, we have proposed a breakthrough fringe tracker concept particularly well suited to extend the limiting magnitude and also the dynamical range of VLTI measures and images (see A4.1). In the next two years we have to complete the prototype that will allow us to negotiate with ESO the construction and installation of such a system on the VLTI. With the appropriate fringe tracker MATISSE will be able to gain several magnitudes and extend its targets to a few dozens of protoplanetary disks, allowing us to better understand the diversity of disk properties.  

The first solids and planetesimals
The analysis of meteorites teaches us that the first solids formed in the disk 4.568Gy ago as Calcium-Aluminum refractory minerals (CAIs); chondrules formed next, throughout a period of 2-3My. However, their formation process remains elusive. In particular, chondrules, 100μm-1mm particles which are the dominant building blocks of meteorites coming from planetesimals that did not experience global melting, have an extremely complex structure, with a non-random mixing of olivine grains, glass, metal inclusions etc. None of the models proposed so far is able to explain the nature of chondrules in all its complexity.

CAIs and chondrules in the Allende meteorite

The expected locations of formation of CAIs and chondrules

Chondrule formation is one of the subjects for which the synergy between planetary science and material science is the key. In fact, to achieve a fundamental breakthrough, it is necessary to analyze in details their microscopic structure (Theme 5.1). By comparing the observed microstructure of chondrules with those of synthetic materials generated in controlled conditions (Theme 5.2) and applying numerical methods developed for the simulation of material evolution (Theme 5.3), it will be possible to deduce the conditions (e.g. pressure and temperature gradients, the nature of the inherited materials) at which the different sub-components formed and got integrated into a chondrule.  For this purpose, the development of a platform for microanalysis of natural and artificial materials (Theme12.3) is a strategic investment for our Center. Another strategic infrastructure is the plasma torch already existing at Laboratoire Persée. This instrument can monitor the condensation of material as the temperature decreases from plasma conditions in a controlled manner (Theme12.1). It will be used to make experiments on the synthesis of condensed materials (Theme 5.6), to be compared with the chondrules’ components.    

The formation of the first planetesimals is also a hot topic of planetary science. The models have radically evolved over the last 10 years. The classic view of a progressive sticking of individual particles to form bigger and bigger objects has lost support because aggregates larger than about 1 mm in size (possibly increasing to ~10cm for icy aggregates) are expected to bounce or break upon collisions, rather than to stick.  Instead, it has been shown that particles can clump due to turbulence in the disk or the so-called streaming instability. Once the clumps of particles are massive enough, the self-gravity can bind all the particles together, forming a large planetesimal (of ~100km in size).
concentration of solid particles due to the streaming instability, leading to the formation of big planetesimals

Although this new view of planetesimal formation is very promising, several unknowns persist. The main open questions are: (i) whether planetesimal formation occurs throughout the protoplanetary disk and all along its lifetime, or only at specific sites or specific phases, (iii) whether turbulence favors or prevents planetesimal formation and (iv) whether chondrule-size particles are large enough to clump, or they first need to glue together in clusters. To bring an answer to these questions, while modeling protoplanetary disks we will model also gas-particles interactions and their feedbacks, looking for conditions for particle clumping (Theme1). We also collaborate on this topic with A. Johansen, in Lund.

Fortunately, meteorites provide us with a huge number of constraints for planetesimal formation models. They show what are the building blocks of planetesimals and their characteristic sizes, when these blocks came together into the same rock, the mixing of different materials in different objects, the size of the first planetesimals and their thermal and collisional history (Theme 5.4).  Thus, the study of meteorites is central to our project, through a strong synergy between chemistry, petrology and material science, but also collaborations with foreign partners. The dialog with the scientists involved in disk modeling and planetesimal formation (Theme 1, Theme 6.1) will be permanent. 

Planet formation
Once planetesimals are formed, planet formation can proceed in two ways. The classic scheme is that planetesimals collide with each other and grow in mass under the effect of their mutual gravity. These growth modes are successful in forming a population of planetary embryos, with masses of the order of the mass of the Moon or Mars. However, they fail forming objects of several Earth masses, as those required to accrete massive atmospheres from the gas of the protoplanetary disk and become giant planets. Thus a different accretion scheme is needed.

This may have been found recently. In has been shown that planetesimals are very effective in accreting the small particles that drift radially through the disk, thanks to a combination of gravitational deflection and gas drag. This growth mode is dubbed pebble accretion and can grow initially Ceres-mass objects to 10-20 Earth masses within the lifetime of the disk, provided the mass-flux in pebbles is large enough (but within realistic limits). Nevertheless a coherent scenario of the formation of planetesimals to planets, based on the pebble-accretion process is still missing. Under which conditions giant planets form only in the outer part of the disk, as in our Solar System, is still debated. Thus, a major goal of this project (Theme 6.1) is modeling pebble accretion combined with the collisional and dynamical evolution of the proto-planets in formation.  
planet migration

A key aspect of the planet-formation problem is that of migration. Planets, due to their gravity, modify the distribution of gas in the disk and, by reaction, migrate towards the central star. Migration becomes more and more relevant with the growing mass of the proto-planets. For Mars-mass embryos it can be neglected, but not for forming giant planets. Migration is a real bottleneck for planet formation models. At the current state of knowledge on migration, giant planets should not exist: they should have fallen onto the central star while growing. Clearly, we are missing something: either some relevant process that limits migration, or a fundamental aspect of the disk’s structure/evolution. We are very active on planet migration, with several relevant contributions in this field (Theme 6.2). We will continue our investigations along multiple lines, and also in synergy with the disk modeling effort. The aspects of giant planet growth and migration are covered by the ANR contract MOJO.
A scheme of the Grand Tack model

Terrestrial planet formation is the last step of planetary growth. In the case of the Solar System the assemblage of the Earth continued for about 100My. We are leaders in modeling terrestrial planet formation, with a dynamical model, dubbed Grand Tack, that reproduces the orbital and mass distribution of the terrestrial planets, the growth timescale of the Earth and its chemical properties (Theme 6.3). However, the Grand Tack model assumes a specific migration history of Jupiter and Saturn. Given what has been said above about our poor understanding of planet migration, this assumption needs to be validated. We will refine and update our model according to the results obtained on giant planet growth and migration. In parallel, we plan to start exploring the geophysical implications of our model in terms of planetary core structure, onset of plate tectonics, low order mantle convection and test the results against the constraints provided by the internal structure of the terrestrial planets (Theme 9.2 and 9.3). The terrestrial planet formation modeling is funded by the ERC grant Accrete.
 
Constraints from the global structure of planets and small bodies
In order to test the formation models described above and give them inputs for improvements, we need to look at the populations of small bodies and to the planets of our system, which we discuss below.

Small bodies
Comet 67P as seen from Rosetta (ESA)

Small bodies are what remains today of the original planetesimal population(s) from which the planets formed. The Gaia mission, currently in operation, will provide a huge astrometric, photometric and spectroscopic database of asteroids. One of us is the responsible of the Gaia mission for small body observations. A number of follow-up observations will be done at the Calern site of OCA (Theme 7). Two telescopes of 1m in diameter will be used combining research and training purposes for Ph.D. students (C2PU program). The capabilities of these telescopes will be incremented by the development of a new concept of wide-field adaptive optics: the CIAO project (Theme 4.3), another synergy between optics and planetary science. We will also follow the transit of asteroids in front of field stars, determining the size of the occulting body and its projected shape. The improved Gaia astrometry of stars and asteroids will allow the accurate prediction of these transit events, which is basically impossible today.  A dozen of events will be observable every night, which imposes the development of robotic platforms (Theme 7).
C2PU telescopes in Calern

Besides this ground-based program, our deep involvement in a number of space missions to asteroids is one of the highlights of our Center. We have a strong participation at the Co-I level in the sample-return missions Hayabusa-II (Jaxa) and OSIRIS-REx (NASA), which will reach their targets in 2018 and bring samples back to Earth in 2020 and 2023, respectively (Theme 8.1). Both theses missions will study primitive asteroids (C-type), with a resolution comparable to that of the Rosetta mission for the comet 67P/Churyumov–Gerasimenko. This will allow establishing the relationship between primitive asteroids and comets, which is of primary importance for our models. In addition, Hayabusa-II will also conduct an impact experiment (SCI), with a 2 kg projectile hitting the asteroid surface at 2 km/s, which will allow us to check our understanding of the cratering physics on a real asteroid (Theme 8.1, Theme11.5). We will have guaranteed access to all Hayabusa-II and OSIRIS-REx data and to the returned samples of OSIRIS-REx (and possibly from Hayabusa-II). The laboratory analysis of these samples will allow us to understand the petrological and chemical properties of the primitive asteroids, with particular emphasis on water alteration and on the presence of organic material (Theme 5.5). Given the analysis of organic material collected at the surface of the comet 67P/C–G (Theme10.1), this will give us another opportunity to compare comets to asteroids. An analysis of organic material at the surface of Mars, expected with the ExoMars mission (Theme 10.2), will then allow linking organic material on the planets to that of asteroids and comets and test predictions of the terrestrial planet formation models (Theme 6.3) concerning the delivery of volatiles and organic material to the planets.

Hyabusa II and Osiris Rex en route towards their asteroid targets

Another ambitious asteroid mission is AIDA (Theme 8.2). This is a joint NASA/ESA technology mission, currently in phase A/B1, and led by one of us for the European component. The goal is to test deflection strategies, in order to acquire the know-how necessary to deviate asteroids found to be on a collision trajectory with our planet. The mission, nevertheless, will make important scientific observations. It will be the first mission visiting a binary asteroid, which will allow discriminating among the main theories proposed for their origin. Moreover, it will allow us to have, for the first time, a direct measurement of an asteroid internal structure by radar tomography. On top of all, the European spacecraft will observe the asteroid from close distance while the American spacecraft is impacting, and this will allow studying the asteroid’s response to an impact at a scale that is inaccessible in laboratory (see Theme 11.4).

Planets
The internal structure of the planets sets strong constraints on how the planets formed.
Supposed Interior structures of giant planets

Concerning the internal structure of giant planets we will focus on Jupiter. Its structure will be constrained using two complementary approaches: asteroseismology and gravitational field mapping (Theme 9.1). Asteroseismology consists in measuring the oscillation modes at the surface of the gaseous object (star, giant planet), which are diagnostic of the interior structure of the body. We are pioneers in asteroseismology techniques for giant planets, with the realization of a first instrument, Sympa, which allowed detecting the oscillations of Jupiter for the first time. Supported by the ANR contract Jovial, we are now constructing a new instrument, which will be operational at one of the C2PU telescopes in Calern, as well as a copy of it to be delivered to the US in order to extend the time of continued monitoring of Jupiter. The mapping of the gravitational field of Jupiter will be provided by NASA’s Juno mission, in which we have a strong participation at the Co-I level. Juno and Jovial will probe the abundance of elements heavier than H and He in the planet’s atmosphere and the mass of the central core. The current core of Jupiter, however, is not necessarily the original solid proto-planet that triggered the accretion of the massive atmosphere. The original core might have been eroded dissolving part of its heavy elements in the atmosphere. Therefore, a substantial modeling work on the evolution of the interior structure of Jupiter will be required to assess information on the original core, which is ultimately the constraint that we need for the formation models (Theme 6.1).

Earth sismic tomography

The Insight mission

Studying the internal structure of rocky planet is a fantastic domain of synergy between geoscience and planetary science. The Earth is clearly our object of reference, for which seismic tomography allows both global and local imaging of the interior. Geophysicists in our Center are very active in developing new methods for improved reconstruction of tomographic images from seismic data and in the use these new methods for improved interior models of our planet (Theme 9.3). The numerical aspects around the problem of wave propagation in solids are a strong point of synergy with material science (Theme 5.3). The Insight mission, a partnership between NASA and CNES in which we are involved at the Co-I level, will bring seismic investigation to Mars (Theme 9.2). Obviously, with a unique receiver, Martian probing will not be at the level of that of the Earth but, nevertheless, it will bring the first information on the deep Martian interior, including the size of its core, its liquid or solid state. Modeling tools developed for the global imaging of the Earth will be essential to set up automatic inversion methods suitable for the Insight mission. The experience acquired on the Insight mission will position our Center for a leading role in future seismic missions proposed for the Moon (possible collaboration between JAXA and CNES).
Lunar laser ranging from Calern

Another powerful tool to constrain the interior structure of terrestrial bodies is mapping the gravity field. For the Earth, this is done through space geodesy, i.e. tracking with high accuracy using laser telemetry the motion of low altitude artificial satellites. Our Center, with current and future laser stations, has a leading expertise in this technique (Theme 12.5). For the Moon, the gravitational field has been mapped with exquisite accuracy by NASA’s GRAIL mission, in which we had a Co-I role. The exploitation of these data is still ongoing (A9.2). For the reconstruction of the lunar interior, there is a clear synergy between the GRAIL data and the Laser lunar ranging data. The MEO telescope in Calern is the only active laser ranging station in Europe; it has collected data for decades and continues to do so with ever improving precision. The Lunar laser ranging constrains the librations of the Moon and the rate at which our satellite migrates away from us, due to tides (Theme 9.4). Both measurements constrain the deep interior of the Moon. Radio-science, i.e. tracking with high precision from the Earth the position of orbiters and landers of planets, is also a powerful tool  to constrain the gravitational fields, rotation and orbital dynamics, all of which provide information on the interior structures. Our Center is involved in the science working group of the BepiColombo mission to Mercury and, at co-I level, in the JUICE mission to Ganymede (the largest satellite of Jupiter). Altogether these data will allow a comprehensive modeling of the interior structures of many bodies, which will provide great constraints to formation models.

Geological evolutions
Once the planets have formed and have acquired their global structure, they continue to evolve throughout the history of the Solar System. Somewhat surprisingly, this is true also for small bodies. The geological evolution is mostly due to plate tectonics (however, only the Earth has a full-developed one), volcanism (all terrestrial planets and the Moon, at least at early times), atmospheric erosion (Venus, Mars), water erosion (Mars), impacts (all bodies) and thermal fatigue (small bodies). These processes need to be studied if we want to understand the current surface morphology of the bodies we observe. In our Center, the synergy among planetary, geo and material sciences brings us to focus on the response of material to mechanical and thermal stresses (Themes 11.1, 8.1) over a wide range of strain rates, and on the effects that this response has on the deformation, damaging and fracturing of the lithosphere, both on Earth (Theme 11.2) and on other planetary or small body surfaces (Theme 11.3).
Landslides on comet 67P

The collisional process, leading to either the formation of craters or to catastrophic disruption depending on the projectile/target mass ratio and impact velocity, will be studied through hydrodynamic simulations that include material strength models, testing different approaches (Eulerian, Lagrangian; see Theme 11.4), equations of state as well as fracture and damage models that are appropriate for the wide range of materials and conditions existing during planetary system formation and evolution. The synergy between planetary, geo and material sciences, as well as computational science, is again striking in this case. The models will be validated, as far as possible, using experiments. Through international collaborations, we have access to hypervelocity impact facilities equipped with high-cadence cameras, which allow monitoring the process of cratering/disruption as it develops and not just the final outcome. The space missions Hayabusa-II and AIDA will provide additional data for higher energy collisions (Themes 8.1, 8.2).
Simulation of collision on an asteroid

The formation of regolith on the surface of small bodies has been the object of the ANR contract Shocks, which identified a previously unconsidered process capable to break rocks into small grains: the thermal fatigue. This process is due to thermal stresses that rocks suffer during the day/night cycle. Laboratory experiments have been conducted and allowed calibrating a thermal fracturing model. However, different materials (even among meteoritic analogs) respond very differently to thermal cycles. Taking advantage of the convergence with material science, a comprehensive study of regolith formation by thermal fatigue will be conducted for bodies of different compositions (Theme 11.5). We will also study the dynamical evolution of the regolith under external effects such as changes in the rotation period of the body or vibrations due to impacts (Theme 11.6). This will allow us to interpret the images of asteroids taken from closely orbiting spacecrafts, gaining insight on how a lander can interact with these surfaces (Theme 8.1). This study requires deep competences on granular materials and computational science, which is another area of synergy between all the partners of our Center.

Extrasolar planetary systems
The extrasolar planets show us completely different realizations of the planet formation process, often leading to final structures without analogues in our Solar System. We know more than 2,000 extrasolar planets. The next big leap forward in extrasolar planet science will be ESA’s Plato mission, which will be launched in 2024. Plato will be a transit-detection mission that will monitor bright stars also observable with complementary techniques (radial velocity, direct imaging). The combination of multiple techniques will provide information on their complete planetary systems. Moreover Plato will be able to detect Earth-mass planets at about 1 AU from their central star, i.e. truly Earth analogues. Our participation in the work packages preparatory to the Plato mission is explained in Theme 3; it concerns modeling in a coupled way both the transiting planet(s) and the host star, to derive information on age and physical radius.  With the additional information on the mass of the planet coming from radio-velocity measurements, these data then allow modeling the composition and the internal structure of the planets.
The ESA Plato mission

However, the Holy Grail remains the direct detection of planets, which is also the only way to obtain direct spectroscopic information on their composition. With today instrumentation, including the chronographers Sphere at VLT and GPI on Gemini, the direct detection of planets remains limited to massive giant planets, quite far from the central star. The discovery of analogs of Jupiter and Saturn in terms of masses and orbital properties would be crucial to test our planet formation models (Theme 6.1) and to get a sense of how rare our Solar System is among all planetary systems. Because of their long orbital periods, Jupiter and Saturn analogs are basically undetectable by transit and radial-velocity techniques. Direct imaging is the only option, but this requires a new generation of instruments for high-dynamics, high-contrast imaging. 
The three outer planets of HR8799

Again, the synergy with research and development in optics is the key in this long-term goal. Our Center will carry forward at least three studies for new techniques or instrumental concepts. KERNEL is a project that aims at enabling every optical and near-infrared telescope to reach its ultimate angular resolution potential, beyond the diffraction limit, at full sensitivity. This project has just been funded by the ERC (Theme 4.2). Further along the timeline, we are getting prepared to play a leading role in the construction of the last instrument scheduled for the European ELT telescope: the planet imager PCS. This is done with a R&D activity for preparing strategies and technol¬ogies for high-contrast instrumentation with segmented telescopes: the SPEED project (Theme 4.4). Finally, we are involved in an international consortium for the development of concepts for a new generation of interferometers working in visible or near-IR lights: the PFI project (Theme 4.5). With PFI, optical interferometry evolves towards large baselines and multiple pupils, similar to what is now done (routinely) for radio interferometers. This ultimate goal, however, requires completely new techniques, such as the use of heterodyne fibers. The revolution in extrasolar planetary science passes necessarily through a revolution in instrumentation. Again, this cannot be done without the contribution of material science for the development of appropriate materials.
The future ESO ELT telescope (39m)