During the 90s, the keystones of the celestial reference system took distance since the community left a stellar realisation for an extragalactic realisation. Very long baseline interferometry [VLBI] is used in this purpose because it determines the absolute astrometric positions of thousands of active galactic nuclei with an accuracy of tenths of microsecond of arc. The realisation of the extragalactic celestial reference frame by a well-chosen set of sources is at the basis of modern geodesy for wich scientific and societal challenges are regularly highlighted.
The VLBI astrometric accuracy stayed unrivaled for the 40 last years. Only the Gaia space mission competes VLBI nowadays. By skiping technical and technological challenges that allowed this feat from the ground, I will explain that this precision makes us sensitive to perturbations linked to the complex and animated physical structure of the active galactic nuclei. Until now, the adopted strategy for the realisation of a hyper-stable celestial reference frame is to put aside the seemingly most turbulent sources. I will give some elements that let us think this strategy will not be good enough at medium term. The challenges of the future for ever more accurate celestial frame will require the study of those sources and their regular monitoring in collaboration with the astrophysical community in order to understand (i) on which sources can we rely on to realize our celestial reference frame and (ii) given a source, can it be sufficiently stable on a finite time to be useful for the realisation of a celestial reference frame.
Better understanding Solar System Giant Planet formation and evolution requires in situ measurements, remote sensing observations either with telescopes or planetary missions, and modeling. While more and more exoplanets are discovered every day and while we will better characterize them with new observatories like JWST, the planets of the Solar System remain our local laboratory for studying formation and evolution of such bodies. The (sub)millimeter domain, owing to the very high spectral resolution of the heterodyne technique and to the ever increasing spatial resolution and sensitivity of new observatories like ALMA, is suitable for determining planetary atmospheric composition and dynamics when coupled with appropriate radiative transfer, photochemical or thermochemical modeling.
In this seminar, I will summarize 10 years of observations and modeling of the Solar System Giant Planets I have been involved in.
I will first show that thermochemical modeling of the deep tropospheres of the Giant Planets can help us establish their deep composition to constrain their formation processes. The next step is the participation in an atmospheric probe proposal for the Ice Giants, and the development of its mass spectrometer, in preparation for a NASA-ESA joint flagship mission to these distant worlds.
I will also show how observations and time-dependent 1D or 2D photochemical modeling have enabled us to improve our understanding of how the composition and chemistry in the stratospheres of the Giant Planets are altered by seasons and external sources. With ALMA, it is now even possible to directly measure winds in the stratospheres of the Giant Planets to constrain their stratospheric circulation.
Finally, I will present how the Submillimetre Wave Instrument of the Jupiter Icy Moons Explorer (JUICE) mission will allow us, in about a decade from now, to monitor Jupiter’s atmosphere, both in terms of chemistry and dynamics, and with spectral and spatial resolutions and temporal coverage never achieved before.
Small bodies have escaped planetary accretion and have best preserved the composition of the matter initially present in the solar nebula. Cosmic dust originates from these small bodies, asteroids and comets. Interplanetary and cometary dust are collected on Earth in places with a low accumulation rate of terrestrial dust, like the polar caps or the stratosphere. Interplanetary dust particles (IDPs) have been collected in the stratosphere by NASA for a few decades. A fraction of IDPs (at least) are proposed to be of cometary origin. Cosmic dust from the polar caps are larger than IDPs and are called micrometeorites. We collect micrometeorite at the Concordia Antarctic station at Dome C since 2000. The Concordia collection contains very pristine samples, including particles that are dominated by organic matter and that are very probably cometary. Spatial missions like Stardust (NASA), Hayabusa (JAXA) and Rosetta (ESA) also gave access to the structure and composition of asteroidal and cometary dust. Stardust brought back dust particles from comet 81P/Wild 2, but the collection occurred at high relative velocity (6 km/s) and the samples were altered during the collection. The Rosetta mission collected dust particles from comet 67P/Churyumov-Gerasimenko at much lower velocity (1-10 m/s), but the analyses had to be performed in situ onboard the Rosetta orbiter by the dust instruments (GIADA, COSIMA, MIDAS). The Hayabusa mission returned samples from asteroid Itokawa, which is an asteroid related to ordinary chondrites. At least two future spatial missions are bound to bring back samples from carbonaceous asteroids: Hayabusa 2 (JAXA, asteroid Ryugu) et OSIRIS-REx (NASA, asteroid Bennu). The CAESAR mission is also currently under study to bring back a sample from comet 67P/Churyumov-Gerasimenko.
The presentation will summarize the present knowledge on the composition of interplanetary and cometary dust, based on the results of laboratory analysis of dust particles collected on Earth, and of spatial missions.