Planetary Sciences

Understanding why Venus and Earth became two different planets using cutting-edge satellite technology

My research activity in Paris Observatory’s LESIA laboratory (Laboratoire d’Études Spatiales et d’Instrumentation en Astrophysique, UMR CNRS 8109) addresses ground-based and space-borne exploration of Venus and ground-based measurements of other solar system objects. It encompasses analysis of wind measurements and trace gases chemistry of sulfur gases in the Venus atmosphere, and participation in international missions to explore Venus proposed in the framework of the United States’ National Aeronautics and Space Administration (NASA) and the European Space Agency (ESA) space programs. My research activity in LESIA also includes participation in international campaigns to observe stellar occultations from distant solar system objects such as Centaurs or Trans-Neptunian Objects.

Surprisingly little is known about our nearest planetary neighbor, not even the basic sequence and timing of events that formed its dominant surface features. NASA/Magellan mission (1990-1994) revealed an enigma: a relatively young surface, rich in apparent geological activity, but with a crater distribution indistinguishable from random. How can a geologically active surface be reconciled with the global stasis inferred from the apparently random impact crater distribution? The surface of Venus is not organised into large plates like Earth’s oceans, but partitioned into areas of low strain bounded by narrow margins of high strain, analogous to continental basins on Earth. Are these regions actively created and destroyed, like Earth’s oceans, or simply mobilised locally? What is the significance of the global network of elevated rift systems on Venus, similar in extent to mid-ocean ridges but very different in appearance? Unique to Venus are coronae, quasi-circular tectonic features, typically 100–500 km across, with a range of associated volcanic features unseen elsewhere in the solar system. Are coronae the surface expression of plumes or magmatic intrusions? What role do they play in global tectonic and volcanic change?

At all scales, there is a huge variety of unexplained features on Venus – from volcanic, tectonic features (e.g. wrinkle ridges), evidence of large flows (canali), aeolian features that we do not understand in terms of a basic sequence of events. Are canali or other specific magmatic features confined to a past regime or still active today? Is there a correlation between mesospheric SO2 concentration and volcanic activity? On Venus, the spatial distribution of impact craters cannot be distinguished from a spatially random population. This indicates a much younger surface than most of the Earth’s continents (less than 600-800Ma). There are open questions about buried features and the nature of volcanic resurfacing. Are crater floors effusively infilled and buried from below? 

Left : Idunn Mons (46 S; 146 W) in Imdr Regio, with its steep-sided dome at the summit, as well as lava flows extending in the surrounding plain, is characterised with high emissivity measurements pointing to recent volcanism; right : a volcanic shield field inside a corona structure, surrounded by a set of younger lava plains. 

Representing about 10 % of the Venus surface, tesserae may represent the oldest terrain on Venus. The processes which have shaped them are unknown. They have never been visited by any lander, so their composition is unknown. They might be either felsic (formed in a water-rich environment) or mafic rock types. Detailed morphology and their tectonic features, coupled with characterisation of surface emissivity and subsurface features, will provide an extensive investigation of these regions and the entire sequence of geological events that have shaped the surface of Venus we know today.

Exploring the distant solar system using light from distant stars

The distant solar system contains largely unaltered material from the primordial circumsolar disk. It also kept the memory of the planetary migrations at the early stages of solar system formationand thus contains essential information on the origin and collisional evolution of our planetary system, the nature and alteration processes of primitive bodies, and the conditions for emergence and sustainability of nitrogen atmospheres such as Pluto’s.The method of observing stellar occultations, when an astronomical body passes in front of a star casting a shadow in starlight on the Earth, provides higher spatial resolution than any other Earth-based observing method. In addition, because the light being observed passes through any atmosphere or obstacle surrounding the occulting body, the method allows direct probing of the celestial body’s environment. This unique investigation requires a combined effort in predictions, astrometrical observations and analysis, portable instrumentation, time-critical ground-based deployment, observation and analysis. 

In the last decade our group in LESIA has led the field by discovering rings around the asteroid-like object Chariklo, predicting and observing large trans-Neptunian objects (TNOs) stellar occultation events (Quaoar, Makemake and Eris), and measuring drastic variations of Pluto’s atmospheric conditions along its seasonal variations. We also developed a distinct approach based on serendipitous detection of very small trans-Neptunian objects (a few hundred meters).

Titania is the largest uranian moon (d = 1576.8 ± 1.2 km). Our analysis of a stellar occultation allowed to constrain diameter, oblateness and density at better accuracy than the Voyager-2 fly-by in 1986. Near-IR spectroscopy has indicated the presence of water ice and carbon dioxide ice on the surface of Titania. While H2O ice is clearly not a candidate volatile, CO2ice stability against sublimation over a seasonal cycle of Titania can be considered. Our analysis allowed to set surface pressure upper limits of 10-20 nbar for a CO2, CH4or Natmosphere (Widemann et al., 2009).

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