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Rosenblatt P.,Royal Observatory of Belgium ROB | Charnoz S.,University Paris Diderot | Charnoz S.,Institut Universitaire de France
Icarus | Year: 2012

We reconsider two scenarios for the formation of Phobos and Deimos from a circum-martian accretion disk of debris: the strong tide regime for which accretion occurs close to the planet at the Roche limit and the weak tide regime for which accretion occurs farther from the planet. We assume a disk with an initial mass of 10 18kg (Craddock, R.A. [2011]. Icarus 211, 1150-1161). In the strong tide regime, the disk loses its material by viscous spreading inward to and outward from the planet. When outward moving material crosses the Roche limit, small-sized moonlets are accreted from gravitational instabilities with a shape and density similar to Phobos and Deimos. Due to the gravitational torque exerted by the disk, the moonlets migrate away from the planet, though they cannot reach the synchronous orbit (lying at 6 Mars' radii). After the disk has lost most of its mass they rapidly fall back onto Mars due to the tidal decay of their orbits. Although, the total mass of moonlets is comparable to the mass of Phobos, their survival time does not exceed 200Ma, which is incompatible with the formation of Phobos and Deimos early in Mars' history. In the weak tide regime, moonlets can accrete near the synchronous orbit with the mass of Deimos in a disk of up to 10 18kg (similarly to planetary embryos formation in the protoplanetary disk). A Phobos-mass embryo can also be formed in the same disk but closer to Mars (at 3-4 Mars' radii) so that it rapidly falls back onto Mars by tidal decay of its orbit. However, several embryos may accrete together in the disk (similarly to the final stage of terrestrial planet formation), and Phobos and Deimos may be the last two remnants of those bodies formed near the synchronous distance to Mars. Further investigations are still needed to understand such accretion mechanism within a circum-martian disk primarily extending below the synchronous orbit. © 2012 Elsevier Inc. Source

Defraigne P.,Royal Observatory of Belgium ROB | Martinez-Belda M.C.,University of Alicante
42nd Annual Precise Time and Time Interval (PTTI) Systems and Applications Meeting 2010 | Year: 2010

R2CGGTTS is a software program dedicated to provide clock solutions for GNSS timetransfer in the standard CGGTTS (Common GPS GLONASS Time Transfer Standard) fromRINEX observation files. This paper presents the upgrade of the R2CGGTTS to includeobservations of the future Galileo satellites; the approach is validated using the GIOVE data. Asecond part of the paper presents the possibilities of the future Galileo E5 signal, of which thevery small noise level promises a large improvement to the GNSS time transfer accuracy. © 2010 by Precise Time and Time Interval (PTTI) - Time Service Department. Source

Noack L.,Royal Observatory of Belgium ROB | Noack L.,German Aerospace Center | Godolt M.,German Aerospace Center | Godolt M.,TU Berlin | And 6 more authors.
Planetary and Space Science | Year: 2014

Motivation: The most likely places for finding life outside the Solar System are rocky planets, some of which may have surface conditions allowing for liquid water, one of the major prerequisites for life. Greenhouse gases, such as carbon dioxide (CO2), play an important role for the surface temperature and, thus, the habitability of an extrasolar planet. The amount of greenhouse gases in the atmosphere is in part determined by their outgassing from the interior. Method: We use a two-dimensional convection model to calculate partial melting and the amount of CO2 outgassed for Earth-sized stagnant-lid planets. By varying the planetary mass, we investigate the evolution of a secondary atmosphere dependent on the interior structure (different ratio of planetary to core radius). We further study the likelihood for plate tectonics depending on the interior structure and investigate the influence of plate tectonics on outgassing. Results: We find that for stagnant-lid planets the relative size of the iron core has a large impact on the production of partial melt because a variation in the interior structure changes the pressure gradient and thereby the melting temperature of silicate rocks with depth. As a consequence, for planets with a large core (a radius of at least 70-75% of the planets radius), outgassing from the interior is strongly reduced in comparison to a planet with the same radius but a small core. This finding suggests that the outer edge of the habitable zone of a star not only depends on the distance from the star and thus the solar influx but also is further limited by small outgassing for stagnant-lid planets with a high average density, indicating a high iron content (e.g. Mercury and the recently detected exoplanets Kepler-10b and CoRoT-7b). This contradicts previous models that have assumed CO2 reservoirs being in principle unlimited for all planets. If plate tectonics is initiated, several tens of bars of CO2 can be outgassed in a short period of time - even for planets with a large iron core - and the outer boundary of the habitable zone is not influenced by the interior structure. It is, however, more difficult for planets with a thin mantle (in our test case, with a thickness of 10% of the planets radius) to initiate plate tectonics. Our results indicate that the interior structure may strongly influence the amount of CO2 in planetary atmospheres and, thereby, the habitability of rocky planets. To obtain better constraints on the interior structure accurate measurements of size and mass are necessary. © 2013 Elsevier Ltd. All rights reserved. Source

Barkad H.A.,CNRS Institute of Electronics, Microelectronics and Nanotechnology | Soltani A.,CNRS Institute of Electronics, Microelectronics and Nanotechnology | Mattalah M.,CNRS Institute of Electronics, Microelectronics and Nanotechnology | Gerbedoen J.-C.,CNRS Institute of Electronics, Microelectronics and Nanotechnology | And 10 more authors.
Journal of Physics D: Applied Physics | Year: 2010

Deep-ultraviolet solar-blind photodiodes based on high-quality AlN films grown on sapphire substrates with a metal-semiconductor-metal configuration were simulated and fabricated. The Schottky contact is based on TiN metallization. The material is characterized by the micro-Raman spectroscopy and x-ray diffraction technique. The detector presents an extremely low dark current of 100 fA at -100 V dc bias for large device area as high as 3.1 mm2. It also exhibits a rejection ratio between 180 and 300 nm of three orders of magnitude with a very sharp cut-off wavelength at 203 nm (∼6.1 eV). The simulation to optimize the photodiode topology is based on a 2D energy-balance model using the COMSOL® software. Simulation performed for different spacing for a given bias between electrodes show that a compromise must be found between the dark current and the responsivity for the optimization of the device performance. The measurement results are in good agreement with the model predictions. © 2010 IOP Publishing Ltd. Source

Rosenblatt P.,Royal Observatory of Belgium ROB | Rosenblatt P.,Catholic University of Louvain | Bruinsma S.L.,French National Center for Space Studies | Muller-Wodarg I.C.F.,Imperial College London | And 3 more authors.
Icarus | Year: 2012

On its highly elliptical 24. h orbit around Venus, the Venus Express (VEX) spacecraft briefly reaches a periapsis altitude of nominally 250. km. Recently, however, dedicated and intense radio tracking campaigns have taken place in August 2008, October 2009, February and April 2010, for which the periapsis altitude was lowered to the 186-176. km altitude range in order to be able to probe the upper atmosphere of Venus above the North Pole for the first time ever in situ. As the spacecraft experiences atmospheric drag, its trajectory is measurably perturbed during the periapsis pass, allowing us to infer total atmospheric mass density at the periapsis altitude. A Precise Orbit Determination (POD) of the VEX motion is performed through an iterative least-squares fitting process to the Doppler tracking data, acquired by the VEX radioscience experiment (VeRa). The drag acceleration is modelled using an initial atmospheric density model (VTS3 model, Hedin, A.E., Niemann, H.B., Kasprzak, W.T., Seiff, A. [1983]. J. Geophys. Res. 88, 73-83). A scale factor of the drag acceleration is estimated for each periapsis pass, which scales Hedin's density model in order to best fit the radio tracking data. Reliable density scale factors have been obtained for 10 passes mainly from the second (October 2009) and third (April 2010) VExADE campaigns, which indicate a lower density by a factor of about 1.8 than Hedin's model predicts. These first ever in situ polar density measurements at solar minimum have allowed us to construct a diffusive equilibrium density model for Venus' thermosphere, constrained in the lower thermosphere primarily by SPICAV-SOIR measurements and above 175. km by the VExADE drag measurements (Müller-Wodarg et al., in preparation). The preliminary results of the VExADE campaigns show that it is possible to obtain with the POD technique reliable estimates of Venus' upper atmosphere densities at an altitude of around 175. km. Future VExADE campaigns will benefit from the planned further lowering of VEX pericenter altitude to below 170. km. © 2011 Elsevier Inc. Source

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