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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.

Defraigne P.,Royal Observatory of Belgium ROB | Baire Q.,Royal Observatory of Belgium ROB | Harmegnies A.,Bureau International des Poids et Mesures
42nd Annual Precise Time and Time Interval (PTTI) Systems and Applications Meeting 2010 | Year: 2010

Measurements from Global Navigation Satellite Systems (GNSS) have been used since theeighties to perform precise and accurate Time and Frequency Transfer (TFT). Two mainapproaches are used presently: the Common View or All in View based on the Common GPSGLONASS Time Transfer Standard (CGGTTS) and the Precise Point Positioning (PPP).Concerning the CGGTTS approach, in order to allow the use of GLONASS data from geodetictypereceivers providing only raw data in RINEX format, the R2CGGTTS software developedinitially for GPS at the Royal Observatory of Belgium (ROB) was updated. The GLONASSnavigation files are used for the determination of satellite clocks and positions, and thecomputation procedure to get the CGGTTS data from the pseudorange measurements is appliedsimilarly as for the GPS satellites. In parallel, we also upgraded the PPP software Atomiumdeveloped at the ROB to allow the use of the Russian GLONASS constellation. The clocksolution is then obtained through the analysis of dual-frequency carrier-phase and pseudorangemeasurements. This study combines GPS and GLONASS observations in PPP in order todetermine the added value of the GLONASS data in geodetic time transfer. © 2010 by Precise Time and Time Interval (PTTI) - Time Service Department.

Robert V.,French National Center for Scientific Research | Robert V.,Laboratoire dAstrometrie Appliquee LASA | Robert V.,Polytechnic Institute of Advanced Science | de Cuyper J.-P.,Royal Observatory of Belgium ROB | And 7 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2011

The astrometric monitoring of the natural planetary satellites is an important step to assess the formation and the evolution of these systems. However, in order to quantify relevant gravitational effects such as tidal forces, it is necessary to have very accurate observations over a long time interval. Unfortunately, the accuracy is decreasing as one considers older observations. To solve this issue, digitizing of old photographic plates is an attractive method, but a high accuracy in the measurement and the reduction of those plates is absolutely necessary. We have developed methods and algorithms adapted to specific plates provided by USNO, using the DAMIAN digitizer of ROB. From a set of 35 plates taken in 1974, we have been able to produce measurements with an accuracy better than 0.08 μm and after reduction using the UCAC2 catalogue, rms residuals of 35 mas (1.7 μm) for intersatellite positions (when the original reduction provided 100 mas) and of 65 mas for equatorial RA and Dec. positions (which were not possible to get with the original reduction). First results on the dynamics of the satellites and of the planet Jupiter are provided. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.

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.

Devos A.,Royal Observatory of Belgium ROB | Verbeeck C.,Royal Observatory of Belgium ROB | Robbrecht E.,Royal Observatory of Belgium ROB
Journal of Space Weather and Space Climate | Year: 2014

The Solar Influences Data analysis Center (SIDC) in Brussels at the Royal Observatory of Belgium (ROB) has been providing daily space weather forecasts for more than a decade. A verification analysis was applied to evaluate the performance of the SIDC forecasts of fundamental space weather parameters such as the F10.7 radio flux, solar flare activity, and local geomagnetic index. Strengths and weaknesses are determined compared to common numerical models. Descriptive model statistics, common verification measures, error analysis and conditional plots related to forecasts and observations are presented. The verification analysis methods have been designed such that future improvements and additions can easily be included, for example with new forecasting models. The SIDC forecast (together with the persistence model) achieves the best performance for forecasting F10.7 on day 1, but has potential for improvement for a larger lead time mainly by applying estimates from the persistence and corrected recurrence models. The persistence model is superior for the forecast of flares, though corrected recurrence models are slightly better in foreseeing M- and X-class flares and the SIDC forecast estimates B- and C-class flares very well. The SIDC forecast scores better than all models in forecasting the local K-index. It best reproduces observations in the range of K = 2-4, but underestimates larger K values. The SIDC forecast provides a distribution that best matches the observations of the K-index. The analysis presented here demonstrates the influence of solar activity on the confidence level of the forecasts, as well as the hinted influence of the forecaster on duty due to the subjective nature of forecasting. The output aids to identify the strong and weak points of the SIDC forecast as well as those of the models considered. Though the presented analysis needs further extension, it already illustrates the opportunity to regularly reevaluate space weather forecasts and to stimulate ideas for improvement and increase the reliability of space weather forecasting. © A. Devos et al., Published by EDP Sciences 2014.

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.

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.

Van Malderen R.,Royal Meteorological Institute of Belgium | Brenot H.,Belgium Institute for Space Aeronomy BISA | Pottiaux E.,Royal Observatory of Belgium ROB | Beirle S.,Max Planck Institute for Chemistry | And 5 more authors.
Atmospheric Measurement Techniques | Year: 2014

Water vapour plays a dominant role in the climate change debate. However, observing water vapour over a climatological time period in a consistent and homogeneous manner is challenging. On one hand, networks of ground-based instruments able to retrieve homogeneous integrated water vapour (IWV) data sets are being set up. Typical examples are Global Navigation Satellite System (GNSS) observation networks such as the International GNSS Service (IGS), with continuous GPS (Global Positioning System) observations spanning over the last 15+ years, and the AErosol RObotic NETwork (AERONET), providing long-term observations performed with standardized and well-calibrated sun photometers. On the other hand, satellite-based measurements of IWV already have a time span of over 10 years (e.g. AIRS) or are being merged to create long-term time series (e.g. GOME, SCIAMACHY, and GOME-2). This study performs an intercomparison of IWV measurements from satellite devices (in the visible, GOME/SCIAMACHY/GOME-2, and in the thermal infrared, AIRS), in situ measurements (radiosondes) and ground-based instruments (GPS, sun photometer), to assess their use in water vapour trends analysis. To this end, we selected 28 sites world-wide for which GPS observations can directly be compared with coincident satellite IWV observations, together with sun photometer and/or radiosonde measurements. The mean biases of the different techniques compared to the GPS estimates vary only between -0.3 to 0.5 mm of IWV. Nevertheless these small biases are accompanied by large standard deviations (SD), especially for the satellite instruments. In particular, we analysed the impact of clouds on the IWV agreement. The influence of specific issues for each instrument on the intercomparison is also investigated (e.g. the distance between the satellite ground pixel centre and the co-located ground-based station, the satellite scan angle, daytime/nighttime differences). Furthermore, we checked if the properties of the IWV scatter plots between these different instruments are dependent on the geography and/or altitude of the station. For all considered instruments, the only dependency clearly detected is with latitude: the SD of the IWV observations with respect to the GPS IWV retrievals decreases with increasing latitude and decreasing mean IWV. © Author(s) 2014.

Lefevre L.,Royal Observatory of Belgium ROB | Clette F.,Royal Observatory of Belgium ROB
Astronomy and Astrophysics | Year: 2011

Context. The variability of the 11-year cycle of solar activity on secular timescales is well established through the sunspot record, but it remains unpredictable. Indeed, the duration of the last solar cycle 23 was exceptionally long and took the solar physics community by surprise. The long minimum was marked by particularly low and often unprecedented levels of the international sunspot index R i and most other solar indices. Earlier in the course of cycle 23, discrepancies appeared between several of those indices, raising a new issue: is there something wrong in the recent index values or is a real physical change occurring inside the Sun? Aims. By exploiting detailed sunspot information, we look for independent evidence of a concrete and significant global change in sunspot properties appearing in the course of cycle 23. Methods. To achieve this goal, we compared existing sunspot indices, such as the international sunspot number (R i), and exploited the most complete information currently available on individual sunspots, obtained by merging two recent and complementary catalogs. Detailed statistics were obtained according to group type and spot size. Results. We find that the Sun has shown an important deficit in small spots since the last activity maximum around 2000. While the number of large-scale spots remained largely unaffected, the occurrence rate of the smallest sunspots, and among them the ones with the shortest lifetimes, was more than halved during cycle 23. This explains the divergence between indices, weighted in favor of the largest active regions/magnetic structures, and sunspot counts that do not include such a weighting. It also confirms an actual intrinsic transition in the magnetic field generation inside the Sun, arising years before the exceptional activity minimum. Conclusions. Our results thus reveal the potential of such detailed sunspot analyses for understanding and predicting future trends in the solar cycle. The change found here in the small individual sunspots suggests that solar and solar-terrestrial proxies should be redefined for the current state of the Sun, replacing the present ones. This scale-dependent change also provides support to dynamo models involving the coexistence of a deep and a superficial dynamo. © 2011 ESO.

Karki S.,Catholic University of Louvain | Craeye C.,Catholic University of Louvain | Mitrovic M.,Royal Observatory of Belgium ROB | Dehant V.,Royal Observatory of Belgium ROB
2016 10th European Conference on Antennas and Propagation, EuCAP 2016 | Year: 2016

A crossed slot circularly polarized antenna array conforming to a conical frustum is proposed for a Mars transponder. The cross-slot antenna element is aperture-coupled by a single stripline feed, and its properties are studied. Also, the entire slot antenna array fed with discrete delta-gap sources to produce circular polarization is also described. Eventually, the slot array would all be replaced by stripline feeds with phase control to perform analog beamforming. © 2016 European Association of Antennas and Propagation.

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