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| Overview of the coordinated ground-based observations of Titan during the Huygens mission |
| Coordinated ground-based observations of Titan were performed around or during the Huygens atmospheric probe mission at Titan on 14 January 2005, connecting the momentary in situ observations by the probe with the synoptic coverage provided by continuing ground-based programs. These observations consisted of three different categories: (1) radio telescope tracking of the Huygens signal at 2040 MHz, (2) observations of the atmosphere and surface of Titan, and (3) attempts to observe radiation emitted during the Huygens Probe entry into Titan's atmosphere. The Probe radio signal was successfully acquired by a network of terrestrial telescopes, recovering a vertical profile of wind speed in Titan's atmosphere from 140 km altitude down to the surface. Ground-based observations brought new information on atmosphere and surface properties of the largest Saturnian moon. No positive detection of phenomena associated with the Probe entry was reported. This paper reviews all these measurements and highlights the achieved results. The ground-based observations, both radio and optical, are of fundamental importance for the interpretation of results from the Huygens mission. |
| Publication date: 27 Jul 2006 |
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| Titan's exotic weather |
| Titan is viewed as a sibling of Earth, as both bodies have rainy weather systems and landscapes formed by rivers. But as we study these similarities, Titan emerges as an intriguingly foreign world. |
| Publication date: 26 Jul 2006 |
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| Bistatic observations of Titan's surface with the Huygens probe radio signal |
| Huygens provided an unanticipated bistatic radio scattering experiment from Titan's surface. After a successful entry and descent on Titan, on 14 January 2005, the probe remarkably survived the landing and continued radioing from the surface to the overflying Cassini, until the orbiter set below Titan's local horizon. Here we report high-quality measurements of the 2098 MHz (14.3 cm) postlanding radio signal, focusing on the striking variations observed in signal strength. The mechanism that creates this fading pattern is physically interpreted as multipath interference between the direct signal and the signal reflected on Titan's surface. The geophysical aspects that could bear on the signal analysis are described. |
| Publication date: 25 Jul 2006 |
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| Winds on Titan from ground-based tracking of the Huygens probe |
| Large radio telescopes on Earth tracked the radio signal transmitted by the Huygens probe during its mission at Titan. Frequency measurements were conducted as a part of the Huygens Doppler Wind Experiment (DWE) in order to derive the velocity of the probe in the direction to Earth. The DWE instrumentation on board Huygens consisted of an ultrastable oscillator which maintained the high Doppler stability required for a determination of probe horizontal motion during the atmospheric descent. A vertical profile of the zonal wind velocity in Titan's atmosphere was constructed from the Doppler data under the plausible assumption of generally small meridional wind, as validated by tracked images from the Huygens Descent Imager/Spectral Radiometer (DISR). We report here on improved results based on data with higher temporal resolution than that presented in the preliminary analysis by Bird et al. (2005), corroborating the first in situ measurement of Titan's atmospheric superrotation and a region of strong vertical shear reversal within the lower stratosphere. We also present the first high-resolution display and interpretation of the winds near the surface and planetary boundary layer. |
| Publication date: 20 Jul 2006 |
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| Nonequilibrium radiative heat flux modeling for the Huygens entry probe |
| An electronic collisional-radiative model is proposed to predict the nonequilibrium populations and the radiation of the excited electronic states CN(A, B) and N2 (A, B, C) during the entry of the Huygens probe into the atmosphere of Titan. The model is loosely coupled with flow solvers using a Lagrangian method. First, the model was tested against measurements obtained with the shock-tube of NASA Ames Research Center. Then, the model was applied to the simulation of Huygen's entry. Our simulations predict that the population of the CN(B) state is lower than the Boltzmann population by a factor 40 at trajectory time t = 165 s and by a factor 2 at t = 187 s and that the population of the CN(A) state remains close to the Boltzmann population for both trajectory points. The radiative heat fluxes, driven by the CN(A, B) states, are lower than predictions based on the Boltzmann populations by a factor 15 at t = 165 s and a factor 2 at t = 187 s. |
| Publication date: 14 Jul 2006 |
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| Limits to the abundance of surface CO2 ice on Titan |
| In the context of the Cassini/Huygens mission, we performed supporting ground-based observations to complement the results from the NASA/ESA/ASI space mission to the Saturnian system with particular focus on Titan. On the nights of 18 and 19 December 2004, we conducted adaptive optics observations with VLT/NACO to search for and map the distribution of CO2 ice deposits on the spatially resolved surface of Titan (65 mas resolution). This experiment became possible because (1) solid CO2 possesses two narrow and strong absorption lines at 2012 nm and 2070 nm that fall into the 2.05 micron window of Titan's atmosphere and (2) the width of these bands matches the band pass of the Fabry-Perot instrument installed in NACO. We do not detect this chemical compound, but we can derive firm limits on the abundance of CO2 ice on the surface of Titan at sub-Earth longitudes 284° W and 307° W. With a spatial sampling of 65 mas, we conclude that a partial surface coverage of segregated CO2 ice does not exceed 7% or 14% for bright or dark surface regions, respectively. |
| Publication date: 06 Jul 2006 |
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| ESA SP-1296: ESA's Report to the 36th COSPAR Meeting |
Scientific editor: R. Marsden
Editor: A. Wilson
The report for the 36th COSPAR Meeting covers, as in previous issues, the missions of the Scientific Programme of ESA in the areas of astronomy, Solar System science and fundamental physics. This year's COSPAR meeting will take place only weeks before the end of the SMART-1 mission to the Moon, a technology project that provided the first European look at our natural satellite from lunar orbit.In October of this year, a new mission will be launched: COROT. ESA, together with a number of countries, is contributing to this unique, French-led project that will provide an insight into the interior of the stars, by means of the asteroseismology technique successfully applied by SOHO. COROT will also perform a systematic search for new extrasolar planets using photometric transits.
The record number of ESA Science Programme missions in operation established at the time of the last report was maintained in 2006 (Huygens having been replaced in the list by Venus Express). Eleven different missions, involving 14 operating spacecraft, are providing excellent science to the worldwide scientific community. The Research and Scientific Support Department (RSSD) is responsible for the science operations of these missions and makes every effort to ensure the best possible science return. The Department also supports the realisation of approved projects in all phases of their development. |
| Publication date: 15 Jun 2006 |
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| First Application of the Huygens Descent Trajectory Working Group Algorithm to Huygens Data |
| The ESA Huygens probe performed a successful Entry, Descent, and Landing (EDL) sequence through Titan's dense atmosphere on January 14, 2005. During all three phases, i.e., the supersonic entry phase, the descent phase, as well as the impact and post impact phases, the probe performed measurements that were used for the reconstruction of its entry and descent trajectory. We first discuss the datasets relevant to the entry and descent trajectory reconstruction. We then provide an overview of the reconstruction strategy, and show preliminary results of the reconstructed entry and descent trajectory, including both position and velocity. |
| Publication date: 01 Jun 2006 |
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| Huygens Attitude Reconstruction based on Flight Engineering Parameters |
| Huygens is ESA's main contribution to the joint NASA/ESA/ASI Cassini/Huygens mission to Saturn and its largest moon Titan. The Probe, delivered to the interface altitude of 1270 km above the surface by NASA/JPL Cassini orbiter, entered the dense atmosphere of Titan on 14th January 2005 and landed on the surface after a descent under parachute of slightly less than of 2.5 hours. Huygens continued to function after landing for more than 3 hours. Data was transmitted and successfully recovered by Cassini continuosly Although the Huygens attitude reconstruction based on the flight engineering parameters was not foreseen during the development phase (no gyros were included), a rough descent under parachute and indications of an anomaly in the probe spin direction make the engineering dataset valuable in the frame of the ADRS (Huygens Attitude Determination and Reconstruction Subgroup) as a complement to the scientific measurements. In addition, several scientific teams have a strong interest in understanding the orientation of the probe for interpreting their data, as DISR (Descent Imager and Spectral Radiometer) and HASI-PWA (Huygens Atmospheric Structure Instrument-Permeability, Wave and Altimetry). In this paper we describe the engineering parameters used for the Probe attitude reconstruction, namely (Clausen et al., 2002) the radio link AGC (Automatic Gain Control), RASU and CASU (Radial and Central Accelerometer Sensor Units) and RAU (Radar Altimeter Unit). We explain the methodology applied to indirectly infer from these sensors measurements the attitude information. We also discuss and present the reconstructed information related to attitude: spin rate and azimuthal position (during the atmospheric descent), and landing orientation. Tip and tilt implications are still being worked. Preliminary data on their behaviour is presented.ess clear but hints on their behavior may be inferred. |
| Publication date: 01 Jun 2006 |
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| Huygens Radio Link In-Flight Performance |
| Huygens is the ESA-provided element of the joint NASA/ESA/ASI Cassini/Huygens mission to Saturn and its largest moon Titan. The spacecraft, delivered to the interface altitude of 1270 km above the surface by NASA/JPL, dived into the dense atmosphere of Titan on 14th January 2005 and landed on the surface after a nominal descent of 2.5 hours. The scientific and housekeeping data was continuously transmitted after the heat shield release to Cassini and relayed back to Earth in a later retransmission through the Deep Space Network. Probably the most challenging activity after launch was the identification and recovery from a design flaw in the communications system that, if not corrected, would have led to major loss of scientific data, accounting up to 80 - 90% of the complete dataset. It was February 2000 and the first in-flight test of the Probe relay link was executed. Although results confirmed the expected carrier level performance, unexpected behavior was observed at data-stream level: in particular, the receiver showed anomalous behavior when working at the mission Doppler. |
| Publication date: 01 Jun 2006 |
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| The Huygens Synthetic Dataset |
| The Huygens probe entered into the dense atmosphere of Titan on 14th January 2005 and landed on the surface after a nominal descent of about 2.5 hours [1]. Huygens is the ESA-provided element of the joint NASA/ESA/ASI Cassini/Huygens mission. The probe was delivered to the interface altitude of 1270 km above the surface by NASA/JPL. The propagation and reconstruction of the trajectory from that point onwards is ESA?s responsibility. An important effort was devoted to the development of an algorithm that aimed at reconstructing the descent trajectory and attitude of Huygens from the scientific instruments and probe sensors measurements ([2], this issue). In order to test this algorithm, the Huygens Synthetic Data Set (HSDS), a simulated mission dataset, was prepared. In this paper we describe the philosophy of the approach for preparing the HSDS, the assumptions made and the limitations of the method. The different tools used for producing the simulated data set are described, mainly a 3 Degree-of-Freedom (DoF) entry and descent trajectory calculation, and 6 DoF entry trajectory and attitude simulator. We report how the scientific and engineering models were used to obtain the most realistic Huygens sensor data, and the latest updates leading to the final v3.1 on the 10th of January, just 4 days prior to the Huygens descent. The different parameters are described, with a special attention to the way the accelerations were generated.
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| Publication date: 01 Jun 2006 |
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| Wind-induced seasonal angular momentum exchange at Titan's surface and its influence on Titan's length-of-day |
| Titan's substantial obliquity and the global extent of the Hadley circulation give rise to seasonal variation in the mean zonal wind speed and direction in the geostrophic lower troposphere, causing an exchange of a substantial amount of angular momentum between the surface and atmosphere. The wind-induced seasonal length-of-day variation calculated using the global wind profile predicted by a Titan general circulation model (GCM) amounts to 30 s in the absence of a deep subsurface ocean decoupling the outer ice shell from the ice mantle and ~400 s in the presence of a deep subsurface ocean. This effect should give rise to longitudinal offsets of surface landmarks by ~10 km and ~100 km, respectively, in comparison to predicted positions based on a constant rotation rate, and may be detectable by Cassini imaging. |
| Publication date: 22 Dec 2005 |
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| Characterization of zonal winds in the stratosphere of Titan |
| Titan has been observed with UVES, the UV-Visual Echelle Spectrograph at the Very Large Telescope, with the aim of characterizing the zonal wind flow. We use a retrieval scheme originally developed for absolute stellar accelerometry [Connes, P., 1985. Astrophys. Space Sci., 110, 211-255] to extract the velocity signal by simultaneously taking into account all the lines present in the spectrum. The method allows to measure the Doppler shift induced at a given point by the zonal wind flow, with high precision. The short-wavelength channel (420-520 nm) probes one scale height higher than the long-wavelength one (520-620 nm), and we observe statistically significant evidence for stronger winds at higher altitudes. The results show a high dispersion. Globally, we detect prograde zonal winds, with lower limits of 62 and 50 ms-1 at the regions centered at 200 and 170 km altitude, but approximately a quarter of the measurements indicates null or retrograde winds. |
| Publication date: 15 Dec 2005 |
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| An overview of the descent and landing of the Huygens probe on Titan |
| Titan, Saturn's largest moon, is the only Solar System planetary body other than Earth with a thick nitrogen atmosphere. The Voyager spacecraft confirmed that methane was the second-most abundant atmospheric constituent in Titan's atmosphere, and revealed a rich organic chemistry, but its cameras could not see through the thick organic haze. After a seven-year interplanetary journey on board the Cassini orbiter, the Huygens probe was released on 25 December 2004. It reached the upper layer of Titan's atmosphere on 14 January and landed softly after a parachute descent of almost 2.5 hours. Here we report an overview of the Huygens mission, which enabled studies of the atmosphere and surface, including in situ sampling of the organic chemistry, and revealed an Earth-like landscape. The probe descended over the boundary between a bright icy terrain eroded by fluvial activity-probably due to methane-and a darker area that looked like a river- or lake-bed. Post-landing images showed centimetre-sized surface details. |
| Publication date: 08 Dec 2005 |
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| Titan's internal structure inferred from a coupled thermal-orbital model |
| Through coupled thermal and orbital calculations including a full description of tidal dissipation, heat transfer and the H2ONH3 phase diagram, we propose a model for the internal structure and composition of Titan testable with Cassini Huygens measurements. The high value of Titan's orbital eccentricity provides a strong constraint on the amount of the tidal energy dissipation on its surface and within its interior since its formation. We show that only models with a few percent of ammonia (and not zero) in the primordial liquid water shell can limit the damping of the eccentricity over the age of the Solar System. The present models predict that a liquid ammonia-rich water layer should still be present within Titan under an ice I layer, a few tens of kilometers thick. Furthermore, we predict that any event linked to convective processes in the ice Ih layer (like the degassing of methane) could have occurred very late in Titan's history. |
| Publication date: 15 Jun 2005 |
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| Release of volatiles from a possible cryovolcano from near-infrared imaging of Titan |
| Titan is the only satellite in our Solar System with a dense atmosphere. The surface pressure is 1.5 bar and, similar to the Earth, N2 is the main component of the atmosphere. Methane is the second most important component, but it is
photodissociated on a timescale of 107 years. This short timescale has led to the suggestion that Titan may possess a surface or subsurface reservoir of hydrocarbons to replenish the atmosphere. Here we report near-infrared images of Titan obtained on 26 October 2004 by the Cassini spacecraft. The images show that a widespread methane ocean does not exist; subtle albedo variations instead suggest topographical variations, as would be expected for
a more solid (perhaps icy) surface. We also find a circular structure, 30 km in diameter that does not resemble any features seen on other icy satellites.We propose that the structure is a dome formed by upwelling icy plumes that release methane into Titan's atmosphere. |
| Publication date: 09 Jun 2005 |
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| Confirmation of Our Predictions, Based on Laboratory and Theoretical Studies, on the Findings of the Huygens Probe on Titan |
| 1. The very low abundances of Ar, Kr and Xe in Titan's atmosphere can be easily explained by our experimental findings. These gases are trapped in the aerosols, which are formed by UV photolysis of acetylene in their presence. When the aerosols fall down to the surface, they clean the atmosphere of these gases. A continuous supply of the radiogenic produced 40Ar from the interior can explain its small abundance in the atmosphere.
2. The originally soft and sticky photochemical aerosols, as found by us experimentally, were calculated to harden by spontaneous and radiation induces chemical cross-linking. Indeed the camera and other detectors were not covered by sticky aerosols and the intake ports were not clogged.
3. As we predicted, no lightning discharges were detected in the quiescent Titan atmosphere. Therefore, Titan's atmospheric chemistry is driven mainly by solar UV irradiation and not by electrical discharges.
4. The mixing ratios of the major gas phase species produced by UV photolysis of acetylene, as found experimentally: methylacetylene ; diacetylene ; divinyl ; and benzene were observed by the Cassini spacecraft in Titan's upper atmosphere, with an agreement within better than an order of magnitude.
5. The N:C ratio in Titan's aerosols was measured by the Huygens probe, but no results were published yet. UV photolysis of gas mixtures containing C2H2:HCN=10 yield aerosols with a ratio N:C=0.007 up to 0.01. Electrical discharges through a N2:CH4~10 gas mixtures yield a much higher N:C ratio.
6. We anticipate mountains not higher than 1900 m on Titan's surface. |
| Publication date: 27 May 2005 |
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| Huygens ASI measurements at Titan: a new insight of Titan's atmosphere |
| During the Huygens probe mission at Titan on 14th January 2005, the Huygens Atmospheric Structure Instrument (HASI) obtained measurements of atmospheric properties from up above 1400 km down to the ground, thus inferring the atmospheric structure. The atmospheric profile along the Huygens probe trajectory during entry phase have been retrieved from the accelerometers data, while below 160 km direct pressure and temperature measurements have been performed. The vertical temperature profile retrieved from HASI data is in very good agreement with the model derived from Voyager's observations, confirms the evidence for a stratopause and the inversion layers in the upper atmosphere as observed during stellar occultations and yielded new details on atmospheric structure. |
| Publication date: 27 May 2005 |
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| Magnetospheric Drift Resonance Effects on Local Time Asymmetry, Injection Events, and Moon Interactions for Saturn as Compared to Earth |
| The magnetospheres of Earth and Saturn have similarities in terms of the highest energy radiation belt components from Cosmic Ray Albedo Neutron Decay (CRAND) but have otherwise been expected to differ on the role of charged particle convection driven by solar wind interactions with these magnetospheres. Saturn's inner and middle magnetosphere has been assumed to be dominated by corotation with little direct penetration by solar wind and magnetotail plasma. Since Saturn's planetary magnetic field characterized by the Z3 model is axisymmetric, although slightly offset northward from the ring plane, it has been difficult to understand previous Pioneer and Voyager measurements of local time asymmetry in energetic particle populations, including just outside the main rings as found by Pioneer 11. Small scale features (microsignatures) of charged particle absorption by Saturn moons and possible 'ghost' clouds of co-orbiting debris show no consistent patterns in the context of symmetric models for longitudinal drift shells. Since the 100-MeV CRAND proton drift shells are highly symmetric, it is apparent that lower energy electrons and ions showing substantial local time asymmetry are influenced by forces other than simple corotation. Cassini Huygens neutral atom observations show clear evidence of substorm injections reaching into the middle magnetosphere of Saturn preferentially on the nightside. |
| Publication date: 27 May 2005 |
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| Radiative Transfer in Primordial Atmosphere of Titan |
| In light of Huygens measurements, we present our improved model of thermal and photochemical evolution of Titan's atmosphere. Atreya et. al (1978) demonstrated that photolysis of ammonia on primordial Titan is capable of producing a nitrogen atmosphere substantially thicker than that measured by Voyager. E. Wilson (2001) carried this calculation one step further by including methane and water vapor explicitly in the ammonia photochemistry model, and arrived at a preliminary estimate of time required to accumulate different amounts of nitrogen. However, both models assumed an isothermal atmosphere. Since chemistry leading up to nitrogen occurs in the stratosphere, both the thermal structure and saturation effects are important for determining the time constants and amounts of nitrogen production. In this presentation, we discuss preliminary results of a radiative equilibrium model for the primordial middle and lower atmosphere of Titan. It includes CH4, NH3 and H2O in solar proportions for its initial composition, and CH4-CH4 pressure induced absorption, which presently controls the thermal structure in the troposphere. The temperature in the stratosphere is controlled by the haze, and we explore the effects of a haze layer at various altitudes for accelerating conversion of ammonia to nitrogen. Furthermore, we include the effects of enhanced solar flux during the T-Tauri phase, which could speed up both the loss of nitrogen and conversion of ammonia to nitrogen. We are in the process of coupling the radiative transfer model to a comprehensive photochemical model (Wilson and Atreya, 2004) to access the roles of trace species other than those included in this calculation. |
| Publication date: 27 May 2005 |
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