ESA Science & Technology - Publication Archive

Spectra from Cassini's Visual and Infrared Mapping Spectrometer reveal the presence of a vast tropospheric cloud on Titan at latitudes 51° to 68° north and all longitudes observed (10° to 190° west). The derived characteristics indicate that this cloud is composed of ethane and forms as a result of stratospheric subsidence and the particularly cool conditions near the moon's north pole. Preferential condensation of ethane, perhaps as ice, at Titan's poles during the winters may partially explain the lack of liquid ethane oceans on Titan's surface at middle and lower latitudes.

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.

Measurements of stratospheric zonal winds on Titan were made in preparation for and during the time of the descent of the Huygens Probe into Titan's atmosphere on 14 January 2005. Fully resolved emission lines from ethane near 11.7 micron were measured on the east, center, and west positions on Titan using the NASA/GSFC Heterodyne Instrument for Planetary Wind And Composition, HIPWAC, mounted on the National Astronomical Observatory of Japan 8.2 m Subaru Telescope on Mauna Kea, Hawaii. Analysis of the Doppler shifts of the emission line shapes yielded mean prograde gas velocity ~60 ± 65 m/s at altitudes below ~120 km (~5 mbar). This result is consistent with retrievals from the Huygens Doppler Wind Experiment and from other observations near this altitude range. Current spectral line shapes, however, differed significantly from those obtained in similar measurements on Subaru in 2004 and on the NASA IRTF in 1993-1996, which retrieved prograde zonal winds 190 ± 90 m/s at 230 km (~0.4 mbar). The cores of the emission lines, which probe the high-altitude region, could not be fitted as before to retrieve wind directly using the accepted atmospheric model for Titan. They imply an approximately tenfold increase in ethane mole fraction (1.2 × 10^-4) with strong wind shear above the stratopause, providing a potential probe of the lower mesosphere and possible evidence of temporal and spatial variability. Results contribute to coordinated measurements of winds by various techniques providing information on the altitude distribution of wind velocity in Titan's atmosphere from near the surface to the lower mesosphere.

Saturn's moon Titan shows landscapes with fluvial features suggestive of hydrology based on liquid methane. Recent efforts in understanding Titan's methane hydrological cycle have focused on occasional cloud outbursts near the south pole or cloud streaks at southern mid-latitudes and the mechanisms of their formation. It is not known, however, if the clouds produce rain or if there are also non-convective clouds, as predicted by several models. Here we show that the in situ data on the methane concentration and temperature profile in Titan's troposphere point to the presence of layered optically thin stratiform clouds. The data indicate an upper methane ice cloud and a lower, barely visible, liquid methane-nitrogen cloud, with a gap in between. The lower, liquid, cloud produces drizzle that reaches the surface. These non-convective methane clouds are quasi-permanent features supported by the global atmospheric circulation, indicating that methane precipitation occurs wherever there is slow upward motion. This drizzle is a persistent component of Titan's methane hydrological cycle and, by wetting the surface on a global scale, plays an active role in the surface geology of Titan.

The presence of dry fluvial river channels and the intense cloud activity in the south pole of Titan over the past few years suggest the presence of methane rain. The nitrogen atmosphere of Titan therefore appears to support a methane meteorological cycle that sculptures the surface and controls its properties. Titan and Earth are the only worlds in the Solar System where rain reaches the surface, although the atmospheric cycles of water and methane are expected to be very different. Here we report three-dimensional dynamical calculations showing that severe methane convective storms accompanied by intense precipitation may occur in Titan under the right environmental conditions. The strongest storms grow when the methane relative humidity in the middle troposphere is above 80 per cent, producing updrafts with maximum velocities of 20 m s^-1, able to reach altitudes of 30 km before dissipating in 5-8 h. Raindrops of 1-5 mm in radius produce precipitation rainfalls on the surface as high as 110 kg m^-2 and are comparable to flash flood events on Earth.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

On the basis of previous ground-based and fly-by information, we knew that Titan's atmosphere was mainly nitrogen, with some methane, but its temperature and pressure profiles were poorly constrained because of uncertainties in the detailed composition. The extent of atmospheric electricity ('lightning') was also hitherto unknown. Here we report the temperature and density profiles, as determined by the Huygens Atmospheric Structure Instrument (HASI), from an altitude of 1,400 km down to the surface. In the upper part of the atmosphere, the temperature and density were both higher than expected. There is a lower ionospheric layer between 140 km and 40 km, with electrical conductivity peaking near 60 km. We may also have seen the signature of lightning. At the surface, the temperature was 93.65 ± 0.25 K, and the pressure was 1,467 ± 1 hPa.

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.

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.

Aerosols in Titan's atmosphere play an important role in determining its thermal structure. They also serve as sinks for organic vapours and can act as condensation nuclei for the formation of clouds, where the condensation efficiency will depend on the chemical composition of the aerosols. So far, however, no direct information has been available on the chemical composition of these particles. Here we report an in situ chemical analysis of Titan's aerosols by pyrolysis at 600 °C. Ammonia (NH₃) and hydrogen cyanide (HCN) have been identified as the main pyrolysis products. This clearly shows that the aerosol particles include a solid organic refractory core. NH₃ and HCN are gaseous chemical fingerprints of the complex organics that constitute this core, and their presence demonstrates that carbon and nitrogen are in the aerosols.

The surface of Saturn's largest satellite – Titan – is largely obscured by an optically thick atmospheric haze, and so its nature has been the subject of considerable speculation and discussion. The Huygens probe entered Titan's atmosphere on 14 January 2005 and descended to the surface using a parachute system. Here we report measurements made just above and on the surface of Titan by the Huygens Surface Science Package. Acoustic sounding over the last 90 m above the surface reveals a relatively smooth, but not completely flat, surface surrounding the landing site. Penetrometry and accelerometry measurements during the probe impact event reveal that the surface was neither hard (like solid ice) nor very compressible (like a blanket of fluffy aerosol); rather, the Huygens probe landed on a relatively soft solid surface whose properties are analogous to wet clay, lightly packed snow and wet or dry sand. The probe settled gradually by a few millimetres after landing.