Huygens Atmospheric Structure Instrument Research Articles

Gravity Waves in Titan's Atmosphere: A Comparison Between Linearized Wave Model Calculations and HASI Observations

AbstractThe Huygens Atmospheric Structure Instrument (HASI) detected wavelike temperature fluctuations from 500 to 1,000 km on Titan. However, these fluctuation structures have not been satisfactorily reproduced by any theoretical model to date. In this study, we construct a full‐wave model to simulate the observed gravity wave structure. The model includes dissipation processes due to molecular viscosity, which increases exponentially with altitude, and eddy viscosity, which dominates the lower atmosphere. Using our model, we reproduced the observed temperature perturbations with the superposition of two gravity wave modes, one with λz = 600 km and λx = 1,600 km while the other with λz = 140 km and λx = 8,000 km. We estimate the thermal effects introduced by gravity waves and find that gravity waves below 900 km may significantly modify temperature structure of the upper atmosphere by tens of Kelvins. The wave‐induced thermal effect is sensitive to the eddy viscosity, which controls the dissipation and thermal conductivity below 900 km. When the eddy viscosity is increased by a factor of five, the thermal effect of gravity waves changes from −89 K cooling to nearly zero. This heating mechanism may contribute to the large temperature variability of 60 K in Titan's upper atmosphere.

Bouncing on Titan: Motion of the Huygens probe in the seconds after landing

While landing on Titan, several instruments onboard Huygens acquired measurements that indicate the probe did not immediately come to rest. Detailed knowledge of the probe's motion can provide insight into the nature of Titan's surface. Combining accelerometer data from the Huygens Atmospheric Structure Instrument (HASI) and the Surface Science Package (SSP) with photometry data from the Descent Imager/Spectral Radiometer (DISR), we develop a quantitative model to describe motion of the probe, and its interaction with the surface. The most likely scenario is the following. Upon impact, Huygens created a 12cm deep hole in the surface of Titan. It bounced back, out of the hole onto the flat surface, after which it commenced a 30–40cm long slide in the southward direction. The slide ended with the probe out of balance, tilted in the direction of DISR by around 10°. The probe then wobbled back and forth five times in the north–south direction, during which it probably encountered a 1–2cm sized pebble. The SSP provides evidence for movement up to 10s after impact. This scenario puts the following constraints on the physical properties of the surface. For the slide over the surface we determine a friction coefficient of 0.4. While this value is not necessarily representative for the surface itself due to the presence of protruding structures on the bottom of the probe, the dynamics appear to be consistent with a surface consistency of damp sand. Additionally, we find that spectral changes observed in the first four seconds after landing are consistent with a transient dust cloud, created by the impact of the turbulent wake behind the probe on the surface. The optical properties of the dust particles are consistent with those of Titan aerosols from Tomasko et al. (2008). We suggest that the surface at the landing site was covered by a dust layer, possibly the 7mm layer of fluffy material identified by Atkinson et al. (2010). The presence of a dust layer contrasts with the dampness measured just centimeters below the surface, and suggests a recent spell of dry weather at the landing site.

AN ESTIMATE OF THE CHEMICAL COMPOSITION OF TITAN's LAKES

Hundreds of radar-dark patches interpreted as lakes have been discovered in the north and south polar regions of Titan. We have estimated the composition of these lakes by using the direct abundance measurements from the Gas Chromatograph Mass Spectrometer (GCMS) aboard the Huygens probe and recent photochemical models based on the vertical temperature profile derived by the Huygens Atmospheric Structure Instrument (HASI). Thermodynamic equilibrium is assumed between the atmosphere and the lakes, which are also considered as nonideal solutions. We find that the main constituents of the lakes are ethane (C2H6) (~76-79%), propane (C3H8) (~7-8%), methane (CH4) (~5-10%), hydrogen cyanide (HCN) (~2-3%), butene (C4H8) (~1%), butane (C4H10) (~1%) and acetylene (C2H2) (~1%). The calculated composition of lakes is then substantially different from what has been expected from models elaborated prior to the exploration of Titan by the Cassini-Huygens spacecraft.

The detached haze layer in Titan's mesosphere

By comparing observations from the Cassini imaging system, UV spectrometer, and Huygens atmospheric structure instrument, we determine an apparent radius of ∼40 nm, an imaginary index <0.3 at 187.5 nm and a number density of ∼30 particles cm −3 for the detached haze layer at 520 km in Titan's mesosphere. We point out that the detached haze layer is coincident with a local maximum in the measured temperature profile and show that the temperature maximum is caused by absorption of sunlight in the detached haze layer. This rules out condensation as the source of the layer. The derived particle size is in good agreement with that estimated for the size of the monomers in the aggregate particles that make up the main haze layer. Calculations of the sedimentation velocity of the haze particles coupled with the derived number density imply a mass flux 2.7 – 4.6 × 10 − 14 g cm − 2 s − 1 , which is approximately equal to the mass flux required to explain the main haze layer. Because the aerosol size and mass flux derived for the detached layer agree with those determined for the main layer, we suggest that the main haze layer in Titan's stratosphere is formed primarily by sedimentation and coagulation of particles in the detached layer. This implies that high-energy radical and ion chemistry in the thermosphere is the main source of haze on Titan.

Sputtering and heating of Titan's upper atmosphere

Titan is an important endpoint for understanding atmospheric evolution. Prior to Cassini's arrival at Saturn, modelling based on Voyager data indicated that the hydrogen escape rate was large (1-3x1028amus-1), but the escape rates for carbon and nitrogen species were relatively small (5x1026amus-1) and dominated by atmospheric sputtering. Recent analysis of the structure of Titan's thermosphere and corona attained from the Ion and Neutral Mass Spectrometer and the Huygens Atmospheric Structure Instrument on Cassini have led to substantially larger estimates of the loss rate for heavy species (0.3-5x1028amus-1). At the largest rate suggested, a mass that is a significant fraction of the present atmosphere would have been lost to space in 4Gyr; hence, understanding the nature of the processes driving escape is critical. The recent estimates of neutral escape are reviewed here, with particular emphasis on plasma-induced sputtering and heating. Whereas the loss of hydrogen is clearly indicated by the altitude dependence of the H2 density, three different one-dimensional models were used to estimate the heavy-molecule loss rate using the Cassini data for atmospheric density versus altitude. The solar heating rate and the nitrogen density profile versus altitude were used in a fluid dynamic model to extract an average net upward flux below the exobase; the diffusion of methane through nitrogen was described below the exobase using a model that allowed for outward flow; and the coronal structure above the exobase was simulated by presuming that the observed atmospheric structure was due to solar- and plasma-induced hot particle production. In the latter, it was hypothesized that hot recoils from photochemistry or plasma-ion-induced heating were required. In the other two models, the upward flow extracted is driven by heat conduction from below, which is assumed to continue to act above the nominal exobase, producing a process referred to as 'slow hydrodynamic' escape. These models and the resulting loss rates are reviewed and compared. It is pointed out that preliminary estimates of the composition of the magnetospheric plasma at Titan's orbit appear to be inconsistent with the largest loss rates suggested for the heavy species, and the mean upward flow extracted in the one-dimensional models could be consistent with atmospheric loss by other mechanisms or with transport to other regions of Titan's atmosphere.

Analysis of the HASI accelerometers data measured during the impact phase of the Huygens probe on the surface of Titan by means of a simulation with a finite-element model

The Huygens Atmospheric Structure Instrument (HASI) [Fulchignoni, M., Ferri, F., Angrilli, F., Bar-Nun, A., Barucci, M.A., Bianchini, G., Borucki, W., Coradini, M., Coustenis, A., Falkner, P., Flamini, E., Grard, R., Hamelin, M., Harri, A.M., Leppelmaier, G.W., Lopez-Moreno, J.J., McDonnell, J.A.M., McKay, C.P., Neubauer, F.M., Pedersen, A., Piacardi, G., Pirronello, V., Schwingenschuh, K., Seiff, A., Svedhem, H., Vanzani, V., Zarnecki, J.C., 2002. The characterisation of Titan atmosphere physical properties. Space Sci. Rev. 104, 395–431] was a very complete instrument suite installed on board the Huygens probe, the planetary lander of the Cassini Huygens Mission to Saturn system, which successfully completed its mission in January 2005. HASI comprised a set of accelerometers, temperature sensors, pressure transducers and permittivity analysers aimed at the investigation of Titan atmosphere, which were fully operative during a several hour-long parachuted descent from an altitude of 157 km to planetary surface. Accelerometers were the only instruments activated earlier, just after Cassini separation, and recorded data during all the mission phases from atmospheric entry to landing, providing essential information for elaborating probe trajectory as well as Titan atmospheric profiles [G. Colombatti, et al. Reconstruction of the trajectory of the Huygens probe using the Huygens Atmospheric Structure Instrument, this same PSS issue]. Although not specifically designed for monitoring very fast dynamic events, HASI accelerometers have also recorded the trace of probe impact with the planetary surface, building up along with the data from Huygens Surface Science Package (SSP) instrument [ Zarnecki, J.C., Leese, M.R., Garry, J.R.C., Ghafoor, N.A.L., Hathi, B., 2002. Huygens Surface Science Package. Space Sci. Rev. 104, 593–611] the only set of direct measurements of the mechanical properties of the Titan soil. Though not considered secondary with respect to SSP data, HASI data provide peculiar information related to the dynamic response of the whole Huygens probe when impacting the surface, whereas the SSP data were collected mainly by sensors located on a small penetrometer below the main probe dome. Therefore, although having a sensibly lower sampling frequency, HASI Accelerometer Subsystem (ACC) data complete the information of SSP data in the study of impact deceleration profiles, which can provide key information for the estimation of the mechanical parameters of the soil and get an insight of its consistency and composition. In the analysis of Huygens mission data some unexpected features were present in the ACC data sets, so a dedicated study was needed to investigate the presence of dynamic interferences during an acquisition and correct the impact signature. The developed method is based on dynamic analysis of the impact through a three-dimensional finite-element dynamic model of the Huygens probe and the results lead to a corrected interpretation of accelerometer readings and provided an improved description of key aspects of the planetary landing. Although some aspects of probe's state after impact need some further analysis, as for final resting attitude which is to date not completely agreed on, this study disclosed that the probe experienced a vertical decelerating action which is compatible with two possible scenarios: the first one implies the penetration in a soft substrate material followed by a lateral bounce out of the generated hole and the second one suggests the displacement of pebbles from the surface into the soil. Numerical elaboration of impact data calculated a 12 cm penetration into the surface, which may have been experienced either by the lower dome of the probe or from pebbles that were situated under the dome when contacting the planet. In either case after a few seconds of motion the Huygens probe finally rested above Titan's surface with a negligible penetration. Since HASI piezo-accelerometer design was driven by the need of high full-scale values to monitor critical events during entry and descent phase, it was not possible to reconstruct horizontal motion after main deceleration phase without consistent uncertainties due to the poor overall accuracy in a low-acceleration range.

Reconstruction of the trajectory of the Huygens probe using the Huygens Atmospheric Structure Instrument (HASI)

The Huygens probe returned scientific measurements from the atmosphere and surface of Titan on 14 January 2005. Knowledge of the trajectory of Huygens is necessary for scientific analysis of those measurements. We use measurements from the Huygens Atmospheric Structure Instrument (HASI) to reconstruct the trajectory of Huygens during its mission. The HASI Accelerometer subsystem measured the axial acceleration of the probe with errors of 3E−6 m s −2, the most accurate measurements ever made by an atmospheric structure instrument on another planetary body. The atmosphere was detected at an altitude of 1498 km. Measurements of the normal acceleration of the probe, which are important for determining the probe's attitude during hypersonic entry, were significantly less accurate and limited by transverse sensitivity of the piezo sensors. Peak acceleration of 121.2 m s −2 occurred at 234.9 km altitude. The parachute deployment sequence started at 157.1 km and a speed of 342.1 m s −1. Direct measurements of pressure and temperature began shortly afterwards. The measured accelerations and equations of motion have been used to reconstruct the trajectory prior to parachute deployment. Measured pressures and temperatures, together with the equation of hydrostatic equilibrium and the equation of state, have been used to reconstruct the trajectory after parachute deployment. Uncertainties in the entry state of Huygens at the top of the atmosphere are significant, but can be reduced by requiring that the trajectory and atmospheric properties be continuous at parachute deployment.

Huygens probe entry dynamic model and accelerometer data analysis

During the first phase of Huygens arrival into Titan's atmosphere the probe is subjected to gravitational and aerodynamic forces in aerodynamic hypersonic regime. Atmospheric drag exerts a strong deceleration on the capsule measured by Huygens atmospheric structure instrument (HASI) servo accelerometer. A 6 DOF (Degree of Freedom) model of the Huygens probe entry dynamics has been developed and used for data analysis. The accelerometer data are analysed and the model allows the retrieval of dynamics information of Huygens probe from 1545 km altitude down to end of the entry phase. Probe's initial conditions (velocity and position) were refined to match the measured deceleration profile resulting in a different altitude at interface epoch with respect to those of the Cassini Navigation Team. Velocity and position of probe at interface epoch are compatible with those used by Descent Trajectory Working Group (DTWG). Measurements acquired before atmosphere detection are used to estimate probe's angular rate, bound attitude and characterise the angle of attack profile which results to be lower than 4 ∘ during the whole entry. Probe's spin calculated (6.98 RPM) is slightly different with respect to DTWG of 7.28 RPM but considering a 2% error in the Inertia matrix these results are inside the 1 - σ error band.

Huygens probe entry trajectory and attitude estimated simultaneously with Titan atmospheric structure by Kalman filtering

Space probes entering planetary atmospheres are used for in situ study of their physical structures. During the entry phase aerodynamic forces exerted on the probe depend on atmospheric density. As a consequence accelerations measured by on-board sensors can be used to derive probe trajectory as well as atmospheric density, pressure and temperature profiles. In this work acceleration data acquired by the Huygens Atmospheric Structure Instrument (HASI) have been used to reconstruct the probe trajectory and the Titan's atmospheric structure from ≈ 1545 km down to ≈ 155 km of altitude. An accurate six degree of freedom model of Huygens during the entry phase has been developed and a new reconstruction technique based on Kalman filtering is presented. This technique estimates simultaneously the probe trajectory, the attitude profile consistent with measured data and the atmospheric density, pressure and temperature.

The 2003 November 14 occultation by Titan of TYC 1343-1865-1: II. Analysis of light curves

We observed a stellar occultation by Titan on 2003 November 14 from La Palma Observatory using ULTRACAM with three Sloan filters: u ′ , g ′ , and i ′ (358, 487, and 758 nm, respectively). The occultation probed latitudes 2° S and 1° N during immersion and emersion, respectively. A prominent central flash was present in only the i ′ filter, indicating wavelength-dependent atmospheric extinction. We inverted the light curves to obtain six lower-limit temperature profiles between 335 and 485 km (0.04 and 0.003 mb) altitude. The i ′ profiles agreed with the temperature measured by the Huygens Atmospheric Structure Instrument [Fulchignoni, M., and 43 colleagues, 2005. Nature 438, 785–791] above 415 km (0.01 mb). The profiles obtained from different wavelength filters systematically diverge as altitude decreases, which implies significant extinction in the light curves. Applying an extinction model [Elliot, J.L., Young, L.A., 1992. Astron. J. 103, 991–1015] gave the altitudes of line of sight optical depth equal to unity: 396 ± 7 and 401 ± 20 km ( u ′ immersion and emersion); 354 ± 7 and 387 ± 7 km ( g ′ immersion and emersion); and 336 ± 5 and 318 ± 4 km ( i ′ immersion and emersion). Further analysis showed that the optical depth follows a power law in wavelength with index 1.3 ± 0.2 . We present a new method for determining temperature from scintillation spikes in the occulting body's atmosphere. Temperatures derived with this method are equal to or warmer than those measured by the Huygens Atmospheric Structure Instrument. Using the highly structured, three-peaked central flash, we confirmed the shape of Titan's middle atmosphere using a model originally derived for a previous Titan occultation [Hubbard, W.B., and 45 colleagues, 1993. Astron. Astrophys. 269, 541–563].

A technique to determine the mean molecular mass of a planetary atmosphere using pressure and temperature measurements made by an entry probe: Demonstration using Huygens data

It is possible to determine the mean molecular mass of a planetary atmosphere using pressure and temperature measurements made by an entry probe descending at terminal velocity. The descent trajectory of an entry probe can be determined from pressure, temperature, and mean molecular mass data. This technique offers redundancy for large entry probes in the event of a mass spectrometer failure and increases the potential scientific yield of small entry probes that do not carry mass spectrometers. This technique is demonstrated on Huygens atmospheric structure instrument (HASI) data from Titan. Accurate knowledge of entry probe and parachute drag coefficients is required for this technique to be useful.

Electron conductivity and density profiles derived from the mutual impedance probe measurements performed during the descent of Huygens through the atmosphere of Titan

During the descent of the Huygens probe through the atmosphere of Titan, on January 14th, 2005, the permittivity, waves and altimetry (PWA) subsystem, a component of the Huygens atmospheric structure instrument (HASI), detected an ionized layer at altitudes around 63 km with two different instruments, the relaxation probe (RP) and the mutual impedance probe (MIP). A very detailed analysis of both data sets is required, in order to correct for environmental effects and compare the two independent estimates of the electrical conductivity. The present work is dedicated to the MIP data analysis. New laboratory tests have been performed to validate or improve the available calibration results. Temperature effects have been included and numerical models of the MIP sensors and electric circuitry have been developed to take into account the proximity of the Huygens probe body. The effect of the vertical motion of the vessel in the ionized atmosphere is estimated in both analytical and numerical ways. The peculiar performance of the instrument in the altitude range 100–140 km is scrutinized. The existence of a prominent ionized layer, and of enhancements in the conductivity and electron density profiles at 63 km, are discussed in the light of previous theoretical predictions.

Vertical atmospheric flow on Titan as measured by the HASI instrument on board the Huygens probe

[1] On January 14, 2005, the Huygens probe descended on the surface of Titan, the largest moon of Saturn and the only moon in the Solar system with a substantial atmosphere. After the deployment of the main parachute and the release of the heat shield at an altitude of about 150 km, the local pressure and temperature were measured by HASI (Huygens Atmospheric Structure Instrument) all the way down to the surface. These measurements have now been used to determine the vertical component of the atmospheric flow along the trajectory of the probe. The average flow direction is rising below about 40 km and sinking above that, with features that loosely match the strong shears in the horizontal wind profile obtained by radio telemetry. The prevalence of sinking motion in the stratosphere is an unexpected result for the given latitude, suggesting a layered atmosphere with a large thermally indirect cell. Wiggles superposed on the main upward flow are evidence of inertial instability characteristic of equatorial regions, pointing to a latitudinal shear of zonal wind throughout the lower atmosphere.

Vertical pressure profile of Titan—observations of the PPI/HASI instrument

The Huygens entry probe descended through the atmosphere of Titan and provided an excellent set of observations of the atmosphere and the surface of Titan. During the 150-min descent the Huygens Atmospheric Structure Instrument (HASI) observed a comprehensive set of variables, including pressure, temperature, density and atmospheric electricity. The atmospheric pressure profile was recorded by the Pressure Profile Instrument (PPI), provided by Finnish Meteorological Institute (FMI). The instrument started measurements at an altitude of 150 km, and produced about 28 bits of data per second. Data were also obtained through the time of 31 min beyond the time of surface impact. The first-order scientific analysis of the PPI results has been performed. The observations together with hydrostatic assumption and in combination with other measurements have provided the first atmospheric pressure profile and the surface pressure (of approximately 1467 ± 1 hPa ) for Titan's atmosphere. To carry out the pressure profile reconstruction we developed a real gas formulation, which is applicable also for other Titan atmospheric investigations. The altitude versus time speed of the descent was calculated and the results were compared with the direct altitude observations by the radar altimeter during the last 40 km of the descent. The fit was excellent demonstrating the high-quality level of the PPI observations as well as the utilized investigation methods.

Titan mission data now available

European Space Agency (ESA) has made public the data from four of the six experiments on the Huygens probe, which landed on the surface of Saturn's moon Titan in January 2005.Data is available from the following instruments: the Huygens Atmospheric Structure Instrument, which measured the physical and electrical properties of the moon's atmosphere; the DopplerWind Experiment, which studied the direction and strength of Titan's winds; the Aerosol Collector and Pyrolyser (ACP), which collected aerosols; and the Gas Chromatograph and Mass Spectrometer, which analyzed data from the ACP and measured the chemical composition of the atmosphere.

Titan's planetary boundary layer structure at the Huygens landing site

Huygens Atmospheric Structure Instrument (HASI) for the first time performed an in situ measurement of the thermal structure in Titan's atmosphere with a vertical resolution sufficient to analyze the planetary boundary layer (PBL). The vertical potential temperature profile reveals the presence of a weakly convective PBL, with a surface layer thickness of 10 m and an outer layer with a depth of 300 m. With a mean eddy diffusivity of only ∼7.4 × 10−3 m2 s−1, the turbulence in the PBL is weak. The turbulent heat flux in the surface layer was upward but tiny (∼0.02 W m−2), indicating that the ground surface was marginally warmer than the air. The surface heat flux is too small to cause a diurnal variation of the PBL except in the lowest few meters, so the observed profile may be a nearly steady state feature within this season at the landing site. In the surface layer the mean wind speed is likely to be less than 0.1 m s−1. Given the tiny surface heat flux, the buoyant production of turbulence is very weak. The PBL structure reveals that the weather condition at the time and place of the Huygens landing resembles a calm, overcast day on Earth but is clearly different from the PBL typical for the polar night.

In situ measurements of the physical characteristics of Titan's environment

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.

Huygens probe entry and descent trajectory analysis and reconstruction techniques

Cassini/Huygens is a joint National Aeronautics and Space Administration (NASA)/European Space Agency (ESA)/Agenzia Spaziale Italiana (ASI) mission on its way to explore the Saturnian system. The ESA Huygens Probe is scheduled to be released from the Orbiter on 25 December 2004 and enter the atmosphere of Titan on 14 January 2005. Probe delivery to Titan, arbitrarily defined to occur at a reference altitude of 1270 km above the surface of Titan, is the responsibility of the NASA Jet Propulsion Laboratory (JPL). ESA is then responsible for safely delivering the probe from the reference altitude to the surface. The task of reconstructing the probe trajectory and attitude from the entry point to the surface has been assigned to the Huygens Descent Trajectory Working Group (DTWG), a subgroup of the Huygens Science Working Team. The DTWG will use data provided by the Huygens Probe engineering subsystems and selected data sets acquired by the scientific payload. To correctly interpret and correlate results from the probe science experiments and to provide a reference set of data for possible “ground-truthing” Orbiter remote sensing measurements, it is essential that the trajectory reconstruction be performed as early as possible in the post-flight data analysis phase. The reconstruction of the Huygens entry and descent trajectory will be based primarily on the probe entry state vector provided by the Cassini Navigation Team, and measurements of acceleration, pressure, and temperature made by the Huygens Atmospheric Structure Instrument (HASI). Other data sets contributing to the entry and descent trajectory reconstruction include the mean molecular weight of the atmosphere measured by the probe Gas Chromatograph/Mass Spectrometer (GCMS) in the upper atmosphere and the Surface Science Package (SSP) speed of sound measurement in the lower atmosphere, accelerations measured by the Central and Radial Accelerometer Sensor Units (CASU/RASU), and the probe altitude by the two probe radar altimeters during the latter stages of the descent. In the last several hundred meters, the altitude determination will be constrained by measurements from the SSP acoustic sounder. Other instruments contributing data to the entry and descent trajectory and attitude determination include measurements of the zonal wind drift by the Doppler Wind Experiment (DWE), and probe zonal and meridional drift and probe attitude by the Descent Imager and Spectral Radiometer (DISR). In this paper, the need for and the methods by which the Huygens Probe entry and descent trajectory will be reconstructed are reviewed.

Huygens/HASI 2002 balloon test campaign: Probe trajectory and atmospheric vertical profiles reconstruction

In the framework of the activities going on in preparation for the mission of the Huygens probe in Titan's atmosphere (January 2005), the Huygens Atmospheric Structure Instrument (HASI) team scheduled and performed several balloon campaigns to test the HASI sensors’ performance in flight conditions in the Earth's atmosphere. In particular, pressure conditions reached during each test are similar to those expected in Titan's lower atmosphere. A 1:1 scaled mock-up of the Huygens probe was launched with a stratospheric balloon in 2001 (Br. Assoc. Adv. Sci. 33 (2001) 1109) and in 2002 (Br. Assoc. Adv. Sci. 34 (2002) 911; Adv. Space. Sci. (2003)) from the G. Broglio base of the Italian Space Agency, located in Trapani Milo (Sicily). In both cases the mock-up was dropped from an altitude higher than 27 and 32 km , respectively, and recovered on the ground after a parachuted descent. In this paper, we describe the results obtained in reconstructing (i) the probe descent trajectory and (ii) the profiles of the physical quantities characterizing the Earth's atmosphere, on the basis of a complete analysis of the data obtained during the HASI 2002 balloon flight experiment. Using temperature and pressure measurements, we are able to reach an accuracy of the order of 0.5% on the altitude reconstruction during the descent. We validate both the models used for trajectory reconstruction and to check the sensors’ performance. We describe the problems faced in determining the Huygens probe descent trajectory in Titan's atmosphere focusing our discussion on the critical aspects of the descent reconstruction (such as the uncertainties due to measurement errors, limited knowledge of the atmospheric composition, etc.) and the validity of the adopted assumptions.

Shumann resonances and electromagnetic transparence in the atmosphere of Titan

Among the multiple questions that the CASSINI/HUYGENS mission tries to answer is the likelihood of electric discharges in Titan's atmosphere. The instruments “Huygens Atmospheric Structure Instrument” and “Radio and Plasma Wave Science” will probe the electromagnetic emissions during the Huygens descent and Cassini flybys, respectively. Although no lightning was observed during Voyager's encounters with Titan in 1980 and 1981, this does not exclude the existence of lightning phenomena. Recent investigations show that lightning discharges could occur in the lower atmosphere, such as the detection of methane condensation clouds in the troposphere and the theoretical prediction of an electric field that would be sufficient enough to cause lightning. We present a numerical model of Titan's atmosphere with the aim of calculating the resonance frequencies and the atmospheric transparency to electromagnetic waves. The detection and measurement of these resonances, Schumann frequencies, by the Huygens probe, would show the existence of electric activity connected with lightning discharges in the atmosphere. As it happens with the Schumann frequencies of Earth, losses associated with the electric conductivity will make these frequencies to be lower than the theoretically predicted, the fundamental one being located between 11 and 15 Hz. An analytical study shows that the strong losses associated with the high conductivity make it impossible that an electromagnetic wave generated near the surface with a frequency of 10 MHz or lower reaches the outer part of Titan's atmosphere. Therefore the detection of electromagnetic waves coming from Titan's lower atmosphere by the RPWS instrument is very unlikely.