G. Bianchini

THE CHARACTERISATION OF TITAN'S ATMOSPHERIC PHYSICAL PROPERTIES BY THE HUYGENS ATMOSPHERIC STRUCTURE INSTRUMENT (HASI)

The Huygens Atmospheric Structure Instrument (HASI) is a multi-sensor package which has been designed to measure the physical quantities characterising the atmosphere of Titan during the Huygens probe descent on Titan and at the surface. HASI sensors are devoted to the study of Titans atmospheric structure and electric properties, and to provide information on its surface, whether solid or liquid.

VIRTIS: An imaging spectrometer for the ROSETTA mission

The VIRTIS (Visual IR Thermal Imaging Spectrometer) experiment has been one of the most successful experiments built in Europe for Planetary Exploration. VIRTIS, developed in cooperation among Italy, France and Germany, has been already selected as a key experiment for 3 planetary missions: the ESA-Rosetta and Venus Express and NASA-Dawn. VIRTIS on board Rosetta and Venus Express are already producing high quality data: as far as Rosetta is concerned, the Earth-Moon system has been successfully observed during the Earth Swing-By manouver (March 2005) and furthermore, VIRTIS will collect data when Rosetta flies by Mars in February 2007 at a distance of about 200 kilometres from the planet. Data from the Rosetta mission will result in a comparison – using the same combination of sophisticated experiments – of targets that are poorly differentiated and are representative of the composition of different environment of the primordial solar system. Comets and asteroids, in fact, are in close relationship with the planetesimals, which formed from the solar nebula 4.6 billion years ago. The Rosetta mission payload is designed to obtain this information combining in situ analysis of comet material, obtained by the small lander Philae, and by a long lasting and detailed remote sensing of the comet, obtained by instrument on board the orbiting Spacecraft. The combination of remote sensing and in situ measurements will increase the scientific return of the mission. In fact, the “in situ” measurements will provide “ground-truth” for the remote sensing information, and, in turn, the locally collected data will be interpreted in the appropriate context provided by the remote sensing investigation. VIRTIS is part of the scientific payload of the Rosetta Orbiter and will detect and characterise the evolution of specific signatures – such as the typical spectral bands of minerals and molecules – arising from surface components and from materials dispersed in the coma. The identification of spectral features is a primary goal of the Rosetta mission as it will allow identification of the nature of the main constituent of the comets. Moreover, the surface thermal evolution during comet approach to sun will be also studied.

PFS:A FOURIER SPECTROMETER FOR THE STUDY OF MARTIAN ATMOSPHERE

The Planetary Fourier Spectrometer PFS has been designed for the study of the atmosphere and soil of Mars. PFS has two infrared channels: a long wavelength (LW) channel with range 250 - 2000 cm-’ and a short wavelength (SW) channel with range 2000 - 8333 cm-‘. The spectral resolution is 2 cm-‘. Both channels work simultaneously. The field of view is 2” which covers 10 km on the Martian surface being observed from the pericenter at 300 km. The signal to noise ratio is better than 100 in a range of particular scientific interest (at 650 cm-’ , for example). The built-in pointing device allows to study the atmosphere over extreme regions like Hellas Planitia or Olympus Mons.

Surface and sub-surface electrical measurement of titan with the PWA-HASI experiment on HUYGENS

Abstract The Permittivity Waves and Altimetry (PWA) experiment is part of the Huygens Atmospheric Structure Instrument (HASI) presently ‘en route’ toTitan as a part of the HUYGENS Probe carried by the Saturn orbiter CASSINI. When HUYGENS lands upon Titan, the Mutual Impedance (MI) probe, part of the PWA experiment will be set to a special mode to measure the complex permittivity of the surface. We discuss here the capabilities of the instrument relative to several possible surface models currently under discussion. For a liquid surface, PWA will provide additional permittivity measurements to the Surface Science Package (SSP) measurements. In the case of a solid surface, with an estimation of the probe penetration from SSP acceleration data and under the assumption of homogeneity, the complex permittivity can be derived. Moreover, if the surface layer is well known up to a few tens of centimeters, we would also have knowledge of the permittivity of the underlying material. Although PWA-MI provides only permittivity measurements it should contribute greatly to identifying Titans surface structure and composition in synergy with the other HUYGENS instruments.

INFRARED SPECTROMETER PFS FOR THE MARS 94 ORBITER

PFS (the Planetary Fourier Spectrometer) covers the range 1.25 – 45 μm with spectral resolution about 2 cm−1 and angular resolution 0.035 – 0.070 rad (10 – 20 km on the Martian surface working at the periapsis). The instrument has two spectral channels: shortwavelength (SW) and longwavelength (LW) with a boundary near 5 μm. The photoconductive detector (PbSe) is used in the SW channel and the pyroelectric in LW channel. The main optical units of both channels are rotating interferometers with cubic mirror corner reflectors. The infrared radiation from Mars is directed to the interferometers by the pointing system that allows to observe selected points on the Martian surface. A “dichroic” plate splits the beam between LW and SW channels. Several hundred spectra will be obtained during one periapsis passage. These spectra will be used for investigation of Martian atmosphere (temperature and pressure vertical profiles, variations of small constituents such as H2O and CO, pressure near the surface, aerosol distribution, composition and optical depth) and some of surface properties (thermal, compositional, textural). Scientific facilities of six countries (Italy, Russia, Germany, Poland, France and Spain) cooperate in the work on this experiment.

S.A.M., the Italian Martian Simulation Chamber

The Martian Environment Simulator (SAM “Simulatore di Ambiente Marziano”) is a interdisciplinary project of Astrobiology done at University of Padua. The research is aimed to the study of the survival of the microorganisms exposed to the “extreme” planetary environment. The facility has been designed in order to simulate Mars’ environmental conditions in terms of atmospheric pressure, temperature cycles and UV radiation dose. The bacterial cells, contained into dedicated capsules, will be exposed to thermal cycles simulating diurnal and seasonal Martian cycles. The metabolism of the different biological samples will be analysed at different phases of the experiment, to study their survival and eventual activity of protein synthesis (mortality, mutations and capability of DNA reparing). We describe the experimental facility and provide the perspectives of the biological experiments we will perform in order to provide hints on the possibility of life on Mars either autochthonous or imported from Earth.

Analysis of dynamic performances of hasi temperature sensor during the entry in the Titan atmosphere

Abstract The performances of the temperature sensors of the HASI instrument ( Fulchignoni et al., 1996 ) have been analyzed using a numerical model, validated by comparison with the results of various experimental tests. The idea of implementing a numerical model of the sensors arose when the tests for the dynamic characterization of the thermometer ( Angrilli et al., 1996a ) showed that the behavior was quite different from that of a first order system. Modeling the sensor only by means of the time constant of the sensing wire does not account for the effect of the mechanical structures thermal inertia. A more accurate dynamic characterization of the sensor requires three parameters, that depend on the thermo-fluid environment. Actually the sensor during the descent in Titan atmosphere changes its behavior dramatically according to the changes in velocity and atmospheric conditions. The most relevant effect is the change of the frequency bandwidth of the sensor, leading to a variation of the actual spatial resolution in the measured temperature profile. Using the numerical model of the sensors together with the profile of temperature, pressure and composition given in literature ( Lellouch et al., 1989 ) for the Titan atmosphere, the dynamic characteristics of the sensor at various altitude have been determined. The choice of using numerical simulation instead of experimental tests on the unit depends on the consideration that to accomplish similitude, both Re and Nu numbers must be tuned. A complete experimental characterization of the sensor would require a series of tests in a wind tunnel with various fluid velocities and thermal conductivity. The most relevant results are related to the large difference between the frequency bandwidth of the sensors at the beginning of the measurements, where the speed is high, but the density is low with respect to those at the end. Due to the changes in density, the response of the sensor is slow when the velocity of the probe is high, at the beginning of the measurements, in this case the sensor could detect only very large atmospheric structures. The maximum in thermometer response speed is obtained during the last part of the descent, when the velocity is low but the foreseen density is high, therefore in this phase the sensor will be able to detect structures of small dimension. Starting from the results of these dynamic performances a new sensor has been devised, the dynamic performances seem to be promising, showing an improvement in the frequency bandwidth of one decade, and though its realization is only at the beginning, it can be considered as a candidate in the planning of future missions.

Planetary Fourier spectrometer: An interferometer for atmospheric studies on board Mars 94 mission

SummaryPFS is a two-channel Fourier spectrometer operating in the infra-red wavelengths between 1.25 and 45 μm. The instrument will be used mainly in the study of the Martian atmosphere. The principal goals are the measurements of the atmospheric temperature and pressure, atmospheric constituents, aerosol and clouds, ground pressure for surface topography, optical and thermophysics properties of the Martian soil. PFS will fly on the Mars 94 spacecraft which should be launched in 1994 and reach the planet in 1995. It is essentially constituted by two different interferometers located in the same box which is divided into two parts. A dichroic placed on the PFS entrance is used to separate the spectral range into two parts, a division needed by the different optical materials which have to be used in each spectral range. The optical layout of the experiment is very compact. Each channel uses two cubic mirrors mounted on an L-structure pivoted on a motor. The motor moves the mechanics and permits the optical-path difference between the arms to be varied. Each interformeter operates in a different spectral range, respectively, between (1.25÷4.8) μm (8000÷2083cm−1) and (6÷45)μm (1666÷220)cm−1). The spectral resolution is 2 cm−1. The entrance aperture area is 30 cm2 per channel and the field of view is 2 and 4 degrees. Every measurement lasts about 4 s. The time and, therefore, the relative optical-path difference for the measurement of every point of the interferogram is given by the zero crossings of the interferogram of a reference monochromatic channel at 1.2 μm which uses a laser diode as source. The two interferograms are double-sided and will have 16384 and 4096 points, respectively, corresponding to spectra of 6250 and 1823 useful points.

Thermomechanical design optimization and acceptance of the wide-angle camera for the Rosetta mission

The WAC is a telescope developed by University of Padova for the OSIRIS experiment, mainly composed by two instruments, Narrow Angle Camera and Wide Angle Camera, and the related electronics. The payload will fly on board of the Rosetta ESA scientific mission, that will be flown to encounter Comet Wirtanen after about 10 years of flight in 2013. WAC main scientific objectives are to follow structure evolution in the coma and monitor their dynamics. To fulfill scientific requirements, the optical characteristics of the WAC telescope may be summarized as follows: wide field of view of 12° X 12°, focal length of 140 mm, operate in the wave-length range 240-1000nm after 10 years in space, Encircled Energy greater than 70% over the entire FoV, contrast ratio of 10-4 to detect coma activities against a bright nucleus, minimum exposure time of 10 msec with a repeatability better than 1/500, scattered light rejection for sources inside and outside FoV. This paper deals with the design optimization of critical parts and acceptance test campaign performed to validate the thermo-structural behavior of the WAC. The functional and performance tests carried out at experiment and system level demonstrated the overall capability of the telescope to satisfy the system and scientific requirements.