Surface Science Package Research Articles

VIRTIS: The imaging spectrometer of the Rosetta mission

The study of minor bodies of the Solar System is acquiring an increasing importance in recent years thanks to new space missions, such as NEAR, and to new ground based observations. However none of the presently planned missions will be able to study targets, such as asteroids and comets with such a complete and state-of-the-art- payload, as will Rosetta mission. Rosetta will study in detail a comet nucleus, the prime target of the mission following it from large distances (more than 3 AU) inbound up to almost 1 AU from the Sun. This will permit the detection of the beginning of the cometary activity, as well as determining the composition of emitted gases. At the same time, modification of the cometary surface will be observed and analysed. Detailed in situ analysis will be performed by the Surface Science Package (SSP), thus allowing not only a detailed analysis of the selected landing site, but also establishing ground truth for the remote sensing experiment. VIRTIS (Visible Infra Red Thermal Imaging Spectrometer) is fundamental to the detection and study of the evolution of the typical spectral bands of minerals and molecules characterising the comet surface as well as those of gases and dust dispersed in the coma. Their identification is a primary goal of the Rosetta mission; this will allow the identification the nature of the main constituent of the comets. Moreover, the surface thermal evolution during comet approach to sun will be monitored. In this paper the VIRTIS design and its detailed science goals are reported.

The Cassini–Huygens SSP refractometer : ref

The refractometer (REF) in the Huygens probe Surface Science Package (SSP) is a linear critical-angle refractometer ; it covers the refractive index range 1.2500–1.4500 with a discrimination of 0.0005, giving an accuracy of about 0.5% in the relative abundance of methane and ethane in the expected mixture on the possibly liquid surface of Titan. Light-emitting diodes at 635 nm provide illumination ; the output image shows a cut-off edge whose position is a linear function of refractive index. The image intensity profile is sensed by a 512-element photodiode array, and digitised ; it is then differentiated numerically, as this is found to enhance the discrimination, and may give information about any suspended matter. The main structure of the instrument is made of titanium, with a synthetic sapphire prism.

Line heat-source measurements of the thermal conductivity of porous H 2O ice, CO 2 ice and mineral powders under space conditions

Measurements of the thermal conductivity of porous loose mineral, porous H 2O ice and porous CO 2 ice samples under low temperatures (77 K < T < 300 K) and pressures (10 −4 Pa < p < 10 5 Pa) are reported. The samples were selected to cover the end members of possible comet nucleus compositions and the ambient conditions were chosen to investigate the samples under space conditions. A transient technique is used for the measurements which is well suited for in situ application. The method is based on the line heat-source technique: a thin internally heated cylindrical sensor is inserted into the sample material. The thermal conductivity is deduced from the observed temperature rise in the sensor and the heating power applied. Depending on sensor dimensions, single experiment runs may be completed within a few minutes. The method proved to be accurate, fast and well suited for an application in the laboratory as well as in situ, e.g. on future comet nucleus or Mars missions. A thermal probe (MUPUS-PEN) which employs the experimental technique discussed here has been proposed for the ROSETTA surface science package “RoLand”. The thermal conductivity of the loose dunite sample is studied as a function of gas pressure. At low pressures, it is almost constant and close to 0.03 W m −1 K −1. At atmospheric pressure, the thermal conductivity is about one order of magnitude higher. Both domains are linked by a pressure region with a strong pressure dependency of the thermal conductivity. Three porous water ice samples with different pore sizes have been investigated. The results are in agreement with theoretical predictions (e.g. Steiner et al., 1991) and reveal a strong increase of the thermal conductivity at temperatures close to the sublimation temperature of water ice (≈ 200 K in vacuo). The increase is due to heat transport by pore filling vapour which is more effective in samples with large pore radii. The measured matrix conductivity is close to 0.02 W m −1 K −1, while maximum values for the effective (= matrix + vapour) thermal conductivity at high temperatures exceed 0.25 W m −1 K −1. Similar results are obtained for one porous CO 2 ice sample.