S. Espinasse

A P/Wirtanen evolution model

Abstract Comet P/Wirtanen is the currently selected target for the international ROSETTA mission, the cornerstone ESA mission to a cometary nucleus that will be launched in 2003. Presently, little is known about this comet, but the successful design of the ROSETTA mission, and in particular of the on-board scientific instruments, requires some preliminary knowledge of the comets physical parameters, such as surface temperature, percentage of active surface, intensity of gas and dust fluxes, and so on. These quantities cannot be determined through ground-based observations, so predictive models of the thermal evolution and differentiation of a cometary nucleus are needed. A thermal evolution model is applied to a comet on the orbit of P/Wirtanen with the aim of obtaining some of the needed information. The numerical code solves the heat conduction and the gas diffusion equations throughout a spherical, porous comet nucleus made of amorphous H2O ice (the dominant constituent), CO2 ice, CO ice and dust particles of different sizes. The equations are coupled via the source terms, which describe the sublimation and recondensation of ices as latent heat or mass exchanges. Amorphous H2O ice can undergo an exothermal, irreversible phase transition to crystalline form. The ejection of dust is allowed only if the grains have been liberated from ice, when the drag exerted on them by the outflowing gas is stronger than the gravitational pull of the nucleus. The behaviour is simulated of a “P/Wirtanen”-like object, that starting from the Kuiper belt is injected, through multiple close encounters, in the present orbit of P/Wirtanen. This has been done with the aim of predicting the present characteristics of surface condition and emission of this poorly known comet. From the results of the simulations it can be seen that surface activity starts at 2.1 AU with gas emission and then reaches its maximum at the perihelion, where it is accompanied by a strong dust emission; the range of the computed comet surface temperatures is, at the equator and at the perihelion, between 130 and 200 K. Differences between day and night temperatures on the surface of the nucleus can reach 50 K at perihelion, with a consequent variation in the H2O and dust emission rate, but the flux of more volatile ice, like CO2 and CO, is not affected. Such ices can be found at depths varying from a few metres for CO2 to hundreds of metres for CO.

Thermal evolution and differentiation of a short-period comet

Abstract The evolution of the subsurface layers of a short-period comet has been studied. The structure and composition of the surface layers due to sublimation recondensation phenomena, to gas diffusion processes through the pore system and to the ejection of dust particles have been investigated in detail. The nucleus has been modelled as a mixture of water ice, CO2 ice and dust in specified proportions. The icy matrix is assumed to be porous and crystalline. The model is based on the solution of two symmetric diffusion equations through the whole nucleus, one describing the transport of matter and the other the transport of heat. These equations are linked by a source term which accounts for production or loss of gas. We assume that the water vapour present in the pore system acts as a perfect gas, and that sublimation and recondensation are instantaneous in order to maintain the local thermodynamic equilibrium between the solid phase and its vapour. Under these assumptions, the source term depends on the variation of the pressure due to vapour diffusion, and on the variation of the saturation pressure of the vapour due to the evolution of the temperature. The diffusion regime, Knudsen or viscous, depends on the mean free path of the molecules of gas through the pore network, considered as a system of cylindrical pipes. The dust particles may be removed from the surface of the nucleus depending on the force balance. The calculations are performed for a nucleus on the orbit of P/Du Toit-Hartley, that was one of the possible targets for the Rosetta mission. Different nucleus compositions with various CO2/H2O ice and dust/ice ratios are investigated. Results are presented on the evolution of the stratigraphy of the nucleus and on the production rates of CO2. H2O and dust particles as a function of the heliocentric distance. Several phenomena are observed, such as the depletion of CO2 ice in the subsurface layers and the possible formation of a dust layer at the nucleus surface.

MA_MISS: Mars multispectral imager for subsurface studies

Abstract The Italian drill “DEEDRI” is going to be the lander based sample acquisition system for the Mars Surveyor Program of the Mars Sample Return mission. DEEDRI is capable to collect core/sand sample of the martian soil down to 50 cm in depth. The MA_MISS experiment belongs to the DEEDRI system and it will be dedicated to observe the wall of the excavated hole in terms of infrared spectral reflectance in the range 0.8–2.8 μm. The spectral sampling is about 20 nm while the spatial sampling is 100 μm over the target. The optical window of MA_MISS is placed very close to the drill tip so that the target view to be observed can span from the soil down to 50 cm. The proximity optics and electronics of MA_MISS have to be very miniaturized since they will be collocated inside the drill tool in a very limited volume of about 25 mm in diameter. On the other side the main electronics will be on the lander and it will communicate through an interface based on slip rings devices. MA_MISS can acquire in different observation modes. The images are scanned by moving the DEEDRI itself. One image ring is built up by acquiring contiguous images of the MA_MISS slit. The study of the Martian subsurface will provide important constraints on the nature, timing and duration of alteration and sedimentation processes on Mars, as well as on the complex interactions between the surface and the atmosphere. This study will permit to infer the history of erosion, transport and deposition of loose material. Alteration processes can dominate the mineralogy of the Martian surface: it will be essential to study the mineralogy of deeper layers, where a more limited alteration took place. MA_MISS can provide very important scientific return from the subsurface of Mars along with a selection criteria for the samples collection.

The international package for scientific experiments (IPSE) for Mars surveyor program

Abstract IPSE is a micro-laboratory for Mars soil and environment analysis. It provides the capability to serve and handle scientific miniaturised instruments accommodated inside its envelope. The instruments have the goal to perform in situ study of the collected martian samples, thus quantitatively characterizing the mineralogy, the composition, the microphysical structure of the materials of the Martian soils down to the depth available to the sampling mechanism. Given the complex structure of the surface material it will be essential to perform in-situ science, both at the surface and at different depths. This is done in order to validate remote sensing observations through specific measurements, identify local characteristics of the selected landing areas, document sample collection both for in situ and sample return. IPSE is an example of a small and flexible lab, that can be integrated on different Landers and Megarovers. IPSE contains: • Scientific instruments • A small robotic arm - with five degrees of freedom - to provide samples to the IPSE instruments. • Power conditioning. • Electronics for system and thermal control, communications and instrument data handling. The Phase A report for all the IPSE instruments has been already provided to the Italian Space Agency. The experiments MAGO, IRMA, MA_FLUX are inherited from previous space qualified instruments and breadboards of them already exist. DOSE is a new experiment, however, a breadboard of the detector and of the photomultiplier is under development.

IPSE: Italian package for scientific experiments

IPSE is a scientific autonomous micro-laboratory for Mars soil and environment analysis. It is designed to provide the capability to serve, handle and manage scientific miniaturised instruments accommodated inside its envelope. These instruments will carry out measurements on soil samples, atmosphere, radiation environment and dust flux. IPSE is an example of a small and flexible laboratory, that can be integrated on different landers and rovers. It contains: scientific instruments; a small robotic arm to provide samples to the instruments; power conditioning; electronics for system and thermal control, communications and instrument data handling.

Italian participation in the Mars exploration program

Abstract Recently agreements have been signed between the Italian Space Agency (ASI) and ESA and NASA for the exploration of Mars. These agreements initiate the participation of the Italian scientific community as well as the Italian industrial community in the international program to explore Mars. ASI and NASA have agreed to co-operate in a long-term systematic program of robotic exploration of Mars sustained by a series of missions to Mars in support of their respective strategic goals. The Mars Surveyor Program is a sustained series of missions to Mars, each of which will provide important focused scientific results. ASI is expecting to participate in the future missions with the provision of two sub-systems: a subsurface drill and a scientific package. The drill will be capable of drilling and collecting several samples and delivering them to instruments located within a scientific package fixed on a landed platform. ASI is also providing scientific instruments placed on a scientific package (IPSE) fixed with on the lander platform. The goals of the investigations are to study physical and mineralogical properties of bulk soil and dust (atmospheric and surface) as well as geochemical, structural, radiation and geophysical properties of subsurface materials to a depth of 0.5 meters.