Roberto Orosei

Radar Soundings of the Subsurface of Mars

The martian subsurface has been probed to kilometer depths by the Mars Advanced Radar for Subsurface and Ionospheric Sounding instrument aboard the Mars Express orbiter. Signals penetrate the polar layered deposits, probably imaging the base of the deposits. Data from the northern lowlands of Chryse Planitia have revealed a shallowly buried quasi-circular structure about 250 kilometers in diameter that is interpreted to be an impact basin. In addition, a planar reflector associated with the basin structure may indicate the presence of a low-loss deposit that is more than 1 kilometer thick.

The organic-rich surface of comet 67P/Churyumov-Gerasimenko as seen by VIRTIS/Rosetta

The VIRTIS (Visible, Infrared and Thermal Imaging Spectrometer) instrument on board the Rosetta spacecraft has provided evidence of carbon-bearing compounds on the nucleus of the comet 67P/Churyumov-Gerasimenko. The very low reflectance of the nucleus (normal albedo of 0.060 ± 0.003 at 0.55 micrometers), the spectral slopes in visible and infrared ranges (5 to 25 and 1.5 to 5% kÅ−1), and the broad absorption feature in the 2.9-to-3.6–micrometer range present across the entire illuminated surface are compatible with opaque minerals associated with nonvolatile organic macromolecular materials: a complex mixture of various types of carbon-hydrogen and/or oxygen-hydrogen chemical groups, with little contribution of nitrogen-hydrogen groups. In active areas, the changes in spectral slope and absorption feature width may suggest small amounts of water-ice. However, no ice-rich patches are observed, indicating a generally dehydrated nature for the surface currently illuminated by the Sun.

Radar Sounding of the Medusae Fossae Formation Mars: Equatorial Ice or Dry, Low-Density Deposits?

The equatorial Medusae Fossae Formation (MFF) is enigmatic and perhaps among the youngest geologic deposits on Mars. They are thought to be composed of volcanic ash, eolian sediments, or an ice-rich material analogous to polar layered deposits. The Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) instrument aboard the Mars Express Spacecraft has detected nadir echoes offset in time-delay from the surface return in orbits over MFF material. These echoes are interpreted to be from the subsurface interface between the MFF material and the underlying terrain. The delay time between the MFF surface and subsurface echoes is consistent with massive deposits emplaced on generally planar lowlands materials with a real dielectric constant of ∼2.9 ± 0.4. The real dielectric constant and the estimated dielectric losses are consistent with a substantial component of water ice. However, an anomalously low-density, ice-poor material cannot be ruled out. If ice-rich, the MFF must have a higher percentage of dust and sand than polar layered deposits. The volume of water in an ice-rich MFF deposit would be comparable to that of the south polar layered deposits.

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.

Exposed water ice on the nucleus of comet 67P/Churyumov–Gerasimenko

Although water vapour is the main species observed in the coma of comet 67P/Churyumov–Gerasimenko and water is the major constituent of cometary nuclei, limited evidence for exposed water-ice regions on the surface of the nucleus has been found so far. The absence of large regions of exposed water ice seems a common finding on the surfaces of many of the comets observed so far. The nucleus of 67P/Churyumov–Gerasimenko appears to be fairly uniformly coated with dark, dehydrated, refractory and organic-rich material. Here we report the identification at infrared wavelengths of water ice on two debris falls in the Imhotep region of the nucleus. The ice has been exposed on the walls of elevated structures and at the base of the walls. A quantitative derivation of the abundance of ice in these regions indicates the presence of millimetre-sized pure water-ice grains, considerably larger than in all previous observations. Although micrometre-sized water-ice grains are the usual result of vapour recondensation in ice-free layers, the occurrence of millimetre-sized grains of pure ice as observed in the Imhotep debris falls is best explained by grain growth by vapour diffusion in ice-rich layers, or by sintering. As a consequence of these processes, the nucleus can develop an extended and complex coating in which the outer dehydrated crust is superimposed on layers enriched in water ice. The stratigraphy observed on 67P/Churyumov–Gerasimenko is therefore the result of evolutionary processes affecting the uppermost metres of the nucleus and does not necessarily require a global layering to have occurred at the time of the comet’s formation.

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.

Chiron Activity and Thermal Evolution

We discuss the results obtained by simulating the thermal evolution of 2060 Chiron, the largest of the Centaur objects. The study of Chiron is extremely important because of the presence of activity in spite of its large distance from the Sun. In this paper we study the thermal evolution of a Chiron-like body under diUerent assumptions concerning the initial composition and taking into account its evolutionary history. The composition of Centaurs and Kuiper belt objects is almost unknown: we can guess that Chirons ii cometary behavior ˇˇ is an indication of a composition dominated by diUerent ices. We have therefore computed the thermal evolution of Chiron using a numerical code developed to study the thermal evolution of cometary nuclei. As in the case of comets, we assume that the ice is mixed with dust and that the body is highly porous. CO is present in the mixture both as ice and as gas trapped in the amorphous ice. Our conclusion is that Chirons activity can be explained only by assuming a composi- tion in which CO is present not far from the objects surface as an ice or as a gas trapped in the amor- phous ice.

MODELS OF P/WIRTANEN NUCLEUS : ACTIVE REGIONS VERSUS NON-ACTIVE REGIONS

Abstract From the current understanding we know that comet nuclei have heterogeneous compositions and complex structures. It is believed that cometary activity is the result of a combination of physical processes in the nucleus, like sublimation and recondensation of volatile ices, dust grains release, phase transition of water ice, depletion of the most volatile components in the outer layers and interior differentiation. The evolution of the comet depends on the sublimation of ices and the release of different gases and dust grains: the formation of a dust crust, the surface erosion and the development of the coma are related to the gas fluxes escaping from the nucleus. New observations, laboratory experiments and numerical simulations suggest that the gas and dust emissions are locally generated, in the so-called active regions. This localized activity is probably superimposed to the global nucleus activity. The differences between active and inactive regions can be attributed to differences in texture and refractory material content of the different areas. In this paper we present the results of numerical models of cometary nucleus evolution, developed in order to understand which are the processes leading to the formation of active and non-active regions on the cometary surface. The used numerical code solves the equations of heat transport and gas diffusion within a porous nucleus composed of different ices—such as water (the dominant constituent), CO 2 , CO- and of dust grains embedded in the ice matrix. By varying the set of physical parameters describing the initial properties of comet P/Wirtanen, the different behaviour of the icy and dusty areas can be followed. Comet P/Wirtanen is the target of the international ROSETTA mission, the cornerstone ESA mission to a cometary nucleus. The successful design of ROSETTA requires some knowledge of comet status and activity: surface temperatures, amount of active and inactive surface areas, gas production rate and dust flux.

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.

Subsurface Radar Sounding of the Jovian Moon Ganymede

This paper provides an overview of the Europa Jupiter System Mission (EJSM) and of its scientific objectives, focusing the attention on the subsurface radar (SSR) instrument included in the model payload of the Jupiter Ganymede Orbiter (JGO). The SSR instrument is a radar sounder system at low frequency (HF/VHF band) designed to penetrate the surface of Ganymede icy moon of Jupiter for performing a subsurface analysis with a relatively high range resolution. This active instrument is aimed at acquiring information on the Ganymede (and partially on the Callisto during flybys) shallow subsurface. The paper addresses the main issues related to SSR, presenting its scientific goals, describing the concept and the design procedure of the instrument, and illustrating the signal processing techniques. Despite the fact that SSR can be defined on the basis of the heritage of the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) and SHAllow RADar (SHARAD) instruments currently operating at Mars, the EJSM mission poses additional scientific and technical challenges for its design: 1) the presence of a relevant Jupiter radio emission (which is very critical because it has a significant power spectral density in proximity of the expected SSR central frequency); 2) the properties of the subsurface targets, which are different from those of the Mars subsurface; 3) the different orbit conditions; and 4) the limited available resources (in terms of mass, power, and downlink data rate). These challenges are analyzed and discussed in relation to the design of the instrument in terms of: 1) choice of the central frequency and the bandwidth; 2) signal-to-noise ratio (SNR); 3) signal-to-clutter ratio (SCR); and 4) definition of the synthetic aperture processing. Finally, the procedure defined for SSR performance assessment is described and illustrated with some numerical examples.