S. Mottola

German Aerospace Center (DLR), Köln, North Rhine-Westphalia, Germany

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Publications (327)606.49 Total impact

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    ABSTRACT: ROLIS (ROsetta Lander Imaging System) [1,2] is an imager with multispectral capabilities onboard the Rosetta Lander Philae. From its location on the instruments balcony and having a down-looking orientation, ROLIS acquired an imaging sequence of the Agilkia landing site during the descent onto comet 67P/Churyumov-Gerasimenko. These images provide the highest resolution available for the first touchdown site. The aim of the ROLIS experiment is to study the photometric properties, the morphology, the texture and microstructure of the comet's surface in order to understand the processes that control the cometary evolution. ROLIS is a compact CCD imager with a 1k x 1k pixel sensor and a 57 • field of view. During the descent it was focused to infinity and acquired panchromatic images. After landing ROLIS was refocused to a nominal distance of 30 cm and performed close-up, multi-spectral imaging of the soil, with help of a 4-color LED illumination device. During the descent, ROLIS acquired images with a cadence of 10 s. Since the exact time of touchdown was not known, and due to storage and uplink capacity limitations, the images were stored in a ring buffer with the capacity of seven images, with the latest image overwriting the oldest. At the moment of touchdown the acquisition sequence was halted, and the last seven images −the ones with the highest resolution− were relayed to the orbiter. The landing occurred on Nov 12, 2014 at 15:34:04 UT. The sun elevation angle of about 30 • provided near-ideal conditions for morphological analysis. The image acquired at the highest altitude (70 m) has a footprint of about 70 m and a resolution of 7 cm/pix, whereas the image closest to the surface was acquired at about 10 m altitude and has a resolution of about 1 cm/pix. The high-resolution images acquired just before touchdown reveal a generally smooth and subdued terrain whose characteristics vary over scales of a few tens of meters. A comparatively uniform background made of particles below or at the resolution limit (about 1 cm on the image at closest range) is superimposed by debris and blocks ranging in size from centimeters to several meters with a regionally varying spatial density. Within the region imaged by ROLIS there appear to be undulated formations, terraces and pitted terrains. The spatial density of blocks varies with the type of terrain: the region with pits shows a significantly smaller presence of blocks than the neighboring regions. The appearance of the blocks themselves also varies considerably, ranging in shape from angular, polygonal to rounded. Some of the largest blocks resemble assemblages of smaller, similarly-sized units. The presence of clusters of block suggests that fragmentation and degradation processes are at work. Some blocks are partly buried by regolith, suggestive of dust mobilization. References: [1] Mottola et al. (2007), SSR, 128, 241-255. [2] Michaelis et al. (1999), proc. SPIE 3794, 115-121.
    EGU, Vienna; 04/2015
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    ABSTRACT: On Nov 12, 2014, the Rosetta Philae lander descended towards comet 67P/Churyumov-Gerasimenko. The onboard ROLIS camera successfully acquired high resolution images of the surface looking down from its vantage point on the instrument platform. ROLIS is a compact CCD imager with a 1024×1024 pixel sensor and a 57 • field of view (Mottola et al., 2007, SSR 128, 241). It is equipped with an infinity lens (IFL), without which the camera focus is 30 cm. At Philae's final landing site, ROLIS removed the IFL and initiated an imaging sequence that shows the surface at the highest resolution ever obtained for a cometary surface (∼0.5 mm per pixel). Illumination of the scene was provided by an onboard array of LEDs in four different colors: red, green, blue, and near-IR. ROLIS acquired one image for each color and a single dark exposure. The images show a unique, almost fractal morphology for the surface below the landing site that defies easy interpretation. However, there are similarities with some structures seen by the CIVA camera. Color and albedo variations over the surface are minor, and individual grains cannot be distinguished. The images are out-of-focus, indicating the surface was further away than the nominal 30 cm. The location of the illumination spot and the change of focus over the image are consistent with an inclined surface, indicating that Philae's final resting position is strongly tilted. In fact, it was inclined so much that we see the local horizon, even though ROLIS is downward-looking. Remarkably, the scene beyond the horizon is illuminated by the Sun, and out-of-focus particles can be seen to travel in the sky. The images suggest the environment of the lander is laden with fine dust, but a final assessment requires careful consideration of possible sources of stray light. Just before Philae went to sleep, ROLIS acquired an additional exposure with the IFL and the red LED. The resulting image is fully in focus. Because Philae had rotated and lifted prior to acquisition, the last image allows us to reconstruct the scene below the lander in 3D. In case Philae wakes up from its slumber in the spring of this year, ROLIS should be able to resume operation. It may acquire additional well-focused images in all colors to better characterize the cometary environment and search for changes.
    EGU, Vienna; 04/2015
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    ABSTRACT: Introduction: After a 10-year cruise, the Rosetta spacecraft began a close exploration of its main target, comet 67P/Churyumov-Gerasimenko, in July 2014. Since then, the Visible InfraRed Thermal Imaging Spectrometer (VIRTIS) [1] acquired hyperspectral images of the comet's surface with an unprecedented spatial resolution. VIRTIS data are routinely used to map the surface composition and to retrieve surface temperatures on the dayside of the comet. The thermal behavior of the surface of comet 67P is related to composition and physical properties that provide information about the nature and evolution of those materials. Here we present temperature maps of comet 67P that were observed by Rosetta under different illumination conditions and different local solar times. The VIRTIS instrument: VIRTIS is a hyperspec-tral imaging spectrometer in the overall range 0.25-5.1 µm (VIRTIS-M) and high spectral resolution capabilities (VIRTIS-H) onboard the ESA Rosetta mission [1]. VIRTIS design fully accomplishes Rosetta's scientific objectives at all targets of the mission. The composition of the uppermost layer of the comet's surface down to depths of tens of microns, in terms of water ice, salts, organics and volatiles, can be revealed and mapped by visual and infrared spectroscopy using high spatial resolution imaging, and high spectral resolution to simultaneously improve diagnostic capabilities. The infrared range longward of 3.5 µm is crucial to reveal the thermal emission of the comet on its daysi-de, which can be used to map surface temperature across different orbits and local solar times (LST), and therefore constrain thermal properties at different spatial scales (Fig. 1). Data set and analysis: Here we show spatially-resolved temperature maps of comet 67P derived in medium-term planning (MTP) phases carried out in the second half of 2014: MTP006-007-008-009-010-011, in the overall spatial sampling range between 23 m/px and ~2.3 m/px, which has never been achieved before with this technique on a planetary body. Data were acquired under variable phase angles, illumination conditions, and heliocentric distances. Local-scale features on comet 67P are currently being investigated by the Rosetta team.
    46th Lunar and Planetary Science Conference, The Woodlands, Texas; 03/2015
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    ABSTRACT: Introduction: ROLIS (ROsetta Lander Imaging System) [1] is an imager with multispectral capabilities onboard the Rosetta Lander Philae. From its location on the instruments balcony and having a down-looking orientation, ROLIS acquired an imaging sequence of the Agilkia landing site during the descent onto comet 67P/Churyumov-Gerasimenko. These images provide the highest resolution available for the first touchdown site. The aim of the ROLIS experiment is to study the photometric properties, the morphology, the texture and microstructure of the comet's surface in order to understand the processes that control the com-etary evolution. The ROLIS Instrument: ROLIS is a compact CCD imager with a 1k x 1k pixel sensor and a 57° field of view [2]. During the descent it was focused to infinity and acquired panchromatic images. After landing ROLIS was refocused to a nominal distance of 30 cm and performed close-up, multi-spectral imaging of the soil, with help of a 4-channel LED illumination device. Landing Sequence: During the descent, ROLIS acquired images with a cadence of 10s. Since the time of touchdown was not predictable with accuracy, and due to storage and uplink capacity limitations, the images were stored in a ring buffer with the capacity of seven images, with the latest image overwriting the oldest. At the moment of touchdown the acquisition sequence was halted, and the last seven images −the
    46th Lunar and Planetray Science Conference, The Woodlands, Texas; 03/2015
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    ABSTRACT: Asteroid (234) Barbara is the prototype of a category of asteroids that has been shown to be extremely rich in refractory inclusions, the oldest material ever found in the Solar System. It exhibits several peculiar features, most notably its polarimetric behavior. In recent years other objects sharing the same property (collectively known as "Barbarians") have been discovered. Interferometric observations in the mid-infrared with the ESO VLTI suggested that (234) Barbara might have a bi-lobated shape or even a large companion satellite. We use a large set of 57 optical lightcurves acquired between 1979 and 2014, together with the timings of two stellar occultations in 2009, to determine the rotation period, spin-vector coordinates, and 3-D shape of (234) Barbara, using two different shape reconstruction algorithms. By using the lightcurves combined to the results obtained from stellar occultations, we are able to show that the shape of (234) Barbara exhibits large concave areas. Possible links of the shape to the polarimetric properties and the object evolution are discussed. We also show that VLTI data can be modeled without the presence of a satellite.
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    ABSTRACT: Images of comet 67P/Churyumov-Gerasimenko acquired by the OSIRIS (Optical, Spectroscopic and Infrared Remote Imaging System) imaging system onboard the European Space Agency's Rosetta spacecraft at scales of better than 0.8 meter per pixel show a wide variety of different structures and textures. The data show the importance of airfall, surface dust transport, mass wasting, and insolation weathering for cometary surface evolution, and they offer some support for subsurface fluidization models and mass loss through the ejection of large chunks of material. Copyright © 2015, American Association for the Advancement of Science.
    Science 01/2015; 347(6220):aaa0440. DOI:10.1126/science.aaa0440 · 31.48 Impact Factor
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    ABSTRACT: 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. Copyright © 2015, American Association for the Advancement of Science.
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    ABSTRACT: Images from the OSIRIS scientific imaging system onboard Rosetta show that the nucleus of 67P/Churyumov-Gerasimenko consists of two lobes connected by a short neck. The nucleus has a bulk density less than half that of water. Activity at a distance from the Sun of >3 astronomical units is predominantly from the neck, where jets have been seen consistently. The nucleus rotates about the principal axis of momentum. The surface morphology suggests that the removal of larger volumes of material, possibly via explosive release of subsurface pressure or via creation of overhangs by sublimation, may be a major mass loss process. The shape raises the question of whether the two lobes represent a contact binary formed 4.5 billion years ago, or a single body where a gap has evolved via mass loss.
    Science 01/2015; 347(6620). DOI:10.1126/science.aaa1044 · 31.48 Impact Factor
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    ABSTRACT: Critical measurements for understanding accretion and the dust/gas ratio in the solar nebula, where planets were forming 4.5 billion years ago, are being obtained by the GIADA (Grain Impact Analyser and Dust Accumulator) experiment on the European Space Agency's Rosetta spacecraft orbiting comet 67P/Churyumov-Gerasimenko. Between 3.6 and 3.4 astronomical units inbound, GIADA and OSIRIS (Optical, Spectroscopic, and Infrared Remote Imaging System) detected 35 outflowing grains of mass 10(-10) to 10(-7) kilograms, and 48 grains of mass 10(-5) to 10(-2) kilograms, respectively. Combined with gas data from the MIRO (Microwave Instrument for the Rosetta Orbiter) and ROSINA (Rosetta Orbiter Spectrometer for Ion and Neutral Analysis) instruments, we find a dust/gas mass ratio of 4 ± 2 averaged over the sunlit nucleus surface. A cloud of larger grains also encircles the nucleus in bound orbits from the previous perihelion. The largest orbiting clumps are meter-sized, confirming the dust/gas ratio of 3 inferred at perihelion from models of dust comae and trails. Copyright © 2015, American Association for the Advancement of Science.
    Science 01/2015; 347(6220):aaa3905. DOI:10.1126/science.aaa3905 · 31.48 Impact Factor
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    Astronomy and Astrophysics 01/2015; 573(A62). DOI:10.1051/0004-6361/201424735 · 4.48 Impact Factor
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    ABSTRACT: The ROSETTA mission has reached the nucleus of comet 67P/Churyumov-Gerasimenko in early August, allowing a detailed mapping of its surface with the onboard OSIRIS imaging system, up to resolutions of 50 cm or even better in some areas. Shape reconstruction techniques have been used since July 2014 to build a very detailed 3D model of the comet surface and to retrieve highly accurate rotational parameters. The attached Figure shows an early 3D shape of the comet. The most striking property of the global shape is the presence of two clearly separated components. We use a combination of images and 3D models to quantitatively characterize their bulk and surface properties: relative position, volume, topography and roughness. The position of the center of mass and the direction of the principal axis of rotation are used to constrain the internal mass distribution. Digital terrain models, slope, gravity, and geo-referenced images of the most interesting features observed at the surface of the comet are presented and consequences for their formation are discussed. If they are detected, topographic changes which could have occured between August and December 2014 are also presented.
    AGU Fall Meeting 2014; 12/2014
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    ABSTRACT: In understanding the composition and internal structure of asteroids, their density is perhaps the most diagnostic quantity. We aim here to characterize the surface composition, mutual orbit, size, mass, and density of the small main-belt binary asteroid (939) Isberga. For that, we conduct a suite of multi-technique observations, including optical lightcurves over many epochs, near-infrared spectroscopy, and interferometry in the thermal infrared. We develop a simple geometric model of binary systems to analyze the interferometric data in combination with the results of the lightcurve modeling. From spectroscopy, we classify Ibserga as a Sq-type asteroid, consistent with the albedo of 0.14$^{+0.09}_{-0.06}$ (all uncertainties are reported as 3-$\sigma$ range) we determine (average albedo of S-types is 0.197 $\pm$ 0.153, Pravec et al., 2012, Icarus 221, 365-387). Lightcurve analysis reveals that the mutual orbit has a period of 26.6304 $\pm$ 0.0001 h, is close to circular, and has pole coordinates within 7 deg. of (225, +86) in ECJ2000, implying a low obliquity of 1.5 deg. The combined analysis of lightcurves and interferometric data allows us to determine the dimension of the system and we find volume-equivalent diameters of 12.4$^{+2.5}_{-1.2}$ km and 3.6$^{+0.7}_{-0.3}$ km for Isberga and its satellite, circling each other on a 33 km wide orbit. Their density is assumed equal and found to be $2.91^{+1.72}_{-2.01}$ g.cm$^{-3}$, lower than that of the associated ordinary chondrite meteorites, suggesting the presence of some macroporosity, but typical of S-types of the same size range (Carry, 2012, P\&SS 73, 98-118). The present study is the first direct measurement of the size of a small main-belt binary. Although the interferometric observations of Isberga are at the edge of MIDI capabilities, the method described here is applicable to others suites of instruments (e.g, LBT, ALMA).
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    ABSTRACT: Aims. Approach observations with the Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS) experiment onboard Rosetta are used to determine the rotation period, the direction of the spin axis, and the state of rotation of comet 67P's nucleus. Methods. Photometric time series of 67P have been acquired by OSIRIS since the post wake-up commissioning of the payload in March 2014. Fourier analysis and convex shape inversion methods have been applied to the Rosetta data as well to the available ground-based observations. Results. Evidence is found that the rotation rate of 67P has significantly changed near the time of its 2009 perihelion passage, probably due to sublimation-induced torque. We find that the sidereal rotation periods P 1 = 12.76129 ± 0.00005 h and P 2 = 12.4043 ± 0.0007 h for the apparitions before and after the 2009 perihelion, respectively, provide the best fit to the observations. No signs of multiple periodicity are found in the light curves down to the noise level, which implies that the comet is presently in a simple rotation state around its axis of largest moment of inertia. We derive a prograde rotation model with spin vector J2000 ecliptic coordinates λ = 65 • ± 15 • , β = +59 • ± 15 • , corresponding to equatorial coordinates RA = 22 • , Dec = +76 • . However, we find that the mirror solution, also prograde, at λ = 275 • ± 15 • , β = +50 • ± 15 • (or RA = 274 • , Dec = +27 •), is also possible at the same confidence level, due to the intrinsic ambiguity of the photometric problem for observations performed close to the ecliptic plane.
    Astronomy and Astrophysics 09/2014; 569(L2). DOI:10.1051/0004-6361/201424590 · 4.48 Impact Factor
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    ABSTRACT: Using our photometric observations taken between April 1996 and January 2013 and other published data, we derive properties of the binary near-Earth asteroid (175706) 1996 FG3 including new measurements constraining evolution of the mutual orbit with potential consequences for the entire binary asteroid population. We also refined previously determined values of parameters of both components, making 1996 FG3 one of the most well understood binary asteroid systems. We determined the orbital vector with a substantially greater accuracy than before and we also placed constraints on a stability of the orbit. Specifically, the ecliptic longitude and latitude of the orbital pole are 266{\deg} and -83{\deg}, respectively, with the mean radius of the uncertainty area of 4{\deg}, and the orbital period is 16.1508 +\- 0.0002 h (all uncertainties correspond to 3sigma). We looked for a quadratic drift of the mean anomaly of the satellite and obtained a value of 0.04 +\- 0.20 deg/yr^2, i.e., consistent with zero. The drift is substantially lower than predicted by the pure binary YORP (BYORP) theory of McMahon and Scheeres (McMahon, J., Scheeres, D. [2010]. Icarus 209, 494-509) and it is consistent with the theory of an equilibrium between BYORP and tidal torques for synchronous binary asteroids as proposed by Jacobson and Scheeres (Jacobson, S.A., Scheeres, D. [2011]. ApJ Letters, 736, L19). Based on the assumption of equilibrium, we derived a ratio of the quality factor and tidal Love number of Q/k = 2.4 x 10^5 uncertain by a factor of five. We also derived a product of the rigidity and quality factor of mu Q = 1.3 x 10^7 Pa using the theory that assumes an elastic response of the asteroid material to the tidal forces. This very low value indicates that the primary of 1996 FG3 is a 'rubble pile', and it also calls for a re-thinking of the tidal energy dissipation in close asteroid binary systems.
    Icarus 06/2014; 245. DOI:10.1016/j.icarus.2014.09.023 · 2.84 Impact Factor
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    ABSTRACT: The NASA Dawn spacecraft acquired thousands of images of asteroid Vesta during its year-long orbital tour, and is now on its way to asteroid Ceres. A method for calibrating images acquired by the onboard Framing Camera was described by Schröder et al. (2013; Icarus 226, 1304). However, their method is only valid for point sources. In this paper we extend the calibration to images of extended sources like Vesta. For this, we devise a first-order correction for in-field stray light, which is known to plague images taken through the narrow band filters, and revise the flat fields that were acquired in an integrating sphere before launch. We used calibrated images of the Vesta surface to construct simple photometric models for all filters, that allow us to study how the spectrum changes with increasing phase angle (phase reddening). In combination with these models, our calibration method can be used to create near-seamless mosaics that are radiometrically accurate to a few percent. Such mosaics are provided in JVesta, the Vesta version of the JMARS geographic information system.
    Icarus 05/2014; 234:99-108. DOI:10.1016/j.icarus.2014.02.018 · 2.84 Impact Factor
  • Stefano Mottola, Paul Schenk, Harald Hiesinger
    Planetary and Space Science 04/2014; DOI:10.1016/j.pss.2014.04.016 · 1.63 Impact Factor
  • Article: 2014 HF124
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    ABSTRACT: Abstract available on the publisher website.
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    ABSTRACT: MASCOT on Hayabusa-2 will carry four instruments, including a wide-angle camera. We describe science goals, instrument design, and performance of the camera.
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    ABSTRACT: The object was discovered in August 2013, displaying a cometary tail, but its orbital elements indicated that it was a typical member of the inner asteroid main belt. We monitored the object from 2013 August 30 until 2013 October 05 using the CFHT 3.6 m telescope (Mauna Kea, HI), the NTT (ESO, La Silla), the CA 1.23 m telescope (Calar Alto), the Perkins 1.8m (Lowell) and the 0.6 m TRAPPIST telescope (La Silla). We measured its nuclear radius to be r ≲ 0.25-0.29 km, and its colours g' - r' = 0.58 ± 0.05 and r' - i' = 0.23 ± 0.06, typical for an S-class asteroid, as expected for an object in the inner asteroid belt and in the vicinity of the Flora collisional family. We failed to detect any rotational light curve with an amplitude <0.05 mag and a double-peaked rotation period <20 h. The evolution of the tail during the observations was as expected from a dust tail. A detailed Finson-Probstein analysis of deep images acquired with the NTT in early September and with the CFHT in late September indicated that the object was active since at least late January 2013 until the time of the latest observations in 2013 September, with at least two peaks of activity around 2013 June 14 ± 10 d and 2013 July 22 ± 3 d. The changes of activity level and the activity peaks were extremely sharp and short, shorter than the temporal resolution of our observations (~1 d). The dust distribution was similar during these two events, with dust grains covering at least the 1-1000 μm range. The total mass ejected in grains <1 mm was estimated to be 3.0 × 106 kg and 2.6 × 107 kg around the two activity peaks. Rotational disruption cannot be ruled out as the cause of the dust ejection. We also propose that the components of a contact binary might gently rub and produce the observed emission. Volatile sublimation might also explain what appears as cometary activity over a period of 8 months. However, while main belt comets best explained by ice sublimation are found in the outskirts of the main belt, where water ice is believed to be able to survive buried in moderately large objects for the age of the solar system deeply, the presence of volatiles in an object smaller than 300 m in radius would be very surprising in the inner asteroid belt.Based on observations collected at the European Southern Observatory, La Silla, Chile (NTT), program 184.C-1143(H), the Canada France Hawaii Telescope, Mauna Kea, Hawaii, and the 1.2 m telescope on Calar Alto, Spain.

Publication Stats

2k Citations
606.49 Total Impact Points

Institutions

  • 1991–2014
    • German Aerospace Center (DLR)
      • Institute of Planetary Research
      Köln, North Rhine-Westphalia, Germany
  • 2011
    • National Institute of Astrophysics
      • Institute of Physics of Interplanetary Space IFSI
      Roma, Latium, Italy
    • William Penn University
      University Park, Florida, United States
  • 2004–2009
    • University of Padova
      • Department of Geosciences
      Padua, Veneto, Italy
  • 2007
    • SETI Institute
      Mountain View, California, United States
  • 1998
    • University of Michigan
      Ann Arbor, Michigan, United States
  • 1989
    • University of Catania
      Catania, Sicily, Italy