Papers by Alessandro Frigeri
Life, 2020
The NASA/Dawn mission has acquired unprecedented measurements of the surface of the dwarf planet ... more The NASA/Dawn mission has acquired unprecedented measurements of the surface of the dwarf planet Ceres, the composition of which is a mixture of ultra-carbonaceous material, phyllosilicates, carbonates, organics, Fe-oxides, and volatiles as determined by remote sensing instruments including the VIR imaging spectrometer. We performed a refined analysis merging visible and infrared observations of Ceres’ surface for the first time. The overall shape of the combined spectrum suggests another type of silicate not previously considered, and we confirmed a large abundance of carbon material. More importantly, by analyzing the local spectra of the organic-rich region of the Ernutet crater, we identified a reddening in the visible range, strongly correlated to the aliphatic signature at 3.4 µm. Similar reddening was found in the bright material making up Cerealia Facula in the Occator crater. This implies that organic material might be present in the source of the faculae, where brines and ...
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Science (New York, N.Y.), Feb 17, 2017
Organic compounds occur in some chondritic meteorites, and their signatures on solar system bodie... more Organic compounds occur in some chondritic meteorites, and their signatures on solar system bodies have been sought for decades. Spectral signatures of organics have not been unambiguously identified on the surfaces of asteroids, whereas they have been detected on cometary nuclei. Data returned by the Visible and InfraRed Mapping Spectrometer on board the Dawn spacecraft show a clear detection of an organic absorption feature at 3.4 micrometers on dwarf planet Ceres. This signature is characteristic of aliphatic organic matter and is mainly localized on a broad region of ~1000 square kilometers close to the ~50-kilometer Ernutet crater. The combined presence on Ceres of ammonia-bearing hydrated minerals, water ice, carbonates, salts, and organic material indicates a very complex chemical environment, suggesting favorable environments to prebiotic chemistry.
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Science (New York, N.Y.), Dec 23, 2016
Carbon dioxide (CO2) is one of the most abundant species in cometary nuclei, but because of its h... more Carbon dioxide (CO2) is one of the most abundant species in cometary nuclei, but because of its high volatility, CO2 ice is generally only found beneath the surface. We report the infrared spectroscopic identification of a CO2 ice-rich surface area located in the Anhur region of comet 67P/Churyumov-Gerasimenko. Spectral modeling shows that about 0.1% of the 80- by 60-meter area is CO2 ice. This exposed ice was observed a short time after the comet exited local winter; following the increased illumination, the CO2 ice completely disappeared over about 3 weeks. We estimate the mass of the sublimated CO2 ice and the depth of the eroded surface layer. We interpret the presence of CO2 ice as the result of the extreme seasonal changes induced by the rotation and orbit of the comet.
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Science, 2016
INTRODUCTIONThe surface of the dwarf planet Ceres is known to host phyllosilicate minerals, but t... more INTRODUCTIONThe surface of the dwarf planet Ceres is known to host phyllosilicate minerals, but their distribution and origin have not previously been determined. Phyllosilicates are hydrated silicates, and their presence on the surface of Ceres is intriguing given that their structure evolves through an aqueous alteration process. In addition, some phyllosilicates are known to bear NH4, which places a constraint on the pH and redox conditions during the evolution of Ceres. We studied the distribution of phyllosilicates across the planet’s surface to better understand the evolutionary pathway of Ceres.RATIONALEUsing the data acquired by the mapping spectrometer (VIR) onboard the Dawn spacecraft, we mapped the spatial distribution of different minerals on Ceres on the basis of their diagnostic absorption features in visible and infrared spectra. We studied the phyllosilicates through their OH-stretch fundamental absorption at about 2.7 µm and through the NH4absorption at about 3.1 µm...
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Nature, Jan 13, 2016
Although water vapour is the main species observed in the coma of comet 67P/Churyumov-Gerasimenko... more 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 v...
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Nature, 2015
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ABSTRACT Vestarsquos spectrum has strong absorption centered near 0.9 and 1.9 $mu$m, indicative o... more ABSTRACT Vestarsquos spectrum has strong absorption centered near 0.9 and 1.9 $mu$m, indicative of Fe-bearing pyroxenes. Data from the Dawn VIR characterize and map the mineral distribution on Vesta, providing new insights into Vestarsquos formation and evolution.
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ABSTRACT 4 Vesta has a surface of basaltic material (1) with strong absorption features centered ... more ABSTRACT 4 Vesta has a surface of basaltic material (1) with strong absorption features centered near 0.9 and 1.9 μm, indicative of Fe-bearing pyroxenes. The spectra of HED (howardite, eucrite and diogenite) meteorites have similar features (1). This led to the hypothesis that Vesta was the parent body of the HED clan (2,3) and the discovery of a dynamical Vesta family of asteroids (Vestoids) provides a further link between Vesta and HEDs (4). Data from the Dawn VIR (Visible InfraRed mapping Spectrometer) (5,6) characterize and map the mineral distribution on Vesta, strengthen the Vesta - HED linkage and provide new insights into Vesta's formation and evolution.
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2008 IEEE Radar Conference, RADAR 2008, 2008
The MARSIS observations are optimized during periods when the pericenter of the orbit is near or ... more The MARSIS observations are optimized during periods when the pericenter of the orbit is near or below zero degrees sun elevation (ldquonightsiderdquo) and the nightside phase, the last of the primary MEX mission, occurs on March-July 2005, in the northern latitude of MARS regions. This paper provides a description of the modeling approach and of the expected performance of the
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2008 IEEE Radar Conference, RADAR 2008, 2008
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Proceedings of the 13th Internarional Conference on Ground Penetrating Radar, GPR 2010, 2010
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2007 4th International Workshop on, Advanced Ground Penetrating Radar, 2007
In this paper we describe an inversion approach in order to analyze data from the MARSIS (Mars Ad... more In this paper we describe an inversion approach in order to analyze data from the MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) instrument on Mars Express. The inversion process allows the dielectric constant of the subsurface material to be estimated provided the dielectric constant of the surface is known. In addition, if impurity are present, it is possible
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ABSTRACT The Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) is the first ra... more ABSTRACT The Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) is the first radar to collect data from Mars orbit. MARSIS is a multi-frequency synthetic aperture orbital sounding radar. MARSIS acquires data in the conventional fashion of a radar in the range 0.1-5.5 MHz, which corresponds to wavelength of tens to hundreds of metres. These long wavelengths maximize the penetration of the signal into the subsurface of Mars. MARSIS has been configured to collect High Resolution Data in order to improve the science investigation of the North Polar Deposits of Mars (Planum Boreum). This special configuration provided data of unprecedented quality that contributed to the reconstruction of the three-dimensional structure of the icy North Polar Layered Deposits and of the enigmatic Basal Unit beneath. The processing of the achieved data show that the deepest MARSIS detection is at the bottom of the basal unit and is estimated to be around 3.3 Km in the vacuum and 2.4 Km in the medium. CONCLUSION: We successfully implemented an advanced setting of the on board software in order to boost the signal-to-noise ratio of measurements and above all to have an adequate set of contiguous and unprocessed data. This gave us the flexibility to improve the ground processing, choosing different size for the Synthetic Aperture Technique. Although this procedure pushed the instrument to the limit of its original design capabilities, performance was flawless.
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ABSTRACT Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) is an orbital subsu... more ABSTRACT Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) is an orbital subsurface sounder aboard ESA's Mars Express spacecraft . It transmits a low-frequency radar pulse that is capable of penetrating below the surface, and is reflected by subsurface dielectric discontinuities. MARSIS has been used to probe both the south and the north polar caps of Mars, revealing their thickness and structure. We report on the results of a campaign of observations of the north polar ice cap of Mars that took place between May and December 2011 in uniquely favorable conditions and produced data of unprecedented quality. The focus of this work is the so-called Basal Unit, a dark, ice-rich, complexely layered geologic unit lying stratigraphically between the polar layered deposits and the Vastitas Borealis Formation, and extending beneath most of Planum Boreum and Olympia Planitia. The objective of this work is the to study the full three dimensional structure of the Northern Polar Deposit and in particular of the Basal Unit (BU). It was recently found that the BU consists of two markedly different units, called the Rupes Tenuis unit and the Planum Boreum cavi unit. The Rupes Tenuis unit appears to be older, horizontally layered, and lacking erosional contacts. It has been thus interpreted as the result of precipitation and cold-trapping of dust-laden volatiles. The Planum Boreum cavi unit displays cross-bedding, indicating dune accumulation. Bright layers within it are interpreted as being made of ice-cemented dust, while dark layers should consist of weathered basalt fines. It seems likely that, in places, the Planum Boreum cavi unit rests directly on the Vastitas Borealis, without the Rupes Tenuis unit in between. Because the two units in the BU have formed much earlier than the north polar layered deposits, and at some interval from each other, they bear evidence of past climatic conditions that were very different from present, so that they "could potentially be a Rosetta Stone for the Martian climate". Subsurface sounding radar investigations by both MARSIS and SHARAD revealed that the BU has radar properties that are different from both the polar layered deposits and the Vastitas Borealis Formation, probably because of a mostly icy composition, but with a larger fraction of impurities than the polar layered deposits above. The upper surface of the BU exhibits significant relief, with features appearing to be erosional cutbacks and reentrants, indicating a complex accumulation history. Higher dust content and the resulting stronger attenuation is thought to be the reason why SHARAD radar signal could not penetrate through the BU and detect its bottom face. However, such a volume fraction cannot be much larger than the polar layered deposits, since MARSIS data revealed strong returns from the BU-Vastitas Borealis Formation interface, implying a relatively low fraction of impurities within the BU. From the summer phase of the Polar Campaign of data acquisition we have selected 161 radargrams. The radargrams were processed in order to cancel the effect of the ionosphere and to align the primary echo to a datum. The post-processed radargrams have been ingested into a Free Open Source software stack for geophysical imaging and interpretation. We find weak echoes within the BU that appear to outline a two-layer structure, perhaps corresponding to the Rupes Tenuis unit and the Planum Boreum Cavi unit. This was found through visual inspection, however, because echoes within the BU are too sporadic to be automatically picked, so further data processing and analysis is needed to confirm the result. As first results we found that the BU has a dielectric constant significantly greater than that of water ice, and thus that it contains a much larger dust fraction than the NPLD above them. We find, however, that no single value can produce the correct topographic relationship over the whole BU, which implies either that the topography of Planum Boreum beneath the polar cap is not a regular continuation of the topography outside the cap, or that the dielectric permittivity of the BU, and thus its dust content, is laterally inhomogeneous. Work to verify these two hypotheses is currently ongoing.
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ABSTRACT This paper describes the detection, the analysis, and the possible solutions of an anoma... more ABSTRACT This paper describes the detection, the analysis, and the possible solutions of an anomaly event that occurred in MARSIS science data: the MARSIS Data Bad Time Stamp.
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This presentation will be about the overall science performance and results of the shallow radar ... more This presentation will be about the overall science performance and results of the shallow radar SHARAD on the MRO mission.
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The Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) on the Mars Express orbi... more The Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) on the Mars Express orbiter obtained data over the North Polar Layered Deposits (NPLD) and found that they were easily penetrated to their base by the radar signal, and that the deposits must be nearly pure ice. In addition, there was no evidence for deflection of the lower contact of the NPLD with the substrate, implying a thick elastic lithosphere beneath that sector of the NPLD. Here we report on new MARSIS observations of the South Polar Layered Deposits ...
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Science, 2018
Liquid water under Mars' southern ice cap Mars is known to host large quantities of water in ... more Liquid water under Mars' southern ice cap Mars is known to host large quantities of water in solid or gaseous form, and surface rocks show clear evidence that there was liquid water on the planet in the distant past. Whether any liquid water remains on Mars today has long been debated. Orosei et al. used radar measurements from the Mars Express spacecraft to search for liquid water in Mars' southern ice cap (see the Perspective by Diez). They detected a 20-km-wide lake of liquid water underneath solid ice in the Planum Australe region. The water is probably kept from freezing by dissolved salts and the pressure of the ice above. The presence of liquid water on Mars has implications for astrobiology and future human exploration. Science , this issue p. 490 ; see also p. 448
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Papers by Alessandro Frigeri